2 Meter Interference Problem

[qslmark]

I currently have a 50 watt, Kenwood TM-241 installed in my vehicle. The antenna, Larsen 5/8 wave, is roof-mounted with an NMO mount. I also have a Dell Laptop computer being powered by a power inverter. Been using this setup for about 2 years with no problems. (Cords/cables for all equipment do not mingle together.)

I needed to change the antenna to a 1/4 wave due to some clearance problems with the 5/8 wave. When I made the change this problem occured: Keying the microphone on med or high power, the inverter powers off/on and the computer restarts. Changing back to the 5/8 wave solves the problem.

I put ferrite chokes on the power line feeding the inverter, the power line to the Dell, and the 2 USB cables plugged in to the computer. The antenna mount cable is low-loss & double-shielded RG-58. Have double checked all connections and SWR is perfect. Problem still exists when using the 1/4 wave.

Any ideas or solutions?

 

[Mudfoot]

Is the 1/4 wave on the old NMO mount with original csble? Or are you using a different mount with new coax?

 

[qslmark]

Is the 1/4 wave on the old NMO mount with original csble? Or are you using a different mount with new coax?

NMO & coax were replaced 4 months ago. The car was recalled for replacement of a side airbag sensor, and the dealer damaged the coax. I replaced it myself. The SWR is perfect with both antennas using this mount/coax. I thought about the mount/coax might be it, but the 5/8 wave is just fine using any power level.

I did try a different radio and the results were the same. I also removed all the USB cables and the power cable for the laptop. Even without any type of cable plugged-in to the laptop, the results were the same. Weird.

I did talk to a radio tech friend who works for the local cop's radio shop. He said that the 1/4's radiation is much closer to the horizon than the 5/8 wave and that could be the cause - he saw this problem before: Key the mike and the patrol car's MDT unit would cycle off/on. Problem was tracked down to the power/control cable for the lightbar/siren. RF was following cable into the car interior. Chokes did not work. Only fix was to use a high gain antenna versus the 1/4 wave.

I'm hoping there is another solution than his answer.

ADDED: Tomorrow I plan to put a temp radio w/magnetic antenna in my wife's car, place the laptop in it and see if it duplicates the problem. I have a buddy that might slide by with his laptop so I can test it in my car.

 

[W5LZ]

That is odd! I honestly doubt if changing antennas was the complete problem, but it sure brings the fact out that there is one. As close as that antenna is, I very seriously doubt if it's because of any differences in the antenna's radiation pattern, no matter what kind of antenna it is. 'RF' may be the problem but not because of the antenna's 'size'.
I'm wondering about a few things. You said an airbag had to be replaced, after it was, did the cable follow the same routing as before, or was it moved any? Sometimes, it doesn't take a lot of difference to make a difference.
I also wonder about those USB cable and what they are 'connecting' to the laptop. Any significance/susceptibility to RF from them? Since they aren't 'connected' tot he radio I don't see why it would affect the radio, but it may 'point' to a cause, sort of.
The 'other' way of RF getting to the radio and the computer especially is through the power lines. (I hate those @#$ inverters, wonder why they don't just do the DC power directly from the DC supply like they used to do?) The power source and lines would seem to be a much more probably entry point that just RF. I use an inverter too, but don't use the car's cig.plug, it's wired directly to a second battery through an isolation device, same for the radio and laptop. I haven't noticed any significant problems so far. (Long story about that, you don't wanna know.)
Very basically there are two routes into things by RF, radiation, and conduction. The problem has to be with one of them. Ferrite 'beads' do work, but are not always a 'cure'. So is distance between cabling for various things. I have to think there's something neither of us are thinking about so the 'trick' is to find that 'thing'.
Good luck! If/when you do find the problem, I really hope you let us know about it.
- 'Doc

 

[Shockwave]

Try placing ferrite on the coax near the antenna. You may have more common mode currents flowing down the braid on this cable with the 1/4 wave than you did with the 5/8 wave. If that doesn't work, try moving the ferrite down the coax about 1/4 wavelength away from the base.

 

[qslmark]

FINALLY!

Long story short. Replaced all the power lines & the antenna mount. Put chokes on everything, and on the antenna cable where suggested. Same problem.

Replaced the inverter. PROBLEM SOLVED. I guess something went wrong with the inverter and it became super sensitive to RF.

Thanks for making me think, thus solving the problem.

Now, let me go out to the garage and put the interior of the car back together.

73 & Happy New Year.