Topband: 1/2 wave inv L

[Guy Olinger K2AV]

If one is not talking about ham implementations of the commercial AM
BC paradigm and their big verticals over very well done dense radial
fields, the models can do quite poorly. Go ask W7EL. But then again,
what other tools do we have to go with.

It's a little dangerous extrapolating between 80 and 160, either way.
W8JI had up a full sized 160 dipole at a half wave, which he says
rarely outperformed a high grade 1/4 wave vertical.

An 80 meter dipole at 120 feet is a really good antenna, and at NY4A a
full sized 4 element 80m wire yagi off a catenary at that height was a
true killer antenna. Been there done that! IF it was in maintenance,
it always seriously beat a well-done 4 square.

An end-fed half-wave L on 80 is a really good antenna, usually better
all around than an 80 inverted vee, apex at 76' feet at my QTH. The L
was always better on DX. Many A/B tests. The models insisted that for
many paths the Vee had a four or five dB advantage, an advantage which
I never, ever heard. This is my experiential reason for distrusting
what the models say about an end-fed halfwave L.

The problem with a big end-supported tee on 160 is all mechanical, the
distance between the supporting ends and the middle gives the weight
of the vertical wire a huge leverage advantage over the horizontal
pull at the ends, usually making it a "Y" antenna instead of a T.

This is why those kinds of top loaded antennas usually wind up
supported in the center with significantly sloping sides, appearing
like an arrow pointed up. This also reduces the effective radiation
from the top, counter-productive to the originally intended
performance of pushing RF current density, hence radiation, up high.

I would definitely top-load a T's center-supported vertical first with
a substantial coil up top and then significantly shorten the T wires.

If you actually had the means to support both the center and ends of a
160 dipole turned into a "T" with current max up top, chances are you
have the room for a good implementation of the commercial AM BC
paradigm. A good commercial quality 160 1/4 wave over dense full size
radials does not disappoint.

Personally, with a 160 3/8 wave L, up 90, out 105 for seven years, I
always thought it covered what otherwise would be holes in the
coverage for a pure vertical.

Where I finally got to with that concept was to top an L with the
electrical equivalent of ~95 feet horizontal. Call it a 65-70 degree
"topper wire".

The effect of that was to place 1/16 wave with the densest RF current
at the top of the vertical, above man-made clutter, and with a much
shorter path through RF absorbent tree tops to low angles of takeoff.
That really was what was going on with my up 90 out 105.

It also reduced the RF current in the antenna near the ground.
The 65 degree topper could be successfully used with as little as 55
feet available horizontally, by dropping down from the far end of the
horizontal as much as 40 feet, making it a sort-of inverted U. And of
course you would put up *as much vertical as was possible*. This DID
usually result in a miscellaneous feed Z that required some amount of
base feeding network.

For the I-want-to-hook-it-directly-to-50-ohm-coax-and-forget-it crowd
this is disqualifying, probably accounting for the lack of popularity.
Can't buy an end feed matcher for an L off anyone's shelf.

The far end of the L or U can be trimmed for X=0, R=whatever, losing
some of the current density up high, but possibly allowing
base-network-less operation

On Sun, Feb 15, 2015 at 5:52 PM, Richard (Rick) Karlquist
<richard at karlquist.com> wrote:
> On 2/15/2015 2:35 PM, Art Snapper wrote:
>>
>> I have been researching the inverted L for 160, and have received much
>> helpful information. - Thanks!
>>
>> Today I stumbled across a document regarding the 1/2 wave inverted L.
>>
>> Has anyone done a side-by-side comparison?
>> Art NK8X
>> _________________
>
>
> I have modeled it and the results are predictable.  About
> half your power goes into likely useless horizontally
> polarized radiation.  If you instead make a top loaded
> ("T" type) vertical where the sum of the height and half
> the top wire is a half wave, then you get a "voltage fed"
> vertical that behaves pretty much like a half wave
> vertical.  Since the drive impedance is high, you MIGHT
> get away with a much less extensive counterpoise.
> There is some controversy about this.
>
> Rick N6RK
>
> _________________
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