Topband: Broadband vertical

[Herb Schoenbohm]

Earl W Cunningham wrote:

>I revisited those model files and here are the corrected values:
>
>single-wire shunt = 90 kHz  BW (2:1 SWR)
>
>two-wire shunt = 130 kHz BW (2:1 SWR)
>
>The increase in BW is about 44%.
>
>  
>
My change from a single wire to a three wire cage unipole certainly made 
my antenna bandwidth useable. I think several NAB tech papers on this 
suggest that the copper wire cage also increases the overall efficiency 
of the radiator.  Before the changed to the cage feed I could only work 
15 Khz with a CLC network. The antenna performance was mediocre.  The 
three wire and finding the sweet point changed everything here.

One very important issue not mention yet is the fact that the "apparent" 
bandwidth due to a poor ground system can cause us to make harmful 
assumptions.  The poor ground and relative ground loss scenario can make 
any shunt fed tower appear to have greater bandwidth. We feel good 
because we have a large bandwidth and the VSWR is against the pin to the 
left!  The bandwidth is real but the performance can be poor, sort of 
like connecting non-inductive resistors from the feed point across the 
coax. resistors. I think Earl can prove this theory from his models. It 
is so very important to begin these test with a very good grounding and 
radial system,. With a poor ground system you could have a low VSWR over 
a good portion of the band but have an antenna that loses nearly half 
its power applied in the ground below. But yet you feel good about the 
antenna for all the wrong reasons. I am sold on a cage wire feed shunt 
feed especially for a short tower like I have.

73

Herb Schoenbohm, KV4FZ