open wire feeders
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Those of you using open wire feed lines. How do you keep RF out of the
shack? 73 George/W2BPI K2/100 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Sssshhh don't tell them, they are supposed to radiate.... I've got three runs, into my shack, and as long as the current in each side of the line is balanced, the radiation is minimal... and of course installation concerns are observed, with spacing away from metal and the like.. They work fine... balanced. Good Luck and Happy New Year!! --... ...-- Dale - WC7S in Wy > From: [hidden email] > Date: Sat, 31 Dec 2011 11:25:54 -0500 > To: [hidden email] > Subject: [Elecraft] open wire feeders > > Those of you using open wire feed lines. How do you keep RF out of the > shack? 73 George/W2BPI K2/100 > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by George
George,
Short question, long answer follows -- Do to constraints at home, I no longer use open wire or ladder line feeders, but when I did use them, I found several things were true if you did not want them to radiate (and create RF in the Shack). My first rule is to use balanced antennas - off center fed antennas are famous for feedline radiation and RF in the shack. The second rule is to run the feedline away from the antenna at right angles for as great a length as you can manage, but certainly for a quarterwavelength - The feedline can pick up radiation from the antenna if this rule is not followed. Third is to run the feedline correctly - use nice gentle bends if you must change direction, support it using as few hangers as possible (if you can put the feedline under tension, you can get away with very few supports) but support it so it is stable even in the wind. Do not run it parallel to other conductors, but you may cross a conductor at right angles if necessary. The line should be spaced away from other objects by at least 3 times the spacing of the conductors. Lastly, If I could, use a true balanced tuner, link coupled is best, so if you see a Johnson Matchbox at a hamfest, get it. If you must use an unbalanced tuner, use a good balun at the output (see K9AY's info on baluns). BTW, do not assume that a 4:1 balun is the thing to use, the feedpoint impedance in the shack can vary wildly from very low to very high. If you do encounter a high impedance feedpoint on any band, that will place a high RF voltage point at the shack end - add or subtract some feedline to bring the feedpoint impedance down. If you do not understand how the feedpoint impedance changes with the length, take a look at the Antenna article on my website www.w3fpr.com 73, Don W3FPR On 12/31/2011 11:25 AM, [hidden email] wrote: > Those of you using open wire feed lines. How do you keep RF out of the > shack? 73 George/W2BPI K2/100 > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Don...
Many hams - as I have done/do - use coax to get out of the house, connect it to a balun, and then connect twinlead from the balun to an antenna. There is a lot of commentary on this setup, but perhaps another time through would be helpful. The questions arise: - if the coax is short, say under ten feet, is this setup more or less equivalent to running the twinlead all the way from the antenna to the transmitter? - again, if the coax is short, will RG8 or 213 be sufficient to the task? - is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]? Happy New Year, and thanks for your contributions here on the Elecraft reflector. ...robert On 12/31/2011 17:42, Don Wilhelm wrote: > George, > > Short question, long answer follows -- > > Do to constraints at home, I no longer use open wire or ladder line > feeders, but when I did use them, I found several things were true if > you did not want them to radiate (and create RF in the Shack). > My first rule is to use balanced antennas - off center fed antennas are > famous for feedline radiation and RF in the shack. > The second rule is to run the feedline away from the antenna at right > angles for as great a length as you can manage, but certainly for a > quarterwavelength - The feedline can pick up radiation from the antenna > if this rule is not followed. > Third is to run the feedline correctly - use nice gentle bends if you > must change direction, support it using as few hangers as possible (if > you can put the feedline under tension, you can get away with very few > supports) but support it so it is stable even in the wind. Do not run > it parallel to other conductors, but you may cross a conductor at right > angles if necessary. The line should be spaced away from other objects > by at least 3 times the spacing of the conductors. > > Lastly, If I could, use a true balanced tuner, link coupled is best, so > if you see a Johnson Matchbox at a hamfest, get it. If you must use an > unbalanced tuner, use a good balun at the output (see K9AY's info on > baluns). BTW, do not assume that a 4:1 balun is the thing to use, the > feedpoint impedance in the shack can vary wildly from very low to very high. > > If you do encounter a high impedance feedpoint on any band, that will > place a high RF voltage point at the shack end - add or subtract some > feedline to bring the feedpoint impedance down. > If you do not understand how the feedpoint impedance changes with the > length, take a look at the Antenna article on my website www.w3fpr.com > > 73, > Don W3FPR > > On 12/31/2011 11:25 AM, [hidden email] wrote: >> Those of you using open wire feed lines. How do you keep RF out of the >> shack? 73 George/W2BPI K2/100 >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > -- Robert G. Strickland, PhD, ABPH - KE2WY [hidden email] Syracuse, New York, USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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The coax will be exposed to whatever SWR is on the feedline. The SWR will
