[Antennas] VHF J-Pole and Slim-Jim on UHF

[Terry Conboy]

At 10:49 AM 2008-11-15, Deon Erwin ZS1ZL wrote:
>A 2m 1/4 wave antenna can be used as a 70cm 3/4 wave antenna.  The antenna
>presents a low impedance on both bands and one only has to consider the
>change in radiation pattern, making it an effective dual-band antenna.
>
>There is a lot of local debate over the use of a 2m J-Pole or Slim-Jim on
>the 70cm band.  In the 2m 1/4 wave example, the antenna is an odd multiple
>of 1/4 waves on 70cm.  The J-Pole is a 1/2 wave on 2m, resulting in
>something completely different on 70cm.
>
>Some local hams claim that the 2m J-Pole or Slim-Jim also functions as a
>dual-band antenna, just like the 1/4 wave example.  Surely this is
>incorrect?

The 2m J-pole used on 70cm is an end-fed 1.5 wavelength radiator (fed 
through a 3/4 matching section), so it will have large lobes at about 
45 degree elevation in addition a smaller lobe on the horizon.  It's 
not surprising that it might have a fair impedance match on 
70cm.  One problem with J-poles is that the feedline shield isn't 
well decoupled, so the pattern, even at the design frequency, can 
have lots of high angle lobes due to feedline (and any metal support) 
radiation.

Ideally, a 3/4 wl radiator over an infinite ground plane would have a 
similar pattern (above the horizon) to a 1.5 wl radiator in free 
space, but in the real world, the pattern will depend significantly 
on what makes up the "ground plane".

Given all the variables involved, it's coin toss which of these 
compromise antennas would work best.

73, Terry N6RY