VHF antenna mounts and Electrolysis

[donradcliffe]

As a newbie on the forum, I'll start with a warmup question. If I mount a SS VHF whip antenna to the masthead, the standard L-bracket mount makes an electrical connection from the ground shield of the antenna wire to the mast. Since the other end of the antenna wire shield goes to the radio and eventually the boat ground, I am concerned about electrolysis damage to the mast. The mast step is wired to the keelbolts for lighting protection, and I think the mast may be at a different potential than the boat ground. Is this a real problem, and does anyone have a suggested fix??

[Brion Toss]

Hello and welcome,
I think your letter assumes ideas that are not accurate. The L-bracket should make no such electrical connection, except that from the juxtaposition of dissimilar metals, if I understand it correctly. And even if it did, the result would not be electrolysis (look it up). Therefore make sure that you don't have live current going to the mast -- it should be contained inside the wire -- and be sure to use Tef-Gel on any dissimilar metals that will be in contact with each other.
As for the keel bolts and lightning protection, do you really want a strike to come into the saloon? Seems like it would just fry you on the way to blowing the keel bolts off.
Fair leads,
Brion Toss

[Auspicious]

Originally Posted by Brion Toss

I think your letter assumes ideas that are not accurate. The L-bracket should make no such electrical connection, except that from the juxtaposition of dissimilar metals, if I understand it correctly.

It isn't often I can contribute here, so I welcome the opportunity.

It depends on the particular antenna. If the VHF antenna is a 20(ish) inch vertical with no coil at the bottom it needs an RF ground at the feedpoint (mast head). The RF ground may be 3 or 4 horizontal pieces (RF) grounded to the L-bracket (itself connected through the RF connector to the coax shield) in which case Brion's answer is correct. Otherwise a solid electrical connection to the mast is necessary to provide the RF ground; this is obviously not desirable from the perspective of connecting dissimilar metals and so those antenna should be discouraged. Some antennas (end-fed 1/2 wave in particular) don't need a ground plane at all (they are inherently balanced) in which case Brion's answer is again correct.

RF grounds and conventional power grounds (AC and DC) are not the same animal. Although I haven't done it myself, there is no reason one couldn't use some appropriately sized capacitors to form the RF ground connection between L-bracket and mast. In practice one would still have to consider the metals in the capacitor leads which gets complex.

In my opinion the easiest, most flexible, and best performance antenna solution is a balanced end-fed unit. Regardless of the vocabulary that may be used in the installation instructions of the antenna it should be clear if a feedpoint ground connection is required.

[JVannice]

As it happens I recently delt with a vessel with a major corrosion issue related to the VHF ant bracket... The vessel's keel coatings had a history of failure all the way back to delivery from the dealer. It is my understanding that the keel was striped and re-BC/bottom painted once by the dealer's agent and twice by an indepentant company (at owners great expence). I was on hand when this vesel most resently hauled out and much to the frustration of those involved...the BC was blistering off the keel. I noted that the lead in the blister locations was bright and shinny an indication that the keel was part of an active galvanic cell. I had the vessel relaunched and perfored a corrosion survey of the vessel. The vessel was set up with a factory lightning suppresion system; mast, shroud chainplates, stay chainplates, and arch all bonded to the keel. The rest of the vessel was unbonded ie zincs on shaft and no electrical connction to any other underwater hardware suposidly. In conversations with the factory electrical enginer he confermed that this is how the vessel was ment to be set-up: electricaly isolated lightning bonding system and engine/prop shaft/ DC neg bonding. Unfortunatly no one told the riggers... The VHF bracket was rivited to the mast, the ant was attacted to the bracket using the supplied nut on the grounded connector, and the cable screwed onto the ant. This created an electrcal path from the keel, to the mast, to the bracket, to the ant, to the cable shield, to the VHF radio, to the DC negative, to the engine block, to the prop shaft. this in turn allowed a paralell galvanic cell to form causing the keel to become cathodic in relation to the shaft zinc and leading to the repeated disbondment of the keel coatings.
After the keel was repared and the vessel in the water again I went aloft and installed plastic isolators to the ant. When I resurveyed the keel for galvanic activity I found the keel to be effectivly isolated and now "inactive." I hope you all can appreciate the irony of such a small item, as far from the keel as you can get causing such an ordeal.

[Robbie.g]

i mostly use 'pacific' antenna's, they're isolated from the mast due to the stepped washer that seperates the antenna from the s'steel bracket, [if you put it the right way round!!!]
Very interesting about the keel issue.... never thought about it....will now.... is there any reason/advantage to not having the antenna isolated from the mast??