vary as a function of line length, characteristics, antenna length/height and frequency of operation. If you know the SWR you will be seeing then you can use coax loss calculators available online to see what the impact of the various coax types and lengths. The use of a balun is generally counterproductive for this purpose. A common mode choke is what you want. Google W9CF for math to show theoretical details and K9YC for practical implementations. A practical CM choke is inexpensive and easy to build. 73 jim ab3cv ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by RobertG
Robert,
When that is done, I recommend doing several things: 1 - prune the balanced feedline until the feedpoint impedance is close to the characteristic impedance of the coax. You may not be able to accomplish that on all desired bands, so plan to switch in extra feedline to accomplish that goal. 2 - If the parallel line feedpoint impedance is in the 150 to 300 ohm range, use a 4:1 balun (make certain it is a common mode choke type as has been recently pointed out. 3 - If the parallel line impedance is in the 25 to 100 ohm range, use a 1:1 balun (or more properly a common mode choke - see Jim Brown's Balun articles. Actually, the only thing accomplished by this technique is that the coax can be run into the shack more readily without regard for keeping the line away from other objects. It is just as easy to use two parallel lengths of coax with the shield grounded and the balanced feedline connected to the two center conductors. That will produce a shielded balanced line that you can bring into the shack just like a single coax. As I said, the best is a link coupled balanced tuner, and no balun is needed at all. The link coupling takes away concerns about common mode current and does a lot to cure RF in the Shack - unfortunately such couplers are "scarce as hen's teeth", and although you can easily build your own on a piece of wood (no need for shielding), they are not readily adaptable to bandswitching. Most of the ones I have built use plug-in coils. 73, Don W3FPR On 12/31/2011 1:40 PM, Robert G. Strickland wrote: > Don... > > Many hams - as I have done/do - use coax to get out of the house, > connect it to a balun, and then connect twinlead from the balun to an > antenna. There is a lot of commentary on this setup, but perhaps another > time through would be helpful. The questions arise: > - if the coax is short, say under ten feet, is this setup more or less > equivalent to running the twinlead all the way from the antenna to the > transmitter? > - again, if the coax is short, will RG8 or 213 be sufficient to the task? > - is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]? > > Happy New Year, and thanks for your contributions here on the Elecraft > reflector. > > ...robert > > On 12/31/2011 17:42, Don Wilhelm wrote: >> George, >> >> Short question, long answer follows -- >> >> Do to constraints at home, I no longer use open wire or ladder line >> feeders, but when I did use them, I found several things were true if >> you did not want them to radiate (and create RF in the Shack). >> My first rule is to use balanced antennas - off center fed antennas are >> famous for feedline radiation and RF in the shack. >> The second rule is to run the feedline away from the antenna at right >> angles for as great a length as you can manage, but certainly for a >> quarterwavelength - The feedline can pick up radiation from the antenna >> if this rule is not followed. >> Third is to run the feedline correctly - use nice gentle bends if you >> must change direction, support it using as few hangers as possible (if >> you can put the feedline under tension, you can get away with very few >> supports) but support it so it is stable even in the wind. Do not run >> it parallel to other conductors, but you may cross a conductor at right >> angles if necessary. The line should be spaced away from other objects >> by at least 3 times the spacing of the conductors. >> >> Lastly, If I could, use a true balanced tuner, link coupled is best, so >> if you see a Johnson Matchbox at a hamfest, get it. If you must use an >> unbalanced tuner, use a good balun at the output (see K9AY's info on >> baluns). BTW, do not assume that a 4:1 balun is the thing to use, the >> feedpoint impedance in the shack can vary wildly from very low to very high. >> >> If you do encounter a high impedance feedpoint on any band, that will >> place a high RF voltage point at the shack end - add or subtract some >> feedline to bring the feedpoint impedance down. >> If you do not understand how the feedpoint impedance changes with the >> length, take a look at the Antenna article on my website www.w3fpr.com >> >> 73, >> Don W3FPR >> >> On 12/31/2011 11:25 AM, [hidden email] wrote: >>> Those of you using open wire feed lines. How do you keep RF out of the >>> shack? 73 George/W2BPI K2/100 >>> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[hidden email] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by RobertG
