[R-390] Preselector

[Cecil Acuff]

Group,

I agree with Lester and others that have commented.

I think the first question that needs to be asked is what problem are we
trying to fix?  Are we trying to increase sensitivity, reduce noise floor,
eliminate adjacent channel interference..etc...?? (probably others that
could be added to this list)

Then determine two things....is the radio fully up to specification and not
the source of, or contributing to, the problem. (try other radio's on the
same antenna system and determine if the problem still exists...even a high
quality modern rig.)

Then when all those questions are answered and the radio is eliminated as
part of the problem...what would be the most appropriate fix for the
problem. Could be as simple as some grounding improvements.  Could be a
local interference source that needs to be located...might be contributing
to the creation of an intermod problem which can present itself as adjacent
channel problems or overloading. (not a front end filter in the world going
to fix an on frequency intermod problem)

As you can see we are shooting in the dark here....

I would say in general, the addition of a preselector to the front end of a
properly working R-390A would probably not be an improvement....even with a
preamp because unless the preamp is a late vintage, well designed preamp it
would probably contribute more to the raising of the noise floor than to the
improving of weak signal responsiveness of the radio. A well designed radio
has the gain well distributed along the signal path to detection...adding
significant additional gain to the front end brings along with it a truck
load of new problems....even when combined with tunable selectivity.

I've heard stories of guys bringing in an R-390A to replace any number of
receivers used in amateur rigs of the last 20 years at annual Field Day
events practiced by the Amateur Community because a complete operating
position was rendered useless due to overloading from a co-located
transmitter and antenna system but on another amateur band.  The R-390A
didn't even know the co-located radio systems were there. (as the story
goes) Makes sense since the same type conditions existed many times in ship
board installations.

Hard to improve on that!

Cecil Acuff
WB5VCE


----- Original Message ----- 
From: "Veenstra, Lester" <lester.veenstra at lmco.com>
To: "Dan Merz" <djmerz at 3-cities.com>; "Bob Camp" <ham at cq.nu>; "Charles B"
<ka4prf at us-it.net>; "R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Friday, August 20, 2004 6:04 AM
Subject: RE: [R-390] Preselector


> With all the tuned RF stages in the front end on the 390(*), it is very
> unlikely that an external preselector is going to add a significant out
> of band protection.
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Dan Merz
> Sent: Thursday, August 19, 2004 11:03 PM
> To: Bob Camp; Charles B; R-390 HF Receiver List
> Subject: Re: [R-390] Preselector
>
>
> Bob,  I've heard this said before - and I always delight when my
> receiver shows noise when the antenna is connected - a good sign.  But
> what if the preselector  that I add has narrower bandwidth than the
> front end of the receiver I'm using but is still wide enough for what I
> want to hear?  Won't I see a benefit by adding this preselector?
>
> another misinformed listener ready to be informed,
>
> Dan.
> ----- Original Message ----- 
> From: "Bob Camp" <ham at cq.nu>
> To: "Charles B" <ka4prf at us-it.net>; "R-390 HF Receiver List"
> <r-390 at mailman.qth.net>
> Sent: Thursday, August 19, 2004 4:28 PM
> Subject: Re: [R-390] Preselector
>
>
> > Hi
> >
> > One place that a pre-amp might help an R-390 is up on 10 meters. The
> > radio is plenty sensitive enough on the lower bands but as frequency
> > goes up the noise level off a typical antenna goes down. The antenna
> > it's self gets smaller and both the man made and natural noise drop
> > off some as frequency goes up. You can come up with a sub one db noise
>
> > figure pre amp for just about any band from 1 GHz on down. That has to
>
> > be significantly better than the front end of most HF radios.
> >
> > A simple way to check if it will help:
> >
> > Listen to the noise out of the radio as you attach the antenna. If it
> > goes up you don't need a pre-amp. Since you have an antenna trimmer
> > involved it's a little more complicated than with a rice box, but the
> > net result is the same.
> >
> > Take Care!
> >
> > Bob Camp
> > KB8TQ
> >
> >
> > On Aug 19, 2004, at 5:04 AM, Charles B wrote:
> >
> > > Hi all,
> > >
> > > What happens if you install a small preselector or preamplifier in
> > > front of an R-390A?  Does it help or hinder?
> > >
> > > Chuck
> > > ka4prf at us-it.net
> > >
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