Where Do You Ground Your Comms?
- Thread starter SSTJ
- Start date
OP
Ok, fair. So then, how does the DC circuit of my Ham radio or CB radio pick up interference from my alternator or spark plug wires? The ‘noise’ that they make isn’t DC is it?
Nope. Its an AC signal riding on top of the DC - its modulating the DC. The radio "should" be able to filter that crap out, but as everybody knows: In theory, there's no difference between theory and practice. In practice, there is!
OP
This came up today while studying for the Ham general license. For them, the answer is still "back to the battery".
The ham radio should have both wires fused and go to the battery. With the CB it is a good idea but not so important. The difference is the Ham radio draws more power.
OP
Understood. But can you explain why it's important to have the negative wire go to the battery, instead of to the nearest location on the tub? It still doesn't make sense to me, and I just want to understand.
It doesn't. A good ground at the nearest point is better than running an antenna for a negative lead - ASS-U-ME-ing that the battery is well grounded in turn.
I was taught at Ft Gordon there are a thousand things you can't control in communications - clouds, rain, trees, hills, solar flares, humidity, etc., so control what you can. Clean power was high on the list. Of course, that was almost 40 years ago.
If you are running pos from the battery, why not run two wires at the same time and have a dedicated ground? We're not talking 25 feet of wire here.
If you are running pos from the battery, why not run two wires at the same time and have a dedicated ground? We're not talking 25 feet of wire here.
I had a TJ and just did the most convenient thing, tapped into a fuse behind the glove box and used a dash screw for ground. Worked great but had intereference with built in LED fog lights in aftermarket bumper. When I moved it all to my LJ I went to the battery with power and ground. CB works exactly the same, great unless I have the led fogs on.
He explains things I didn't know about - like why not to fuse the negative, and VERY interesting historical information.
HOWEVER
I still say the SHORTEST possible wire from the radio to the NEAREST "good" ground point on the chassis/body/tub is best. Even that article says you want the lowest possible ground impedance - you won't get it by running feet of wire to ground - esp. in an RF sense. Inches only, and as few as possible.
Keep in mind that the TJ's frame does not constitute a "good" ground from the factory. I fixed that on mine, but still suggest using a tub ground instead of the frame when possible.
Head Lice
TJ Addict
The ground wire from the factory is designed for all of the factory systems including accessories that are expected to be about 20 amps total. If you add a high power transceiver obviously the factory wiring wasn't designed for that and you need a wire to deal with the grounding loads to do it by the book, following all the rules. It's that simple. I have a 1960 Ford with a 25 amp charging system, it's designed around that with suitable wires. If I put a high power transceiver that draws 50 amps I am way over the total rated current of my ground system transmit worst case. It's an extreme example but exactly why that is what they suggest. You can do the hand waving argument that the starter isn't always running but you design a system around worst case and not the best case. Hope that makes sense.
There is absolutely no difference between the copper wire that ships with a radio and the wire in your Jeep when it comes to RF. The only thing that matters is the route the wire takes getting to the radio. This is exactly why I suggest to try the easy route first and simply connect the radio to the nearest source of power able to supply enough current for your needs (obviously these radios under the ACC current ratings don't need a completely new circuit). Most of the time it'll work fine. If it doesn't you can simply re-route the path with the new wire but there's no need to jump to that step for low power radios like CBs unless you need to. If you do need to run a new wire, no big deal, it's not difficult and doesn't mean "I'm wrong" giving this advice. Most people will save time doing it the easy way.
OP
That was great. I will need to re-read when not on my phone, but I think it answers my question.
OP
I admit that I read it quickly, since I’m on a phone at the moment (forgive me), but I don’t think the author disagrees with you here, do they?
Partially. He wants you to run a long negative wire to where the battery is grounded to the body.
My Kenwood TM-V71A dual band radio manual says to connect both positive and negative wires directly to the battery with a fuse on both connections. Rather than make a big research project out of it, I decided to take the advice of the radio engineers that made the radio and have a vested interest in making it work to it full capacity. - it works wonderfully.
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