Re: VHF Antennae (Attn: Bob L)


Bob Larkin
 

Hi Guys -
1-A quarter-wave wire coming out of the coax is basically poor. As you say, John, it needs another half of the antenna, the vertical dipole going down or several "radials" going out horizontally.

2-The half-wave dipole, end fed, is a different deal.  It must have a matching network, usually put into a small tube at the bottom.  It also needs a "ground element" at the base.  BUT, and this is big, for this special case, that ground element can be almost anything.  A quarter wave of wire would be very adequate.  So would a metal mast or even the outside of the feed line.   Because of the very high impedance at the antenna end, almost anything serves as a ground.  It is a very forgiving.

3-The AIS signals are strong, especially from freighters, tankers, and Washington State Ferries---the stuff we are really scared of!  They put the AIS antennas high on the vessels.  I built an AIS dipole antenna into the wall  of the Birdwatcher and also the Scamp.  Both have seemed to be very adequate.

I'm not sure that counting AIS reports at different times is too meaningful.  One might see what the maximum range is on a WS Ferry, or something.

Cheers, Bob

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