[Elecraft] SWR safe levels question

[Fred Jensen]

I worked my way through college on the engineering crew of the local 
[only] TV station.  Twice a year, we had to do a Proof of Performance 
and log it.  It included assuring that the HP frequency/modulation 
monitors were correct.  The CE assigned the modulation monitors to me 
and said, "You can't use any method that relies of the calibration of 
any of our test equipment."

Very long story short, I finally discovered that if I fed the aural 
transmitter a known audio frequency and slowly increased the amplitude 
while monitoring the amplitude of the FM carrier, it would go to zero at 
a known modulation index determined by the zero-order Bessel function of 
the first kind.  I could get a known frequency audio signal from the 
sync generator which was phase locked to the network which was phase 
locked to the NBS standard.  And the key ... I could use my SX-28/VHF 
converter with the narrow crystal filter to exclude the FM sidebands to 
measure the amplitude on the S-meter ... but ONLY because I was looking 
for zero on the meter.  That's the the key.  The meter is accurate and 
requires no calibration at zero.  Elsewhere, it's anyone's guess.

And that's the key to your issue, Don.  SWR bridges are notoriously 
inaccurate at all readings, except zero [i.e. 1:1].  When the bridge is 
balanced [i.e. the load is 50+j0 ohms], it produces zero volts and all 
meters are inherently accurate at zero.  At any other value, your guess 
is as good as any of theirs.

Fred K6DGW
Auburn CA

On 8/28/2011 6:15 AM, goldtr8 at charter.net wrote:
>
> Getn's
>
> I have been searching for the "safe" swr level for the k3.   The reason
> that I ask this is because the SWR reading on the k3 is differnt than my
> MFJ 259 and the meter levels on the front of my tuner.