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



OK, but my point is that having a crappy detector, a crappy power meter and 
a crappy signal generator and tuning for minimum reflection is better than 
not tuning the antennas at all.

One thing I've always hated about the "microwave ham" community is the 
"everything must be perfect or don't bother doing it" mentality.  It is a 
good thing that I've ignored that, and just started building.  My initial 
stuff was, of course, deaf compared with my later stuff (but still 90 dB 
better than nothing!), but it got me:  a) started, interested, and more 
knowledgeable.  Now I can build commercial quality or better equipment.  If 
I would have listened and decided that I was afraid of 2.4 GHz or 10 GHz or 
whatever-- I wouldn't be here right now. And as I learned more, I went back 
and tweaked and redesigned and now that early stuff works well too.

I've found that for good power transfer and decent circularity all you 
absolutely need is a VSWR of about 1.25:1 or better which can be done with a 
simple setup.

I agree that if you need NIST tracablilty and/or measurements of return loss 
down to -40 dB that you'll need better equipment than that.  Although I can 
get pretty close with the out of range Boonton and the directional coupler 
(-30 db) that I have for that band.

You can build a pretty decent power meter from scratch if you use a good 
diode, microstrip techniques and good chip components.  Good diodes are, 
what $2 instead of $0.21?  Chip components in 2002 are cheap and easy to 
get.  I'd rather encourage people to do this than nothing.  If the signal 
generator is still in calibration, then it's simple to go and calibrate the 
homebrew meter, and check for linearity.

HP couplers are hard to find in hamfests & expensive.  But there are good 
couplers readily available at 'fests like Dayton (always have LOADS of them 
there) from other manufacturers.  I've had good luck with Narda...($25-$75 
buys you a good Narda at Dayton usually-- most I ever payed was $50)

Fred W0FMS


>From: "i8cvs" <domenico.i8cvs@tin.it>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>,   "AMSAT-BB" 
><amsat-bb@AMSAT.Org>
>Subject: R: R: [amsat-bb] SWR testing of AO-40 S-band antennas
>Date: Sat, 16 Mar 2002 04:16:30 +0100
>
>
>----- 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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>
>
>
>
>
>
>
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