[TenTec] Preselector

Edward A. Feustel efeustel at hughes.net
Wed Oct 23 05:29:05 EDT 2013


How many tuned circuits are in your home built preselector? At one point 
I had an RME DB-22a preselector which had 3 tuned circuits with two 
active stages: http://www.ohio.edu/people/postr/bapix/db_22a.html. It 
really helped with a Hallicrafters SX-71 and an SX-28a, but would be 
overlarge for a modern rig!  If I recall you get about 6 db attenuation 
per octave with each tuned circuit (element?).
Now most rigs seem to use octave bandpass filters instead.

Ed, N5EI

On 10/22/2013 3:57 AM, Rick - DJ0IP / NJ0IP wrote:
> Seems you didn't get much response on that.
> Doesn't matter, most responses are usually just opinions.
> Let me  "show you" what a preselector can do, even for a good radio.
> In the link below, it shows me switching my pre-selector in and out on an
> Here in Europe, before the broadcast stations moved out of the ham band,
> almost every radio on the market had its front end overloaded to the point
> that it created intermodulation.  The intermodulation produced "phantom
> signals".  The phantom appear to be signals, usually carriers, but in fact
> they do not exist.  They are being generated within the RX itself.
> A pre-selector will reduce the signal strength of all signals outside of its
> passband.  How much depends on the quality of the preselector. The damage
> done to reception is due to the sum of all the voltages hitting the front
> end, not just the signals within the radio's own filter.  30dB of
> attenuation can make a significant difference.  Unfortunately all
> pre-selectors also have loss.  Typically they inject about 6dB of loss on
> their own.  My preselector (home brew) has a 6dB amplifier to compensate for
> the loss.  It can be switched in or out.
> In the example below, on about 7005 kHz, what you hear is a Phantom Carrier
> about S7 or S8.  Of course any station you wish to work that is less than S7
> or S8 will be covered up by this carrier.  The job of the preselector is to
> prevent overload of the receiver's front end remove the phantom signals.
> Setup:
>   - QTH Munich about 5 or 6 years ago, at the Contest Station site of DL1A
>   - 40m at night (around 8pm German time)
>   - Antenna:  3 element monoband yagi, 105 ft. high,, pointing North East
> Here are the results:
> http://www.bavarian-contest-club.de/reviews/omni-vii/omni7.mpg
> What you witnessed is, the preselector with its 6dB amp on dropped the
> phantom to about S4.
> But with the 6dB amp off, the phantom disappears completely.
> I went on to show how the OM7's built in attenuator affects the phantom.
> 6dB made no improvement.  12dB began to reduce the phantom.  With 18dB of
> attenuation the phantom was about S1.
> Unfortunately all of the signals on the band that you wish to copy are also
> reduced by 18dB.
> So if they were S3 before, they would be S0 with 18dB attenuation.
> On the other hand, with the pre-selector the desired signal would still be
> S2.
> And for comparison, you might just want to see how an Orion performed under
> the exact same circumstances:
> http://www.bavarian-contest-club.de/reviews/omni-vii/ormain.mpg
> http://www.bavarian-contest-club.de/reviews/omni-vii/orsub.mpg
> As you will see and hear, the ORION Main is superior; you basically are not
> troubled by the phantom (just barely audible).
> The Sub RX performs about the same as the OMNI VII.
> BTW, the Eagle (which was not available at the time of the test) performs
> identical to the ORION MAIN RX.
> So in answer to the question of what is the difference between an OM7 and an
> Eagle:
> The answer is SHOWN in these examples.
> Under normal operating condx you would not see or hear any difference, but
> at a Multi-Multi Contest site, or at a Field Day site, especially where you
> have 2 transmitters on the same band (one in CW, one in SSB), the benefits
> of the Orion and Eagle will be significant.
> UNFORTUNATELY, I don't know of any source for good pre-selectors these days.
> My pre-selector in the example is home brew, copied from the design of the
> Braun SWF-5-40 Pre-Selectors.
> The SWF-5-40 was possibly the best pre-selector ever built for amateur use.
> Currently the only source of pre-selectors that I know of is MFJ.
> The pre-selector is not as good as the one I used in the demo but it will
> still make a difference in cases with strong interfering signals on other
> frequencies.
> If your problem is the broadcast band, then I recommend a high pass filter
> like the one sold by Dunestar.  It will certainly clean up the 160m band for
> you.  Dunestar Model 400-HPF.  Costs $100.
> Probably the best solution now days is the Dunestar 800-BPF.  Cost $449, but
> this one has band pass filters for all 6 hf classical bands (not WARC) and
> may also be used for transmitting.
> Hope this helps, Mike.
> 73
> Rick, DJ0IP

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