Antenna matching
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Hi All,
Here's a proper radio question. I'm still learning about this so apologies for any gaffs. I have an Elecraft K2 (with which I'm extremely happy) running into a full size G5RV. The G5RV consists of a horizontal dipole with an impedance matching ladder line dropped from the centre. It currently terminates in a BNC connector and thence to a length of 50 ohm co-ax to the transceiver. It works tolerably well on 40 but is hopeless on other bands. I'm uncomfortable about going from the unbalanced co-ax straight into the ladder line without a balun though it's more intuition than having done the sums. I'm mean and impecunious so I want to use what I have to hand. I have an ex-military ATU built by Redifon. This takes a 50 ohm unbalanced co-ax feed and matches to an arbitrary dipole. BNC connector on the front, two terminals on the back for the antennae and a further terminal for a ground stake. Of course the best place for the ATU would be at the base of the antenna but this is impracticable so the ATU must reside in the shack. I'm only using 10W so I want as much of the transmitter power to be coupled to the antenna and waste as little as possible in the feeds. My guess at the moment is that what I need to do is to connect the 50 ohm output of theK2 into the ATU, take the ATU outputs and feed into a balun to drive the coax feed and then have a further balun at the base of the G5RV to match to the antenna. Does this sound sensible or am I completely barmy? Paul M3CRQ _______________________________________________ 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: Antenna matching
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Paul,
There is a good series of articles on the G5RV at http://www.cebik.com -- reading those would probably help a lot with the balun questions. A classic G5RV should have low SWR and exhibit gain on 20m but requires a tuner for other bands. Perhaps yours is double length? _______________________________________________ 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 Paul Bartlett
Paul,
If your "impedance matching ladder line" is of the correct length, the impedance looking into it can be close to 50 ohms on ONE band! That may well be 40 meters in your particular case. Your 'double balun' idea MAY work, but you will still be running the coax at a high SWR and that can cause significant loss, especially with the smaller diameter coax (OTOH, if you are using VERY low loss coax - like hardline - it will do just fine) For multiband operation, you will need an ATU, and it could be mounted at the current junction of your coax and ladder line (assuming that is physically possible). If that is not possible, scrounge some more of the ladderline material and run it all the way from the antenna to the tuner location. It should work FB, although you may have difficulty with some lengths of feedline on some bands (which lengths and which bands depends on the length and the feedpoint impedance of the antenna). There are several resources that you can use to predict the impedance seen at the shack end of your transmission line. For the G5RV antenna length, there are several instances of published antenna feedpoint impedance charts, and I believe there is one on L B Cebik's website www.cebik.com. Then to estimate the transformation effect of your transmission line, you can use several computer programs to do the calculation for you - TLW is available from the ARRL website, and it will do the job nicely. If you must use your current feedline configuration, use a balun at the junction of the ladder line and the coax. Whether you want to use a 1:1 balun or a 4:1 balun depends again on the antenna feedpoint impedance and the length of the ladderline, so you still need to work through the calculations to make an informed decision. Should you desire a bit of information on the transformation properties of antenna feedlines without delving into the math, take a look at my website (www.qsl.net/w3fpr) article on 'Antennas, Transmission Lines and Tuners'. 73, Don W3FPR ----- Original Message ----- > > I'm still learning about this so apologies for any gaffs. > > I have an Elecraft K2 (with which I'm extremely happy) running into a full > size G5RV. > > The G5RV consists of a horizontal dipole with an impedance matching ladder > line dropped from the centre. > > It currently terminates in a BNC connector and thence to a length of 50 > ohm > co-ax to the transceiver. > > It works tolerably well on 40 but is hopeless on other bands. > > I'm uncomfortable about going from the unbalanced co-ax straight into the > ladder line without a balun though it's more intuition than having done > the > sums. > > I'm mean and impecunious so I want to use what I have to hand. I have an > ex-military ATU built by Redifon. This takes a 50 ohm unbalanced co-ax > feed > and matches to an arbitrary dipole. BNC connector on the front, two > terminals on the back for the antennae and a further terminal for a ground > stake. > > Of course the best place for the ATU would be at the base of the antenna > but > this is impracticable so the ATU must reside in the shack. > > I'm only using 10W so I want as much of the transmitter power to be > coupled > to the antenna and waste as little as possible in the feeds. > > My guess at the moment is that what I need to do is to connect the 50 ohm > output of theK2 into the ATU, take the ATU outputs and feed into a balun > to > drive the coax feed and then have a further balun at the base of the G5RV > to > match to the antenna. > > Does this sound sensible or am I completely barmy? > _______________________________________________ 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 Paul Bartlett
Good grief, guys. Paul has the solution already. A G5RV is simply a
