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
>tower.)
>
>
>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?
>
>Thanks,
>
>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
work.
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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