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Author Topic: RFI Every 80 Khz  (Read 300 times)

AJ8MH

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RFI Every 80 Khz
« on: January 31, 2021, 07:49:40 AM »

I have some interference in the neighborhood, and it's every +/- 80 kHz starting at 1684 kHz.  It's 20-30 khz wide and every other blip is really strong.  The third signal falls on 1844 kHz, which is an FT8 frequency on 160 meters.  The interference disappears around 7000 kHz.  Since I've been watching it, it hasn't shut off.

The K9AY loops and the MFJ-1026 can't really reduce it much when I'm pointed right at it.  It's very strong when the loop is favoring southwest, so the 1026 really struggles with it.  I can null it out when I point northeast.

I walked around the neighborhood with a portable (antenna retracted and atten on), and it appears to be coming from a residence about a 150 yards away, but I'm not walking to the door without backup.  Ha...
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W1VT

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Re: RFI Every 80 Khz
« Reply #1 on: January 31, 2021, 07:56:49 AM »

The high frequency suggests a device like high resolution computer monitor, but who knows with all the possible devices out there.

One way around this issue is to place another flag or loop antenna in a location where the offending residence is no longer in line with the direction you wish to listen.
I was able to work Japan on 160 by moving my NW loop so that it avoided a noisy residence.  I have loops every 45 degrees and moved half of them to quieter locations.
With the new locations I rarely need to use the null canceller though occasionally it it is useful for hearing stuff like Europe before sunset on 80M. 
I have no chance of breaking a European pileup on another European at that time of day.  I'd be barely above the noise level while the locals are booming in.

Zak W1VT
« Last Edit: January 31, 2021, 07:59:32 AM by W1VT »
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W6QW

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Re: RFI Every 80 Khz
« Reply #2 on: January 31, 2021, 08:56:51 AM »

I endorse W1VT's creative approach with small loops. 

A possibility to think about is that the RFI sense antenna for the MFJ-1026 may not be adequate enough.  The usual root-cause of inadequate notching from signal phasing  boxes like the MFJ-1026 is insufficient noise pick-up by the RFI sense antenna.

Another idea to consider would be to build or buy two magnetic loop's - use one as the sense antenna for the MFJ-1026 and point it to for peak RFI.  Use the other as the RX antenna and broadside it towards the RFI source to produce a deep null of the RFI. The only downside is that you'll also null wanted signals in the direction of the RFI source. 

Building or purchasing a small loop would also allow you to triangulate where the noise source is emanating from.  Once you establish a prime suspect, perhaps a friendly letter left in their mailbox could break the ice without a person-to-person conflict situation.

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AJ8MH

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Re: RFI Every 80 Khz
« Reply #3 on: January 31, 2021, 09:34:40 AM »

My sense antenna is actually my K9AY loop system and usually works well.  I've been playing with it this morning and it appears that my main antenna (the 160 meter inverted-L) isn't hearing the noise enough to give me a deep null when phased with the loops.  I get about a 2 S-unit reduction.  The inverted-L is broadside to the noise.

If I switch to the 80 meter inverted-L, I get a deep null of 6 1/2 S-units.  The noise is off the back of the 80 meter inverted-L.

My sense antenna configuration is:  Rf from the loops pass through a high RF sense detector that will ground the antenna input, through the K9AY loop filter, amp and switch box, then on to the MFJ-1026 sense port.

The main antenna input on the 1026 comes from the micro-tune port of the rig. The output of the 1026 goes back to the other micro-tune port on the rig.  With this configuration, transmitted RF doesn't go through the 1026.

I emailed the FCC a video of the noise being received and got the expected canned response.
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W6QW

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Re: RFI Every 80 Khz
« Reply #4 on: January 31, 2021, 01:35:09 PM »

Interesting that there is an increase in the signal-to-noise through the MFJ-1026 phasing process with your 80M RX inverted-L.  That suggests that the RFI magnitude into the MFJ-1026 noise-input port is insufficient to counterbalance the magnitude of that same RFI received at the RX port with the 160M antenna
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