Interesting posting with good info, Jerry - thanks. As I tried to make
clear, my points were based upon assumption that the SWR instrument
truly measured SWR correctly and that is was not being influenced by
such extraneous factors as common-mode current.
I have used a number of meters of varying provenance over the years,
and I keep coming back to the Bird as the most practical and most
likely to produce usable, if not 100% correct results in the real
world.
I recently have acquired an HP 436A Power Meter which has given me a
means for checking the calibration of my Bird 43 HF slug. I was
pleased to see that it was well within tolerance. My next step is to
evaluate the directivity since the 436A with one of the sensors is
capable or +/- 0.5% accuracy at 0.3 microwatts.
If the findings are interesting, I drop you a note . . .
Fortunately, the Icom PRO "SWR" meters appear to be done "right" in
that they are placed appropriately in the antenna tuner circuitry and
use a microcomputer (part of the internal automatic tuner control) to
calculate "SWR" based upon current and voltage readings taken on a
controlled-impedance line. Resistance and reactance are
measured/calculated separately as is forward and reverse power. I
don't know what the effective directivity is, but I suspect that it is
more than adequate for the task. Incidentally, the PROs, like the
preceding generation IC-765, still use motor-driven variable caps for
the actual tuning operation.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489 IC-765 #2349 IC-756 PRO #2121 IC-756 PRO2 #3235
----- Original Message -----
From: "Dr. Gerald N. Johnson, electrical engineer"
<geraldj@isunet.net>
To: "George, W5YR" <w5yr@att.net>
Cc: <tentec@contesting.com>; <AC5E@aol.com>; "Walter Maxwell"
<w2du@iag.net>
Sent: Wednesday, January 29, 2003 7:12 PM
Subject: Re: [TenTec] SWR, and meters
> Many so called SWR meters are rough approximations and their error
> depends to a great degree on the local voltage and current on the
line.
> A good meter would use those to derive power, but the monimatch
style
> (copied for most varieties of 11m compatible SWR meters), does show
> different results at different points of the line.
>
> As you said, SWR meters tend to cause some mismatch. Bird recommends
> adding a length of 50 ohm coax to their meter to make the total
length
> exactly a half wave. Then the impedance seen by the Bird is the same
as
> seen by the radio. The greater the sensitivity of the meter for
working
> at low power, the tighter the coupling required and the more the SWR
> meter will change the impedance as seen through it. Even Birds will
> introduce a SWR with the low power closely coupled slugs. And Bird
uses
> a 30 microamp meter to allow loose coupling. Bird also keeps the
length
> of the coupled line short to minimize wavelength effects.
<snip>
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