Marine VHF wire antenna

[JOHNNY BE GOOD]

Can anyone tell me if I can make a wire type antenna for a marine VHF radio on my boat similar to the ones they make for cb use? What would be the dimensions for the wire and how important is swr on marine radio's? Thanks John

[Big B]

SWR is SWR, no matter what frequency you work. High SWR is bad, Low is good. To calculate the length of a FULL WAVELENGTH antenna, divide 300 by the operating frequency expressed in megahertz. The answer is the length in meters. Simply convert meters to feet, and there you go.

Generally a ¼ wavelength wire antenna (dipole) loads a little better, so just divide your answer from above by 4. This ¼ wave dimension is for EACH SIDE of the dipole. One side (leg) of the antenna will connect to the coax's center conductor, the other side will connect to the coax's shield and this leg will provide the 'counterpoise' for the antenna system. On an automobile, the sheetmetal of the car body is the 'counterpoise'.

If your antenna has to be mounted horizontally (horizontal polarization), you will not communicate very well with other stations who use vertical polarization. The signal loss will amount to about 20dB, or about 3½ 'S' units. If you are fairly close to the other stations, this signal loss may not be much of a problem.

BTW, your SWR can be adjusted by lengthening or shortening the antenna. MAKE SURE any adjustments are made to both legs. Their lengths need to be the same.

[JOHNNY BE GOOD]

Thanks BIG B for the info very help full just got back on line that's why I haven't replied sooner thanks again John

[Big B]

No Problem..............I hope it all works out.

[PA629]

Years back I made a 1/4 wave ground plane for my marine radio using an SO-239 connector and some brass brazing rod. Use the following for dimensions. Vertical radiator length (in feet) 234/f(MHz), the four radials are 240/f(MHz), (slightly longer). Bend the radials so they droop at aprox. a 45 degree angle respective to the mounting mast and your feedpoint impedance will be close to 50 ohms and SWR should be close enough. This will obviously be an omni-directional antenna with vertical polarization. Simple, cheap, and works well.

I also made a 10 element quad/yagi hybrid for the same band, but that's another story.