Hello Tor,
Although the Comtek (and others) are "50 ohm devices" - the antenna elements
that connect to them are not. The Z of a mutually coupled system at the
Comtek box end of the element feedlines is no where near 50 ohms. Several
factors are involved:
The feedpoint Z of each element (includes mutual)
The impedance of the element feedlines (should be 1/4 wave - 75 ohms). This
is a transformer too.
The quality of the ground system (affects the real part of Z)
The Z of the waster load (dummy load) - yes this does matter - must be 50
ohms.
The spacing between elements.
Symmetry and sameness among the elements has a significant impact on pattern
control (as K9YC pointed out). Make sure all of your elements are resonant
at the exact same frequency and have the exact same resistance - for best
performance. Typically the elements are adjusted lower in frequency to
obtain very little power in the 50 ohm waster load.
For example, at K3LR my individual elements are resonant (36 +/- j0) at
3.510 MHz. This results in zero watts in the waster load at 3.650 MHz. At
3.500 and 3.800 there is about 7% of the power going into the waster load.
Not bad for 300 KHz of coverage.
73,
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of RT
Clay
Sent: Friday, September 23, 2016 2:15 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 4 square for 80
I thought that the hybrid-type controllers (at least the Comtek I have) were
really meant to work at 50 ohms. Then for a fullsize vertical, 36 ohms +
some ground loss is not too far from 50. Obviously they seem to work ok over
a reasonable range.
Tor
N4OGW
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