On Tue, 04 May 2004 15:08:08 -0000, Larry W4CSC
wrote:


The square of the turns ratio is 9:1 so the antenna's impedance is
somewhere around 6 ohms or so at the feedpoint. The 650 W amp melted the
solder joints on the core using #12 wire for the turns, so I went to #10

If one were to forego the old untuned wire/tuner marine antenna
configuration and go with a real tuned vertical, this toroid
autotransformer will very efficiently match the very low base impedance to
the 50 ohm transceiver across the 2-30 Mhz HF band.


Larry W4CSC

Some day I might try this antenna using the handrails of the boat as ground
plane. It's gotta work better than the stupid untuned backstay and
inefficient antenna tuner. It certainly results in much better signal
reports.


Your feed point resistance may be 6 ohms but about 5.8 to 5.9 ohms of
that are coil resistance. The radiation resistance of the 15 foot whip
on 3.5 mhz is in the order of .1 ohm.

So about 97% of your power is going up in heat in the coils. Only a
couple percent of the power is making it to the antenna to be
radiated.
Of 650 watts only around 20 watts makes it to the antenna.

A full quarter wave length vertical has a radiation and feed point
resistance of around 36 ohms. Much easier to get power into than a .1
ohm 15 foot antenna.

Oh, don't forget to add in all the ground loss resistance too. Less
power to the antenna yet.

If you can get your feed point resistance down to around 1 ohm then
you will get about 10% of your power into the 15 foot antenna!

Regards
Gary