[TowerTalk] 160m. Inverted L question

[David Gilbert]

Pretty much this.  I modeled K9OM's setup for him, and although I 
couldn't accurately copy his ground conditions and radial field, the 
model said there was virtually no interaction at all.  He doesn't have a 
lot of top loading on his 57' tower so that makes qualitative sense.  He 
is now planning to put his effort into upgrading his radial field and I 
referred him the N6LF's work on that.

I was, however, able to significantly distort the model to show a major 
effect on his pattern, but only by adding enough inductance into the 
tower to essentially tune it to 160m.  With the right value of 
inductance the pattern turns cardioid with about a 20 db notch in the 
direction away from the tower.  That also makes qualitative sense, but 
I'm not sure that's a typical condition.  A tower that isn't 
approximately close to resonance shouldn't have any more effect on an 
adjacent Inverted-L than a 15m element would have for 20m on a 
tribander.  The fact that the tower is grounded is irrelevant.

I think hams often like to generalize far too much without bothering to 
investigate further, especially in the case of antennas.  We have tools 
available to us now that can turn rules of thumb (at least some of them) 
into science, but somehow the myths still perpetuate.

73,
Dave   AB7E


On 1/16/2019 10:43 AM, Rob Atkinson wrote:
> Where does the idea come from that a grounded tower holding up an
> inverted L is a problem.  Is this just some notion that took hold?
> The tower is some how an RF sponge?  Hams have been doing this for
> decades.  Of course the tower and ground system are bonded to each
> other so the tower provides a return current path as with a ground
> system.  How is this different from an AM broadcast skirt fed grounded
> tower.  I have a 50 foot mast holding my inverted L--the L wire is out
> about 3 feet from it.  The mast has a 15 foot stinger on top to make
> it a 1/4 wave 80 m. vertical.  I can ground the mast or float it above
> ground when using the inverted L on 160 but I see no difference in
> performance.  Anyone really concerned about this could always drop a
> second wire on the opposite side from the inverted L, and tie the two
> together at top and bottom with the single inverted L horizontal wire
> continuing on, but I doubt if it would make much difference.
>
> 73
>
> Rob
> K5UJ
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