[Milsurplus] ARB Antenna capacitor...and the winner is...

[Mike Hanz]

Well, I'm not sure...

But the N4FS "suckout" thought seems to have a tenuous support by some 
words in the manual, which I finally recovered from the iced up radio shack.

The input capacitance to the ARB is augmented by a paralleled 68pF 
capacitor, in an apparent attempt to swamp out resonance frequency 
changes caused by connection to different capacity aircraft antennas, up 
to 180pF.  The lack of an antenna trimming capacitor evidently caused 
this particular choice in input design - obviously if you change the 
resonant frequency you start to perturb the gain through the 
transformer.  The choice of component values is such that an antenna of 
more 180 pF should not be connected to the Af (fixed antenna) terminal.  
If either a long wire trailing antenna was used, or two or more 
receivers needed to be connected to the same antenna, then the idea is 
that they would be connected through the At (Trailing wire antenna) 
terminal, which has a 180pF cap (C101) placed in series with the 
receiver input.  The combination would then bring the effective antenna 
capacitance presented to each receiver Af terminal below the 180pF 
design limit, at the cost of some attenuation considered negligible by 
the writers of the manual.  At least that was the rationale presented in 
the manual.

On the other hand, slapping a receiver with such a high input 
capacitance across an antenna that was also connected to the command 
receiver antenna posts sorta alters their own input characteristics a 
bit drastically.  What would seem reasonable to do in that situation 
would be to reduce the size of At capacitor C101 to some compromise 
figure, or, as it appears they chose, mount an external series capacitor 
on the Af post input.  It wouldn't take long to determine a compromise 
capacitor size empirically with three command sets also connected to the 
test point.

On 12/9/2013 2:46 PM, Mike Feher wrote:
> See my response to Hue regarding "suckout". Very technical indeed, HI. 73 -