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  #1 (permalink)  
Old 12-27-2012, 10:21 AM
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Default 2 Meter Interference Problem

I currently have a 50 watt, Kenwood TM-241 installed in my vehicle. The antenna, Lasen 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 occurs: 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?
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Old 12-27-2012, 6:43 PM
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I wonder if your NMO mount is properly grounded to your roof. Have you tried tightening it up? It could be that you have always had some reflected power coming back toward you, but with a poor quarter wave you don't have a loading coil to absorb some energy.

Try putting ferrites right next to the NMO mount, not near the radios (where it's convenient). Also consider that some ferrites don't work very well at VHF frequencies. Are you certain what kind of ferrites you have?

Finally, check the whole things for short circuits. You never know. You might have had one all along and you never noticed.

That's where I would start...
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Old 12-27-2012, 8:21 PM
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Check the DC voltage going to the Inverter. When you changed the antenna it may have changed the loading on the radio a little making it pull more power in transmit. This could make the supply voltage drop if you have a bad connection somewhere. Slim chance this is it but I have seen this happen before. You may also want to put a bypass capacitor across the power leads right at the inverter. Use something like a .01 or .1 uf disk capacitor. Radio Shack has them and there cheap. Keep your ground lead as short as you can and also try grounding the inverter case if its metal. Does the antenna cable run near (1 foot or closer) to the inverter. If so seperate them.
It sounds like the RF from the antenna is strong enough to get back into the inverter and cause problems in it. When dealing with RF a good DC ground may not be a good ground to RF. This is why I said to keep all the ground leads as short as you can. Fixing these type of problems is a real pain. You just have to keep trying things.

Good luck and let us know what you did to fix the problem.
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Old 12-28-2012, 8:10 AM
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If the installation were done properly, you could eliminate the "rig draws more power" possibility. All radio power in a car should come directly from the battery, not from the car's wiring (like an accessory socket). I've been out of the business too long to have installed MDTs (laptops), but I'd suspect that the problem is the same - they don't like the car's electrical system any better than radios do.

Run the radio power leads directly to the terminals on the battery and you'll probably eliminate the immediate problem. Then check the SWR on the antenna and if it's not 1:1 somewhere in the band (the area you operate in most of the time, or 147.0 if you want a compromise), trim the antenna to get a better match. I've never run a 241, but some of these rigs will tolerate quite a bit of reflected power, at the expense of drawing a lot more current from the battery.

(The other possibility is that, due to a high SWR, the feedline is radiating a lot of power, and that's getting into the inverter. Trimming the antenna for a low SWR would solve that too. Throwing bypass caps and grounds around willy-nilly usually creates ground loops and more problems than it solves.)
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Old 12-29-2012, 9:12 AM
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Default Finally!

Due to everyone's input I ended up removing & re-installing everything.

1. The radio & inverter have seperate feed lines connected directly to the battery, which is located in the trunk. Checked all those connections, looked fine, but I replaced them with new lines & connectors. Did not change the routing of the lines - they are not routed together to the battery.

2. Stripped the headliner out (real pain in...). Checked the NMO mount, nice & tight, no rust or indication of leaks, looked at the grounding location and it looked fine. Had another mount w/cable, so I replaced the antenna mount and used the same route to the radio.

3. Placed choke on the antenna feed line as suggested (closer to the antenna).

4. Placed chokes on the power leads for the radio & inverter.

5. Checked the SWR on the 1/4 wave with the inverter & laptop turned off. SWR is perfect.

6. Turned on the inverter & laptop - keyed mike, same problem.

7. Took an hour off - went to Walmart & bought another inverter. Old inverter is 2 years old.

8. Put new inverter in the car. Turned everything on - keyed mike. PROBLEM SOLVED!

Now I know very little about inverters, but I can only speculate that something happened to the inverter that allowed it to become super sensitive to RF.

At first I was a little ticked that I had spent the entire day replacing all the power lines and antenna mount. Taking apart the headliner and other parts of a Charger is a real pain. But after I thought about it I am glad I did do all that work because I did eventually, by process of elimination, find the true problem.

I posted this problem here because I knew the Hams, and other radio gurus, would chime-in with suggestions. Your comments helped me solve the problem. I do appreciate that.

Thanks for all the input. Now, let me go to the garage and put the car back together.

73.
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Old 12-29-2012, 10:14 AM
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Quote:
Originally Posted by qslmark View Post
Due to everyone's input I ended up removing & re-installing everything.

1. The radio & inverter have seperate feed lines connected directly to the battery, which is located in the trunk. Checked all those connections, looked fine, but I replaced them with new lines & connectors. Did not change the routing of the lines - they are not routed together to the battery.

2. Stripped the headliner out (real pain in...). Checked the NMO mount, nice & tight, no rust or indication of leaks, looked at the grounding location and it looked fine. Had another mount w/cable, so I replaced the antenna mount and used the same route to the radio.

3. Placed choke on the antenna feed line as suggested (closer to the antenna).

4. Placed chokes on the power leads for the radio & inverter.

5. Checked the SWR on the 1/4 wave with the inverter & laptop turned off. SWR is perfect.

6. Turned on the inverter & laptop - keyed mike, same problem.

7. Took an hour off - went to Walmart & bought another inverter. Old inverter is 2 years old.

8. Put new inverter in the car. Turned everything on - keyed mike. PROBLEM SOLVED!

Now I know very little about inverters, but I can only speculate that something happened to the inverter that allowed it to become super sensitive to RF.

At first I was a little ticked that I had spent the entire day replacing all the power lines and antenna mount. Taking apart the headliner and other parts of a Charger is a real pain. But after I thought about it I am glad I did do all that work because I did eventually, by process of elimination, find the true problem.

I posted this problem here because I knew the Hams, and other radio gurus, would chime-in with suggestions. Your comments helped me solve the problem. I do appreciate that.

Thanks for all the input. Now, let me go to the garage and put the car back together.

73.
That is known as a nearfield problem. Some electronics is adversly affected by strong RF fields
as it is not designed to shield out strong RF fields and the 1/4 antenna just made that nearfield
stronger. The 5/8 antenna simply put radiates better and gets the strong nearfield outward better
than a 1/4 wave antenna does. I have seen this happen many times with 50W and higher power
radios many times.
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Old 12-29-2012, 12:05 PM
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May I suggest simply eliminating the inverter?

Dell makes an AC/DC laptop "travel" power supply for most of their computers. The one I bought for my Latitude D620 includes cables and inputs for 120VAC, 12VDC cigarette lighter plug, and whatever plug the airlines use. It's about half the size of the standard AC power supply. Since I can plug it right into the cigarette lighter socket or hard-wire it to 12 VDC in the car, I can eliminate the inverter.

I recently replaced the Dell with a Lenovo X230. The travel power supply I bought for it consists of a cigarette lighter plug and a "wart" on the cable about 1.5" x 1" x 0.5". Again, no inverter necessary.
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Old 12-30-2012, 2:15 PM
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I often wonder how susceptible modern switching supplies might be to RF. Nobody spends much time measuring how much energy can be coupled in to the unit at what frequency.

I'm tempted to take a dip meter to one some day and see for myself how bad it is, if for no other reason than to know what to watch for on HF.

Good find! And by the way, cleaning up the antenna installation is not a wasted effort. You may be pleasantly surprised by some improved performance...
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