[TowerTalk]Tuner Losses & Bird ratings

[K7GCO@aol.com]

In a message dated 11.05.00 04:58:24 Pacific Daylight Time, 
w8ji@contesting.com writes:
<< 
 The most accurate way to measure loss in a tuner with different 
 source and load impedances is to measure the temperature rise in 
 the tuner, not the power levels. >>
  
 . 73, Tom W8JI
 w8ji@contesting.com
 
Right on Tom.  Basically, if a tuner can match the Z at the end of the 
feedline (1:1 SWR is obtained at the input of a tuner), 100% transfer of 
power occurs to the antenna less the losses in the feedline which are 
predictable and the losses in the tuner as measured by the "heat test."  
There are exceptions.  I had a load that the Johnson MtatchBox matched 1:1 
but wasn't transferring all the power without getting warm--with 100 W as the 
inductors were heavy.  With 1 KW I had arcing which tipped me off of a 
problem but no apparent heating with short tests.  A continuous carrier for 5 
minutes may be needed in marginal cases as SSB or CW short transmissions may 
be not enough for a heat test.  The arcing was the main tip off something 
wasn't right with what I thought was proper spacing of the variable Xc.

I reversed the differential variable Xc, still obtained 1:1 SWR, the arcing 
went away
and the output RF meters read higher.  I'll recreate the situation and from 
the RF ammeter current meter difference, I can determine the dB difference.  
The reversing of the differential resulted in less inductance used in the 
tuner tank coil and other favorable power transfer of power factors.  I have 
a well insulated knife switch that reverses the differential Xc connections 
in my MatchBoxes.

QRP'ers should first test their tuners with a 1 KW carrier heat test for 5 
minutes, turn the rig off and feel the conductors quickly with the fingers.  
I've had conductors stone cold with this test on 10M.  I had one end of a 
home made 20M coil get warm.  A 50 year old solder joint was the problem.  
K7GCO

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