[Antennas] Hanging more wire.

[Terry Conboy]

At 05:39 PM 2004-02-17, Christopher Boone wrote:
>For minimum results, the 1/2wl dipole or equiv in length of wire is what 
>you want to shoot for on 80....going from 88 to 132ft will make some 
>difference in matching...it will change the RF current distribution on the 
>antenna to a better pattern than what you have now...

Matching considerations aside, the pattern of an 88 foot dipole (1/3 
wavelength) and a 132 foot half-wavelength dipole will be very 
similar.  The gain is only about 0.5 dB lower.  W4RNL has an article on the 
88 foot dipole at http://www.cebik.com/88.html which shows patterns and 
typical impedances.

However, as Dan discovered, his 746's internal tuner isn't up to the task 
of matching the short dipole on 80m.  Even at 4 MHz, after 60 feet of 450 
ohm line, the SWR is probably over 100:1 with either a 1:1 or 4:1 balun.

A simple solution would be to add a 22 foot wire hanging straight down from 
each end of Dan's existing dipole to make it a half-wavelength 
"inverted-U".  On 80m, the end wires wouldn't contribute much to the 
pattern, but will bring the feed impedances closer to the range of the 
internal tuner.  EZNEC shows the patterns on the higher bands to be fairly 
well behaved, with the vertical wires helping to fill in the pattern nulls 
that would appear with a purely horizontal wire.

With this approach, or with a full half-wavelength dipole on 80m, the 
feedpoint impedances  may require adjusting the feedline length to get the 
impedance after the balun into the matching range of the internal 
tuner.  The 450 ohm line is now about 60 feet long, so it's a 1/4 
wavelength on 80m, which will step up the low feed impedance of a resonant 
dipole on 80m into the thousand ohm range at the balun connection.  A 4:1 
balun is probably the best choice.

Horizontal loops ("SkyWires") are also attractive.  Even though they seem 
like they would be cloud warmers at their lowest frequency, the actual gain 
is within 1 dB of a dipole at the same height, but more 
omnidirectional.  The feed impedance of a horizontal loop is well behaved, 
with a moderate impedance at all harmonics, not just at the odd harmonics 
when using a dipole.  See W4RNL for a thorough discussion at 
http://www.cebik.com/atl1.html and check out the Yahoo!Group [SkyWires] at 
http://groups.yahoo.com/group/SkyWires/

At a previous location, I used a stealth 80m loop made of #22 wire fed with 
a short 300 ohm line and matched with a link-coupled balanced tuner on all 
bands, including the shortwave BC bands.  It performed quite well.  When I 
lived in the San Francisco Bay area, the best 40m signal (other than my 
next-door neighbor) I ever heard was from a ham in Seattle with a 160m 
horizontal loop.

73, Terry N6RY


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