[R-390] R-390A sensitivity measurements
Bob Camp
ham at cq.nu
Fri Mar 4 18:26:04 EST 2005
Hi
If when you *use* the radio the noise out goes up when the antenna is
attached then you do not need to do anything at all. The front end of
the radio is not limiting what is going on.
If this is not the situation then the best thing to try is an antenna
tuner. This will allow you to match things up better, add a little
front end selectivity, and generally will not create distortion. There
are a variety of outboard tuners out there that allow you to do this.
Since you really need this kind of thing at the higher end of the
radio's range you can also build one fairly easily.
To the extent that the antenna tuner is "tuning the radio" then it's a
perfectly reasonable thing to do.
If you have an idea that your antenna looks like 300 ohms broad band,
then by all means align your radio out of 300 ohms. The main effect
will be to better center up the antenna trimmer for use with a near 300
ohm antenna. Since the range of the trimmer is limited it makes sense
to center it up as best you can. The radio will not provide a 300 ohm
"match" but then it doesn't supply a 50 ohm or 125 ohm match either.
If you want to go really nuts:
1) Grab a "correct" coax connector for the balanced input and a chunk
of "correct" coax.
2) Run it over to a nice little box and mount a properly designed 1:1
balun to drive the coax.
3) Put in a "bypass" switch and a simple T match tuner for 10 to 30 MHz
On receive the tuner is not so much providing a proper load to the
antenna as taking what you get from the antenna and doing the best job
of shoving it into the radio.
I would guess that if this won't peak up the antenna to override the
front end noise then you have a really rotten antenna (or the band is
*very* dead).
Take Care!
Bob Camp
KB8TQ
On Mar 4, 2005, at 11:42 AM, John KA1XC wrote:
> 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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