dipole with some magic consisting of a combination of balanced and coax line which makes it look like 50 ohms on certain frequencies. But Paul has a really nice balanced tuner. Now if it were me, I would eliminate the coax, extending the open-wire feed down to the balanced tuner. Now he will be able to tune the antenna for any frequency he wants. Open wire or ladder line has far less loss than coax, and because of this SWR really doesn't matter. In this case it is the SWR between the tuner and the radio that needs to be matched, since that is where the lossy line is. Paul, it looks like you have the solution at hand, it allows you to maintain your impecunious image and get the operating capabilities you desire. On Sep 26, 2004, at 7:53 AM, Paul Bartlett wrote: > Hi All, > > Here's a proper radio question. > > I'm still learning about this so apologies for any gaffs. Questioning things is the right way to go about things. No apologies needed, just ask! Be sure to question the answers too. Sometimes we get bad advice, but more importantly as you research and validate the advice, you give yourself the chance to learn why! > It works tolerably well on 40 but is hopeless on other bands. I have a horizontal delta loop at 30 feet that works reasonably well on all bands. It is, of course, fed by open wire line as are all my other antennas at present. > I'm mean and impecunious so I want to use what I have to hand. I have > an > ex-military ATU built by Redifon. This takes a 50 ohm unbalanced co-ax > feed > and matches to an arbitrary dipole. BNC connector on the front, two > terminals on the back for the antennae and a further terminal for a > ground > stake. Is it automated, or do you have knobs that need adjusting to get the tuning? > Of course the best place for the ATU would be at the base of the > antenna but > this is impracticable so the ATU must reside in the shack. With open-wire feed this is not necessarily bad. Open-wire or ladder line has such low loss characteristics that you could literaly run miles of it before you get the same loss figures as a relatively short length of coax. The problem with Coax is that at medium to high SWR levels you get heating of the dielectric, resulting in faulty cable and higher losses. At 10 watts this is not much of a concern, but then at that power level you want as much RF as possible getting to the antenna. This makes an even stronger case for balanced feed. > My guess at the moment is that what I need to do is to connect the 50 > ohm > output of theK2 into the ATU, take the ATU outputs and feed into a > balun to > drive the coax feed and then have a further balun at the base of the > G5RV to > match to the antenna. Nah. get rid of the coax completely. Replace it with more ladder line and you will be happy. Just remember that the key is to experiment. If you want to try something go for it. If it doesn't work, try something else. You might also want to pick up some reference books to study when you aren't operating. My favorites at the moment were written by Mr Moxon. Hopefully they will be more available on your side of the pond than mine. .. - Jack Brindle, WA4FIB ------------------------------------------------------------------------ --------------------- _______________________________________________ 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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I'll add this to Jack's excellent summary: The open wire feed line on a G5RV
is never balanced, since it's an off-center fed wire. That's good, because that feed line contributes a lot by providing vertical, low angle radiation on the lower frequencies. A horizontal wire needs to be up near 100 feet or more to produce good, low angle radiation on 80. Most hams are happy if they can get anywhere near half that. So on 40 and 80 the radiating open wire feeder is a real bonus. The higher-angle radiation from the "flat top" provides shorter skip contacts for good all-around coverage. So there's no reason to jump through hoops trying to use baluns to provide a balanced feed. I always avoid extra devices in the antenna system because ALL gadgets have some loss, it's just a matter of trying to figure out how much. In this case, I'd forget the baluns. If it would be possible to bring the open wire to the tuner, Jack has the right answer. If not, then the lowest-loss coax you can get and the shortest possible run are next best. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Jack Brindle Sent: Sunday, September 26, 2004 1:13 PM Good grief, guys. Paul has the solution already. A G5RV is simply a dipole with some magic consisting of a combination of balanced and coax line which makes it look like 50 ohms on certain frequencies. But Paul has a really nice balanced tuner. Now if it were me, I would eliminate the coax, extending the open-wire feed down to the balanced tuner. Now he will be able to tune the antenna for any frequency he wants. Open wire or ladder line has far less loss than coax, and because of this SWR really doesn't matter. In this case it is the SWR between the tuner and the radio that needs to be matched, since that is where the lossy line is. Paul, it looks like you have the solution at hand, it allows you to maintain your impecunious image and get the operating capabilities you desire. O _______________________________________________ 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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On Sun, 2004-09-26 at 13:44, Ron D'Eau Claire wrote:
> I'll add this to Jack's excellent summary: The open wire feed line on a G5RV > is never balanced, since it's an off-center fed wire. That's good, because A G5RV is centre-fed. -- 73, Brian VE7NGR _______________________________________________ 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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You are SO RIGHT, Brian!
I was reading G5RV and thinking "Windom". Thanks! Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Brian Mury Sent: Sunday, September 26, 2004 1:53 PM To: [hidden email] Subject: RE: [Elecraft] Antenna matching On Sun, 2004-09-26 at 13:44, Ron D'Eau Claire wrote: > I'll add this to Jack's excellent summary: The open wire feed line on > a G5RV is never balanced, since it's an off-center fed wire. That's > good, because A G5RV is centre-fed. -- 73, Brian VE7NGR _______________________________________________ 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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