Marine vhf aerials

[Shearwater]

It seems a lot easier to buy a competitively priced radio than the required aerial. Does anyone have a link to a sensibly priced mast top aerial c/w bracket and cable as I'm having trouble finding anyone willing to ship abroad.

 

[Chrusty1]

Try this.....

http://www.seamarknunn.com/cgi-bin/sh000...18_2dAIS_2dMAST

 

[Shearwater]

is that seamark nunn's site really dark blue text on dark blue background or has my machine gon ick ?

 

[Chrusty1]

Ugh!!.....It's a white page with loads of Masthead antennas on it.

Google Seamark Nunn, then type masthead antennas in quick search box, should give you the same result, without the blue page /forums/images/graemlins/smile.gif

 

[Bodach na mara]

It is still over 50 quid! The radio does not cost much more. Why does it cost so much for a bit of wire and some co-ax cable?

 

[William_H]

Hi Colin I use a homemade VHF antenna mounted in some plastic piping on the pushpit that works very well. If you go for a mast head antenna you might still need a pushpit mounted one for emergency. PM me for details.
or look at www.whitworths.com.au
good luck olewill

 

[Chrusty1]

Well that's what they cost.......... There is a bit more to them than a bit of bent wire and some coax.

 

[johnphilip]

Start with a Stainless aerial,add the most unreliable connection system possible to a length of wire, add to that a thick coating of marine leisure market mark-up.Hey! One VHF Aerial!

 

[LORDNELSON]

What distance can you transmit/receive over? I always thought that a VHF aerial should be sited high up in order to obtain good distance performance..

 

[Salty John]

Yes, VHF transmitting distance depends on the two antennas "seeing" each other. The lower the antenna the more quickly the horizon gets in the way. A receiver height of 10' and a transmitter height of 10' gives a theoretical communication distance of 8nm. At 40' for each antenna this goes up to 16nm.
In fact this formula is based on D = 1.23 times the root of the combined heights whereas in reality the figure is closer to 1.4 times under average conditions.
Because coast guard stations have antennas mounted hundreds of feet above sea level they can communicate at much longer distances than yacht to yacht, and this is also why an antenna on the pushpit can appear to have a good transmission distance when checking with the CG.
A pushpit mounted antenna works for AIS because the transmitting ships have their antennas very high on their superstructure and also very long distance isn't an issue - 10m is usually ok. The pushpit mounted AIS antenna can provide emergency cover for the masthead radio antenna.

 

[Bilgediver]

Yes the site is crap and the antenna is for AIS so check its spec re transmitted power in case it is active

 

[Bilgediver]

The Slim Jim is easily made from 300 ohm tv/fm feeder and then installed inside plastic pipe. I have one tuned for MArine and Ham 2 metres as an emergency antenna .

 

[Richard10002]

Here's one with bracket:

http://www.mailspeedmarine.com/aerials-antennas-accessories/stainless-steel-whip-antenna100385.bhtml

I think it's the one I've got, and it works fine now that I have my underdeck connectors sorted /forums/images/graemlins/smile.gif

I bought RG58 cable and PL259 connectors, (for underdeck - 2 x PL259, + 1 Male-Male connector), from Maplins. My deck gland was a largish rectangle which took 3 or 4 wires through the deck.

http://www.mailspeedmarine.com/electrical-equipment/v12f-deck-gland558669.bhtml

http://www.maplin.co.uk/Module.aspx?ModuleNo=256

Maplin cable at £0.89 per meter, (mailspeed price £2.99 per metre!!)

You can probably do the lot for about £80 inc. aerial.

 

[rob2]

If you're into building an emergency aerial - I second the Slim Jim design. I built a few some years ago, using the dipole aerial wire (300 ohm). The dimensions can be adjusted using the fractional wavelength dimensions given on the website, so as to centre the matching to channel sixteen. Gives excellent results and can be hung from the spreaders with great effect. The assembled aerial fits inside the commonly available oval conduit and the ends can be sealed off with mastic. Find a radio ham to check the SWR and you won't fry the radio.

 

[Anonymous]

Richard, we seem to be talking about the same thing on two threads...

Your emergency antenna should be pluggable straight into the back of your VHF radio, bypassing all existing RF cabling and socketry. KISS, that's the whole point of it. It must be easy to deploy with clear instructions for non-technical crew (the techie-one might be incapacitated). You just feed the emergency cable through a hatch, light, or companionway, and lash the antenna part to something non-metallic.

 

[William_H]

Yes theoretically you are correct. However within my sailing environment I have never run out of range of both our club station and the local sea search and rescue. I have however lost 3 masts to various failings and accidents so consider the pushpit mounting desirable. Apart from the fact that I remove the mast every winter.
The design I use is called a coaxial antenna and uses the exposed inner of the coax as the 1/4 wavelength radiator (17inches). A section of larger sized braid again 17inches is solder to the top end of the coax braid and this fits around the coax from the end of the coax braid downwards.
This all means that the only actual join or solder of the whole system is at the plug at the radio apart from the join of braid onto the braid. So reliability is very good.
Also being just a 1/4 wavelength the radiation pattern is not focussed to t he horizon making it more usable at high angles of heel.

This antenna could be used at mast top however in stead of using additional braid for a ground plane you make a good connection to the Al mast. olewill