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Author Topic: FT-8800R Mobile Installation RFI?  (Read 2133 times)
N1ZHE
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Posts: 68




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« on: March 14, 2011, 10:16:38 AM »

I have a new Yaesu FT-8800R feeding a Diamond SG-7900R antenna mounted with a Diamond K600M trunk lip mount.

The radio is mounted remotely in the trunk of my 2007 Toyota Corolla. I am using the YSK-8900 separation kit.

My SWR is 1.4 or less on 2 meters as well as 440.

The radio plays very well on 2 meters, however it's an entirely different story on 440.

When I first transmit on 440 all is well, good carrier and audio. But if I say more than a few words, the audio becomes at best garbled and sometimes just a few syllables get through.

I've only briefly tried this on a few different 440 machines and always at the high power level, I always had the same results with my audio on 440.

Someone suggested today that RFI might be getting into my radio, probably via the separation cable which looks like a network cable for your computer, with the modular jacks and all.

Has anyone seen this or something similar? I know I need to do some more experimentation to be sure, I intend to try lower power levels and see if it reoccurs.

If lower power levels "solve" this problem, would you suggest some kiind of ferrite cores for the separation cable? At both ends? I know nothing about ferrite cores so if this is the solution, specifics about where to buy and what exactly what to buy would be appreciated.

David




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K0BG
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« Reply #1 on: March 14, 2011, 01:42:53 PM »

I'd bet it is common mode current. No matter how well you think you mounted the antenna, clip mounts just are secure enough in come cases. This even causes some folks to sand the paint to the metal surface which is a very bad idea. Since the vehicle has a zinc dip before it is painted, when you sand through the zinc coating, it allows rust to form further exacerbating the issue. This is why it is always best to drill a hole, and use an NMO mount which doesn't leak if installed correctly. Remember too, Pacific Rim antennas advertised as NMO are not compatible with real (New Motorola) NMO mounts.

About your only recourse is to install about 5 or 5 turns of the feed line around a mix 31, 3/4 inch ID split bead, and hope for the best.
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N1ZHE
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« Reply #2 on: March 14, 2011, 11:22:28 PM »

I have a question. I'm not disputing what you say, I'm just trying to learn.

Why does this only effect 440 and not 2 meters?

David, N1ZHE
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K0BG
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« Reply #3 on: March 15, 2011, 06:59:44 AM »

If you understand how impedance works, you'd have an Ah Ha moment. There are so many factors involved, you can't point a finger, and say with certainty why 2 meters isn't affected too. Possibly, the length of the coax between the antenna, and the radio is just the right (or wrong) length.

Lots of folks just can't bring themselves to drill a hole in sheet metal. Instead they use clip mounts, mag mounts, etc. to keep from doing so. If they have problems as a result, they either aren't affected by them and/or they don't cause any odd behavior to occur.

You might get yourself a few mix 31 split beads, and put a few turns through one close to the antenna, and see if that cures or lessens the problem. If it does, it is indeed common mode.
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N1ZHE
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Posts: 68




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« Reply #4 on: March 20, 2011, 04:11:37 AM »

Alan,

I checked over my antenna and mount and discovered those 4 little Allen screws that are supposed to ground against the body and keep the mount tight were loose. Duh!

I tightened them and 440 works as advertised.

After learning what I have (a lot from your web site, thanks! It rocks!) I am not so pleased with this antenna setup. I normally keep a vehicle until I "drive the wheels off" so drilling a hole in the trunk lid is not a problem.

I'm going to have to "live with what I got" for the next month or so, then I will replace my current antenna and mount with an NMO mount and antenna.

I don't know anything about that company in California you warned me about, I usually stick with the "big brand names" such as Diamond, Larsen, etc.

Thank you VERY much for all your help.

David, N1ZHE
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AD7C
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« Reply #5 on: March 24, 2011, 11:32:04 AM »

Since you fixed it... congrats, however, as additional notes:

Make sure your coax cable, power cables, and separation cable are not run close together.  A lot of people run all 3 along next to each in the vehicle and that is just begging to couple RFI into your radio.  The main suggestion would be to run coax on one side and power/control cable on the other side.

Rich
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K0BG
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« Reply #6 on: March 25, 2011, 09:39:31 AM »

Rich, just about every text on the subject says the same thing you did, about running power, and coax cables together. However, the truth lies elsewhere.

If the antenna in question is mounted properly, what little common mode there may be, is easily choked. For HF, that's about 5 or 6 turns through a 3/4 inch ID, mix 31 split bead. For VHF, mounted with an NMO in sheet metal, you won't need any choking. Further, it isn't the transceiver's power cable you have to worry about if there is common mode; it is all the other cabling inside the vehicle.

I have often used under-sill wiring troughs for all three, along with the vehicle's wiring, and never had a problem. But then again, I don't use clip mounts, and other mounting styles which lead to common mode in the first place.
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