A warning that this is a bit long-winded... please bear with me... I'm
curious to get some feedback...
Now that I have finally gotten my Omni V operating better and greatly
improved my antenna deployment, I finally can see what has given the Omni V
its reputation for being a "great set of ears". My best antenna is a
G5RV-length dipole fed with ladder line from a Johnson Matchbox. This
antenna is sloped between thirty feet and seventy five feet off the ground.
I'm getting wonderful sensitivity now, but the downside is that I'm getting
quite a bit of power line QRN at my QTH.
I knew this problem was likely before I set up this station given that my
lot is on a hilltop just adjacent to a utility company right of way for high
tension towers. Among the advice I got beforehand was that given the rain
here (Oregon), one of the chief sources of trouble - dirty power line
insulators - might be somewhat diminished. Truthfully, the noise is not
intolerable, I do get DX sources, but with my RF gain at maximum and all
extra filters off, I am seeing a noise floor between S5 and S6. With the
wonderful crystal ladder filters in this rig (I now have the 217, 287, and
285 added, still looking for a 282), I can do wonders with CW stations, but
it is harder to deal with SSB (yes I know, some of you will say "what's SSB
(hi hi)", but I do enjoy listening to phone as well). I can hear a lot of
voice down in the noise that is just not copyable.
I am investigating several approaches to treating the noise problem and
would welcome any and all comments and suggestions:
One thought that immediately came to mind was to experiment with a DSP
solution. I just happen to have a Motorola 56002 EVM board that I've
discovered has been the object of much amateur radio experimentation in DSP
filtering and data modem development. So I've been reading and downloading
things from a very good web site that Motorola directed me to for
ready-to-use software and application notes for this board. I have not
gotten very far with that yet, and don't mind getting some education in
DSP's in the process; but I'm no expert in the field, and I'm sure this is
going to take some time to learn.
I've also heard from two sources that another treatment may be effective.
That is a technique for receiving the QRN with a device that is essentially
an out of band receiver which nonetheless receives essentially the same
power line QRN which is then inverted and with careful phase adjustment,
used to cancel the QRN as a common mode rejection. I have found what appears
to be one such device from JPS Communications, their ANC-4 for around $190.
(MFJ has something that looks very similar (MFJ1025 or MFJ1026), but I think
it actually uses a different technique, that of nulling signals from a given
direction. That wouldn't be very useful in my case since the power lines are
along a broad line to the east of me where much of my listening would be
directed.) I understand from an old Motorola commercial communications
systems engineer that they once used that technique, but one flaw in the
theory it had was that in a crowded metropolitan area you could have trouble
locating the necessary alternate frequency band that was devoid of traffic.
Any signal other than the QRN in the alternate receiver would not cancel
properly with the QRN signal in the primary band and would therefore become
an inverted spurious QRM in the primary band.
I have several questions:
1. What kind of noise floor do others with a comparable rig experience?
2. Does anyone have experience with the Motorola 56002 EVM or independently
developed DSP algorithms?
3. Does anyone have experience with the JPS ANC-4 device (or the MFJ 1025/6)
or something similar, and what do you think of its effectiveness for this
problem? Does the JPS technique seem like a worthwhile approach to try?
4. Does anyone have experience with any other DSP device or technique for
treating this level of power line QRN?
5. Any other suggestions / comments I haven't mentioned?
If you got this far I have to thank you very much for your time and
patience. I really value the collected wisdom I've found from this group.
Thanks!
-Tom
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Tom Scott -- KD7DMH
14623 SW Peak Court, Tigard Oregon 97224
503-603-1931 - Day 503-684-6620 - Eve
503-504-1583 - Cell 503-604-8651 - Pager
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