"Owen Duffy" wrote in message
...

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 dispute part of that. On receive, the diodes are not conducting, so they
won't generate IMI products. If they were a problem, then every receiver
front end in the world would be a worse problem.

In message . com,
writes

Antonio Vernucci wrote:
Because active SWR meters do not usually suffer
from this effect, they can normally be left in without causing such
problems

Can anyone clearly explain what is an "active" SWR meter? I would
presume that
the line is decopled from the diode by means of an amplifier. Is that
what they
mean?

73

Tony I0JX

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.
A "passive" meter is one that uses capacitors to give the meter
a "hang time" to sort of read peaks. But it's not as accurate, and
almost always reads low. "20-25% worth" for the average meter.
The cross needle meter in my MFJ-989C is an example of a
passive meter circuit. I call it "psuedo peak reading". :/
I also built a passive circuit into my old green cube heathkit
meter. It has a switch on the back to switch the caps in or out.
Naturally, the passive meters require no power to operate.
MK


Driving the diode with a wideband RF amplifier would have the following
advantages:
(a) allow a lower level of RF sample to be used (higher tap-off ratio).
(b) further minimise the possibility of diode distortion products from
getting back into the RF through-signal.
(c) drive the diode at a higher level, thus improving the RF-in to
DC-out linearity.
Do any SWR meters do this?
Ian.
--

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

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.
--

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