[TowerTalk] Topband: 160m Sloper on Crank-up tower

[K4SAV]

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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