Darn it. I did it again, sent it to the wrong reflector. Please disregard.
Jerry
K4SAV wrote:
> KQ0B wrote:
>
>> I am trying to use a 160m sloper on a TRI-EX 54 foot crank-up tower.
>>
>>
>>
> It's difficult to make a 160 sloper work very well with only a 54 ft
> tower. (That's a quarter wave sloping wire, not a half wave sloping
> dipole.) The configuration that works the best is one in which most of
> the currents are confined to the top part of the tower above the
> attachment point of the sloping wire, and the sloping wire itself, while
> very little current flow is present in the bottom part of the tower
> below the attachment point. You want the currents in the bottom part of
> the tower to be low because the tower is connected to the dirt, plus the
> sloping wire makes a sharp angle with the lower part of the tower and if
> the lower part of the tower has a lot of current this will cause field
> cancellation to occur because of that sharp angle, which will reduce the
> radiation resistance and cause even more ground loss. You could add a
> lot of radials to the tower and reduce the ground loss, but that lower
> radiation resistance really hurts.
>
> I ofter hear of the noobies putting up a 40 meter sloper on a push-up
> pole. That's the same a putting up an inverted vee with a very small
> included angle and attaching one end of the antenna to a ground rod.
>
> Notice that to be efficient the top part of the tower above the
> attachment point needs to be close to resonant on 160 meters. There
> aren't many tower configurations that fit that description. Of course a
> compromise will work, it just won't be as good. If you do an analysis
> of this antenna you will discover that when significant current flows in
> the lower part of the tower, and there is also significant top loading,
> multiple resonance points will occur. That is because it becomes a
> three leg antenna. This can produce some interesting SWR curves but
> efficiency wise, current in the lower part of the tower always hurts. A
> while back I did an experiment with EZNEC, looking for a tower
> configuration that would work well with a sloper. Here is what I found:
>
> One configuration that should work very well on 160 is a 150 ft tower
> with a full size 3 element 40 meter Yagi on top 5 ft above the top of
> the tower, and no other wire antennas hanging off the tower. A 140.5 ft
> wire attached at the 120 ft point on the tower and the far end pulled to
> 30 ft off the ground. All the cables exiting the tower have their
> shields tied to the tower at the bottom. Assuming Phyllistran or
> polyrod guys. You should have at least a minimal radial system to
> decouple any cables exiting the tower. This system doesn't depend on a
> good radial system, a poor one will do OK since the major currents in
> the system are mainly confined to the sloping wire and the Yagi at the
> top. Ground dissipation should be minimal even with a poor radial
> system. EZNEC says that this system should perform as well as a full
> size 1/4 wave 160 vertical which has a good radial system, except in the
> direction opposite the sloping wire. However it will have a narrower
> bandwidth.
>
> Usually you can find lots of other options that will work better than a
> sloper and be a lot less trouble (unless you just happen to have the
> right configuration tower already sitting there waiting).
>
> Jerry, K4SAV
>
>
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