SSB Antenna on an Aluminum Yacht ?

[atoll]

hi
i have an icom m700 with an at120 tuner,that came off a previous vessel.

i will be fitting the unit to an aluminium vessel,due to electrolysis issues i am not sure if it is a good idea or not to use the hull as a ground?

i would be interested in any input on this issue?

atoll

[capt.bobfm]

Aluminum is an excelent conductor and makes a very good ground. Everything on your boat should be bonded and grounded to the outside water. The hull,though in the water, is probibly not in good electrical contact with the water due to bottom paint and any sealeant to protect the aluminum. Most boats make ground contact thru a "Grounding plate". I would recomend this. If you're getting good conductivity to the water thru the hull, your aluminum is exposed to salt water somewhere and subject to electronic galvation,corrosion and electrolosis. You need to be extra carefull with an aluminum hull especially in a marina environment as aluminum is almost as active as zinc acting as a sacrifical annode.

[atoll]

thanks for the reply,as far as i can see electolysis would only be a problem when transmitting.
also nothing is bonded to the hull,all electronics have double pole switches from the dc main power panel so as to isolate both neg and pos from the hull.
only the ac power is bonded to the hull.
grounding plate sounds like a good idea though generally these are copper/bronze,which could also create issues if in close proximity to the alumilium?

would be interested to hear from someone with an aluminium vessel in this respect.
thanks for input

[noelex 77]

An alumium hull will act as a great ground so there is no need for a separate grounding plate.
You need however to prevent a DC connection to the hull. This is usually done by connecting the SSB ground to the hull via a capacitor this allows the SSB ground to function but stops the electrolysis that would result from a direct connection.
If you Google it I think you will come up with some detailed instructions.
Sorry I cannot do the search, but my internet connection from the boat is very slow.
If you get stuck post again and I will try and dig up some more specific information. I think I saved some articles, but I have had no luck finding them so far.

[Wotname]

Noelex 77 is correct in saying the Al hull is a great ground plane. The jury is still out regarding the DC isolation between the antenna tuner and the hull but as it is fairly easily achieved, it is probably a good idea. You will find details on this forum; as suggested, use the google custom search function in the pull down search menu.

For the record, you do not need worry about a DC connection between the hull and the water; only an RF (AC) connection is required, so the bottom paint etc is not an issue.

An easy way to understand this is to consider the hull and water as two plates of a capacitor with the paint as the insulating dielectric. Given the size of the "plates" and RF frequencies involved, this "capacitor" offers a low reactance path to the seawater.

[Jon Hacking]

FWIW, I just sold a friend with an AL boat my old AIS receiver & VHF antenna. When he installed it, he found that the VHF antenna ground (shield) was connected to the battery (V-)! He's now isolating the VHF antenna from the hull, at the mounting point. So be careful with VHF products as well as HF/SSB.