[R-390] R-390A sensitivity measurements

[John KA1XC]

I have to disagree here, I think aligning this receiver front end to a (freq
dependent) antenna load is falling off the deep end of the RF sensibility
curve; for one thing you can kiss your RF deck coil tracking goodbye. Is
anyone actually doing this? (the aligning part, not the kissing part!).

The 390x antenna trimmer circuit is designed to tune out only *small*
amounts of  XC or XL present at the antenna input, and it can do nothing to
compensate for a mismatch in the R component because the turns ratio in the
RF transformer Primary to Secondary windings is fixed and so is the degree
of coupling between them.

If you really have a bad antenna mismatch and you really really want that
last dB of performance from your setup then put an antenna tuner/transmatch
device inline and tune it up with a TX or MFJ-259B, they work as well in the
receive direction as well as in the transmit direction ya know........

73,
John


----- Original Message ----- 
From: "Veenstra, Lester" <Lester.Veenstra at intelsatgeneral.com>
To: "Bob Camp" <ham at cq.nu>; <DJED1 at aol.com>; "R-390 HF Receiver List"
<r-390 at mailman.qth.net>
Sent: Friday, March 04, 2005 7:50 AM
Subject: RE: [R-390] R-390A sensitivity measurements


"See if the noise out of the radio goes up. If it does then the whole
front end match thing is not an issue"

Again Bob has come back to the key point. If the interest is in coming
up with numbers to make a radio look good, then there needs to be a
commonly used "interface" pad (or no pad at all).

However if the interest is in getting the most sensitive receiver
operationally, than it should be connected to the antenna it will be
operated on, and with signal injected via an independent antenna, the
front end adjusted for optimum.

In the absence of that, and in recognition of the fact that most of us
use multiple antennas, an alignment with a 50 ohm source signal
generator into the standard configuration of one side the standard
balanced to external input and the other side of the balanced line to
ground is most practical. Then, in the few cases where the ambient noise
from external sources is not significantly in excess of the radio's
internal (input terminated) noise, and when the trimmer adjustment does
not produce a peak within its range, you might want to consider an
antenna specific front end only re-alignment.

------------------------------------------------------------------------
--
The antenna is what we care about. If you could set up the generator to
duplicate the antenna then you might be able to directly measure what
is really going on. If you run a vector network analyzer into the
antenna you can get a pretty good idea of what it looks like. That
sounds like a lot of work though ....

The easy test is to hook the antenna to the radio. See if the noise out
of the radio goes up. If it does then the whole front end match thing
is not an issue. With a reasonable R390 and even a fairly short antenna
I pretty much always seem to pass this test. The only time it can be a
problem is above about 16 MHz after the band dies.

Take Care!

Bob Camp
KB8TQ

Lester Veenstra
Senior Engineering Program Manager
Intelsat General
6550 Rock Springs Drive, Suite 450
Bethesda Maryland, 20817
+1-301-571-1212
e-mail: lester.veenstra at intelsatgeneral.com


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