Can anyone tell me why I get a worse SWR at
100w than at 5w? I know the reflected power
goes up but I'm using a cross needle meter here
so I'm referring to the actual ratio.

Which SWR should I trust?

Ken

three possible reasons..

the meter is not as accurate at low power levels. this is common on meters
driven by line power. below some level the simple diode rectifier just
doesn't drive the meter movement, since this usually happens on the
reflected side first due to the lower voltage the readings can't be trusted
at low swr's or low power levels.

the other possibility is that you have something breaking down causing a
change in swr. while this is unlikely at only 100w, it is possible. this
would normally cause a sudden jump in swr as power is increased and often
results in erratic indications.

and lastly, the transmitter is poorly adjusted. as power is increased you
may be generating more harmonic content. if the antenna reflects the
harmonics back they can add to the swr. this is often seen in tube finals
or high power amps, when being tuned the swr reading after the amp changes
as the tune/load controls are adjusted, even if the power indicated stays
constant.


"Ken Bessler" wrote in message
...
Can anyone tell me why I get a worse SWR at
100w than at 5w? I know the reflected power
goes up but I'm using a cross needle meter here
so I'm referring to the actual ratio.

Which SWR should I trust?

Ken

On Mon, 18 Oct 2004 09:31:41 -0500, "Ken Bessler"
wrote:
Which SWR should I trust?

Hi Ken,

The reading at the power you intend to use. Lower powers have
problems of scale linearity and poor accuracy; however, low powers
have very little to do with actual damage that may follow from SWR.

73's
Richard Clark, KB7QHC

"Ken Bessler" wrote in message
...
Can anyone tell me why I get a worse SWR at
100w than at 5w?

You probably don't.
The SWR meter is probably not accurate, at low power.
Try a different meter.


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On Tue, 19 Oct 2004 00:49:42 -0400, "Hal Rosser"
wrote:
Try a different meter.
Same problem.

In message , Ken Bessler
writes
Can anyone tell me why I get a worse SWR at
100w than at 5w? I know the reflected power
goes up but I'm using a cross needle meter here
so I'm referring to the actual ratio.

Which SWR should I trust?

Ken



It's all to do with the knee of the diodes. The rectification efficiency
of diodes drops off rapidly when the level of the applied RF signal is
low.

Assuming that the SWR is fairly good, but not 1:1, with 100W the reverse
detector will get a reasonable level of RF, and will be reasonably
efficient. The forward detector will, of course, get lots of RF, and
will be very efficient. The result of this is that you get a pretty
accurate (if slightly optimistic) indication of the SWR.

At 5W, the reverse detector will get less than a quarter of the voltage
than at 100W, and this will make it fairly inefficient - so inefficient
that the diode hardly works at all. However, even though the forward
detector will also get less than a quarter of the 100W voltage, this
will still be sufficient for it to be reasonably efficient. As a result,
you still get a good reading for the forward power, but essentially
nothing for the reverse. As you are not really seeing the reflected
power, the SWR looks decidedly optimistic.

Ian.
--

Ian Jackson wrote:
In message , Ken Bessler
writes

Can anyone tell me why I get a worse SWR at
100w than at 5w? I know the reflected power
goes up but I'm using a cross needle meter here
so I'm referring to the actual ratio.

Which SWR should I trust?

Ken



It's all to do with the knee of the diodes. The rectification efficiency
of diodes drops off rapidly when the level of the applied RF signal is low.

Assuming that the SWR is fairly good, but not 1:1, with 100W the reverse
detector will get a reasonable level of RF, and will be reasonably
efficient. The forward detector will, of course, get lots of RF, and
will be very efficient. The result of this is that you get a pretty
accurate (if slightly optimistic) indication of the SWR.

At 5W, the reverse detector will get less than a quarter of the voltage
than at 100W, and this will make it fairly inefficient - so inefficient
that the diode hardly works at all. However, even though the forward
detector will also get less than a quarter of the 100W voltage, this
will still be sufficient for it to be reasonably efficient. As a result,
you still get a good reading for the forward power, but essentially
nothing for the reverse. As you are not really seeing the reflected
power, the SWR looks decidedly optimistic.

Ian.
One other possibility, could be mismatched diodes simple test is to reverse
your swr meter (The Antenna to the Transmit connector, the Transmiter to
the Antenna connector, and see if you get the same results. this should at
least test the knee of the "square law response" of low level voltage from
the diodes . for a dual , crossed meter, this jist is to make sure that
the pointers (opposite) are at the same relative positions as measured
in the correct direction, as obviously, the swr curve on the meter will
not be valid-- Do this on both low power, and high power. Tests not
only the diodes, but the coupling links in your swr bridge. Jim NN7K

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"Richard Clark" wrote in message
news
On Tue, 19 Oct 2004 00:49:42 -0400, "Hal Rosser"
wrote:
Try a different meter.
Same problem.

Then check the darn thing with an antenna analyzer
sheesh!



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