Interference issue

lanbergld · #1 (**permalink**) 10-13-2009, 10:43 AM

When plugging headphones into my Yaesu FRG-8800, I get a strong distorted signal from 4.000 to about 4.080 MHz. The distorted signal sounds like its from a local FM broadcast station. This is a huge problem for me at this frequency, because I use a 4 MHz upconverter for LF monitoring. So therefore my reception of 20 kHz to 80 kHz is really hindered when using headphones (which I need for working LF). The headphone-induced overload also plagues, to a lesser extent, the 136 kHz band.

I verified it isn't an issue if my headphones are not plugged in.

Note: Sometime, but only sometimes, if I ball-up the headphone wire in my hand and hold this close down to my thigh, the interference is reduced. Also, if I lay back in my chair, at an almost horizontal position, this interference is reduced. But needless to say, those are both difficult measures when slow-tuning CW and beacons and such.

Has anybody experienced a similar issue when using headphones? And how did you solve it?

Thanks.

Larry Lanberg

Richmond VA

ka3jjz · #2 (**permalink**) 10-13-2009, 12:34 PM

Larry, while I'm not big into LW, I understand that MW bleedthrough - not FM - can be an issue when using some upconverters. A MW bandstop filter may be the answer - I am going to assume you have a loud AM station somewhere nearby to you that is likely overloading the upconverter.

If memory serves, Kiwa sells such, as well as others. See if you can parallel the interference with a local station - once you find it, then you should be able to devise a solution. There are, I think, FM filters out there as well, if you find you need one.

Homework time. 73 Mike

lanbergld · #3 (**permalink**) 10-13-2009, 12:48 PM

Hello Mike, Thanks but its definitely a local FM station. And it invades 4.000 MHz whether or not the LF upconverter is attached -yet- only when my headphones are connected to receiver.

Seems that the headphone wires act as an unwanted FM antenna, but only at that HF frequency. It wouldn't bother me so much except that I spend most of my time on longwave, and, my LF upconverter uses a 4.000 MHz crystal. So naturally my work down there is thus affected.

I'm going to try attaching ferrite cores to the headphone wire, when I get home from work. I'll post the results if it works.

Larry Lanberg

Richmond VA

comsec1 · #4 (**permalink**) 10-17-2009, 11:52 AM

I had a frg8800 that had an issue with interference, at the time I live within eyesight of a 50kw am station. I tried several things such as am broadcast filters and nothing helped. after reading a interference handbook I found the answer, I don't remember exactly as this was 15-20 years ago but this is what I found: the 8800 has some circuitry that rectifies the powerful signals and was causing my interference problem. this interference also clobbered my r390a's as well as any HF receiver I had, as soon as I unplugged the 8800 from the wall everything went quiet. my interference was generated by the radio itself because of the powerful am signal mixing with the circuit.

kb2vxa · #5 (**permalink**) 10-18-2009, 01:32 PM

Since the headphone cord is acting as an antenna it seems to me you have grounding and equipment bonding issues to look into. I'll bet you have other problems with your interconnections yet undiscovered.

lanbergld · #6 (**permalink**) 10-21-2009, 01:07 PM

Thanks comsec and Warren. Well, so far I can attenuate it by balling-up the headphone wire and clutching that at a certain spot in my chair. Have to do what I have to do, for now. Comsec: I think the Yaesu FRG-100 also has the same circuitry issue you mention. Regardless, I love Yaesu's. (Even the FRG-9600) I'm not sure why I do, but I think its because they are less than perfect.

Warren: Hopefully I'll solve it one day. I might be wrong but I believe you're referring to ground loop issues, right?

Larry Lanberg

Richmond VA

kb2vxa · #7 (**permalink**) 10-21-2009, 05:55 PM

Ground loops are one possibility but improper equipment bonding and/or ungrounded (broken continuity) shielding is more likely. Do you have a bonding wire connecting each individual item and a common point ground? Are your interconnects excessively long perhaps with an open ground lead or ungrounded shield somewhere? You have RF where it doesn't belong, cabinets, chassis and interconnects seem hot to me, it's leaking into the circuitry that should be shielded and connecting an "antenna" only makes them hotter contributing to the leakage.

kc2rgw · #8 (**permalink**) 10-21-2009, 06:31 PM

You could try changing the length of the headphone cable and/or wrapping it around a toroid core near to the plug end.

I've had HF rigs that were horrible for RFI when I had headphones in and fine otherwise. I think the headphone circuits don't have much isolation in them.

lanbergld · #9 (**permalink**) 10-21-2009, 07:13 PM

Warren, I'm going to look further into "broken continuity of shielding". Interconnection length is interesting and probably a good thing to focus on too. (Up until now I've tinkered with the geometry & path of the interconnections) There's a lot of things I'm looking at, experimenting & tinkering with.

On a related note, I've heard that hand-capacitance indicates a need to balance something, somewhere in the system. So another project I have is to build an impedance matcher for low frequency work. Sure, I've seen plans & schematics for these, but obtaining the required parts is a greater challenge. I don't spend much dough. I tear apart old surplus electronics at work & get parts like that.

As far as my RF environment goes, well, I'm pretty resourceful with the situation I have. I'm way, way up in a downtown high-rise & surrounded by equally tall buildings. Some believe its impossible to Dx (reliably) from a dense urban location, but actually I do pretty good I think. The challenge of getting low-power NDBs from a long distance away makes it all worth it.

Kc2grw: I just saw your post now. Thanks. I haven't tried toroids yet, but I can report that standard ferrite 'capsules' did not work at all for this. Again, length of cords is something worth focusing on too.

Larry Lanberg

Richmond VA

kb2vxa · #10 (**permalink**) 10-24-2009, 06:33 PM

"You could try changing the length of the headphone cable and/or wrapping it around a toroid core near to the plug end."
That's like putting a Band Aid on a gaping wound. He has design issues to resolve quite unlike the plug 'n play radio you're talking about. Still what you describe is a grounding problem and balling up the cord as I said early on won't solve the underlying problem, only mask it.

Larry, you're on the right track, I believe I may have asked the right questions and started you thinking and tinking. This reminds me of the never ending battle of engineers vs. technicians and the R&D work I used to do. An engineer would bring me a prototype to analyze and as always what works on paper never works on the bench. I'd fuss around with it until I got it working properly not really understanding the science behind it, all I cared was getting this mess into production to justify my paycheck. When the engineer asked me how I got it to work all I could do was show him the mods I documented and say "ah dunno, it jus' werks" and watch the deer in the headlights look I got back. One time he said "All I wanted you to do was analyze it." and was he ever stunned when I replied "Oh, I thought you said ANALize it!"

Oh it's so much fun when you know how the game is played!