Mast head VHF aerial connection

[BabaYaga]

Is it essential or preferable that the shield of a VHF mast head aerial is in electrical contact with the aluminium mast in order for it to function as ground plane?
Or could the aerial be electrically isolated from the mast without reducing its efficiency?
Asking because my aerial terminal (brass I think) makes contact with the new bracket (aluminium) and there could be corrosion if not isolated.

Any input much appreciated!

 

[alahol2]

My aerial has a glass filled nylon (or similar) base and as such has no possible means of connection to the mast. It works fine so, no you don't need the mast as a ground plane.

 

[Salty John]

Most marine vhf whip antennas don't need a ground plane and work even on a plastic bracket completely isolated from the mast.

 

[VicS]

Is it essential or preferable that the shield of a VHF mast head aerial is in electrical contact with the aluminium mast in order for it to function as ground plane?
Or could the aerial be electrically isolated from the mast without reducing its efficiency?
Asking because my aerial terminal (brass I think) makes contact with the new bracket (aluminium) and there could be corrosion if not isolated.

Any input much appreciated!

Use what is supplied fitted according to the instructions.


my aerial has a brass retaining nut. It has been mounted on an aluminum bracket for 30 years without any corrosion. I do try to keep the top of the mast well clear of the water though.

 

[BabaYaga]

Thanks for your replies.

Use what is supplied fitted according to the instructions.


my aerial has a brass retaining nut. It has been mounted on an aluminum bracket for 30 years without any corrosion. I do try to keep the top of the mast well clear of the water though.

My aerial is also 30 years old, so instructions are long lost. Up till now it has been sitting on a s/s bracket, which I intend to change to an aluminium one. There has been some corrosion on alu masthead fitting, under the bracket footprint.

I find it surprising that you had no corrosion in the brass/alu interface over 30 years. Perhaps my worries are unfounded.

 

[nigelmercier]

Do be aware that if you do make contact with the mast where none existed before, you can open up a whole new can of worms. Now your mast may be connected to the keel, and the latter to the battery negative.

 

[Alpha22]

Most marine vhf whip antennas don't need a ground plane and work even on a plastic bracket completely isolated from the mast.

The laws of physics dictate that a dipole is the most perfect of omnidirectional antennas. BUT, to achieve this, the counterpoise would have to be 'tuned' to the same length of as the antenna vertical, which in turn is 'tuned' to the frequency of the transmitter.

In practical terms it is just not good seamanship to chop your mast down to 2 meters long to improve the RF prorogation as it generally hampers the boats ability to sail upwind efficiently.

Thankfully for us the performance of a dipole is not too drastically reduced if the counterpoise is removed.... especially if you are over water, as most of us generally are.