[HCARC] Antenna Tuners a PS to my earlier.

[Lee Besing]

W2IK knows his stuff... He's proven it to me, many times over.  :-) 


Lee Besing
210-771-7075 ( voice/text)



-----Original Message-----
From: Kerry Sandstrom <kerryk5ks at hughes.net>
To: ALoneStarYank at aol.com, galeheise at windstream.net, bob.k5yb at yahoo.com, HCARC at mailman.qth.net
Sent: Mon, 08 Oct 2012 3:57 PM
Subject: Re: [HCARC] Antenna Tuners a PS to my earlier.

Gang,

Listen to W2IK!

SWR under about 2:1 just doesn't matter that much.  And remember, just because your SWR meter says 1:1 doesn't mean its 1:1.  Most of the current SWR meters use a pickup coil and a capacitor connection to the transmission line.  The Heathkit HM102 is an RF power meter/SWR bridge.  It lists the accuracy of the power meter as +/- 10 percent of the full scale power reading.  Note the power sensor is what is used to make the SWR measurement, actually 2 separate circuits, one for forward and one for reverse.  Heathkit doesn't give an accuracy for the SWR measuremnts.  It has two scales, 200 W and 2 kW.  The SWR scale is only calibrated up to 3:1.  On the 2 kW scale the possible error is +/- 200 W.  

These are pretty typical specs.  If you have a ham power/SWR meter it is probably close to the same.  The accuracy of the SWR function is partly limited by the basic meter and your ability to read it  and partly limited by the "balance' between the forward and reverse measurement circuits.  What are the chances that the diodes resistors and capacitors were matched?  I think you may be able to tell when you have a minimum in SWR, but I don't believe you can reliably tell whether that minimum is 1.2:1 or 1.1:1.  

If your power meter is accurate to +/- 10 % of full scale and full scale is 2000 W, how much power can you legally run if the limit is 1500 W?  The answer is 1300 W as indicated on your power meter!  

Note that digital meters are no more accurate than analog meters.  Many will have the same +/- 10 % of full scale +/- 1 count in the final digit.  Accuracy varies with temperature.  Measurements you make in a hot room in the summer will not necessarily match the same measurements made on a cold winter morning.  Proffessional instruments aren't really any better.  Typically they are listed as +/- 5 to 10 % of full scale.  The old precision VOM's with the mirrored scale to eliminate parallax were typically +/- 2 % of full scale.  You can accurately calibrate instruments but it isn't easy and it has to be rechecked frequently.   

Don't worship measurements and the instruments that make them, they are just not that good. 

Kerry

Oh, Yes,  MFJ is a trip!  I have a couple MFJ gadgets and they work fine.  I don't think there is a screw in them, however,  everything is attached with hot glue instead of nuts and bolts.  But you aren't paying a HP or Collins or Tektronix price either.  I think they are reasonable for the price and will do OK if used with care until you can afford what you really want.  

One of the beauties of the old ultimate transmatch design is there are no switches, just two variable capacitors and a variable inductor, you can twiddle to you heart's content as long as the transmitter output stage can handle it.        
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