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Re: Topband: Sunset / Sunrise and low dipole question

To: "Gilles RENUCCI" <ve2tzt@arrl.net>, <Topband@contesting.com>
Subject: Re: Topband: Sunset / Sunrise and low dipole question
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Tue, 27 Nov 2007 18:52:18 -0500
List-post: <mailto:topband@contesting.com>
> Different authors are saying that a low dipole (id 30 to 
> 60 feet high) on 160m can in certain circumstances be 
> better than a vertical during the short period of 
> sunset/sun rise ducted propagation.

True enough. In certain circumstances.

Until recently I had two dipoles and a vertical, all a good 
distance apart:

High dipole at ~300 feet broadside east and west.
Low dipole at ~120 feet, broadside east and west
Vertical at about 190 feet with 100 radials about 200 feet 
long

In hundreds of A B C tests with VK3ZL at sunrise we 
generally find a few days a year when a dipole at 120 feet 
(which has about the same pattern as one at 60 feet) ties or 
beats my 200 foot vertical. This most often occurs at 
sunrise or during geomagnetic storms. The same was true into 
Europe.

But.....the dipole I have at 300 feet, which has a null 
straight up, is always as good as or better than the low 
dipole at the same time!

So is it wave angle or polarization? Your guess is as good 
as anyone's, but almost never was the low dipole ever better 
for anything except contacts within a few hundred miles. I'd 
say in the high 90 percent period the vertical is equal or 
better at any distance beyond a few hundred miles to a few 
thousand, and beyond that any dipole....even one at 300 
feet....is worse except for very special occasions.

When the band "likes the dipoles", it likes one at 300 feet 
or 120 feet nearly the same.

A dipole certainly can be useful, especially under disturbed 
conditions or if the vertical isn't so good! I wouldn't say 
it is a really high wave angle thing as much as the band 
just likes the polarization. If it was a really high angle, 
the lower dipole should have been better. It almost never 
was better, although at times it was the same.

When the band liked the vertical best,  it also liked the 
high dipole 1 or 2 S units better than the low dipole. So is 
it wave angle or polarization? You make the guess!

> This question is for the choice of the orientation of the 
> dipole, because it is a matter of fact that over 60 
> degrees the dipole have no more directivity and it can be 
> interesting to install it in a direction minimizing the 0 
> degree neighbors qrm or very local broadcast station.

Minimum groundwave or local  radiation of a dipole is 
BROADSIDE to the dipole. Maximum groundwave radiation is 
near the antenna ends. Keep that in mind!! I always thought 
the maximum radiation would be broadside, but that's true 
only for horizontal polarization which does not propagate 
along the earth. So when I wanted to null my transmitter 
from a receive dipole antenna, I had to orient the receive 
dipole nearly broadside to the transmit vertical.

73 Tom 


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