Omni directional tv antenna

[killick]

I would like to make an effective tv antenna for use on my boat moored in Durban harbour. What I had in mind was a circle connected at the two ends to the cable. To assist me advice on the following will be appreciated:
Which metal will be best, stainless steel, copper or aliminium? How do I determine the diameter? How far should the two ends be appart? Should it be horizontal? Any possible alternatives? All advice will be greatly appreciated.

 

[hartcjhart]

time+materials+frustration would it not be cheaper to buy one?

 

[funk-an-bord.de]

If you are lucky and there is a TV transmitter near by, then a simple cupper wire will show a result.
If you are not lucky, you have to by a directional antenna, like seen on all roofs.
Wilhelm

 

[Rigger Mortice]

It sounds like you are wanting to make a round dipole aerial, in which case the diameter of the aerial will be determined by the length of the two elements (i.e the elements form the circumference of the aerial). If you bung the frequency into this calculator http://www.csgnetwork.com/antennaedcalc.html then you will get an idea which will at least give you something to play around with. I'm sure that it will get a bit more complicated if you make a normally straight aerial into a round one, but you've got to start somewhere.

The choice of metal is not vitally important. TV aerials are usually aluminium but that has more to do with keeping the weight down so that the mountings can be lighter. FWIW I made a simple DAB dipole using stainless welding wire and that worked very well.

Whether it is horizontal or not largely depends on the polarisation of the transmitter. If the signals are transmitted horizontally then the aerial would normally be horizontal. If the signals are vertically polarised then I doubt that a round aerial will have any benefit and a simple straight dipole will do the job perfectly well, but I'm now at the very limit of my knowledge and others will be better equipped to answer your question.

By the way, whilst a round aerial is supposed to give reception through 360 degrees, a simple dipole will also give reception over a fairly wide range. If you add reflectors and directors to a simple dipole you end up with a typical TV aerial. The more elements it has the more directional it becomes but the better the signal that it puts down the line.

Personally, I would play around with a simple dipole first and see how that works in both vertical and horizontal orientations. You could throw one together in just a few minutes. My DAB aerial used wire which was threaded into fibreglass tent poles to keep it straight. It was glued together in the middle, waterproofed with self amalgamating tape and fixed onto standoffs on the back stay. The difference it made to reception was fantastic.

Google "dipole" and you'll find lots of geeky sites to explore.

 

[Billjratt]

Remember, most of the masthead omni-directional loops you see are active - ie, have an amplifier incroporated.
If you're in a harbour, on a stable pontoon, why not just get a normal aerial - even if you have to mount it on a pile.

 

[killick]

Thank you all for your advice. I appreciate the alternatives but this is actually more a matter of wanting to do it myself and make it work well.