Topband: Half Slopers

K4SAV RadioIR at
Mon Feb 4 18:39:40 EST 2008

WS6X wrote:

>I have some questions about the top loading in shunt-fed tower systems in
>general, and more specific to my case, half slopers (which have been
>described as "a lazy man's shunt-fed tower").
>1. Other than the tower itself, exactly which components contribute to the
>radiated field? I understand that the minimal contribution of the sloping
>wire is high angle radiation, but what about the yagis used for top loading?
>Do they actually radiate?
>2. Is there a difference in the amount of loading (or radiation?) between a
>yagi with elements bonded to the boom and yagis with isolated elements? (My
>mast is bonded to the tower with a flexible ground strap at the top of the
>3. I understand how to decouple or detune a classic shunt-fed tower for use
>with a nearby RX antenna. How would you best detune a tower-fed half sloper?
>I could easily switch in additional length to the bottom end of the sloping
>wire, or switch in a reactive component at the tower feedpoint. Would this
>do it?
>Jim - WS6X
 I like Tom's name for this antenna best, "half slopper".  You can think 
of the sloping element as a matching network for feeding the tower.  The 
exact configuration of the tower determines how well the technique will 

1.  Everything on the tower, or connected to it contributes radiation to 
some degree.  The exact amount depends on the exact configuration of the 
tower.  If the system is optimally designed, the top Yagis contribute a 
lot to the radiation.  If it is poorly configured, the top Yagis may 
contribute very little, and in some cases it may work like a large dummy 
load. The sloping wire also contributes.  Depending on the exact tower 
configuration the lack of radials may contribute huge losses, or maybe 
only minor losses.  Are you beginning to get a feeling for the 
description, slopper?

2.  Yes the effect of the Yagi elements being connected to the boom, or 
not, makes a considerable difference, the same as with any shunt fed 
tower.  The balun used at the antenna also makes a difference.  Many 
baluns supplied with Yagis, don't really look like good chokes at 160 
meters, so they often provide very little isolation on 160 and the 
driven elements then contribute to top loading.  Where you can get into 
trouble is when the balun happens to have a mid-range impedance on 160.  
Then it can dissipate considerable power due to the currents in the 
tower.  It doesn't happen very often, but some have burned up baluns 
this way.  The most likely situation for this to happen is for a very 
large antenna at the top of the tower with no antennas lower on the tower.

3.  Detuning the tower may be tricky.  If the system has large currents 
in the tower base, you can detune it for receiving the same way you 
detune a shunt fed tower.  But in this case unless you have many radials 
the antenna will be poor anyway, so why bother. If you have a design 
where the sloping wire and the top section of the tower are resonant and 
the bottom section isn't, then the best way is probably by creating a 
stub by using the feedline, such that on receive the feedline looks like 
a high impedance at the feedpoint of the antenna.  Of course you will 
also have to investigate where the tower itself is resonant when the 
feeline is open.  Multiple resonances will be very tricky.  Changing the 
length of the sloping wire, or inserting reactance at the feedpoint 
won't do it.

The only way to be sure of how a sloper is going to perform is to 
analyze the entire tower and everything on it with NEC.  For best 
performance the currents should be confined mostly to everything above 
the point of attachment of the sloping wire, and the sloping wire 
itself.  If you discover a way of doing that without analyzing the whole 
thing let us know.  Of course you may have to completely reconfigure the 
entire tower to get this to happen.  Another option if you have 
considerable currents in the bottom of the tower, is to add many radials 
to the tower.  This may provide acceptable performance.  A third option 
is do what most people do, put it up and take what you get.  That may be 
anything from very good to very poor.  Hence the name slopper.

Jerry, K4SAV

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