[ARC5] A simple receiver improvement

[Ian Wilson]

Ken,

The input circuitry of the receivers has a small capacitor from the antenna
terminal to the top of the input tuned circuit (with a small variable
capacitor
to ground to provide adjustment for different antenna capacitances).

A couple of schematics - sorry, didn't have time to check for the R-26
specifically - show values for C1 of 8.5pF and 11pF. At 3.5MHz, 11pF
looks like 4.1k of reactance.

So, your balun is effectively working as a voltage stepup transformer.
I would expect this to help significantly. It will not only raise the input
level,
it will probably improve the S/N ratio of the receiver (since you now have
almost noiseless gain ahead of a somewhat noisy pentode RF stage).

I would guess that the input circuitry of these radios was designed to work
with 'almost any' antenna. There is no attempt made to match the radiation
resistance of the antenna. In return for some power gain, you get a receiver
that will work pretty well with many different types of antenna. Another
good
piece of design, methinks.

73, ian K3IMW


On Tue, Jan 14, 2014 at 11:34 AM, Kenneth G. Gordon <
kgordon2006 at frontier.com> wrote:

> On 14 Jan 2014 at 10:27, Glen Zook wrote:
>
> > Although definitely not an "exact" match, television baluns can work
> > wonders in matching the usual 50-ohm coaxial cable antennas to older
> > receivers.
>
> What I don't understand in this case is that the impedance of the original
> antennas on the aircraft is supposed to be on the order of 5 to 20 ohms.
>
> So, why does this help the receiver so much?
>
> Is the input impedance of the receiver really so high that the 300:75 ohm
> transformer really and actually improves matters as much as it appears to?
>
> BTW, in answer to a question I received off list, I connected my balun with
> the 300 ohm side connected to the receiver antenna connector, and the 75
> ohm side connected to my antenna. I presumed that the receivers' input
> impedance was "high".
>
> Ken W7EKB
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