Long wire balun question
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I put up a 125' long wire antenna at 35' this weekend. It tunes up well
10m-80m running directly to the KAT2. I am wondering if I should ad an unbalaced feedline and a 4:1 current balun between the tuner and antenna. Would this reduce potential RF in the shack or change the antenna's efficiency? John K5XTX _________________________________________________________________ Dont just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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John,
There is not much you can do about the antenna efficiency short of changing the physical characteristics of the antenna itself - if it works, it works and the voltage/current relationship at the feedpoint will determine its impedance and how easily it can be matched. A balun does not make sense to me with a long wire antenna fed directly out of the tuner, but an efficient and effective RF ground does make a lot of sense. If you suspect any problems with RF feedback, try adding a 1/4 wave wire for each band at the shack (tuner output) location. The wires should be separated at the ends and since they may have a high RF voltage on them, be certain that they are well insulated and cannot be contacted by persons unaware of the danger. 73, Don W3FPR ----- Original Message ----- > I put up a 125' long wire antenna at 35' this weekend. It tunes up well > 10m-80m running directly to the KAT2. I am wondering if I should ad an > unbalaced feedline and a 4:1 current balun between the tuner and antenna. > Would this reduce potential RF in the shack or change the antenna's > efficiency? _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by John Rader
Don,
I currently have the antenna attached to a good earth ground and will add radials for each band per your suggestion. I understand that radials can significantly improve the efficiency of monopole antennas. Should I try to add more then one radial per band? Thanks, John K5XTX _________________________________________________________________ Overwhelmed by debt? Find out how to Dig Yourself Out of Debt from MSN Money. http://special.msn.com/money/0407debt.armx _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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John,
For your long wire situation, one set should be sufficient to provide adequate RF grounding at your tuner loaction - or if you prefer you could use an artificial ground instead (see recent discussion on the reflector). The situation with a monopole antenna is quite different than feeding a long wire -- For tuned radials on a vertical, you CAN use only one radial per band, but the radial will radiate (and pick up signals too - it works both ways) which will add a horizontally polorized component to the radiation pattern. Adding a second radial physically opposite the first will create the same phase currents in the two radials (relative to the base of the vertical) and the radiation will cancel, IF and only if they are physically opposite. No real magic involved!!! 73, Don W3FPR ----- Original Message ----- > Don, > I currently have the antenna attached to a good earth ground and will add > radials for each band per your suggestion. I understand that radials can > significantly improve the efficiency of monopole antennas. Should I try to > add more then one radial per band? > Thanks, > John K5XTX _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by John Rader
Have used vertically polarized antennas extensively "in the field" with added
radials/counterpoise wires. Usually laid on the ground. Primarily QRP operations, where a small change can be detected readily! I've found TWO radials the best compromise. Big improvement over one! Adding more give slight changes for the better, but nothing of a large difference. I'd say four would be the maximum needed. More is always better as regards radilas, but after 4 the returns increasingly diminish. 73, Sandy W5TVW ----- Original Message ----- From: "John Rader" <[hidden email]> To: <[hidden email]> Cc: "Elecraft Discussion List" <[hidden email]> Sent: Monday, August 02, 2004 10:03 AM Subject: Re: [Elecraft] Long wire balun question | Don, | I currently have the antenna attached to a good earth ground and will add | radials for each band per your suggestion. I understand that radials can | significantly improve the efficiency of monopole antennas. Should I try to | add more then one radial per band? | Thanks, | John K5XTX | | _________________________________________________________________ | Overwhelmed by debt? Find out how to ‘Dig Yourself Out of Debt’ from MSN | Money. http://special.msn.com/money/0407debt.armx | | _______________________________________________ | Elecraft mailing list | Post to: [hidden email] | You must be a subscriber to post to the list. | Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft | Help: http://mailman.qth.net/subscribers.htm | Elecraft web page: http://www.elecraft.com | | _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by John Rader
