[Collins] 51J series input coil antenna requirements.

[Dr. Gerald N. Johnson]

On Fri, 2008-05-02 at 17:34 +0000, Bill Kirkland wrote:
> Agreed.  Generally it is good to terminate in the correct impedances.
> 
> That antenna trimmer cap has caused me a lot of grief though.
> You work on a receiver, have it all tuned up, come back a week later
> and wonder why the sensitivity is so poor.  You check everything twice
> and are all set to pull it apart and if you are lucky you remember that
> you twisted the antenna trim.  Even more fun is when you hook the
> 51j-3 up through a transmatch and decide to play with both the transmatch
> and antenna trim.

Ah, not used to the ham receiver with a tunable high frequency
oscillator, I take it. There when tuning the antenna trimmer it warped
that oscillator so peaking the antenna was a two handed operation, one
on the antenna trimmer, one on the fine tuning. ;<)

Yes the transmatch can be confusing because the antenna trimmer only
affects the resonant frequency compensating for antenna reactance and
errors in antenna coil tracking that reactance causes. While the
transmatch can vary both its reactance and its resistance. And for best
signal transfer both need to be adjusted. Worse the effective input
impedance of the 51J/2 is much higher than that expected by the adjacent
transmitter even if a 1:4 balun is used, while the effective input
impedance of the J3/4 is closer to 50 ohms (the book) says, but is
reactive trying to inherently match the short whip, not 50 ohms. It
might have been better for many applications had they all been designed
for 50 ohms input impedance (resistive) with auxiliary boxes for the 300
ohm and the short whip applications.

But then maybe the first purchasers that made the products survive for
us wanted those odd input characteristics. Maybe these collections of
inputs are like the use of the current regulator tubes on the oscillator
tube heaters in the 390 and 390A. The existing competition (with high
frequency oscillators) couldn't work without them and the military
reviewers wouldn't accept a receiver without them though today we can't
detect their benefit in the 390 and 390A.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer