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Ground loop between transmitter and tuner?

Mar 15th 2016, 01:27


Joined: Jun 22nd 2000, 00:00
Total Topics: 0
Total Posts: 0
I am installing my first transmitter, an Icom m802, in a 30 foot sailboat. The instructions say I should ground the transmitter and tuner to each other at the same ground, yet the transmitter and tuner are already connected to one another through the shield of the coax connecting the two. My question is: Why isn't this a ground loop? Why doesn't half of the RF current travel on the shield and half on the ground between the two units? Why doesn't this cause common mode RF radiation from the ground connection and from the coax shield?

Because the negative power, ground terminal and coax shield are connected internally within the transmitter (and within the tuner), there are actually three other potential ground loops:

1) transmitter negative power --- tuner negative power.

2) negative power of the transmitter --- negative power of the VHF radio --- shield of the VHF antenna --- VHF antenna=== sailboat mast --- sailboat ground system --- tuner. (I know I can fix this one by isolating the antenna from the mast, but it is really hard to get to right now.)

3) negative power of transmitter --- the frame of the diesel engine (for the starter and alternator) --- the propeller shaft --- the propeller --- salt water (about 12 ohms here) === sailboat ground plate --- tuner.

I am a computer engineer, but I seem to be missing something here. Is there some magic that keeps the RF energy on the coax? I could use an intuitive explanation here.

Mar 15th 2016, 03:37


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Doug -

In a properly set up antenna circuit, no RF current flows on the outside of the coax shield. If there is current (caused by inductive coupling with the antenna, for example, or some other imbalance in your antenna system), it can affect your radiation pattern and possibly raise the RF potential of your rig (causing unpleasant shocks, feedback issues, etc.) People often put ferrite chokes on the coax to reduce those shield currents, when needed.

But the main point is that the RF is meant to be completely contained inside the coax. Current flowing one way on the inner conductor is balanced by an equal and opposite current flowing the other way on the inside of the shield. Very little current or voltage should be present on the outside. (That's because of the RF skin effect. RF currents flow only on the surface of conductors, generally.) In any case, there's probably no "ground loop" issue for the RF.

For DC, 60 Hz AC, and for nasty surges (lightning?), you do want to provide a ground path that does not depend on the coax shield, partly (mainly?) to protect your electronics. But of course DC will divide among all the available conductors.

One reason you want to avoid ground loops is that a loop acts as an antenna, picking up RF currents from the antenna. You can minimize this by running the ground wiring close by the coax, minimizing the area of the "loop". I doubt this is going to be a big issue for a simple (non QRO) sailboat installation.

I haven't tried combining sailing and hamming, so take all this with a grain of salt!

73 Martin AA6E

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