DIY vhf antenna

[saltyanchor]

A few years ago, PBO ran an article on how to make your own emergency vhf antenna. I lent the article to someone and no longer have it. It is pretty simple to make, but the measurements of wire have to be exact. Do you have the instructions or anything to help?
Thanks

 

[firstascent2002]

This is developed from the amateur radio assembly called a slim jim. Any type of wire can be taped to a 1425mm length of insulating material. i used domestic ftal twin insulated flex tped around a dowel.

The flex should run all the way up one side of the dowel, on the otherside there should be a gap of 25 mm which is 467 mm from one end and resultingly 933 mm from the other. 50 ohm co ax cable is soldered, core on one side and sheath on the other side 175 mm from the end of the dowel( the end closest to the gap in the domestic flex.) usually produces and S.W.R or 1 : 1.3.

not easy and fairly effective if sent up on the burgee halyard

 

[alahol2]

The way I was shown was to strip the sheathing from elbow to fingertip off the co-ax. Pull the centre core from the shielding and turn the shielding back on itself. You should end up with an aerial about a metre long that should be hung vertically.

 

[Howardnp]

I was also shown the stripped co ax method. About 17 inches I think... is what you need, central bit one way and the screening the other and you end up with a half wave di pole (?)

 

[Birdseye]

Given that you are most likely to use an emergency aerial in an emergency when you want good reception of your signal at the other end, I would be inclined to buy a pukka one. Instead, save money by doing the DIY bit on things like boathooks

 

[Ships_Cat]

I agree with that Birdseye.

We carry a spare VHF antenna which is exactly the same as the masthead one (so can serve as a non emergency spare for that too).

A separate coaxial cable transmission line is run from behind the radio out into a cockpit locker and a bracket to take the antenna, if ever required, is permanently mounted on the pushpit.

John

 

[Salty John]

Although not quite what you asked, birdseye and ships-cat have it right in my opinion. You can make a VHF antenna out of a piece of coat hanger wire 18 inches long. Push it into the antenna socket, being careful not to short it to the radio body, and it will act as an antenna. Unfortunately it will have little range, being below deck, so will be quite useless just when you need it. If you lose the rig you will have no halyard upon which to hoist an emergency antenna. A good quality whip antenna with a rail mount and the coax pre run to a location behind the radio where it can be easily accessed for connection is a good idea. If you can detach the whip and stow it where you can quickly find it in an emergency - behind the radio with the emergency coax connection - then the body can be permanently rail mounted without being obtrusive. Such an antenna will not have the range of a properly installed masthead unit, but if it is 8 feet above sea level it should give 10 to 15 miles depending on the height of the receiving unit.