IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

" 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

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

"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.

IMD products from VSWR meters
 

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.
--

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

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.
--

IMD products from VSWR meters
 

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

IMD products from VSWR meters
 

ARRL members can look at:

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