Topband: 160m Sloper on Crank-up tower

[K4SAV]

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