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Old 28-04-2009, 09:37   #1
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VHF antenna - insulate or not?

I have some stray RF issues on the boat, but before I track them down, I'd like to know the recommendations for the mounting of VHF antennas and their electrical connections with respect to bonding/grounding.

I have marine VHF antennas atop the main and mizzen - a Metz Manta 6 and a Shakespeare 5215. Both appear to be shut-fed, as their DC resistance between shield and center-conductor is much closer to zero than infinite. The Metz 2m ham antenna, as measured from the coax, seems to be series-fed, and also insulated from the mast (no connection between coax shield).

If this is the case, then, unless there's a serious problem, then instinct tells me that I should have a single-point ground with respect to each VHF radio's chassis to the boat's main DC ground point - yet my Raymarine Ray240 radio internally connects the chassis and SO-239 connector to the power input's negative lead, as do both my West Marine VHF 550 and my Icom 706MkIIG HF/VHF/UHF ham radio. So I basically have a multi-point ground for my marine VHF radios (at the radio and at the antenna), and a single-point ground for the ham radio.

My RF theory is rusty, so pointers and explanations would be appreciated. Once I know the preferred/proper method (e.g., insulating the marine VHF antennas on the mastheads), I'll be able to begin tracking down the real gremlins in the system...
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Old 28-04-2009, 11:26   #2
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I wouldn't both insulating either antenna mount, although a ground plane is NOT required with either the Shakespeare 5215, nor the Metz Manta-6 antennae.

The Shakespeare will read “open” (high Ohms) circuit, and the Metz will read “closed” (low Ohms) circuit.

One of our radio guhrus (Bill, Midland, ...) should be able to offer more expert information.
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Old 28-04-2009, 11:44   #3
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Right - none of the three antennas require an external RF ground plane to radiate efficiently. I already verified their status after reading your earlier VHF response regarding shunt vs. series fed antennas. I just wanted to make sure the fixed-mount VHF radios were bonded correctly and not contributing to the problem...

The RF gremlins I'm trying to track down cause my handheld VHF to break squelch when inside the main salon, or near the Raymarine E-120 chartplotter at the helm. I've since re-terminated the E-120's power cable shield connector, and now I have to hold the handheld within 6" for it to break squelch at its lowest setting.

In the salon, the suspects are a Magnum Energy sine-wave inverter/charger (MS-2812), two Iota DLS-75 smart chargers, a small Statpower MSW inverter, an AC-powered desktop radio, and some PC equipment. I know for a fact that my el-cheapo Linksys 5-port ethernet switch has birdies in the HF spectrum - can't power that off at the moment or my wife will lose her VPN connection to her work (yeah - she's still a wage slave!).

Back to the radios, I do have some Alpha Delta lightning protectors which I want to wire into the lightning bonding system when I get a chance.
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Old 28-04-2009, 12:28   #4
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Jon,

No need to ground the VHF radios. As you noted, they're "grounded" thru the negative power wire.

Also, the VHF radios are highly unlikely to be a source of your "RF problems", since they don't transmit any appreciable RF while in receive mode.

Much more likely candidates are the inverters, the computer, the ethernet switch, the chartplotter, and anything else aboard with a motor or a computer.

The Iotas are not likely to be a primary source. They're very RFI quiet.

Liberal use of ferrites on the power and signaling wires of all suspect devices can help a lot.

Beyond that, it's a real sleuthing problem which is not easily solved.

Good luck,

Bill
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