As others have noted two radials is a minimum. They should be used in
balanced numbers, pairs. You also may benefit to have one radial in each direction you wish to work, as that increases the signals from that direction when you have only one radial in that direction. Pairing radials is supposed to balance return currents and then the radials do not radiate, but that depends on the actual balance which is a combination of the radial and its ground together as a system. -Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Sandy W5TVW
OK, it's my turn -
On the long wire/vertical antenna topic: For field day, we often use vertical antennas for 80 and 40 meters. These are full size verticals, made from aluminum tower sections plus tubing "stingers" to get the right length. There is no reason that these same ideas cannot be used for the higher bands as well. Each vertical antenna is surrounded by a batch of 16 to 24 radials, made from plastic jacketed multistrand wire about 16 gauge (exact gauge unimportant), and cut to some random size between 1/8 and 3/8 wavelength, laid on the ground. We use nails or weights on the far ends to keep the wires more or less flat on the ground. We then put an antenna coupler box right at the base of the antenna, and use the MFJ antenna analyzer to adjust the system for resonance at the desired operating frequency. BTW - if the MFJ box is connected directly to the antenna, it usually shows about 35-40 Ohms impedance at resonance - exactly what a 1/4 wave vertical should read! All of the radial wires are connected to a common ground stake at the antenna base, of course. The ground stake need not be very long - we use it just to provide a common mechanical tie point. If you are using an end fed wire (frequently mis-named a long wire), the principles are exactly the same. Any end fed antenna, if operated against earth ground, needs a fairly good ground system to be effective. Antennas with one radial (or two, or three or four) may tune up easily and show good SWR, but then so does a dummy load. An interesting aside - to be a true "long wire", an antenna must be at least one wavelength long, preferably several wavelengths. The field day antennas are series fed (the base is insulated from ground) via the antenna coupler. For KW levels, we use a Drake MN-2000 or similar. For QRP Levels, any of the small tuners should do the job equally well. The proof, as they say, is in the doing. These antennas really kick butt and take names. Remember, on Field Day, we here in Alaska are fighting almost 24 hours of Summer daylight, and we still manage a respectable number of contacts on both 80 and 40. You don't have to build your antenna out of tower sections -- hanging wire from a tree or other support will work just as well. But, there really is a difference between one (or two) radials and 16 to 24 - there are some charts in the antenna books that detail how much "return on investment" you get from differing numbers and lengths of radials. The way I read these charts, 16 to 24 random length radials represents the best compromise between performance and effort/cost. Yes, when trying to fit all this into a pack, it can be challenging, but small gauge wire can be wrapped around a stick or other "media" (discarded fishing leader spools are ideal), and kept in a zip-lock bag. And you don't necessarily need to pack an antenna coupler - the one in the K2 (or whatever you have) will handle the task just fine. Using a Balun might work, and it might not. In this situation, I would opt for not using the Balun and just go for a direct connection. If the antenna feed point is some distance away, use coax cable, and be sure to connect the distant end of the coax to ground. Unfortunately, this makes for an antenna that works well only on odd-numbered 1/4 wavelengths. A better approach for portable operations would probably be direct feed (the antenna attached directly at the rig) - which gives the possibility of multiband operation with an antenna tuner, such as the one built into the K2 or other rig, assuming you have that option installed. However, for semi-permanent installations, there is no reason you cannot use the old "multiple element" trick connecting two (or more) 1/4 wave elements for different bands to the same feed point. If you do this, try to keep the "far" ends separate from one another by at least a few inches, if possible. Antennas using traps are also a good possibility. Traps can be made very small for low power operation, and a cleverly designed antenna for portable use can use some of all of the traps to operate on some of all of the bands for which it was built. Good hunting! 