Re: [TenTec] Simple Answer - Why and how is Orion better

[Bill Tippett <btippett@alum.mit.edu>]

W0JSB wrote:
>I can attest to that - W1AW isn't so far south of me - when the
teleprinter broadcasts are going out at 1500 watts I can squeeze the
passband down to 300Hz, then use the PBT tuner to shift up 300Hz and my
front end is able to receive the very weakest signal on the band as if
W1AW isn't even there.

>I have witnessed an S9+44dB signal from W1AW on my Jupiter be completely
taken out of the picture - if you can't hear it, it's because the Jupiter
has taken it out, not because it's been masked over.

        Jason, please don't take my comments the wrong way, I'm only trying to
explain a commonly misunderstood point.  I've gotten some questions to the
effect that "My XXXXX radio has no problems eliminating a very strong signal
just a few kHz away, why is Orion any better?"  The answer to this is that
intermodulation distortion (IMD) products are caused by TWO or more very strong
signals within the receiver's roofing filter.  These signals mix together and
cause phantom signals (IMD products) that will appear in your passband and
sound like "bleeps and bloops" on CW or simply noise for wideband signals
(like SSB).  Most radios will easily handle ONE very strong signal within their
roofing filter, but the IMD problems come when there are TWO or more, and this
is why IMDDR3 is very important to contesters and DX'ers who want to hear weak
signals closely surrounded by many strong signals.

        I believe Jupiter uses a 15 kHz roofing filter (BTW it is identical to
Orion's Sub-Receiver), and will be subject to closely-spaced strong signals
creating IMD products just like every other radio on the market today which
uses 12-15 khZ roofing filters.  Elecraft's K2 and the Omni VI are the sole
exceptions since both use narrow roofing filters.  This is also why both the
K2 (at 91 dB) and Omni VI (at 86 dB) have the best IMDDR3 performance at 5 kHz
spacings (5th column of data) in this list:

http://www.elecraft.com/K2_perf.htm

        When ARRL did Jupiter's Product Test in June 2001, they were not 
yet doing
IMD and BDR tests with 5 kHz spacings, and unfortunately they did not do an
Expanded Test Report, so I cannot tell you what Jupiter's actual 5 kHz IMD 
results are.  However, the 14 MHz IMDDR3 result was 85 dB using interfering
signals spaced at 20 kHz.  My guess is that Jupiter's IMDDR3 for interfering
signals at 5 kHz would be in the ballpark of ~70 dB, versus Orion's Main RX at
101 dB.

        Does this mean it is a bad receiver?  Not at all.  But just don't 
expect it
to perform like an Orion if W1AW AND ANOTHER S9++ signal happen to come within
+/- 7.5 kHz of your listening frequency.  I hope this helps explain what 
this IMD business is all about and why it is mainly of concern to 
contesters and
DX'ers (especially us low-band types).


                                                        73,  Bill  W4ZV



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