Earlier, VK1OD wrote:

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?

Sorry for this delayed reply...


The question about harmonics (as distinct from IMD) from VSWR meters
came up in my Radcom column in 1997.

The following text is based on what I could find out at the time. I
can't reproduce the schematic of an old-style VSWR meter here (called
"Fig 39" in the original article) but it is similar to the schematic in
the Wikipedia article. R2 is the resistor that terminates the sampling
line, with the diode detector at the opposite end of the line. RV1 is
the "sensitivity" pot between the diode detector and the moving-coil
meter.

*****************************

Q: IN MY HF STATION, should I put the lowpass filter between my in-line
VSWR meter and the antenna, or between the transmitter and the VSWR
meter? Local club members are split on the issue!

A: IT MATTERS LESS than some people think. There is an old, old story
that the rectifier diodes in the VSWR meter cause harmonics, and that
therefore the lowpass filter should come after the VSWR meter to prevent
these harmonics from reaching the antenna and being radiated. That may
once have been true, but it required some special conditions that rarely
apply any more.

The older style of HF VSWR meter used coupled sampling lines, parallel
to the main conductor of the coaxial cable (Fig 39). The coupling
between the main line and the two sampling lines increases with
frequency, so a sensor designed to cover all HF bands would be very
strongly coupled on 21 and 28MHz - all the more so if the moving-coil
meter was not very sensitive and the coupling had to be increased even
more. On the higher HF bands, the sensitivity control RV1 would be
turned down so as not to 'pin' the meter, but there is no doubt that the
diodes were being driven quite hard. Because the coupling increases with
frequency, any harmonics generated in the sampling lines would be
coupled even more effectively back into the main line. Under these
circumstances, interference to Band 1 TV on the second or third harmonic
would be quite possible.

Modern HF VSWR meters are quite different. They use a transformer
coupling system that is essentially independent of frequency, and in
most instruments the moving-coil meters are also more sensitive than in
days of old [1]. Therefore the diodes are driven nothing like so hard,
and there is much less likelihood of significant harmonic generation.
Even with the older-type of VSWR meter, harmonic generation was by no
means universal. G3RZP has grossly overdriven his old 'Labgear' meter
until the terminating resistor R1 bubbled hot wax, but still found no
sign of harmonics above -70dB with respect to the main carrier.

It therefore seems that, especially in modern times, the EMC hazards of
connecting a VSWR meter straight to the antenna are over-rated.

*****************************

[1] Inside a Directional Wattmeter: 'In Practice September 2002 (written
5 years later than the above):
http://www.ifwtech.co.uk/g3sek/in-pr...-of.htm#bruene


The "harmonics" story is an old one, but the accusation that VSWR meters
also contribute significant IMD is new to me. However, since we don't
observe significant levels of harmonics generated by the diode detectors
in modern VSWR meters, it's also very unlikely that we would observe
significant levels of IMD since the two processes are very closely
related.

As Owen and others have remarked, the "IMD" story seems to be nothing
more than a new twist on an old (but persistent) piece of ham folklore.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek