[R-390] Preselector
Bob Camp
ham at cq.nu
Fri Aug 20 11:54:14 EDT 2004
Hi
I agree that it's better to have a destination in mind before you pull
out of the driveway ...
One of the things that we all are putting up with a lot more of these
days is man made crud. Seemingly simple stuff like toasters all of a
sudden are full of strange RFI inducing gizmos. This not the world that
the R-390 was designed for.
There are several paths that RF can take to get into an R-390. The
antenna connectors are the obvious ones, but the power cord is an
equally good path under some conditions. Almost any wire that attaches
to the radio *can* act as an antenna.
For those of us who live on multi hundred acre estates with several
rhombics to choose between for any particular direction and band this
is a particularly significant issue. It also impacts those of us who
live in the real world.
The line filter on the R-390 is there for a purpose. On a lot of radios
this filter has not survived very well. Replacing it with an outboard
filter is a reasonable solution to the problem.
Isolation of any lines you have off of the radio may be as simple as a
couple of resistors and maybe a coil or two. It may be a lot more
complicated depending on how much crud you have in the same room as the
radio.
How does this all relate to preselectors?
If the crud is coming from your VGA through the audio cable to the
speaker no amount of preselection on the antenna lead is going to help
you. You first need to be sure that the radio is quiet *without* the
antenna. This is a bit easier on something like an R1051 with it's
monster shield of a case than it is on a R-390.
One nasty thing can be an RF loop that runs from a local crud source
out to the antenna ground. The radio sounds fine with no antenna so the
assumption is that the curd is not local in origin. A balanced antenna
connection can help this out. Also an RF isolation transformer may be
of help.
Until you are sure that the problem is definitely coming from the
antenna any work on preselection will be a very frustrating exercise.
Take Care!
Bob Camp
KB8TQ
On Aug 20, 2004, at 11:24 AM, Cecil Acuff wrote:
> 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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