[Collins] 51J series input coil antenna requirements.

[Dr. Gerald N. Johnson]

On Thu, 2008-05-01 at 09:09 -0400, jeremy-ca wrote:
> Could someone refresh my memory as to what antennas and impedances these 
> various models were designed for?
> 
> I seem to remember that the 51J3/R388 and J4/R388A were different?
> 
> Carl
> KM1H 
> 
As far as I know, the 51J-3 and 51J-4 differ only by the mechanical
filter module. And there was a conversion kit to add the mechanical
filters to a 51J-3.

I have downloaded military R388 manual and I own a Collins printed 51J-4
manual. They both say 50 ohms 100 pf for a short whip. The schematics
are IDENTICAL in that area. Even to the component numbers and general
layout.As if the same draftsman drew both schematics or made one from
the other.

That's not to say there haven't been variations in that area. I know my
round emblem J-3 (probably a new front panel on a R388) is squirrelly on
the 3 and 4 MHz bands. Peaking the antenna trimmer makes the RF stage
oscillate with a few feet of wire for the antenna. So the antenna isn't
loading the RF stage. There may be a mod to improve that. I don't know
that this receiver front end hasn't been modified since its previous
owner used it a lot in the 3 and 4 MHz bands for MARS. He did put in
product detector and AVC mods.

I can't tell from the changes I downloaded
from /downloads/collins/r388co/ at BAMA (bama.sbc.edu) whether there
were antenna circuit changes that affected the input impedance.

388C4-07.gif from that list shows an added RF triggered relay to
disconnect the antenna input to protect the input coils from transmitted
RF. Its a diode detector, a transistor amplifier and a relay. Probably
not a benefit to intermod performance with the diode on the antenna
connection. But surely would be a benefit to protect fragile and rare
antenna coils.

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