Would it were that all "baluns" were the same. There is a limit to how
much blocking can be put between the balanced line and the coax. What most people fail to account for in the balanced line business is how much COMMON MODE current and voltage there can be. Unbalanced current on the feedline is always common mode current. Most will never see it in a model because they DO NOT add the single conductor that gives the model the common mode path that exists in reality. I had a blocking need at the end of a long 450 window line run. I was doing very poorly in the matching, balancing business for this antenna. When I modeled everything literally, EZNEC said that with a PERFECT block there was 955 volts RF RMS across the common connection in the balun !!! Gads. There is NO balun ANYBODY makes that will stand up to that and work right. It will (and did) smoke ferrites. Conversion to an isolation transformer (which is not a balun...not inherently broadband) stopped all the issues. Of course an isolation transformer is a single band solution without a tuner, as the series inductance has to be tuned out somehow. Running the balanced line to an isolation transformer (not a balun) IMMEDIATELY followed by a tuner will handle common mode current. But then, who talks about isolation transformers any more. 73, Guy. On Sat, Dec 31, 2011 at 1:40 PM, Robert G. Strickland <[hidden email]>wrote: > Don... > > Many hams - as I have done/do - use coax to get out of the house, > connect it to a balun, and then connect twinlead from the balun to an > antenna. There is a lot of commentary on this setup, but perhaps another > time through would be helpful. The questions arise: > - if the coax is short, say under ten feet, is this setup more or less > equivalent to running the twinlead all the way from the antenna to the > transmitter? > - again, if the coax is short, will RG8 or 213 be sufficient to the task? > - is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]? > > Happy New Year, and thanks for your contributions here on the Elecraft > reflector. > > ...robert > > On 12/31/2011 17:42, Don Wilhelm wrote: > > George, > > > > Short question, long answer follows -- > > > > Do to constraints at home, I no longer use open wire or ladder line > > feeders, but when I did use them, I found several things were true if > > you did not want them to radiate (and create RF in the Shack). > > My first rule is to use balanced antennas - off center fed antennas are > > famous for feedline radiation and RF in the shack. > > The second rule is to run the feedline away from the antenna at right > > angles for as great a length as you can manage, but certainly for a > > quarterwavelength - The feedline can pick up radiation from the antenna > > if this rule is not followed. > > Third is to run the feedline correctly - use nice gentle bends if you > > must change direction, support it using as few hangers as possible (if > > you can put the feedline under tension, you can get away with very few > > supports) but support it so it is stable even in the wind. Do not run > > it parallel to other conductors, but you may cross a conductor at right > > angles if necessary. The line should be spaced away from other objects > > by at least 3 times the spacing of the conductors. > > > > Lastly, If I could, use a true balanced tuner, link coupled is best, so > > if you see a Johnson Matchbox at a hamfest, get it. If you must use an > > unbalanced tuner, use a good balun at the output (see K9AY's info on > > baluns). BTW, do not assume that a 4:1 balun is the thing to use, the > > feedpoint impedance in the shack can vary wildly from very low to very > high. > > > > If you do encounter a high impedance feedpoint on any band, that will > > place a high RF voltage point at the shack end - add or subtract some > > feedline to bring the feedpoint impedance down. > > If you do not understand how the feedpoint impedance changes with the > > length, take a look at the Antenna article on my website www.w3fpr.com > > > > 73, > > Don W3FPR > > > > On 12/31/2011 11:25 AM, [hidden email] wrote: > >> Those of you using open wire feed lines. How do you keep RF out of the > >> shack? 73 George/W2BPI K2/100 > >> > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[hidden email] > > > > This list hosted by: http://www.qsl.net > > Please help support this email list: http://www.qsl.net/donate.html > > > > -- > Robert G. Strickland, PhD, ABPH - KE2WY > [hidden email] > Syracuse, New York, USA > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by RobertG
I have a balanced fed balanced length dipole. The dipole sits atop
40' of rope guyed fiberglass pole. 55 feet on each side. About 45 feet of 450 ohm balanced line comes into the far end of the back side of the house into one of my 7 , um, junk/storage rooms! There I have a 4:1 balun, which then hits a LDG RT-11 antenna tuner. The RT-11 is a remote controlled tuner. A little control sits in the shack. I tap the tune button on the K3, tap the tune button on the little LDG box and the tuner works. I don't know what the experts or Enzec says about this antenna, but I'll tell you that it works, and very well. So much so that I have a second copy of it in a plastic box in the garage that is used on field day. Anyone who uses that antenna at field day is very impressed. I have about 60 feet of rg-8x from the shack to the remote antenna tuner. Zero RF in the shack, ever. No chokes on any feed line. And it has worked K5C and VP6DX and others on all bands from 80 to 10. I have no idea how the success ratio is divided between the antenna and K3! Now, maybe we can build a simple interface to the KAT500 tuner to convert its need to communicate to the K3 to a balanced rs-485 configuration. That would allow us to remote the tuner hundreds of feet. I would then use that at my remote base site. But that antenna does work. Tom K8TB ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Don Wilhelm-4
Ron,
Thank you for the references. I found the information very interesting to say the least. Parts here would be an issue, but the construction and methods to check and adjust the tuner were most valuable. Now just to have some drawer space! As Don had mentioned in an earlier post, getting the balanced feed line to the shack can be quite a challenge, especially for those of us who have rebar enforce concrete with no simple method to feed through the wall. I think even the double coax systems to be a difficulty especially when having to drill into the side of the house and miss all the surrounding materials, esp. the rebar. This may not be my last shack so I have filed the project and will see what I can find for parts now before this stuff gets extinct. That copper tubing is really becoming scarce and I am not even sure how much copper really is in the tubing and what impact it has on the coils if not of the same purity. I did locate some surplus Comet vacuum capacitors in the $800+ range??!!! 73, Bill K9YEQ -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Ron D'Eau Claire I have often had a voltage loop at the rig end of my open wire feed line and have never experienced a problem with stray RF, even though a neon bulb lying near the ATU blinked as I sent CW. HOWEVER - I do use a homebrew link-coupled ATU with open wire line. It lacks the dual-differential tuning capacitors that Johnson used to adjust virtual coil taps for the feed line in the Matchbox, but mine has a much wider matching range using actual taps on the coil. (The Johnson Matchbox is only rated to match balanced lines with impedances up to 1200 ohms). Link coupling has a number of advantages including a high order of suppression of stray signals since there is no direct electrical connection between the antenna and the rig. Everything coming in (or going out) has to pass magnetically across the link coupling and through the tuned circuit. That's especially handy if you live near a broadcast station. And of course, you can use the same tuner with unbalanced lines (e.g. coax). Just connect the shield to the center of the coil and tap the center conductor out to either side. WZ5Q has a great example of a beautifully link coupled tuner at: http://www.wz5q.net/index/shack_data/tuna.htm It's bigger and neater than mine but the circuit is the same right down to the RF current sensors. Ron AC7AC -----Original Message----- If you do encounter a high impedance feedpoint on any band, that will place a high RF voltage point at the shack end - add or subtract some feedline to bring the feedpoint impedance down. If you do not understand how the feedpoint impedance changes with the length, take a look at the Antenna article on my website www.w3fpr.com 73, Don W3FPR Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Hi,
Wow, gorgeously constructed tuner by WZ5Q. I wish I had one. But is the center tap of the secondary really connected to the coax input ground as indicated in the schematic? AB2TC - Knut
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Actually, that is a *brute* tuner, capable of mega kilowatts! Something
a bit scaled down is more practical IMHO. BUT the design is exactly what I was referring to. The design principle is the same, but I have always used dual section capacitors so the shaft is at RF Ground potential (no insulators required). Number 12 or 14 wire in the coil should be all that is required for normal legal power levels. WZ5Q would probably consider it "wimpy", but for practical purposes, it should be just fine. 73, Don W3FPR On 12/31/2011 5:01 PM, ab2tc wrote: > Hi, > > Wow, gorgeously constructed tuner by WZ5Q. I wish I had one. But is the > center tap of the secondary really connected to the coax input ground as > indicated in the schematic? > > AB2TC - Knut > > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Don Wilhelm-4
On 12/31/2011 11:34 AM, Don Wilhelm wrote:
