I see in the Wikipeadia entry for SWR meters the following:

"When not actually measuring VSWR, it is best to remove the ordinary type
of passive SWR meter from the line. This is because the internal diodes of
such meters can generate harmonics when transmitting, and intermodulation
products when receiving. Because active SWR meters do not usually suffer
from this effect, they can normally be left in without causing such
problems."

I have seen this advice from time to time, and having done some back of the
envelope calcs, I wonder at the accuracy of it.

Has anyone seen reports of measurements of the level of harmonics and IMD
products in the main line caused by the diodes of VSWR meters?

Owen

On Fri, 22 Dec 2006 07:46:15 GMT, Owen Duffy wrote:

I have seen this advice from time to time, and having done some back of the
envelope calcs, I wonder at the accuracy of it.

Hi Owen,

It sounds like sophistry. In years past when the FCC was more
vigorous in chasing spectrum trash, they would have nailed this down
for the transmit side. Even barring that argument, nearly every piece
of gear produced in the past 30 years has a built in SWR meter. Any
IMD from those (and, by abstraction, external units) is probably 50dB
to 60dB down.

73's
Richard Clark, KB7QHC

Hi Owen,

It sounds like sophistry. In years past when the FCC was more
vigorous in chasing spectrum trash, they would have nailed this down
for the transmit side. Even barring that argument, nearly every piece
of gear produced in the past 30 years has a built in SWR meter. Any
IMD from those (and, by abstraction, external units) is probably 50dB
to 60dB down.

73's
Richard Clark, KB7QHC

Someone I used to work with spent a long time trying to suppress the
harmonics from a model aircraft radio control transmitter, only to find that
they were generated by the detector diode in the o/p power meter.

Also in the UK it is a licence requirement for CB that power/vswr meters are
only used for setting up and are removed for normal operation.

Jeff

Any
IMD from those (and, by abstraction, external units) is probably 50dB
to 60dB down.

And considering that 3rd order transmit IMD from some newer rigs is
coming in as bad as just -20dB, I wouldn't worry about SWR meter IMD!

It seems like the loose coupling of the diodes to the transmission line
would limit the effect to whatever the coupling ratio is as an upper
bound.

If you're applying a signal to the diodes that's already 20-30dB weaker
than the power going to the antenna you're already doing pretty well
compared to, say, an FT-857's transmitter output.

Dan

On 22 Dec 2006 10:37:11 -0800, "
wrote:

It seems like the loose coupling of the diodes to the transmission line
would limit the effect to whatever the coupling ratio is as an upper
bound.

Exactimundo

73's
Richard Clark, KB7QHC

" writes:

And considering that 3rd order transmit IMD from some newer rigs is
coming in as bad as just -20dB, I wouldn't worry about SWR meter IMD!

Care to name them? You'd be doing the community a favour. It's also
pretty hard to evaluate a claim about "some rigs", what's being claimed
exactly?

regards,
Ross ve6pdq

Well, the FT-857D comes in at -21dB according to ARRL lab tests.

A FT-1000MP is only 6dB better at -27dB.

An IC-746 is -23dB, as is an IC-756 (non pro, pro is -31dB)

An IC-703 is -24dB

The absolute best I can find in the ARRL comparison charts is the
TS-950SD which comes in at -42dB

So we don't need to worry about SWR meters that can, at most, cause -40
or -50dB IMD products.

Dan

ARRL members can look at:

http://www.arrl.org/members-only/pro...sum/pr-sum.pdf

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

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
down between 512 MHz and 20 MHz. Coupling decreases to 70 dB down
between 10 MHz and 2 MHz.

The 30 microamp d-c meter and its associated circuitry need negligible
r-f power from the line for nearly all ranges.

60 dB is a power ratio of one million to one.

Best regards, Richard Harrison, KB5WZI

(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