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Creating a RF choke...on purpose?

Freightlinerbob

Freightlinerbob

Member
Mar 20, 2012
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I have a Texas Star 350HD installed in a Freightliner truck. It works really well when the engine is off.

When I key the mic with the engine running, the dashboard gages cycle, the warning lights and buzzers come on as if I've just turned the ignition off and on again. The advice I've received was proper bonding of the antenna mount, door and cab. If that didn't cure it, I was advised to- on the outside just before the antenna- "coil the coax over a 3 inch diameter pipe, 5 or 6 times taping it up really well, creating an RF choke to keep the signal from feeding back into the cab."

What do you all think of this?
 

Similar RF chokes are commonly used on base antennas. I have one on mine right now. These chokes are there to minimize common mode currents on the outside of the shielding, as well as assist with decoupling the antenna from pretty much everything else attached to it. They will not affect your transmitted signal by any appreciable amount if any at all.

I would tie such a device down pretty tight or the constant high winds from highway driving might loosen it causing it to flop around.


The DB
 
Interesting. I just now tried it, here's what I found:

I took a spare length of coax, created the choke and hooked it up. SWR went down ever so slightly. 1.4:1 from 1.5:1 on 40. Ch. 1 stayed at 1.1:1.

The one giving me the advice tested the amp and showed me that running it past 11 O' clock on AM (100 w) and 1 O'clock on SSB (125w) really distorted the signal. At these settings, it sounded very clear.

So without the RF choke, I was able to keep it at the above settings without affecting the dashboard, from what I could tell.

With the RF choke, I was able to set the amp a little higher before the dashboard was affected. But I really shouldn't be running it that high anyways.

What is happening here? Is the distorted signal from an amp set too high causing the problem or is it something else?

If it is the distorted signal, am I safe running the amp at lower output?
 
Two possibilities there. Yes, over driving the amplifier will cause distortion and some unwanted RF. The other possibility is that your truck is just getting too much RF into it. That doesn't mean it's 'bad' RF, just more than it can handle. You could go to a LOT of trouble shielding the dash etc, or just turn the power down.
Are there other possible causes of what you are seeing? Sure, but the two above are the most likely.
- 'Doc
 
1) Braided wire connected from the amp to the closest point of the frame.
2) Add Low Pass filter after amp.
3) Check modulation level in radio.
4) Make certain amp isn't being over-driven.

Few thoughts . . .
 
Thanks to both of you.
Edit: all three of you...

In your opinions, am I correct in concluding that setting the amp at a lower level than would cause the above problems is safe?

Thanks.
 
Last edited:
My conclusions were premature. The dashboard still cycles with the RF choke installed. I think I wasn't speaking loud enough or the dynamike was turned down when I tested it earlier today. Bummer.
 
Freightlinerbob said:
With the RF choke, I was able to set the amp a little higher before the dashboard was affected. But I really shouldn't be running it that high anyways.

What is happening here?
Click to expand...

You have a poor RF ground and the increase in power is showing this up. When the RF hasn't a good path to ground it uses what it can which usually means the coax. This means your radio becomes RF hot as do the power cables and it gets into the electronics of the vehicle.

The RF choke you're using isn't good enough. For CB band it should be 5 turns wrapped around a 4.5" former. Either that or get a couple of clip on 31 Mix ferrites and wind the coax through one of them as near to the antenna a few times and likewise with the power cable for the CB and amp.

I take it you're using a mirror bracket to mount the antenna on? Its just about the worst possible place so you're going to suffer to some extent if you run power.

www.k0bg.com if you want some reading on it.
 
Could it be cheap coax causing this? I ran what I believed to be quality- one piece -coax, but I do also get my voice coming through the stereo when talking barefoot.
 
