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Re: SWR testing of AO-40 S-band antennas

Get a directional coupler that covers the bands of interest (I see several 
every year at Dayton), make a power meter using a microwave diode (or pick 
up something like that surplus) and a good 50 ohm termination.  A simple 
meter could be a PCB microstrip with a good diode and a 50 ohm chip resistor 
with very small lines on an SMA connector.  The voltage out of that circuit 
can then either drive a analog meter directly, or better drive a OP-AMP 
circuit and an analog meter.  Simple and cheap to build.

To measure VSWR with a directional coupler, put the power meter on the "-10 
dB" (or -20 or -30 depending on the coupler) port (if running power here is 
the place to put the attenuator).  Put the antenna on the forward direction 
first (arrow towards the antenna).  Take power reading "X". Then switch it 
around so the arrow points to the signal generator.  Take power reading "Y". 
  The reflection coefficient will be "X"/"Y".

Take the ratio of the measurements (X/Y-- as long as they are linear, 
arbitary, non-calibrated, readings are ok).  From this voltage ratio 
(reflection coefficient) there are ways to convert this to VSWR.  Or simply, 
tune the reflected for minimum and the same effect will be realized.

When done tuning, if you want to know real VSWR make sure that the readings 
are done after minimizing reflection, as both readings will vary with 
different SWR.  (I.E. The more power that is reflected, the less goes 

(For coversion to VSWR, return loss, etc.  I found this site with Google:  
http://www.mogami.com/e/cad/vswr.html )

Fred W0FMS

>From: Jason White <jason.white@saintleo.edu>
>To: <amsat-bb@AMSAT.Org>
>Subject: [amsat-bb] SWR testing of AO-40 S-band antennas
>Date: Fri, 15 Mar 2002 10:01:16 -0500
>Hello list,
>I am lucky enough to have just come into ownership of a calibrated HP 8616A
>S band signal generator. I would like to learn how to use it to generate
>test signals and measure the SWR of my various AO-40 antennas (G3RUH helix,
>patches, etc.). I know I'll need a coupler or slotted line as well as a
>meter. Does anyone know where I could find the "basics" of this sort of
>thing at microwave frequencies? I understand the basic procedure as
>illustrated in the ARRL manual, but there isn't much about microwave
>frequencies. I assume it's similar.. Is there any information that would be
>of help to me in the Microwave experimenters book from ARRL, or the Mode S
>book from Amsat?
>N1XBP - Jason White
>EL88qi - Hudson, FL
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org


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