In message , Antonio
Vernucci writes
In this neck of the woods "active" usually means the meter has
peak reading circuitry that requires power to operate. In general,
as far as ordinary peak reading meters, this is the most accurate
type as far as trying to read voice peaks. The better ones use
fast acting LED's as the meter.

If by "active" one just means that the meter requires power, then I
would see little difference between an active and a passive circuit as
far as intermodulation is concerned.

As a matter of fact if, in an "active" meter, the amplifier (or the
active display) follows the diodes, there would be no difference at all
(as to intermodulation) with respect to a passive meter.

If the amplifier is instead put before the diodes, the intermodulation
products generated by the diodes (if any) can get coupled to the line.

In summary the distinction is between a circuit in which the diodes are
directly coupled to the line and a circuit in which there is an
isolating stage between the line and the diodes. And not between a
passive and an active circuit.

73

Tony I0JX



The belief that SWR meters produce harmful harmonics probably dates from
the 1950s, when TV coverage was low, and many viewers had to drag in
weak signals from a considerable distance. I'm in the UK, and the signal
level on UK Channel 5 (around 65MHz) was only about 30 microvolts. Oh
yes, the picture was snowy!

Obviously, the 2nd and 3rd harmonics from transmitters operating on the
higher HF bands could easily wipe out TV reception, especially lowband
(our 'Band 1').

As well as using the usual 30MHz lowpass filter, some amateurs went to
considerable lengths to suppress harmonics emanating from the
transmitter itself (with particular attention to the screening and
bonding of the metalwork, and the filtering of all leads (mic, key,
power). Without this, there was little point in even thinking of fitting
a highpass filter etc to the TV set.

I believe there were one or two cases where the final vestiges of
interference were traced to harmonics being generated in the SWR meter.
This was cured by ensuring that the line-up between the transmitter and
the antenna was TX - SWR Meter - LPF - ATU (if used) - Antenna. Doing it
this way ensured that any harmonics from the meter were removed in the
LPF and (to some degree) the ATU.

The line-up above is the one which I always recommend should be used if
possible. Generally, there is no reason not to. It is good engineering
practice. But as to whether it is always essential? I suspect that often
it is not.

Ian.
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