[Paddlewise] VHF Antenna

Peter Rattenbury wrote:

>I have made up such an item for my handheld, using good quality coax cable,
>perhaps RG-58.  You strip back the outside protective covering and the metal
>shielding to expose the inner core at the 'high' end of your antenna.   I am
>going on memory here, [ I am at work ] but I think the length of the strip
>back is 17 and three quarter inches,  in your imperial measurements.
>Perhaps one of the other radio enthusiasts on Paddlewise can confirm that
>length of stripback for the marine frequencies.
>
What you want to do is take the center frequency- which would be about 
157 MHz- and convert that to a wavelength:

3x10^6 m/sec / 1.57 x10^6 Hz =  1.91082803 meters.

Now you want the elements of your antenna to be 1/4 wavelength: 1.91 / 4 
=  0.4777 meters

Since radio waves travel more slowly in copper wire than in free air, we 
multiply by the velocity factor, 0.86:

= .41 meters, or 16.14 inches.

The antenna Peter describes is a simple full sized quarter-wave antenna, 
which is more efficient than a stubby duck, but you can make it much 
more efficient by giving it two quarter wave elements to make it 
symmetical, and that's very simple to do.

Start by preparing the cable as Peter described earlier. At one end 
attach a BNC connector. At the other, strip off around 18"-20" of 
insulation.

Now push the shield back , which will enlarge its diameter, and work it 
OVER the insulated part of the cable.

When you get it worked all the way over the cable, trim it to 16.14", 
secure it with a piece of electrical tape, and trim the simgle insulated 
conductor at the and of the cable to 16.14"

Now slip a piece of heat shrink tubing over the end and shrink it to 
fit. You can use anothe rpieve to stiffen it up.

What you've created is a half wave coaxial antenna that is several times 
as efficient as a duck, and can be rolled up and stashed anywhere. If 
you make a loop in the end rather than just rimming it to length, it's 
easy to hang.

-- mike
 -------------------------
 Michael Edelman
 medelman_at_ameritech.net
 http://www.foldingkayaks.org
 http://www.findascope.com
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