RFI caused only by 146.205 MHz and not adjacent repeater pairs

[K4EET]

I need some Radio Frequency Interference (RFI) advice...

My XYL (wife) has a "home office" in the Master Bedroom (MBR). My hamshack is located directly underneath the MBR. She is experiencing RFI to her Dell Monitor with a symptom of wavy lines and to her Plantronics Headset used during telephone conferences that has the symptom of garbled audio when I transmit on a very specific repeater; namely the W3CU repeater on 146.205 MHz (input) and 146.805 (output). When I do test transmissions on repeater frequencies that are below and above the 146.205/146.805 MHz repeater pair (see table below), she does not experience any interference.

My mobile radio used as a base station is an Icom IC-208H (see attached brochure below) that has selectable power for transmit of 5/15/55 Watts on VHF. The temporary location for the antenna, similar to MFJ's MFJ-1724B, is on top of a 4 drawer steel filing cabinet directly underneath her desk that has the affected equipment on it. At this time, the location of the antenna is fixed and cannot be moved to another location which would probably mitigate the RFI. Once I get an outdoor antenna, a Cushcraft AR-270B, that antenna will be much further away from the XYL's "home office" in both the horizontal and vertical directions. If I get the same RFI from the outdoor antenna, well, let's just say I'll cross that bridge when we get there.

The RFI occurs on all three transmit power levels (5/15/55 Watts on VHF) on the 146.205/146.805 MHz repeater pair.

So here are my questions:

1. Why is the RFI caused by a single frequency of 146.205 MHz and not any adjacent frequencies separated by 0.015 MHz?
2. In my current inventory, I have two (2) Model 916, 2X43-151 Ferrite Clamps with a permeability of 850 for general RFI blocking (according to the package's label). The frequency range for this RG-8 sized clamp is 25 MHz to 300 MHz. I will not be able to test this clamp on the affected equipment for a week or so. Being disabled and in a wheelchair, I cannot install these clamps because I cannot get to that floor. I have to wait until our son comes over one day in the future. So the question is this: Is the ferrite clamp that I have in inventory a decent choice to try on the power lead, video/audio leads, etc. or would a ferrite clamp of a different size (I know it will be a loose fit since it is made for RG-8 cable) and/or permeability be better? I know the 917-Ferrite Clamp for RG8X/RG58 coax is better for smaller cable diameters.
3. Is there anything that I can do in my shack to mitigate the RFI on my XYL's computer accessories (monitor and headset)? To not transmit on 146.205 MHz is a non-starter. The W3CU repeater is my primary repeater to use.
4. Is there anything else (thinking outside the box without changing the location of the mag-mount dual-band antenna) that I may be overlooking?

Thanks for any assistance and/or ideas that you might have which will be greatly appreciated.

73, Dave K4EET

Repeater Input Frequency (MHz)	Repeater Output Frequency (MHz)
146.010​	146.610​
146.025​	146.625​
146.040​	146.640​
146.055​	146.655​
146.070​	146.670​
146.085​	146.685​
146.100​	146.700​
146.115​	146.715​
146.130​	146.730​
146.145​	146.745​
146.160​	146.760​
146.175​	146.775​
146.190​	146.790​
146.205​	146.805​
146.220​	146.820​
146.235​	146.835​
146.250​	146.850​
146.265​	146.865​
146.280​	146.880​
146.295​	146.895​
146.310​	146.910​

 

[jonwienke]

It's common for RFI to vary by frequency. A cable might be the right length to resonate, or perhaps a chassis component or circuit trace in the computer somewhere.

Ferrites are probably a waste of time. Move the antenna so it isn't directly under the computer. Get a cookie sheet and stick the antenna to that, then place in a better location.

 

[prcguy]

Ferrite might help and a #43 mix should be ok for VHF but there is a mix that Laird uses which seems to be better. I found that one snap on bead does very little and three in series or two turns through a single bead can make a very noticeable difference at VHF. My experience is from wrapping AC power leads in ceiling can lights with LEDs that were wiping out VHF and UHF reception here. I went from not being able to receive a nearby strong repeater on a hand held inside the house to hearing weak distant repeaters with only a slight amount of interference.

This required three beads on 11 different lights for a total of 33 snap on beads which is usually expensive but I found a deal on Ebay similar to these. 50 LAIRD 28A2736-0A2 Ferrite CLAMP-ON CORE FILTER 220ohms 100MHz Hinged Snap--on | eBay

I would try three beads on every lead in and out of the computer and you have to test it at the computer end and the device end to see what works best. It might end up being direct radiation into the monitor causing some of the problem and ferrite would not fix that.

 

[K4EET]

It's common for RFI to vary by frequency. A cable might be the right length to resonate, or perhaps a chassis component or circuit trace in the computer somewhere.

Thanks for that comment! I have more experience with HF RFI where if I transmit on any frequency in a given band, my signal is demodulated to some extent by a clock/radio, the landline telephones on twisted pair, etc. I have never encountered a RFI problem with my 2M/70cm equipment in my 47 years as a ham. Maybe I have been fortunate in that area. LOL!

Ferrites are probably a waste of time. Move the antenna so it isn't directly under the computer. Get a cookie sheet and stick the antenna to that, then place in a better location.

As I said:

At this time, the location of the antenna is fixed and cannot be moved to another location which would probably mitigate the RFI.

I say that because of several reasons:

1. Being in a wheelchair, I cannot reach the antenna.
2. With assistance, we have moved the antenna previously (before this RFI problem) in attempt to access the '805 repeater. Where the antenna is now was the only spot that I can access the repeater with a full quieting signal. So moving the antenna would cause me to once again have a noisy signal into the repeater. What I really need is to get my outside antenna installed. Then I'll be able to hit the repeater full quieting with a 500 milliwatt signal.
3. Lastly, most of the other locations we tried previously where I had a noisy signal into the repeater would cause me to have RFI problems with my electronics in the shack. Indoor antennas, even while temporary, tend to be a bad idea.

As soon as our son can come over here, I'll do some experiments again to see if we can find a better location for the antenna using the cookie sheet ground plane that you suggested.

73, Dave K4EET[/QUOTE]

 

[K4EET]

Good morning @prcguy. Thanks for your comments! A ham friend of mine may have some clamp-on ferrites that I could borrow. I'll ask to see if they know what the composition is of their ferrites. 73, Dave K4EET

 

[XTS3000]

It's common for RFI to vary by frequency. A cable might be the right length to resonate, or perhaps a chassis component or circuit trace in the computer somewhere.

Ferrites are probably a waste of time. Move the antenna so it isn't directly under the computer. Get a cookie sheet and stick the antenna to that, then place in a better location.

You may want to relocate the coax first. If the coax is routed past a sensor or computer or anything electronics. Can’t hurt to try.

 

[jaspence]

I have a DMR radio that has a circuit design flaw and creates interference on one of the UHF repeater frequency pairs. It is otherwise a good radio, but no firmware fix, and it is a frequency used in my area, Hopefully you will have some luck with the suggestions.