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[TowerTalk] Re: Shortened Monopoles

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Subject: [TowerTalk] Re: Shortened Monopoles
From: (L. B. Cebik)
Date: Fri, 16 May 1997 08:42:29 -0400 (EDT)
> What happens if the top hat is not perpendicular to the ground, but is at a
> 45 degree angle to both the monopole and the ground? This would allow the
> hat to become part of the guy structure of the monopole. If you haven't
> already modelled this, would it be possible to do so?


Your question has general interest, so I'll post my notes to you.  Hats
need never be perpendicular to ground.  They work on the ends of dipoles
at 90-degrees to ground; I have constructed 2-element hatted Yagis that
performed to specification.  The question is always the orientation of the
hat with respect to the antenna main element.

Yes, you can certainly use uninsulated guys as a top hat, even if sloping
at 45 degrees.  (I shall not pretend to comment on the advisability of
this from a mechanical perspective.)  In most cases, such top-hat guys
will not be resonant on other am bands, but be sure to check this if you
implement it.

If the tower is in the 50-70% of a full 1/4 wl vertical range, the
guy-wire top hat at 45 degrees will require hat wires about 25% longer
than if the hat is flat.  There will also be two performance penalties,
one very small, the other fairly important.

If the guyed tower is 70% full height, there is a gain loss of less than
0.1 dB; if in the 50% range, it goes up over 0.1 dB--and grows as the
tower gets shorter.

The feed Z also goes down relative to a flat-top hat.  At 70% height it
drops from 29 to 25 ohms, not too serious.  At a 50% height, it drops from
about 19 ohms to about 10-11 ohms, more serious in terms of overall system
efficiency, but not an impossible matching task.

I do not give absolute gain numbers, since these models were done over
perfect ground, and no one has that.  However, comparative numbers remain
reliable.  My models were for 4-wire hats, corresponding to one typical
guying system.  3 wire systems will also work, but the required hat wire
length is considerably longer than with 4 wires.  To shorten wire lengths,
one can run a perimeter wire from one guy end to the next.  To get into
the length ballpark, but imprecisely, consider the length as running from
the top junction to the radial guy end to the midpoint of the perimeter
wire to the next guy end.  That will give you a guesstimate on how much
length you can save (which will gain a tiny increment of gain, but more
importantly, will save a few ohms of feed Z).

If one goes to guys that are at a 30-degree angle relative to the tower,
the effects are larger; if the angle is greater than 45 degrees to the
tower, the effects grow smaller.

I have seen this form of hatting used in commercial installations, and it
has appeared in some literature for hams.  However, hats--in general--are
best designed for individual installations and are never so cookie cutter
as simple dipoles.

Hope this includes something useful for you.



L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
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