> It is just as easy to use two parallel lengths of coax with the shield > grounded and the balanced feedline connected to the two center > conductors. That will produce a shielded balanced line that you can > bring into the shack just like a single coax. In the days when the FCC's mobile direction-finders consisted of a rotatable "Finch" loop, the feedline was Twinax, two conductors within a single shield resulting in a cable about the size of RG-11, fed into the balanced antenna input of the receivers in use. I don't know if the cable or its connectors are readily available these days. -- 73 de K2ASP - Phil Kane Elecraft K2/100 s/n 5402 >From a Clearing in the Silicon Forest Beaverton (Washington County) Oregon ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: open wire feeders
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In reply to this post by Guy, K2AV
Hi Guy,
OK I'll own up. I have used "isolation transformers" as recently as this year before leaving Scotland, in the feed system of a Laport Rhombic (dual rhomboid). I used this antenna, aimed towards your side of the pond, to listen for VHF signals between 88 MHz and 148 MHz in the pious hope that a 2m contact (terrestrial) could be made across the Atlantic. To return to the topic of Open Wire Feeders. The 4 wire cross-connected type of open wire feeder is a viable and less costly alternative to low loss coax, especially when used at VHF and the runs are long. I have used this type of line during the past decades. Without going into the maths or the results of controlled measurements, in terms of "noise pickup" and radiation from the feeder the 4 wire line provides much better performance than its 2 wire cousin. Also it is practical to dimension and build a 4 wire line (wire spacing etc) whose Zo is 200 ohms - a useful value for transforming to 50 ohms at the Tx/ Rx end of the line. However on the downside this type of line should be kept under tension, which obviously allows the use of fewer spacers, and it does require some "housekeeping" to remove any wind blown twigs if there are trees nearby. Happy New Year. 73, Geoff LX2AO On December 31, 2011 at 8:45 PM, Guy Olinger K2AV wrote: <snip> > But then, who talks about isolation transformers any more. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Don Wilhelm-4
On 12/31/2011 9:42 AM, Don Wilhelm wrote:
> My first rule is to use balanced antennas - off center fed antennas are > famous for feedline radiation and RF in the shack. RIGHT! Some important fundamental principles here. 1) A two wire line will be balanced ONLY if what is connected at each end is balanced. 2) Any imbalance in the antenna or the turner will cause un-equal currents in the two conductors. The DIFFERENCE between those two currents is called common mode current, and it will radiate, just like the current on the outside of coax. Feeding an antenna off-center creates a LOT of imbalance, which, as Don has said, puts a lot of common mode current on the line, and thus a lot of RF in the shack. 3) Most practical ham antennas, even those that we try to build as balanced antennas, like center-fed dipoles, are unbalanced a bit by their surroundings -- trees, buildings, unequal height, sloping ground under the antenna, etc. 4) This unbalance may not hurt the antenna's performance too much, but because antennas work essentially the same on both TX and RX, that feedline picks up noise and couples it to the receiver. THAT'S why we need a really good common mode choke at the feedpoint of ANY antenna. There's another HUGE problem with off-center-fed antennas -- the unbalanced current on the feedline -- ANY feedline, including any form of parallel wire line -- can be quite large, and can easily cause even a very good common mode choke to fail with high power. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by RobertG
The only LARGE problem that rears it's ugly head is a large amount of
inductive or capacitive reactance often times present that the actual balun has to cope with. This dissipates power no mater what the VSWR on the open wire line happens to be. You WILL NOT be transforming a 50 ohm line to a 200 ohm line (4:1 transformer) OR a 50 ohm line to a 450 ohm line (9:1) There will be always some reactance present. If you run higher power (500-1000 watts or more) this may actually ultimately destroy the balun transformer itself! This effect doesn't seem to be as radical with "choke" type (ferrite beads over a run of coax) compared to a "transformer" type balun. I think you would be better off in the long run, in this instance with a "choke" type balun and use a coupler between to rig "power source" and the "load" Choke balun/ladder line/open wire feeder. The idea is to keep the coax part as short as possible and let the tuner deal with the oddball reactances that occur on the line. Over the years I have had troubles and seen other with same syndrome trying to let a transformer balun compensate for a impedance transformation under the duress of a HIGH reactance present which seems to destroy things eventually, AND radiate less useful power rather than it would other wise if the reactance was "tuned out". I hope I am making myself clear. In my old setup before my XYL had a stroke, causing me to stop using a homebrewed balance line tuner (ladder line feeder entering the shack directly) worked most effectively. The dipole was 135' long at 50' and fed with about 110 feet of 450 ohm ladder line. I am unable to erect a similar antenna from the master bedroom where the rig is now and had to resort to using an end fed wire again. 