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




----- Original Message -----
From: Frederick M. Spinner <fspinner@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, March 15, 2002 11:38 PM
Subject: Re: R: [amsat-bb] SWR testing of AO-40 S-band antennas


>
>
>
> >From: "i8cvs" <domenico.i8cvs@tin.it>
>
> >Hi Jason,
> >
> >For serious SWR measurements at 2400 MHz you need a high directivity
> >directional coupler like for example an HP 11692D or similar type and a
> >very
> >well calibrated  power meter like an HP 435 A
> >
> >You will find the theory and the basic procedure in "Fundamentals of RF
and
> >Microwave Power Measurements" HP Application Note 64-1A wich is free
> >distributed by Hewlett Packard
> >
> >This HP's classic 76 pages application note on power measurements is now
> >completely revised and updated.
> >
>
> Why, for amateur use of just tuning up antennas does he need a "very well
> calibrated power meter" or a "high directivity" coupler?  As long as the
two
> relative readings are ok (linear)-- VSWR is a ratio--  "calibration"
cancels
> out.  True if one of the readings is on the non-linear "knee" of the diode
> detector this might pose problems.
>
> I normally just use a Boonton RF Millivoltmeter w/50 Ohm adapter that is
far
> out of its range for accurate measurements at 2400 MHz, an old klystron
type
> HP generator and a Narda coupler at home.  Works fine.  I've compared what
> I've done at home with the Network Analyzer here and found that this junky
> home setup is close enough for what we are doing...
>
> True, I've gotten to do most of my 2.4 GHz tuning on the N.A. at work, but
> its just so much easier and faster.  And then I can post better results to
> my web site.  The "home setup" works just fine though.
>
> Fred W0FMS
>
>

Hi Fred,

Directivity is a measure of how well a directional coupler can isolate two
signals and therefore,sets the limits on how accurately a coupler can
perform a specific measurement.

As an example, when making reflection measurements a dual directional
coupler is required and directivity of the test port is the most important
parameter.

Ideally one would like to measure the magnitude of the reflected signal
alone.

However,because of directivity,the reflected signal is combined
with a small portion of the incident signal .

It is important to enphasize that the magnitude of the portion of the
incident signal (that combine with the reflected signal ) is dependent
strictly on the directivity of the coupler.

The small portion of the incident signal wich combines with the reflected
signal is a phasor that is directly additive to the measurement
uncertaninty.Thus,the higher the directivity the higher the measurement
accuracy.

Because the portion of the incident signal that mixes with the reflected
signal is very small it adds a negligible amount of uncertainty when
measuring large reflections. But as the reflected signal becomes smaller the
small portion of the incident signal become more significant.

When the reflected signal in dB (return loss ) equals the directivity of the
coupler the measurement can result in a -6 dB to + infinite dB error.

This is why if  we want to measure a return loss in the range of  20 dB or
SWR =1.2222  for example,a directional coupler with 40 dB directivity is
required for a serious and accurate measurement.

The power meter and its measuring head must be wery well calibrated because
it can be completely correct  at  -10 dBm  and completely wrong at - 40 dBm
for example expecially  those using diodes working in the square law region.

Why to say  the return loss of my antenna is 25 dB  and SWR =1.1192 when
using a directional coupler with 25 dB directivity only for example and a
power meter showing correctly  0 dBm on forward power and a completely
wrong - 25 dBm on reflected power ?

 73" de i8CVS Domenico


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