Rick,
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.
73,
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
OMNI VII.
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:
ORION MAIN RX:
http://www.bavarian-contest-club.de/reviews/omni-vii/ormain.mpg
ORION SUB RX:
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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