Gentlemen:
Answers to my post asking about a top fed 160m vertical almost convinced =
me to forget it & shunt-feed the 60ft tower with beam. But further =
study & the burning desire to get as much high elevation radiation as =
possible from this forest of thick, tall trees, has given me the =
incentive to send a final post on the subject.
The following is what seems to be given:
1. Install as high & as large a top capacitance hat as possible. =
Spread out losses.
2. Taller radiators usually yield more radiation resistance & average =
higher elevation radiation.
3. Top hat is one plate, ground is the other. The closer to ground the =
top hat is, the more capacitance.
4. The higher & larger a top hat is, the more difficult it is to =
support, & to prevent wind damage.
5. Linear loading is virtually lossless, & the reverse second (& third) =
conductor(s) also radiate.
6. It is easy to add an aluminum tubing "whip" to the beams mast at 65 =
ft, for total of 95 ft physical height.
7. Whip will survive winds alone, but not with any form of capacitance =
hat at its top.
>From the above, the plan is to start at a 14 ft elevated feed at =
rooftop, run a #12awg insulated copper wire to the top of the whip, with =
stand-off separation of 3 ft from the tower & a tapered stand-off of 1 =
ft, to 6 inches from the whip. The #12 wire is connected to the top of =
the whip. The tower resonates near 80m so there should be no =
interaction with the wire.
The theory of operation is that the beam will exhibit much more =
capacitance with the earth, at its lower mounted position, & draw more =
current up higher into the whip/wire area where I want more radiation. =
Current reaching the top & descending back down the whip to the top hat =
beam should add even more high radiation in linear loading fashion. =
Finally, while the tower is presently grounded, theory holds that a =
capacitance hat shunts most of the current into itself, making isolation =
of tower from ground, as with a Moxon linear trap, unnecessary. Trap =
will be added if tower couples with #12 wire, or if current flows into =
ground.
The total conductor length to center (top) hat is 116 ft, but electrical =
length will be shorter due to linear loading effect. Additional linear =
loading can be added along the upper tower area, if needed, to give an =
electrical 1/2 WL with elevated radials & avoid adding a load coil.
I will try this contraption out as soon as ZX0T operation is over. =
Being a novice antenna experimenter & not yet knowing how to model, I am =
leaving myself wide open for mental error. But if any worthwhile =
improvements evolve, it will have been well worth it for me.
73, Doug / Nx4d
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