Could it be cheap coax? Sure, but it's not very likely. It amounts to 'where' is that objectionable RF coming from. And, there are two sides to that, which means it could be a problem with the device receiving the problem, the radio receiver you are hearing your voice over. So how about the 'where' thingy? What's the strongest source of RF, the antenna. It's supposed to produce RF so it really isn't the antenna's 'fault', right? How about the feed line? There's always some 'leakage', but in most cases it's seldom the source of the problem. Even with really 'leaky' coax the amount of 'leakage' isn't that much, especially when compared with what's coming out of the antenna.
Howz all this going to help you 'cure' the problem? By making you consider that the most likely source for the problem isn't always what you think it may be.
- 'Doc

(OR! Park the thing in my driveway for a week and I'll fix it. You might wanna leave your debit card and pin number too. And if you're dumb enough to take me up on that it'll surprise the poo-poo out of me.)
 
the cheap coax may/maynot cause your problem. but for a short piece id definately
get a good piece. id get LMR-195UF OR LMR240UF. the uf stands for ultra flex u will
want this a a mobile install.remember your antenna and coax are your most important
thing in a setup.which leads to nother question what kinda antenna?next is what
type of radio and microphone? make sure your radio has the audio limiter IN PLACE
if its not this allows the radio to over modulate and cause the amp to have all kinda
harmonics and splatter. this could be your problem. another is too much audio from
a power mic but in a big rig this is a no no.so i will assume you use either a stock mic
or noise cancelling mic from here. back 2 the radio i have same amp i use only a 1-2
watt dead key [carrier] you will need a power meter for this, if your radio has a
variable id check this if not take it to your shop and tell them what ya want. last
is too much mic gain now assuming your only using a stock mic or noise cancelling mic
try turning your mic gain down a little between 1 oclock and 3 oclock. while all this will
not promise to solve your issues its definately were id start and make sure your not
overdriving your amp ,and keep your signal clean
 
For those that want to know:

Side mic 148gtl , NOT clipped
636 mic with 5 pin plug, no adapter
The radio is tuned but clean.

I had some small progress today. I had a chance to stop by Bob's CB in PA and they wanted to sell me new power cables, antenna mount, stud, and on.... Double shielded coax seemed the logical and easiest place to start. so thats what I did and am now able to run the amp without affecting the dash, but not at 100w.
For reference, 11 O'clock on the knob is about 100w and very clear. I'm able to run it at 9.5-10. The knob starts at about 7 O'clock.

But this was with the cable passed through the vent window. I'm sure everything could change again once I feed it through the mirror and door.
 
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Chokes on the power leads might also help. Try moving the antenna away from the stereo antenna. If your dash and instruments aren't shielded from the antenna you may never be able to keep 100 watts out all together.I installed some cranes a few years back and had to mount the antennas through the metal roof to keep the rf out of the electronic scales. With the mirror mounts the scales where getting a" LOOK "at the antenna through the windsceen. The coax on the stereo might need replacing as well. I once spent a day trying to stop a commercial twoway from turning a customers stereo on and winding the volume up full ( love cpu's) The GM factory coax had no shielding on it just 1 strand of wire, replaced the cable, problem fixed.Apart from the huge bill! As the power goes up so dose the problems.Hope some of this helps. Merry Christmas
 
Freightlinerbob said:
For those that want to know:

Side mic 148gtl , NOT clipped
636 mic with 5 pin plug, no adapter
The radio is tuned but clean.

I had some small progress today. I had a chance to stop by Bob's CB in PA and they wanted to sell me new power cables, antenna mount, stud, and on.... Double shielded coax seemed the logical and easiest place to start. so thats what I did and am now able to run the amp without affecting the dash, but not at 100w.
For reference, 11 O'clock on the knob is about 100w and very clear. I'm able to run it at 9.5-10. The knob starts at about 7 O'clock.

But this was with the cable passed through the vent window. I'm sure everything could change again once I feed it through the mirror and door.
Click to expand...
yes the double shielded coax was the way to go.bobs cb is a good place ive bought alot
of stuff there.did ya put a rf choke on the NEW coax? also next time ya there see if
he has any ferrite chokes they go on your power cords that might help
 

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