Tom,
I run an 89 meter dipole fed through ladder line and an MFJ-989C roller inductor
tuner to my Omni VI+. I also experience some power line noise. Subsequently, I
put up some band specific dipoles fed with coax. I have noticed an appreciable
reduction in the power line noise with the coax fed antennas. I say power line
noise, but that is just my best guess. That is what it sounds like. The
situation also improved dramatically when I replaced my Delta 580 with the Omni
VI+ and all its filters.
Pete, NO2D
Pewee Valley, KY.
Tom Scott wrote:
> 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
> _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
> 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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