[R-390] Preselector

[Bob Camp]

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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