On Saturday, April 5, 2014 8:40:31 AM UTC-5, Fred McKenzie wrote:
In article ,

Ian Jackson wrote:



I understand that my 2m 5/8 mobile antenna (on a magmount) is

essentially electrically a 6/8 (ie a 3/4 wave - hence a good match). The

actual whip is around 5/8, and the other 1/8 is the 3-turn spring steel

'loading' coil at the bottom end.



Again my understanding is that a 5/8 gives the maximum broadside gain (a

tiddly bit more oomph than a 1/2 wave), and if you make the antenna

longer, the predominant broadside lobe collapses, and most of radiation

moves to the higher angle lobe.



I read somewhere that the maximum gain toward the horizon, was obtained

with a 0.58 wavelength vertical element rather than 5/8 (0.625).

I'm not sure if that gives the maximum gain vs a .64 wave, but
it gives the cleanest pattern, with the least radiation skewing
upwards. So most of the AM broadcasters that use 5/8 radiators
prefer them on the shorter side from what I've read.



The advantage of a 5/8 antenna with a flat ground plane, is that its

impedance is capacitive with a 50 Ohm resistive component. A small

inductor (loading coil) cancels the capacitance.

I didn't see much if any changes in the loading coil no matter
what I did for radials. IE: I used the same coil for all of them,
and never had to change it when I added longer radials, etc.
I did not use a grounded coil. I just ran it in series. That way
I was able to use the antenna on 30m as a 1/4 wave.

And the same when I had the full size 40m 1/4 GP. I also used
it on 17m as a 5/8 GP, and used a 24v relay to bypass the coil
for 40m. That way I could switch from the shack.
Kinda hard to reach when the base of the GP is 36 ft up.. lol..
I used a full size 32 ft aluminum radiator, with the tip made of
car whip to reduce weight and wind load. The lower tubing was
double walled to be stronger. Almost 70 ft to the top of that thing.
It kicked butt on 40 and 17 both. Good on 40m DX late at night,
and I was running a KW+ on top of that, just to make sure I browned
the food across the ponds. I also had a 40m horizontal dipole at 36 ft,
and to DX, the vertical was usually about 2 S units better to the
U.S. coasts, and often 4 S units better across the pond. The longer
the path, the better the vertical did. Also had a good ground wave,
which was often noticed in the daytime.