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Old 12-02-2020, 15:16   #1
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Isolating the vhf antenna from the mast

I just discovered that my mast mounted Shakespeare stubby antenna was providing DC negative ground to the mast by way of the outer ring of the pl239 connector. Is this a problem? If yes, has anyone isolated the antenna attachment from the mast?
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Old 12-02-2020, 17:15   #2
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Re: Isolating the vhf antenna from the mast

It should have continuity to the mast. Even if it didn’t, both are grounded. If you are worried about whether that affects the antenna radiation pattern, don’t be - DC continuity isn’t synonymous with RF isolation.
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Old 13-02-2020, 06:07   #3
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Re: Isolating the vhf antenna from the mast

Actually because the mast is bonded to an underwater plate, and i have 2 masts (ketch) i was concerned about electrical consequences from such a connection. I was aiming to create a floating ground (I think that's the term) where DC-neg connects to battery neg post. FYI, I have removed all AC from the boat.

Should I still explore isolating the antenna connection from the mast?
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Old 13-02-2020, 08:08   #4
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Re: Isolating the vhf antenna from the mast

You could use a plastic mount. Or use a rubber gasket and plastic bolt sleeves. To isolate the metal mount from mast.
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Old 13-02-2020, 09:33   #5
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Re: Isolating the vhf antenna from the mast

Quote:
Originally Posted by andrzej0nl1ne View Post
I just discovered that my mast mounted Shakespeare stubby antenna was providing DC negative ground to the mast by way of the outer ring of the pl239 connector. Is this a problem? If yes, has anyone isolated the antenna attachment from the mast?
If the base of the antenna is connected to the tip of the mast it should improve transmission output, these antennas use the "ground" as a reflection of the active vertical portion and the aluminum mast will do this very well. If insulated you will only have the cable shielding for reflection.
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Old 13-02-2020, 10:09   #6
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Re: Isolating the vhf antenna from the mast

Andrze, have you asked Shakespeare's tech support about this, including the model number of the antenna and grounding situation of the mast?
I have a VHF antenna on my car that tip to sleeve probes out as being 'shorted' but that 'probing' does not account for the loading components in the base of the antenna nor for the capacitive coupling between antenna and vehicle.
After we got done with the multimeter, the real failure was discovered to be the crimped shield on the plug end. A new plug and the antenna was working normally again.
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Old 13-02-2020, 11:49   #7
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Re: Isolating the vhf antenna from the mast

Our rigger was able to add a non-conductive bushing to isolate the antenna from the mount. Why? We go through a lot of zincs even though we are at a mooring. One suspected cause is a battery that is set up when bottom paint and barrier paint expose the raw keel to seawater. Keel -> Mast Bonding -> Mast -> VHF bracket -> VHF antenna -> VHF ground/shielding -> VHF radio ground -> boat DC ground -> engine -> shaft -> prop -> zinc. We have seen some benefit of this isolation. The other alternative for this loop is to use a DC block. The VHF only requires AC. A DC block allows the AC of the signal to pass through the DC block, not a DC ground, thus breaking the DC ground circuit. It is important to match the type of DC block with the frequency. I had trouble confidently finding one so went with the bushing approach.

Shakespeare used to sell a plastic mast bracket like this. https://www.smgeurope.com/shakespear...ket-black.html I couldn't find one in stock when I went looking. Or if you have access to a 3D printer with the right kind of plastic you could make one or have one made.




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Old 13-02-2020, 12:41   #8
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Re: Isolating the vhf antenna from the mast

Quote:
Originally Posted by andrzej0nl1ne View Post
Actually because the mast is bonded to an underwater plate, and i have 2 masts (ketch) i was concerned about electrical consequences from such a connection. I was aiming to create a floating ground (I think that's the term) where DC-neg connects to battery neg post. FYI, I have removed all AC from the boat.

Should I still explore isolating the antenna connection from the mast?
You are very wise to use 2 pole dc electrics
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Old 13-02-2020, 14:41   #9
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Re: Isolating the vhf antenna from the mast

Hlec00's comment was the area of my concern. Not sure that I have a problem but I'll fabricate that bushing for the peace of mind aspect. Thanks all for your advice.
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Old 13-02-2020, 14:46   #10
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Re: Isolating the vhf antenna from the mast

If it is not a problem it is a potential problem--(I know--I cannot help myself)

You must never let any metal equipment on a boat that is aluminium c become part of the negative rail of a battery system, the salt sea spray being a good electrolyte.

I would mount it on an insulated platform, such as Tufnel or something similar in sheet form, with a porcelain insulating base mounted on that.

You can cast your own base bracket using fibreglass--which is a fairly good insulator if you paint it using acrylic enamel where it is in the way of sunlight. To make these gadgets frames etc I use 3 mm plywood faced with plastic laminate as a mould. It takes release agents easily, and after the release compound of a couple of polished layers of Carnauba wax, I use the blue wash-off stuff after that.

You can make almost ANYTHING using this faced plywood stuff, glass and epoxy resin. Anything from hatches to shower bases One of the great boat repair and construction secrets. I once used it to repair a large amount of hull damage to a trimaran I owned. It allows a great finish.
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Old 13-02-2020, 14:56   #11
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Re: Isolating the vhf antenna from the mast

The easiest way to isolate a VHF antenna is to buy a lightning arrestor that does not couple DC. Any of the well-known vendors of these devices can supply them in three "flavors." One, the most common, has a DC path in common with both the center conductor of the coax and the shield. The two other types will isolate only the center conductor or both the center conductor and the shield. This is a common problem on aluminum boats.

Depending on the design of the antenna, insulating it from the mast might interfere with its operation. Isolating the coax at the base of the mast eliminates this.
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Old 19-02-2020, 15:50   #12
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Re: Isolating the vhf antenna from the mast

Thanks for the advice. A quick search retuned an information overload priced from $8 to $400. Can you suggest a specific model for insulating the coax at base of mast. Any guidance would be appreciated.
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Old 23-02-2020, 22:12   #13
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Re: Isolating the vhf antenna from the mast

I know of only one vendor who still makes inner-outer-DC-blocking coax surge arrestors for VHF antennas that bond the outer braid to the mast:
Arraysolutions.com. Order AS-302U, specify Outer DC-block (inner is intrinsic), optimized for 156 MHz.
This is an excellent design that protects your equipment and DC-isolates your VHF coax with DC-Negative outer shield on equipment side from the mast/lightning grounding.
Engine drive shaft should be the only DC-Negative ground point in the water: not SSB ground, not electric-windlass frame/anchor chain.
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Old 24-02-2020, 05:03   #14
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Re: Isolating the vhf antenna from the mast

Greetings and welcome aboard the CF, tomsailor.
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