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On Thu, 13 Jan 2005 18:56:36 -0600, Cecil Moore
wrote: W9DMK (Robert Lay) wrote: (ZZZPK) wrote: capacitance is prop to gap between plates. I think you meant that it's inversely proportional to the gap between the plates. I've made the same mistake - said "capacitance" when I really meant "capacitive reactance". No way, Cecil - you haven't made any mistakes this whole year! Bob, W9DMK, Dahlgren, VA Replace "nobody" with my callsign for e-mail http://www.qsl.net/w9dmk http://zaffora/f2o.org/W9DMK/W9dmk.html |
W9DMK (Robert Lay) wrote:
No way, Cecil - you haven't made any mistakes this whole year! Actually, it was probably a mistake to quote the "IEEE Spectrum" magazine article regarding cellphones and brain tumors. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Walter Maxwell wrote: But a ground plane antenna suffers no ground losses, so using as many as possible only applies to radials on or in the ground. Walt, W2DU Or low to the ground in terms of wavelength. But you wouldn't have to use "as many as possible". With most VHF/UHF ground planes, they are usually several waves up off the ground, and ground loss is very low. But if you take a low band ground plane, say 1/8 wave off the ground, you will need quite a few more radials than 4, to equal the benchmark of 120 radials on the ground. According to charts I've seen, and also backed up with real world results, a ground plane 1/8 wave off the ground will need appx 50-60 radials to equal the 120 on the ground. At 1/4 wave up, about 8-10 or so. At 1/2 wave up, 4, 3, or just 2 radials will all work fairly well. Of course, I would always prefer four, over two. Even if ground loss was not a factor due to being high up, the decoupling of the feedline is better with four, than two. In real world tests on VHF, I've noticed a noticable difference going from 4 radials, to say 8 , or even 10. And that was several waves up...So adding more radials does continue to improve the antenna. Probably more due to the improved decoupling of the feedline, rather than lower ground loss I would suspect...I've heard many a tale of disappointed hams having poor results with "low" ground planes, and not enough radials. But thats usually on 160,80, or 40 meters. Actually, I don't know of any ground plane users on 160, but I have heard of quite a few on 80m..Four radials at 10 ft up on 80m, is better than four radials on the ground, I think, but not by a large amount... Ground loss will be fairly substantial if the ground quality is mediocre. In my HF experience with them, the lowest I would use four radials, and expect *good* performance, would be at 1/4 wave up. When I would lower the mast to 1/8 wave up, you could see quite a difference. That was on 40m, where I ran a full size GP at 36 ft at the base, on a pushup mast. BTW, that was a great DX and late night antenna on 40m...I also had a 24 volt relay to switch a base loading coil in for 17m use as a 5/8 GP. I changed bands here in the shack, by unplugging the transformer, for 40m use..That bypassed the 17m coil. MK |
Joel Kolstadt wrote:
"Hmm...how about...three! ground radials?" Three radials should be fine where they are elevated to such height that they capture all the electric lines of force from the vertical radiator.Radials are balanced and their currents travel in offsetting directions. The radial system does not radiate itself because of its offsetting balances. Elevated radials must shield the earth from induced current. This requires few radials when the radials are far above the earth, but where the radials are near to the earth, many radials are needed to capture all the electric lines and shield the earth from lossy currents. The number of radials and their effect on pattern and efficiency of radiation from a vertical antenna is well addressed by ON4UN in "Low-Band DXing". This is found in Chapter 9 of my 2nd edition. Choose the efficiency and elevation angle you are willing to accept ftom the graphs and tables presented. Best regards, Richard Harrison, KB5WZI |
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(Robert Lay) wrote:
: : If not, what reason does he give for the technique? : : capacitance is prop to gap between plates. : : : I think you meant that it's inversely proportional to the gap between : the plates. : its still proportional.... |
ZZZPK wrote:
"Capacoitance and capacitive reactance are two separate things." Yes. But, they are inextricably related by: Capacitive reactance = 1 / 2 pi f C where pi = approx. 3.1416, f = frequency in Hertz, Capacitance is in Farads, and capacitive reactance is in Ohms. Best regards, Richard Harrison, KB5WZI |
On Sat, 15 Jan 2005 15:15:31 GMT,
(ZZZPK) wrote: (Robert Lay) wrote: : : If not, what reason does he give for the technique? : : capacitance is prop to gap between plates. : : : I think you meant that it's inversely proportional to the gap between : the plates. : its still proportional.... That doesn't qualify as worthy of a response. Bob, W9DMK, Dahlgren, VA Replace "nobody" with my callsign for e-mail http://www.qsl.net/w9dmk http://zaffora/f2o.org/W9DMK/W9dmk.html |
(Robert Lay) wrote:
: : I think you meant that it's inversely proportional to the gap between : : the plates. : : : : its still proportional.... : : : That doesn't qualify as worthy of a response. a relative bearing is a bearing. inverse proportional is a proportion. a is a subset of b get the idea ? |
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