73, Sandy W5TVW -----Original Message----- From: Robert G. Strickland Sent: Saturday, December 31, 2011 12:40 PM To: [hidden email] Subject: Re: [Elecraft] open wire feeders Don... Many hams - as I have done/do - use coax to get out of the house, connect it to a balun, and then connect twinlead from the balun to an antenna. There is a lot of commentary on this setup, but perhaps another time through would be helpful. The questions arise: - if the coax is short, say under ten feet, is this setup more or less equivalent to running the twinlead all the way from the antenna to the transmitter? - again, if the coax is short, will RG8 or 213 be sufficient to the task? - is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]? Happy New Year, and thanks for your contributions here on the Elecraft reflector. ...robert On 12/31/2011 17:42, Don Wilhelm wrote: > George, > > Short question, long answer follows -- > > Do to constraints at home, I no longer use open wire or ladder line > feeders, but when I did use them, I found several things were true if > you did not want them to radiate (and create RF in the Shack). > My first rule is to use balanced antennas - off center fed antennas are > famous for feedline radiation and RF in the shack. > The second rule is to run the feedline away from the antenna at right > angles for as great a length as you can manage, but certainly for a > quarterwavelength - The feedline can pick up radiation from the antenna > if this rule is not followed. > Third is to run the feedline correctly - use nice gentle bends if you > must change direction, support it using as few hangers as possible (if > you can put the feedline under tension, you can get away with very few > supports) but support it so it is stable even in the wind. Do not run > it parallel to other conductors, but you may cross a conductor at right > angles if necessary. The line should be spaced away from other objects > by at least 3 times the spacing of the conductors. > > Lastly, If I could, use a true balanced tuner, link coupled is best, so > if you see a Johnson Matchbox at a hamfest, get it. If you must use an > unbalanced tuner, use a good balun at the output (see K9AY's info on > baluns). BTW, do not assume that a 4:1 balun is the thing to use, the > feedpoint impedance in the shack can vary wildly from very low to very > high. > > If you do encounter a high impedance feedpoint on any band, that will > place a high RF voltage point at the shack end - add or subtract some > feedline to bring the feedpoint impedance down. > If you do not understand how the feedpoint impedance changes with the > length, take a look at the Antenna article on my website www.w3fpr.com > > 73, > Don W3FPR > > On 12/31/2011 11:25 AM, [hidden email] wrote: >> Those of you using open wire feed lines. How do you keep RF out of the >> shack? 73 George/W2BPI K2/100 >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > -- Robert G. Strickland, PhD, ABPH - KE2WY [hidden email] Syracuse, New York, USA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.1901 / Virus Database: 2109/4715 - Release Date: 12/31/11 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Phil Kane-2
(:-)) I actually have a few "Twinax" connectors, both male and female.
Same as PL-259 / SO-239's, except two pins. HAPPY NEW YEAR, and ... 73! Ken On Sun, Jan 1, 2012 at 12:41 AM, Phil Kane <[hidden email]> wrote: > On 12/31/2011 11:34 AM, Don Wilhelm wrote: > >> It is just as easy to use two parallel lengths of coax with the shield >> grounded and the balanced feedline connected to the two center >> conductors. That will produce a shielded balanced line that you can >> bring into the shack just like a single coax. > > In the days when the FCC's mobile direction-finders consisted of a > rotatable "Finch" loop, the feedline was Twinax, two conductors within a > single shield resulting in a cable about the size of RG-11, fed into the > balanced antenna input of the receivers in use. > > I don't know if the cable or its connectors are readily available these > days. > -- > 73 de K2ASP - Phil Kane > Elecraft K2/100 s/n 5402 > > >From a Clearing in the Silicon Forest > Beaverton (Washington County) Oregon Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by w5tvw
Sandy,
