[R-390] Preselector

Fri Aug 20 22:01:01 EDT 2004

Bob,  ok,  I am listening and becoming informed.  My idea probably has no
bearing on making a 390a work better.  I do remember an article by D.
Langford where he put a 6 khz filter in a 390a in front of all the other
filters to improve the set,  closer to the front end.  My thinking started
extending to the idea that if you didn't want to tune the radio and were
happy with a fixed channel,  how close to the antenna could you put a
crystal or mechanical filter,  and derive much improved performance.  Have
such receivers been built?  It would seem that whispering is one of the
earliest forms of hiding your conversation from another listener.   Has this
principle been extended to radio transmission by making the receiver capable
of hearing such weak signals that no one else,  except someone with the same
type of receiver,  can hear the deliberately weak signal.  Or is this
wishful thinking that such a concept would work?  I suppose it's simpler to
just encode the information by other means.    Dan

----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: "Dan Merz" <djmerz at 3-cities.com>; "R-390 HF Receiver List"
<r-390 at mailman.qth.net>
Cc: "Charles B" <ka4prf at us-it.net>
Sent: Friday, August 20, 2004 7:35 AM
Subject: Re: [R-390] Preselector

> Hi
>
> The balanced input to the R-390 has some fairly good filters on it
> already. In order to do better you would have to go to a fairly complex
> filter. The exception to this would be a notch filter for something
> like broadcast band overload.
>
> So far pretty straightforward ..... now off the deep end .....
>
> Passive LC filters are not quite as simple as they look. As you add
> sections of a filter together they interact with each other. A simple
> example is  to whip up two identical three element pi section 1 MHz low
> pass filters. Check them out to make sure they 3 db right at 1 MHz.
> Next put them in series and check out the result. For most filters you
> now have something that peaks significantly and may or may not cut of
> anywhere near 1 MHz. If you do the math for a proper five element
> filter you will find that the parts values are not the same as for the
> two three sections put together. The two filters interacted in a
> fashion that is predictable (the math works) but not intuitive  (1 + 1
> does not = 2). Filters can be cascaded but you can't design them to run
> into and out of a resistive load and then go and run them into
> something way different ....
>
> Now back to reality .... sorry for the drop off into theory land.
>
> If you have a passive filter on the front end of the R-390 *and* its
> narrower than the filter that's already in the R-390 front end then
> they probably are going to interact. The result may be a filter with
> more loss or a wider pass band than you expected.
>
> A solution to the problem is to isolate the two filters. That way they
> won't interact. The good old way to do this was to slap a tube in
> between the two filter sections. That keeps each filter so it runs like
> it should. Another equally good solution is to slap a 10 db attenuator
> in between the two filters. Either way the filters each see a proper
> resistive load and they work right.
>
> If you go with gain to isolate the two filters then you have a real
> possibility of overload and distortion. If you put in a pad then you
> have cut your sensitivity. Either way you are trading off one thing for
> another. I'm not suggesting that you can't do a better job than was
> done on the radio originally. All I'm saying is that it's a fairly
> complex thing to do.
>
> Take Care!
>
> Bob Camp
> KB8TQ
>
>
> On Aug 19, 2004, at 11:03 PM, Dan Merz wrote:
>
> > 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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