You have it right. Up to 5/8 wavelength a vertical can be an excellent
antenna. Beyond 5/8 wavelength more and more RF radiates at a high angle and
the low angle lobe becomes secondary and continues to decrease in strength as
the frequency goes up.
So, for a 43' vertical:
Length: 43.00
Frequency: Wavelength: Wavelength: Degrees:
1.825 0.085 4/47 30.7
3.550 0.166 16/97 59.7
5.368 0.251 1/ 4 90.3
7.050 0.329 27/82 118.5
10.120 0.473 26/55 170.1
14.050 0.656 21/32 236.2
18.150 0.848 39/46 305.2
21.050 0.983 58/59 353.9
24.950 1.165 1 16/97 419.5
28.100 1.312 1 5/16 472.4
Since 5/8 == 0.625 you can see that on 20 meters the antenna is just a bit
over 5/8 wavelength. Beyond 20 meters the antenna gets pretty long.
Most of the commercial 43' antennas have a UNUN at the feedpoint (bad idea in
a reactive circuit), suggest 150' of coax (the losses make the SWR measured at
the rig look "better") and suggest a tuner in the shack.
I's suggesting that all three of those ideas are serious compromises made for
the sake of convenience. Of course, in that scenario the antenna will radiate
some RF, it will work. It's just that it won't work to anywhere near it's
potential.
And, to call the antenna an "all band" setup is a serious disservice to the
radio community. Above 20 meters the performance will be very poor as the
majority of the radiated RF will be Lost In Space.
73,
-Doug, W7KF
http://www.w7kf.com
Damien Mannix wrote:
A most interesting thread. However may I ask for a clarification please?
I have always understood, mainly from Bill Orr's wonderful but out of
print books, that a 5/8 vertical had the lowest angle main lobe and
hence was superior for DX to a 1/4 wave vertical, all othrer factors
being equal. Is it only when longer than 5/8 that the antenna is
transformed into a 'cloud warmer' or have I got it all wrong?
Hope I am not being too stupid or pedantic.
Damien G3XER/C6AXE
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