Topband: Interaction between 160m antennas

[Steve Ireland]

W8JI said:
>
>I think the biggest problem for an absolute answer is a lack of 
>reliable verification data. Very few amateurs have enough room to 
>install a textbook perfect vertical, let alone a horizontal at a few 
>hundred feet. 
>
>All that aside, everything I have read from propagation experts 
>indicates the polarization effects of ionospheric absorption are 
>small compared to potential changes in antenna field strength at 
>different radiation angles. We have a human tendency, however, to 
>assume that trends somehow indicate repeatable absolute results. 
>
>To further complicate matters, we almost always compare one less-than-
>perfect antenna to another less-than-perfect antenna. The antennas 
>are almost always electrically and physically so close to each other 
>and other structures, feedlines, and antennas we have no idea what is 
>actually radiating. (In many circumstances antennas interact 
>noticeably with even a wavelength of separation, so you can imagine 
>the coupling at .1wl or less spacing.)
>
>The saving grace is at propagation peaks virtually any antenna will 
>work pretty well, even a low dipole. The "peaks" we often experience 
>near sunrise or sunset tend to equal things out.
>

VK6VZ replied:

G'day 

My approach to dealing with the the problem of whether to use horizontal or
vertically polarised antennas has been to use both.  Although the low
geomagnetic latitude of my QTH and its poor earth conductivity favours a
horizontal antenna, my Marconi-T vertical is still useful on occasions,
providing me last night with a new country in the form of K1B - who was
totally inaudible/unworkable on the inverted vee dipole at 90'.

My QTH is very small for a 'serious' topband station - slightly less than
half an acre - and minimising interaction between the two antennas is
achieved by placing them electrically in the same spot - on the same tower
- and mounting them at 90 degrees to each other.  This configuration was
arrived at through modelling by Earl K6SE, for which I am very grateful.

Whilst clearly interaction takes place, the presence of the second (the
Marconi-T) antenna only affects the resonant frequency of the first by a
couple of KHz and there is no noticeable effect on the latter's
performance.  The Marconi-T performs like a vertical antenna, generating a
huge ground wave and shows 10dB or so (sometimes more) improvement on the
inverted vee when working local stations (i.e. within 20km of this QTH) who
use vertical antennas.

Don't let fears about interaction stop you from putting up a second
differently polarised tramsit antenna for 160 on a small lot - if you can
mount them in a similar manner to mine.  An even better way to go is to use
a similar set-up to KK4TR/KN4LF and use an antenna which can be switched
from vertical to horizontal polarisation, and vice versa.

A couple of years ago, my friend VK6LW used a delta loop on 80m, which he
could relay switch from corner feed (vertical polarisation) to bottom
centre feed (horizontal polarisation).  This was a fascinating antenna to
use, with excellent DX results.

Vy 73,

Steve, VK6VZ