[R-390] (no subject)

[Tisha Hayes]

Quote:
"To all - It's important to note that the impedance on the C connector
(J103) originally connected to P205, which is the hi impedance input is
about 1 meg ohm.  This connection was designed to connect a whip antenna to
it through a very short piece of coax.  The whip became part of the tuned
circuit input to the rf amp and did not diminish the effectiveness of the
secondary of T201-T206.  This allowed the Q of the antenna circuit to
reduce unwanted images, adjacent stations and noise from entering the rf
amp as much as possible for a whip antenna.  This also reduces IMD produced
in the rf amp.  However, by connecting a low impedance antenna to this
connection, the Q is severely reduced and the benefits of the tuned circuit
are greatly removed."
EndQuote:

Larry,
Very well put, that is exactly the purpose of the C connector. It is a
point to attach a compromise antenna like a whip. This is shown in many of
the user manuals for the radio in the first few pages. The radio is
supposed to be fed from the balanced connector with balanced feedline all
the way up to the antenna.

If you find yourself a bunch of IBM ThickNet coax cable with the connectors
on the end it is exactly what you would want to use from the back of the
radio, all the way out to the antenna. Then you either use an antenna that
is impedance matched to whatever antenna design (and it's impedance) or you
make a BAL-BAL out there to perform an impedance transformation. It will be
the lowest loss, quietest way of feeding the radio. Those Collins guys in
the 1950's knew what they were doing. Many other receivers were set-up in a
similar way. The Hammarlund SP-600 was originally found with the balanced
connector on the back end of the RF deck. Of course most were modded to put
a UHF connector there and the first RF stage was realigned to optimize with
that.

The R-390A has that oddity that you really only see if you stare at the
schematic. Where the first RF stage is bypassed when using the C connector.
I like to use the balanced connection because it is another stage
of permeability tuned RF before we get to the first RF amplifier tube.

Along the way someone suggested swapping the connections off of the antenna
relay to put the C connector through the first RF stage. This suggestion is
difficult to find, it is in some of the Y2K documentation, or the
supplements or maybe in the Hollow State News. Right now I cannot remember
where I saw it but it made sense at the time so I did that before I
homebrewed my first Bal-Un and then later bought the German made Bal-Un
(fantastic little device).

Going in at 50 ohms to the 125-150 ohm balanced connection can be done, you
just need to align the first RF permeability tuned stages for a better
match with that impedance. It is not beyond the means of anyone with a
signal generator.. Heck, if you have your favorite antenna and balanced
feedline set up, you can probably even do a better job of matching with
some sort of known signal (how about a signal generator sitting in the yard
with a short piece of wire as a transmitting antenna while you calibrate
your radio/feedline/antenna setup? you are only going to get a few
milliwatts of transmitter power from a signal generator).

-- 
Ms. Tisha Hayes. AA4HA

*""It is not because things are difficult that we dare not venture. It is
because we dare not venture that they are difficult." -Seneca"*