73 - Jim. KL7CC Sandy W5TVW wrote: >Have used vertically polarized antennas extensively "in the field" with added >radials/counterpoise wires. Usually laid on the ground. >Primarily QRP operations, where a small change can be detected readily! >I've found TWO radials the best compromise. Big improvement over one! >Adding more give slight changes for the better, but nothing of a large difference. >I'd say four would be the maximum needed. More is always better as regards >radilas, but after 4 the returns increasingly diminish. > >73, >Sandy W5TVW >----- Original Message ----- >From: "John Rader" <[hidden email]> >To: <[hidden email]> >Cc: "Elecraft Discussion List" <[hidden email]> >Sent: Monday, August 02, 2004 10:03 AM >Subject: Re: [Elecraft] Long wire balun question > > >| Don, >| I currently have the antenna attached to a good earth ground and will add >| radials for each band per your suggestion. I understand that radials can >| significantly improve the efficiency of monopole antennas. Should I try to >| add more then one radial per band? >| Thanks, >| John K5XTX >| >| _________________________________________________________________ >| Overwhelmed by debt? Find out how to ‘Dig Yourself Out of Debt’ from MSN >| Money. http://special.msn.com/money/0407debt.armx >| >| _______________________________________________ >| Elecraft mailing list >| Post to: [hidden email] >| You must be a subscriber to post to the list. >| Subscriber Info (Addr. Change, sub, unsub etc.): >http://mailman.qth.net/mailman/listinfo/elecraft >| Help: http://mailman.qth.net/subscribers.htm >| Elecraft web page: http://www.elecraft.com >| >| > >_______________________________________________ >Elecraft mailing list >Post to: [hidden email] >You must be a subscriber to post to the list. >Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft >Help: http://mailman.qth.net/subscribers.htm >Elecraft web page: http://www.elecraft.com > > > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: Long wire balun question
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Ya' make my drool, Jim, thinking that I'd have a yard nearly big enough to
stick up a 60 or 120 foot full-size vertical plus a bunch of 60 to 120 foot radials around the base for 80 or 160 meters. I even have to dream of having enough room to do that on 40 at some future date. If an end fed wire is 1/2 wavelength long, the ground is unimportant for efficient operation. The impedance at the feed end will be in the thousands of ohms. Unfortunately, the whole rig is likely to be at the same voltage as the end of any dipole and that's not so good unless you relish the feel of RF burns and a tendency for logic circuits to go "freaky" with stray RF <G>. I look at an antenna system as three resistors in series - which it is once the reactance is tuned out. One resistor is the radiation resistance of the antenna. All the power consumed in this resistor becomes electromagnetic waves - the good stuff! The other two resistors are the ground resistance and the resistance in the conductors of the antenna, including the conductors in the antenna tuning network, any loading coils, feedlines, etc. In most cases, the conductor resistance is small - a total of a couple of ohms. Still that can be significant if hundreds of amps of RF are flowing through the wire, such as can happen on a feedline at very high SWR's. That's basically why open wire feedline is so much better than coaxial feedlines. It has a higher impedance than coax, so the currents flowing at the current loops is proportionately less at the current loops and so the resistance losses are less. The ground resistance is a different matter, and where most of the losses in an antenna working "against ground" come in. Of course, the easiest way to eliminate the ground resistance is to use a balanced antenna that does not require an RF ground, but sometimes that isn't practical. If a ground connection needed, how good it must be depends a great deal on the radiation resistance of the antenna. For example, if you drive a stake in the ground, or throw a hunk of wire on the ground, you can expect to see a ground resistance of at least a couple of hundred ohms. Say 300 ohms for a working number. If the antenna is 1/2 wavelength long, it might show a radiation resistance of say 3000 or 4000 ohms. Remember, those resistances are in series. So, for a given RF current, about 10% of the power is dissipated in the ground and 90% of the power is dissipated in the radiation resistance and becomes the electromagnetic wave headed off to that DX we're trying to raise. 90% is an very efficient antenna! On the other hand, if the antenna is only 1/8 wave long, or less, the radiation resistance might be as low as 2 or 3 ohms. Now, with a 300 ohm ground, about 99% of the RF power is dissipated in the ground connection and only 1% or 2% becomes the electromagnetic wave "tickling the ionosphere". "Tickling" is the right word unless you're running a huge amount of power. Of course, most antennas are somewhere in between, but all end-fed antennas live by the same rules. Divide the ground resistance into the radiation resistance, and the bigger the number the more efficient the antenna. Unfortunately for most of us, when we use an end-fed wire we're restricted to a length of 1/4 or less, so the ground resistance is crucial for decent efficiency. Most of us have a hard time measuring ground resistance, but one way to estimate it is just like Jim did. He knows that a 1/4 wave radiator should show a radiation resistance of about 35 ohms. When he sees a total resistance of about that using an antenna analyzer at the feed point, he is sure the ground resistance is a fairly low value. One might ask, why not use a half wave end fed wire on 20 meters and above if it's so efficient? That's a wonderful idea if your shack is up on the second floor or higher and the wire is out in the clear. Remember, the maximum radiation is taking place at the current loop on the antenna, and at 20 meters that's only 16 feet from the voltage loop. It doesn't make much sense to have an antenna that's 99% efficient but so close to the house, fences, trees and the ground that 95% of what it radiates is absorbed before it can get over the back yard fence! When an end fed wire like that is used on the higher bands, it's common to work out a way to put it up in the clear and feed it with a transmission line. One technique that used to appear in all the handbooks that hasn't been seen a lot lately is to put the half-wave antenna up in the clear, then couple one end to a parallel-connected capacitor and inductor resonant at the operating frequency. A low-impedance feedline is link-coupled to this "tank" circuit. It works FB, even though nothing is connected to the "ground" end of the tank circuit. Another very famous variation is to connect a 1/4 wave length of open-wire feedline to the end of the 1/2 wave long radiator. One side of the feedline connects to the end of the antenna wire and the other side of the open wire line connects to nothing at all. It is insulated. The 1/4 wavelength of feeder converts the very high impedance at the end of the antenna to a low impedance at the rig end, where it can be fed by any low impedance balanced source. This configuration is famous as the "Zepp" (short for Zeppelin) antenna used on the big airships of the 1930's where the wire trailed behind the dirigible and was fed at the end. In theory, if the antenna is exactly 1/2 wave long, the impedance at the end approaches infinity so no current is drawn; It's a strictly "voltage fed" wire. If there was no current, then there'd be no current flowing in either side of the open wire line at that feed point, so the feeders would be balanced and would not radiate. However, a real antenna doesn't approach infinite impedance and does draw some current. But neither are insulators perfect, so there's some leakage from the unconnected side. In practice the balance is good, provided the antenna is exactly 1/2 wavelength long. The problem with both of these configurations is that the antenna length or matching network up at the end needs to be adjusted when changing frequencies! That's not a popular idea today. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Jim Wiley ...For field day, we often use vertical antennas for 80 and 40 meters. These are full size verticals, made from aluminum tower sections plus tubing "stingers" to get the right length. There is no reason that these same ideas cannot be used for the higher bands as well. Each vertical antenna is surrounded by a batch of 16 to 24 radials...We then put an antenna coupler box right at the base of the antenna, and use the MFJ antenna analyzer to adjust the system for resonance at the desired operating frequency. BTW - if the MFJ box is connected directly to the antenna, it usually shows about 35-40 Ohms impedance at resonance - exactly what a 1/4 wave vertical should read! ... If you are using an end fed wire (frequently mis-named a long wire), the principles are exactly the same. Any end fed antenna, if operated against earth ground, needs a fairly good ground system to be effective. Antennas with one radial (or two, or three or four) may tune up easily and show good SWR, but then so does a dummy load. An interesting aside - to be a true "long wire", an antenna must be at least one wavelength long, preferably several wavelengths. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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