R: [amsat-bb] Minimizing de-sense on Arrow and other closely spaced antennas

["i8cvs" <domenico.i8cvs@xxxxxx>]

----- Original Message -----
From: Bill Acito <w1pa@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, October 10, 2002 9:50 PM
Subject: [amsat-bb] Minimizing de-sense on Arrow and other closely spaced
antennas


> Considering a set-up like many of you have used for portable work on the
> LEO's (an Arrow antenna, and a dual band HT), what causes the de-sense
> on the 70cm side during transmit: the 2m power in close proximity, or
> the third harmonic, 3 MHz away (using UO-14 as an example)?
>
> Would a 1/4 stub coax notch, "T"'d into the 440 side, cut for the
> offending frequency, help? Or since it's also effective on multiples of
> the cut frequency, it's a moot point (since it may also be close enough
> to null out the downlink frequency)?
>
> Just an idea I had rolling around in my head.
>
> Bill
> W1PA
>

Bill,

The desense on the 70 cm side during transmit on 2 meters in close
proximity is mostly due by gain compression,overloading,
and intermodulation on the 70 cm receiver but the third harmonic
contribution to this effect is also significant.

If  you connect to your  70 cm RX input a 1/4  wavelenght long coax line
resonant on 2 m and open at the other end the impedance at the
open end will be very high  and it will be very low at the receiver
input creating a notch for the offending signal on 2 meters.

Unfortunately  the above coax open line will be also about 3/4
wavelenght long open line for 70 cm and since 3 is a odd numbar
of 1/4 electrical wavelenght for 70 cm a unwanted  notch  will be
created as well  at the 70 cm receiver input.

You can see what happen using the Smith chart wich represent
a 1/2 wavelength transmission line long  displyed  along  360
degrees.

Put the open end of your 1/4 wavelenght stub for 2 meters  on the
infinit point of the chart wich is located at the right end of the real axis
of the chart

Rotate clockwise toward the generator for 180 degrees or 1/4 wavelenght
folloving  the lover half outer circunference of the chart until you arrive
at the left point of the real axis of the chart were the real part of the
impedance is zero and  good for you for 2 meters because it's a notch
to the unwanted frequency.

Since the same stub is 3/4 wavelenght long for 70 cm you must continue
to rotate clockwise toward the generator for another 360 degrees and you
will arrive to the same point on the left real axis where the impedance is
zero and bad for you on 70 cm because it make again a notch to your wanted
frequency.

I hope this help,

73" de i8CVS Domenico


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