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Help - Hum on 10m/6m AM


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I have an FT-8900R with a new Diamond K400s mount and a Diamond CR8900A antenna.

The radio is acting like I have a ground loop issue, but I can't rectify it.  On 10m and 6m, AM mode, I get very bad alternator hum and when my climate control fan or engine cooling fans are running, they also cause a hum. All of the noise goes away when the antenna is disconnected.  With the radio manually placed in AM mode, the problem does not exist on 2m or 70cm while the antenna is connected.  It only happens on 10m and 6m.

I moved the antenna and transceiver from the back of the vehicle, to the front.  I bolted the antenna mount to the front fender, just inches away from the battery, and then grounded both the fender and the mount to the negative battery post with a 1 inch wide ground strap that was only 7 inches long.  I connected the transceiver power lines directly to the battery, with both the negative and positive leads under 18 inches in length.  The issue still exists.

I thought it may be a bad alternator, but my son is an auto tech, bench tested the alternator as well as load testing it in the vehicle.  He said everything looks fine and there is only a 0.02v AC ripple; well withing normal tolerances.  I tested the ripple voltage myself and found the same results.  Also, a bad alternator would not explain the two electric motors also causing a hum.


As a side note, neither of my other two HF AM radios (Ham and CB) experience this issue.  Also, if I hook up a proper 1/4 wave whip, it gets worse, including testing with a different mount... implying the antenna and mount are not the issue.

Any advice on further testing and possible remedies would be greatly appreciated.

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Have you tried a different coax?


Yes.  I tried a different mount, coax and antenna.  The cables I tried are RG8u and RG58 with studded whips on a direct chassis mount, and RG316 that is built into the K400s mount.


I might move the whole setup to my car, instead of the truck, to see if it is vehicle specific or if its the radio.

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This could very well be noise caused by PWM variable speed motor controllers in your vehicle.


Try installing a 1000uF 25V electrolytic capacitor right across the motor wires of each of the offending motors.  Make sure you get the pos-neg correct.  Electrolytic capacitors will EXPLODE if you hook them up backwards.


Also, be sure to put the capacitors as close as possible to each motor.  Putting filter capacitors 5 or 6 feet down the power line will do nothing for radio noise.


You might also try putting large ferrite chokes on the leads where they come out from the motor controller modules.


I also do NOT recommend hooking the ground wire of your radio straight to the battery negative, or to some random point on the chassis.  This just by itself can cause noise from the vehicles onboard devices.  Don't listen to what the "old timers" blab about this.  That sloppy installation worked in the 60s and 70s, but not on modern cars.  There is always a "Common Ground" point somewhere on the car that has a lot of different circuits all grounded to that one central point, usually within a foot or two of the battery.  THAT is where you should hook your radio negative wire.  THAT point is where the engineers designed your car to have the minimum noise for electronic circuit returns.  Also, do NOT put a fuse in the black wire in a negative-ground auto installation.  Argue with me about that later if you must. 


The red wire can go right to the battery, BUT there is most often a good power distribution take-off point to hook that up as well, right at the end of the main positive cable.  It won't corrode over time if you hook it up to the power distribution point instead of messing with the battery.  (DO put a fuse in the RED wire, as close as possible to the power source.)  I just don't EVER hook anything but the car itself up directly to the battery.  The engineers who designed your car made it that way on purpose.


AGAIN: That old garbage of "just hook it straight to the battery" is just that... garbage.  It is NOT 1960 anymore, and you are NOT running a 235 inline 6 cylinder with points and a condenser.

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  • 1 month later...

One more point. Are you using a single or multiple ground connections? It is possible there is a difference between grounding points even on the same frame member. I'm speaking about the radio grounds primarily but you may need to run a line between your antenna mounts to the single ground point as well.One building I worked at had a 9 V difference between ground rods at each end of the building.

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Oh, wow... that is a huge difference. Basically everything is grounded separately. I didn't think it would be a ground loop after discovering is coming in over the antenna like recieved signals, not over the power lines.


Also I had a battery go tango uniform while winching. I figured I stressed my alternator.


I really think it's either this specific installation or the radio. I have had 3 other am hf radios in it, including a current dedicated 10m am rig currently installed, and it doesn't happen on any other radio.

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