To: <towertalk@contesting.com>
>Date: Sun, 22 Nov 1998 19:03:31 -0500
>From: "w8ji.tom" <w8ji.tom@MCIONE.com>
>
>Hi Eric,
>
>You seem to be saying by choosing the proper attachment point,
>the tower radiates and the sloper does not.
No. Just that the tower radiates and this is more important (and
certainly more useful) than the radiation from the sloper.
Unfortunately, we are talking in generalities about a subject
that is very specific to the circumstances of the particular
tower to be used. That almost stopped me from posting anything
in the first place.
>
>Since the sloper has to carry exactly the same common mode
>current as the tower, perhaps you can explain exactly what
>prevents the sloper from radiating?
Nothing. But there are a number of configurations where the
_horizontal_ component of this radiation is a very minor
contribution to the pattern total. These seem to be the ones
wher the owner's perception of the antenna is that it works.
Like you say, the one that's best is the one you think works for
you. Some of these (on taller towers) also have the
characteristic that the magnitude of the current in the tower
below the feedpoint is much lower than the current magnitude in
the tower segment above the feedpoint.
>
>I also understood you to say feedpoint impedance is secondary to
>finding this "sweet spot" on the tower. I think most people
>have no idea what the pattern is, and are mostly concerned with
>SWR. SWR is a function of the self-impedance of the tower at
>the point where the sloper is attached, since the tower is the
>other half of the feedline termination. The feedline "pushes"
>against both the tower and sloper.
I don't know why SWR is the thing that most are concerned about.
But I'm convinced that you are right that it is. However, as I
said before, leaving the design of a sloper to chance is quite
risky. It should be attached where it will make the system work
best and matched from whatever that feed impedance is to 50 ohms.
It is usually more like finding the "dead" spot and avoiding that
particular location. Almost anywhere else works OK and the
criteria for selecting the location away from the dead spot is
indeed based on feedpoint impedance. I normally pick a spot that
drives at around 20 to 30 ohms so that I can match to 50 ohms
with a shunt inductor. This has the side benefit of requiring
less sloper length for resonance and keeps the center conductor
at DC ground.
I also like to end load the sloper wire with a "hat". This makes
the system even shorter and provides easy tuning if the hat can
be clamped to the sloper and moved a bit for tuning. The shorter
it is, the more nearly vertical it can be placed.
>
>> A bit of modeling of the tower system and the proposed sloper
>> will quickly reveal a reasonable attachment point (if one
>> indeed exists... and it usually does) which will produce an
>> effective antenna system.
>
>Slopers indeed can be planned, but:
>
>1.) Most people expect an out-of-box 1/4 wl sloper to work by
>just grounding the shield of the cable to the tower.
This is unfortunate. It isn't usually true. But it does happen.
>
>2.) If they model the system, they need to model ALL the
>elements (and boom) of any antennas attached to the tower, as
>well as all uninsulated or poorly "detuned" guy lines.
This is true but not difficult to do. And fortunately, for this
purpose, hair splitting accuracy is not required. The top
loading of the yagis does not contribute significantly to the far
field radiation. We DO have to know which elements are connected
to the boom and how the connection is made. But again that is
not difficult. Fortunately, if the biggest antenna is the lowest
one on the mast, you can pretty much ignore the ones higher up.
I have been having pretty good luck just using a 4 radial hat
with the radial lengths set to give the same "total conduction
length" as the yagi elements. It simplifies the model a lot and
has been running within +/- 10% or so of actual measurements made
on towers in the field (predicted tower resonance vs actual
resonance).
>
>3.) If they are successful in modeling the tower and all
>attachments on the tower properly, and if they look for a "sweet
>spot" that has a low self-impedance at the operating frequency
>of the sloper, it can be up to 1/4 wl away from the poorest
>points. In some cases, there might not even be a "sweet spot"
>without adding counterpoises.
>
>73 Tom
>
I don't doubt this is true. But so far a counterpoise has not
been necessary for any of the ones I've done. We did use a
second opposing sloper wire on one to more completely cancel the
horizontal component and circularize the pattern. That worked
quite well (and further shortened up the sloper wire extension
from the tower). I suppose you could call it a kind of
counterpoise.
73, Eric N7CL
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