Charlie K4UWH Wrote:
> I have an aluminum vertical element that is 67' tall. I want to top-load it
> on 160 meters and would like to use a three wire approach for physical
> symmetry in support of the vertical element.
> 160? I was guessing about 40' but that was a wild guess.
>
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>
>
Charlie remember any top loading connected that far (20') below the top
reduces the over all vertical element height above that point and you
will then end up with a 40' top loaded vertical rather than a 67' foot
top loaded vertical. I would recommend guying with two sets of guying
with insulated material such as Philystran at 40 feet. Put the top
loading set from the top sloping down at an angle (with respect to the
pole) of no less than 45 degrees and no more than 25' where insulators
are placed. (Top loading sloping wires have a point of cancellation if
they are to long or the angle is acute.)
Break up any remaining wires to ground at least every 40-50 feet. You
can get about 10% more top loading by connecting the top three wire
spiders together end to end (inside the insulators) to form a skeleton
cone. This should increase the amount of top loading and bring you
closer to 1/4 wave. These are approximations but if you know someone
who can model this you can have more assurances that what you are
putting up will give you the maximum efficiency at the radiation angle
lobe you are looking for.
If the pole is insulated and you have a directed feed the 20 to 30 ohms
resulting impedance, maybe even a bit lower if you have a near perfect
ground, can be matched with an Un-Un or simple L network. If the pole
is grounded you can shunt feed it with a tap about 30 to 40 feet but you
will need to tune out the inductive reactance of the shunt with an ATU.
Remember that resonance is not crucial. What is more important is
matching what you end up with and the ground system this Marconi will
need to make it efficient.
Good Luck with your project.
Herb Schoenbohm, KV4FZ.
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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