Larry. I think you want to try subtracting the termination end earth
resistance from you likely Beverage Z and then terminate the antenna in that
resistance.
Because I use fairly short copper earth rods here I've ended up
terminating my Beverages in 330ohms. 73 Clive GM3POI
----- Original Message -----
From: "Larry Molitor" <w7iuv@earthlink.net>
To: <topband@contesting.com>
Sent: Sunday, January 27, 2002 9:24 PM
Subject: Topband: Shorty Beverages
> After reading the tech notes on K9AY and W8JI web sites, I decided to
> revisit the short Beverage. In the past, my shorty Beverage lengths were
> always determined by the property lines rather than science. Performance
> was always very poor. This time I would do it "by the book".
>
> After much measuring and pacing, I found exactly one place where I could
> run a 270 foot wire without going through the house windows or tower legs.
> I wound up with a 270 foot run, 30 inches above ground, and pointing SSE.
>
> The termination resistance was first determined by using a pot and
> adjusting for minimum signal off the back from a local AM BC station. With
> 900 ohms I was able to get about 23 dB F/B. When replacing the pot with a
> fixed carbon comp resistor of the same value, that unexpectedly dropped to
> about 12 dB. An attempt to measure the surge impedance resulted in a value
> of 450 ohms. With all the 50 KW AM BC stations in the area, that
> measurement is somewhat suspect, but none the less, about what you would
> expect. With 450 ohms for the termination, F/B on the AM station was only
6 dB.
>
> The contest gave me a chance to really see what this wire would do, or not
> do, as it turned out. With either value of termination resistance, there
> was little or no F/B on sky wave signals. While it was somewhat better
than
> the TX antenna, it was not quite as good overall as the small loop and way
> behind the rotatable flag I normally use. The pattern appeared to be
> roughly a figure 8 instead of the expected cardioid.
>
> By the way, ground resistance between ends appears to be on the order of
> 100 ohms or so and there seems to be no coupling/re-radiation from the TX
> antenna. A isolation transformer was used, and beads were placed on the
> coax as an after thought with no effect.
>
> Any one with short Beverage experience have an idea why it does not work
as
> expected?
>
> Thanks & 73,
>
> Larry - W7IUV
>
>
>
>
>
> _______________________________________________
> Topband mailing list
> Topband@contesting.com
> http://lists.contesting.com/mailman/listinfo/topband
|