[Milsurplus] ARB Antenna capacitor

[Mike Hanz]

On 12/9/2013 12:09 AM, Hue Miller wrote:
> My thinking was that this condenser would have minimal effect in 
> attenuating
> stray oscillator inputs. I was thinking that the overall antenna 
> capacity as seen
> by the ARB with multiple receivers connected plus the circuit through 
> the ATC
> might be deemed ?too? far off the design "standard" capacity.  A 
> condenser
> of 36pF would pretty well define that antenna capacity, since it is a 
> multiple
> lower than would be seen by the rest of the outboard wiring. Would a high
> capacity at the ARB antenna post detune the antenna's resonant circuit? I
> do not know. 

I believe you are looking at the wrong end of the problem if you are 
focusing on the impact on the antenna characteristics, Hue.  What I was 
referring to was the *potential* for creating a de facto filter with the 
addition of the small 36pF capacitor in series with the input impedance 
of the ARB, which is composed of both resistive, capacitive, and 
inductive components.  I was only speculating, however, as most of us do 
from time to time on this reflector.

> More wisdom (more inputs from others) is required.

You're probably right.  My explanatory capability is rusty after all 
these years.

> The impedance of the 36pF at 4495 kcs (for example :) is about 1000 ohms.
> Yes, that would make for a filter effect, considering the low 
> impedance of the
> ARB input circuit to off-resonance frequencies. However for stray 
> oscillator
> inputs at the same frequency the ARB is receiving, 1000 ohms, I think, is
> like, forgettable, considering the ARB input impedance.

Might you be neglecting the current vector relationships of the 
connected components if you are *just* talking about the 36pF 
capacitor?  If you take a look at the ARB schematic at 
http://aafradio.org/docs/ARB_schematic.png you will note that there is 
quite an input network between the Fixed Antenna post and in front of 
the first stage tube, starting with a 68pF capacitor from the antenna 
post to ground.  Then you have C-116, T-103 tuned by C-109 and C-180A, 
not to mention wiring inductance and capacitance. All these interact 
with that 36pF capacitance in one way or another.

> It was worth the question for me to learn that this was not a hammy mod.

Then my work here is done...:-)

> -----Original Message----- From: Mike Hanz
> Sent: Sunday, December 08, 2013 8:34 PM
> To: Hue Miller ; milsurplus at mailman.qth.net
>
> On 12/8/2013 7:02 PM, Hue Miller wrote:
>> So the series capacitor was to somehow discriminate against, how? ( 
>> Not getting it. )
>> And not to diminish the total antenna capacity, nor to somehow 
>> protect the receiver?
>
> Why would one want to decrease the antenna capacity seen from an ARB's
> perspective, Hue?  I haven't found the mica transmitting cap on the ARB
> antenna input used in any other system other than where multiple
> receivers were attached to a common receive antenna.  That should tell
> you something about its deployment.  What we do know is that it is
> possible to reduce interaction through de facto bandpass filters, using
> inherent set antenna input capacities and inductances.  What we do not
> know at the moment is whether the intent was to diminish interference
> from the ARB or the command sets.  Further research is needed.