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Owen Duffy · December 22nd 06, 09:15 PM posted to rec.radio.amateur.antenna

(Richard Harrison) wrote in news:495-458C28AB-
:

Owen Duffy wrote:
"Because active SWR meters do not usually suffer from this effect, they
can normally be left in withoout causing such problems."

For clarification, I didn't say those words and I don't represent that
position at all. They are a partial quote by Richard Harrison of my quote
from Wikipeadia.

Ordinary meters needing only r-f to power their indicators can also be
left in the line with no visible effect because their demand for r-f is
very small.

Insertion loss is specified in the Bird Model 43 Instruction Book
between the 50-ohm line and sensing circuitry. Specification is 53 dB

I think "insertion loss" has a different meaning, but I agree that the
coupling loss to the sampler is typically very high.

My back of the envelope calculations we

Looking at typical Breune style ham external VSWR meters which have a
voltage divider of 1:60, the coupling loss (voltage sampler + current
sampler) is probably in excess of 30dB. Conversion loss (power into the
sampler to harmonic/IMD out is probably in excess of 40dB. Total round
trip loss is probably 30+40+30dB, which is far better than the
performance of a typical PA.

If there was substance to it, it is a bit of a worry as others have
noted, most transceivers have a built in coupler for at least protection
purposes.

I know that there were versions of external SWR meters with a "power"
measurement function that indeed had a diode receifier connected directly
to the through line, and the coupling loss term I discussed above would
not apply. Similary, an "RF Power Out" using similar circuitry was
incorporated in a lot of low power transmitters (eg CB). Nevertheless,
conversion loss (as an RF harmonic/IMD generator) in the rectifier
circuit is still probably enough to ensure products are below normal PA
IMD products.

But... the question was really has anyone seen credible measurements that
might support the Wikipeadia line?

It is probably yet another archtypal ham myth about VSWR, and since it is
rarer, it would be a real gem for the "knowing" to trot out.

Owen

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