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Re: (K3) SWR Consistent - low or high power

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alsopb

Re: (K3) SWR Consistent - low or high power

Some of this effect hqs to do with the reflected power diode.
Depening upon the sense circuity, the reflected power diode may not see
enough voltaqe to be fully conducting. Thus it reads less current in
the reflected position and hence lower SWR that at higher powers.

Also the type of diode used is important. I believe the K3 use Schotky
diodes which conduct at lower voltages than say silicon diodes.
Germanium diodes are even better but fragile and harder to get these days.

73 DE Brian/K3KO

Tom Boucher wrote:

> My theory in these cases is that it's nothing to do with bad antenna connections, but more due to the rig putting out a higher harmonic content at higher power levels. The SWR meter sees a bad SWR at the harmonic frequency so the reading is slightly worse.
>
> 73
> Tom G3OLB
>
> N3OI wrote:
> <*One observation:* I use low power to set SWR with my TenTec manual tuner.
> My FT-920's SWR meter would usually shift a bit when I increased power to
> 100w and I would then quickly tweak it back down. With the K3's SWR meter
> (no ATU installed), after setting the tuner at 10w or so, the reading stays
> the same when power is increased to 100w.>
>
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Vic Rosenthal

Re: (K3) SWR Consistent - low or high power

There are several causes for varying SWR readings with power level:

1) Poor diode linearity -- diodes are less sensitive at low levels, and reflected power
may be very low at QRP levels. This effect shows as a low indicated SWR at low power
levels, which then becomes more accurate past a given power level.

2) Harmonics -- as described below by G3OLB.

3) Parasitic oscillations -- a misadjusted amplifier can oscillate a few hundred kHz from
the operating frequency, where the antenna is not resonant (or a tuner is not tuned). VHF
parastics are also possible. Any significant off-frequency output can cause a high SWR
reading. Needless to say, this is very serious and must be corrected.

4) Common-mode current -- antenna currents on the outside of a coax feedline, caused by
feeding a dipole (etc.) without a balun, or by coax running parallel to antenna wires even
with a balun, can upset SWR readings. This effect changes with the feedline length as well
as the power. SWR indicators may vary in their susceptibility to this effect.

On 10/10/2012 2:20 AM, Tom Boucher wrote:

> My theory in these cases is that it's nothing to do with bad antenna connections, but
> more due to the rig putting out a higher harmonic content at higher power levels. The
> SWR meter sees a bad SWR at the harmonic frequency so the reading is slightly worse.
>
> 73 Tom G3OLB
>
> N3OI wrote: <*One observation:* I use low power to set SWR with my TenTec manual
> tuner. My FT-920's SWR meter would usually shift a bit when I increased power to 100w
> and I would then quickly tweak it back down. With the K3's SWR meter (no ATU
> installed), after setting the tuner at 10w or so, the reading stays the same when power
> is increased to 100w.>

--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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w8fn

Re: (K3) SWR Consistent - low or high power

In reply to this post by alsopb

It's pretty unlikely you'd see harmonic content on any kind of broadband
power sensor like a SWR meter. Even if the transmitter was really bad
and had harmonic levels only 40 dB down, this would mean for 100 Watts
output you would be getting 10 mW in harmonic energy, which wouldn't
wiggle anybody's meter very much. And if the total harmonic power at 40
dB down was just equal to the reflected power from your antenna
mismatch, this would mean the antenna would have a VSWR of 1.07. My
money's on diode nonlinearities at low power levels.

73...
Randy, W8FN

On 10/10/2012 04:20, Tom Boucher wrote:
> My theory in these cases is that it's nothing to do with bad antenna connections, but more due to the rig putting out a higher harmonic content at higher power levels. The SWR meter sees a bad SWR at the harmonic frequency so the reading is slightly worse.

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Don Wilhelm-4

Re: (K3) SWR Consistent - low or high power

Randy,

What you say is true only if working into a load that is not frequency
selective - like a dummy load.
If working into an antenna, that will be frequency selective and any
harmonics will increase the SWR indication.

73,
Don W3FPR

On 10/10/2012 8:17 PM, Randy Farmer wrote:

> It's pretty unlikely you'd see harmonic content on any kind of broadband
> power sensor like a SWR meter. Even if the transmitter was really bad
> and had harmonic levels only 40 dB down, this would mean for 100 Watts
> output you would be getting 10 mW in harmonic energy, which wouldn't
> wiggle anybody's meter very much. And if the total harmonic power at 40
> dB down was just equal to the reflected power from your antenna
> mismatch, this would mean the antenna would have a VSWR of 1.07. My
> money's on diode nonlinearities at low power levels.
>
>

riese-k3djc

Re: (K3) SWR Consistent - low or high power

In reply to this post by alsopb

WOW

Bob K3DJC

On Wed, 10 Oct 2012 20:32:58 -0400 Don Wilhelm <[hidden email]>
writes:

> Randy,
>
> What you say is true only if working into a load that is not
> frequency
> selective - like a dummy load.
> If working into an antenna, that will be frequency selective and any
>
> harmonics will increase the SWR indication.
>
> 73,
> Don W3FPR
>
> On 10/10/2012 8:17 PM, Randy Farmer wrote:
> > It's pretty unlikely you'd see harmonic content on any kind of
> broadband
> > power sensor like a SWR meter. Even if the transmitter was really
> bad
> > and had harmonic levels only 40 dB down, this would mean for 100
> Watts
> > output you would be getting 10 mW in harmonic energy, which
> wouldn't
> > wiggle anybody's meter very much. And if the total harmonic power
> at 40
> > dB down was just equal to the reflected power from your antenna
> > mismatch, this would mean the antenna would have a VSWR of 1.07.
> My
> > money's on diode nonlinearities at low power levels.
> >
> >
>
> ______________________________________________________________
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>

w8fn

Re: (K3) SWR Consistent - low or high power

In reply to this post by Don Wilhelm-4

My example assumes a worst case of 0 dB return loss at all harmonic
frequencies (i.e., the harmonic energy is all reflected). Any radiation
or other loss mechanism at the harmonic frequencies will just decrease
their reflected power and lessen their contribution to the total
broadband reflected power seen by the meter. The real situation is even
more complicated, as the directivity of the wattmeter coupler as a
function of frequency must also be taken into account to determine the
actual indicated VSWR.

Also, I should have said in the original post "... your antenna would
have a VSWR of 1.07 AT THE FUNDAMENTAL FREQUENCY."

73...
Randy, W8FN

On 10/10/2012 19:32, Don Wilhelm wrote:
> Randy,
>
> What you say is true only if working into a load that is not frequency
> selective - like a dummy load.
> If working into an antenna, that will be frequency selective and any
> harmonics will increase the SWR indication.
>
> 73,
> Don W3FPR

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