At 07:04 AM 2007-02-24, Carsten DL1EFD wrote:
>I want to model interaction between a Tribander and a 40m-Antenna on
>the same mast.
>I can model both antennas and make sure that the wire elements align
>of course. Is
>this alone a sufficient test, or do I have to model the seperated
>nature of the
>feedpoint of the antennas - or the coax as a transmission line
>(grounded or open)?
I agree with all of Guy K2AV's comments. I have even crudely modeled
the wire mesh backing the stucco walls of my house. 8-(
In addition, it is good to check what happens to the tribander
pattern and feedpoint impedance when the 40m antenna feedpoint is
both open-circuit (e.g. a 10 megohm load) and short-circuit. In many
cases, one or the other will have an effect. It is probably also
worth seeing what happens when you place a +j50 ohm load at the 40m
feedpoint. Repeat with a -j50 ohm load.
An alternate way of doing this is to connect a transmission line to
the 40m feedpoint and vary it's length from 1/8 wl to 1/2 wl in 1/8
wl steps. Either open or short the far end of the line and you will
cover most of impedances you may see at the 40m feedpoint. Be sure
to use electrical transmission line wavelengths at the frequency of
interest (20m, 15m, or 10m).
You could also do this with the tribander when looking at the 40m
pattern, but it's much less likely to be a problem.
73, Terry N6RY
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