----- Original Message -----
From: "Al Williams" <alwilliams@olywa.net>
To: <towertalk@contesting.com>
Sent: Friday, March 18, 2005 9:11 PM
Subject: [TowerTalk] crosstalk between beams on a common boom (such as
c31xr)
> For example, transmitting on a 20 meter beam while listening on a 15 meter
beam.
>
> How great is the risk of damaging the front end of a receiver listening on
one beam while transmitting on a second beam
> when both are close spaced?
>
> I used EZNEC briefly and was surprised that a parasitic element of the
same dimension results in current in the parasitic
> almost as high as the driven element (for a spacing of about 4 feet)
.However, changing the parasitic length to another band
> dropped the current down to about 20 % of the driven element.
>
> But I don't have any idea of the relationship of current in the parasitic
element to that obtained when the parasitic element
> is actually the driven element on another beam. I tried putting a 50 ohm
load in the parasitic element and that dropped
> the parasitic element current even more.
Think about it this way.. there's nothing really special about horizontal
arrangements. If you put an element (i.e. the driven element of the 15 m
beam) that happens to be the same length as a parasitic element on the 20m
beam, the same distance from the 20m driven element as the 20m parasitic is,
then the current will be roughly the same (except that the 15m element
essentially has a 50 ohm load in the middle, right?)
This is how the driven elements of the Force 12 C3, etc. are fed. The
feedline attaches to the 20m DE, which couples to the 10 and 15m DEs.
They're pretty close in that case, so the coupling is pretty strong.
Unfortunately, there's no nice simple equation for the mutual coupling of
different length dipoles. You can run an antenna modeling progarm (like NEC)
to find out, or run a simplified version of the same calculation (by
numerically integrating, etc.). Orfanidis has a great online antennas
textbook at the Rutgers website that covers this, and if you have Matlab (or
Octave) or have a software bent, he has Matlab source code for all manner of
useful stuff like this (specifically, in this case, the routine imped.m).
http://www.ece.rutgers.edu/~orfanidi/ewa/ He's a DSP guy, so a lot of the
material on antenna pattern synthesis comes at it from that orientation,
which is different than the classical electromagnetics approaches like
Balanis.
>
> But, since the frequency of the signal on the 15 meter beam is 20 meters,
I presume that the SWR is high and a
> voltage amplification may result depending on the transmission line
length.
Depends on a lot of things..
You can model this fairly easily.. Put both beams in your model, excite the
20m beam per usual, connect a TL to the 15m model with a 50 ohm load at the
other end of the TL, and see what the voltages wind up being. If you don't
have the matching networks in the model, then put in a TL at a scaled
characteristic impedance (i.e. if the feedpoint impedance is 20 ohms,
instead of 50, use a 20 ohm TL, and a 20 ohm load.
(note that a 50 ohm load (the receiver input) at the end of a 50 ohm
transmission line is perfectly matched, and will present a 50 ohm load to
the antenna)
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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