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Remote antenna/tuner recommendations
I plan to put up an 80m dipole fed with open wire feeder for multiband
operation. It is down the other end of the house from the shack (50 feet away), so am thinking of mounting a remote tuner under the eaves near the antenna and coax from there back to the shack. Anyone tried this combo or similar? Would appreciate recommendations/hints etc. Damien VK3RX Email: The obvious anti-spam measure; replace mycall_above with my callsign :-) |
Andy Cowley wrote:
I think you'd be better of with 102' or 135' rather than a resonant 80 m aerial as these are easier to match on some of the other bands. If he were using 300 ohm or lower Z0 feedline, you might have a point, but there's nothing to worry about with 600 ohm feedline. A resonant 80m dipole may have a feedpoint impedance of 60 ohms. An 80m dipole used on 40m may have a feedpoint impedance of 6000 ohms. In both cases, the SWR on the 600 ohm open-wire line is 10:1. A 60 ohm antenna fed with 600 ohm feedline and a 6000 ohm antenna fed with 600 ohm line are equally mismatched. If one doesn't know the length of the feedline, one can't tell the difference at the transmitter. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Andy,
Is there any particular reason why you didn't use a balun on the _output_ of the SGC rather than on the input? 73, Bob AD3K Andy Cowley wrote: ... http://www.camuw.demon.co.uk/web_sit...na/sgc230.html Which describes a similar setup in more detail. vy 73 Andy, M1EBV -- Robert L. Spooner Registered Professional Engineer Associate Research Engineer Intelligent Control Systems Department Applied Research Laboratory Phone: (814) 863-4120 The Pennsylvania State University FAX: (814) 863-7841 P. O. Box 30 State College, PA 16804-0030 |
Cecil Moore wrote:
Andy Cowley wrote: I think you'd be better of with 102' or 135' rather than a resonant 80 m aerial as these are easier to match on some of the other bands. If he were using 300 ohm or lower Z0 feedline, you might have a point, but there's nothing to worry about with 600 ohm feedline. A resonant 80m dipole may have a feedpoint impedance of 60 ohms. An 80m dipole used on 40m may have a feedpoint impedance of 6000 ohms. In both cases, the SWR on the 600 ohm open-wire line is 10:1. A 60 ohm antenna fed with 600 ohm feedline and a 6000 ohm antenna fed with 600 ohm line are equally mismatched. If one doesn't know the length of the feedline, one can't tell the difference at the transmitter. All true for the two bands. But - I did know the length of the feedline. I did do the EZNEC simulations for the impedance seen looking into the feedline for all bands and the lengths chosen do result in the 'easiest' tuning solutions. The SGC is a good bit of kit but it won't tune just any old thing, there are limits to the amount of L and C it can supply. AFAIK it uses a simple 'L' solution always. If you get the wrong feedline length and aerial length combination you can quickly get solutions which, even when in range, produce excessive voltages at 100W. The SWR on open wire line is not the significant factor here, losses are very low even at very high SWRs, but the impedance transformation required in the tuner. The SGC has fairly low voltage limits so I think 'tis best to treat it as gently as possible. I hope you don't think I'm recommending a G5RV here. The only good G5RV is one without the coax, in which case I believe the design with those lengths is due to Arthur Collins, (W9CXX?) in a prewar QST. Thanks for the input anyway, Cecil. vy 73 Andy, M1EBV |
Damien,
It's a fairly common practice, if you have a remote tuner handy. How well it will work depends a lot on the tuner, if it can handle the impedances presented to it. Not having a remote tuner, I do about the same thing except the coax run is very short and the parallel feed line is very long (lol). Are there any 'problems' with doing it this way (or your way)? Sure, the same ones you see in almost any antenna installation. There are frequencies where the RFI is more objectionable than others. What those frequencies are is going to be different for every installation / situation, so you'll just have to try it and see. Good luck... 'Doc |
Andy Cowley wrote:
If you get the wrong feedline length and aerial length combination you can quickly get solutions which, even when in range, produce excessive voltages at 100W. My point was that the wrong feedline length yields the same results for a 60 ohm antenna as it does for a 6000 ohm antenna. If you feed that 60 ohm antenna with an odd multiple of 1/4WL of feedline, the impedance seen at the tuner will be 6000 ohms. If you feed the 6000 ohm antenna with a multiple of 1/2WL, the impedance seen at the tuner will be 6000 ohms. The impedance of the antenna doesn't really matter as long as it is 1/2WL on the lowest frequency of interest. What matters is the SWR on the feedline and the length of the feedline. The feedpoint should be in the ballpark location of the current maximum point, not the voltage maximum point. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Cecil Moore wrote:
Andy Cowley wrote: If you get the wrong feedline length and aerial length combination you can quickly get solutions which, even when in range, produce excessive voltages at 100W. My point was that the wrong feedline length yields the same results for a 60 ohm antenna as it does for a 6000 ohm antenna. If you feed that 60 ohm antenna with an odd multiple of 1/4WL of feedline, the impedance seen at the tuner will be 6000 ohms. If you feed the 6000 ohm antenna with a multiple of 1/2WL, the impedance seen at the tuner will be 6000 ohms. The impedance of the antenna doesn't really matter as long as it is 1/2WL on the lowest frequency of interest. What matters is the SWR on the feedline and the length of the feedline. The feedpoint should be in the ballpark location of the current maximum point, not the voltage maximum point. My statement is assuming 600 ohm open-wire feedline as specified earlier. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Thanks for all the good info. gents.
Actually 80m is not critical for me. I was really looking for something that will work nicely on 40 - 10m (including WARC would be nice), and have been looking over the 44' design on the W4RNL page http://www.cebik.com/aledz.html The plan was to mount an SGC tuner or similar under the eaves near the antenna and run coax back through the roof, because running open wire feeder just isn't practical in the physical situation. I run 250w PEP SSB, but it would also be nice if the system could handle 120w of AM because I'm into that as well. Unfortunately that means one of the high power tuners. I have tried a Carolina Windom (40m "Beam" version") here, but while it works well on 40m it gets progressively deaf above that. H'm.... all food for thought. Damien VK3RX |
Open wire feedline is an excellent alternative and is much lower loss than
coax, especially at high SWRs. This way you can use a tuner in the shack. 73s, Evan |
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