[ARC5] ATD/ARB on the air

[WA5CAB at cs.com]

Meir's idea is a good one as it works, and in retrospect, it's obvious that 
it would work although I can't recall anyone else ever writing that they had 
figured that out.  But the load resistance of a short whip or wire is low, not 
high.  I've seen the load resistance of the standard 15' whip quoted as 5 to 20 
ohms, with an average of 13 over the HF range of 2-12 MC..  This has been 
pointed out here and elsewhere dating back to at least the early 40's.  Recall 
that the majority of the WW-II aircraft sets only went up to 9 MC.  The BC-191 
and 375 went up to 12.  Only the ATC went to 18,  Of the tactical ground sets, 
only the SCR-399, SCR-499 and modified SCR-299 went to 18.  The others, except 
for the BC-191, topped out at 6 or less.

Anyway, because the expected antenna lengths were electrically short, the 
output impedance of the WW-II vintage aircraft and most of the ground 
transmitters was low, not high.

In a message dated 10/6/2008 8:56:49 PM Central Daylight Time, 
neilb at ihug.co.nz writes: 
> >The antenna matching on the ATD is very tricky.
> >I used a quick and dirty method: I connected the ATD output into the  
> >antenna output of a manual MFJ antenna tuner, while the 50 ohm input  
> >was used for the output.  Of course I pretuned the ATD/tuner into a 50  
> >ohm dummy load.
> 
> I had often thought that this might work well for hi-impedance milsurplus
> transmitters. Are the forward and reflected power readings on the meter
> transposed?
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480   
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