Take the information on K9YC's balun comments to heart. Jim has done a wonderful job of measuring the isolation effect of baluns (more specifically common mode chokes). Yes, the reactive and resistive components of the feedpoint impedance play a large part in how the whole system reacts, but that can be varied with a change in the feedline length (see antenna article at www.w3fpr.com). Keep the feedpoint impedance within range and all will be well for baluns, tuners and all other parts of the antenna system. Keep in mind that the length of an unmatched feedline is critical to the success or failure of such a system. That is a factor that is seldom mentioned in posts that say a particular antenna "works well" and "loads well" - the simple fact is that the feedline type an length are critical elements in that antenna system - to say that a 100 foot dipole works great is not sufficient, one needs to state the type and length of the feedline as well as the length of the antenna. 73, Don W3FPR On 12/31/2011 9:59 PM, Sandy wrote: > The only LARGE problem that rears it's ugly head is a large amount of > inductive or capacitive reactance often times present that the actual balun > has to cope with. This dissipates power no mater what the VSWR on the open > wire line happens to be. You WILL NOT be transforming a 50 ohm line to a > 200 ohm line (4:1 transformer) OR a 50 ohm line to a 450 ohm line (9:1) > There will be always some reactance present. If you run higher power > (500-1000 watts or more) this may actually ultimately destroy the balun > transformer itself! This effect doesn't seem to be as radical with "choke" > type (ferrite beads over a run of coax) compared to a "transformer" type > balun. I think you would be better off in the long run, in this instance > with a "choke" type balun and use a coupler between to rig "power source" > and the "load" Choke balun/ladder line/open wire feeder. The idea is to > keep the coax part as short as possible and let the tuner deal with the > oddball reactances that occur on the line. > > Over the years I have had troubles and seen other with same syndrome trying > to let a transformer balun compensate for a impedance transformation under > the duress of a HIGH reactance present which seems to destroy things > eventually, AND radiate less useful power rather than it would other wise if > the reactance was "tuned out". > > I hope I am making myself clear. In my old setup before my XYL had a > stroke, causing me to stop using a homebrewed balance line tuner (ladder > line feeder entering the shack directly) worked most effectively. The > dipole was 135' long at 50' and fed with about 110 feet of 450 ohm ladder > line. I am unable to erect a similar antenna from the master bedroom where > the rig is now and had to resort to using an end fed wire again. > > 73, > > Sandy W5TVW > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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On Sat, Dec 31, 2011 at 9:35 PM, Don Wilhelm <[hidden email]> wrote:
Keep in mind that the length of an unmatched feedline is critical to the > success or failure of such a system. That is a factor that is seldom > mentioned in posts that say a particular antenna "works well" and "loads > well" - the simple fact is that the feedline type and length are critical > elements in that antenna system - to say that a 100 foot dipole works > great is not sufficient, one needs to state the type and length of the > feedline as well as the length of the antenna. > Lots of good comments. I think part of the original post asked about keeping RF out of the shack. Assuming one does a reasonable job of keeping both antenna and open-wire feedline balanced, there is a fairly easy answer. Operative word here is "reasonable"--perfection is the enemy of good enough. Don't let all the horror stories stop you from trying. People, including me, have been using balanced feeders and center-fed dipoles with good results for a very long time. Here's how to minimize RF in the shack. Make the length of one leg of the dipole plus the length of the feedline as close as possible (practical) to an odd multiple of a quarter wavelength at the frequency of interest. With a dipole, there is an RF voltage maximum at the far end. One quarter wavelength away there is a voltage minimum, another quarter wavelength away, a voltage maximum. Etcetera. An odd multiple of quarter wavelengths puts a voltage minimum in the shack. In addition to minimizing RF in the shack, this approach puts minimum RF voltage and maximum RF current in the shack. This means relatively low impedance in the shack which means relatively easy job for the tuner. This approach is easy for a single band. For multiple bands, it can be tricky. Again, perfection is the enemy of good enough--just try it. There is a useful chart in the 1974 ARRL Antenna Handbook. Chart is posted at https://picasaweb.google.com/StarrGarretson/Fig353#5692717146409405762 . L in the chart is total length of one leg of center-fed dipole plus feedline or total length of antenna plus feedline for an end-fed antenna. When using the chart, keep in mind, that 1974 was before WARC bands. After all of the above, now my experience. For fifty-six years at eleven locations, my antenna has been an eighty-meter dipole (sometimes very contorted) or inverted-V put up with whatever supports I could muster. Open-wire feedline was whatever length was convenient. Using a Johnson Viking KW Matchbox with either 700 or 1,500 watts output, I rarely had RF in the shack on all contest bands eighty through ten. The few times I did, it was simple to add a bit of feedline to solve the problem. Be a ham, try it, and have fun. 73, Hank, W6SX Mammoth Lakes, California Elevation 8083 feet in John Muir's Range of Light ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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