[TowerTalk] 40/80 meter vertical

[jimlux]

On 3/18/16 9:26 AM, Marv Shelton wrote:
> It seems like it could be done.  You will need the right-sized
> aluminum pieces for the lower section and something for the upper,
> which I would think you could also do with wire. To calculate the
> distance to the 40 meter trap, you could model it in EZNEC if you can
> get the electrical specs on the trap itself.

and that's the tricky part. I've got some 6BTVs and I spent some time 
trying to model them and have the Z modeled match the Z measured 
(unsuccessfully).

There have been some folks who have built a measurement fixture for 
traps, and you can get some data from that.

While it's nice to think of the usual trap as a parallel LC, it turns 
out that there's some other effects: for instance, on the 6BTV (and it's 
brethren), the outside of the trap is part of the antenna. This is fine, 
but if you put the measured LC in as a point load in a model, the 
resulting model doesn't match the measurements very well.

The other thing is that some (not very rigorous) measurements convinced 
me that the traps do NOT behave as a single LC resonance: the Z doesn't 
follow the expected slope.

I think what people do is use a model (or other design technique) to get 
"close" and then they fiddle around with the antenna in the field to get 
the match "good enough".

I would venture to say that nobody has actually measured the current 
profile along a multi-trapped vertical, or measured the 3d radiated 
field across a wide frequency range.

My own solution was just to run an autotuner at the base, and not worry 
about precise match.  In my opinion, the advantage that the traps give 
you over just matching a plain untrapped vertical of some height (43 ft 
is popular for some reason) is that you don't get weird lobes due to 
radiating a 10m signal from a more than 1 wavelength long antenna.


  Come to think of it, its
> probably a good idea to model the basic antenna and your 40/80
> version to see what advantage if any, you get from removing the traps
> for the other bands.  Some might argue its not worth the trouble, and
> if it ain't broke, don't fix it.
>
> Marv wa2bfw
>
> Sent from my iPad And YES it's cool!
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