Hi Tom,
I have only a few comments to add.
To: <towertalk@contesting.com>
>Date: Fri, 20 Nov 1998 13:54:59 -0500 (EST)
>From: "w8ji.tom" <w8ji.tom@MCIONE.com>
>
>Hi All,
>
>I'm pretty sure there are many sloper replies in the archives
>Bill posted, but here goes one more for the pile.
>
>>In a message dated 98-11-19 19:09:48 EST, k2fr@juno.com writes:
>>
>
Snip... (K2FR's description of non-functional sloper)
Then Tom replies:
>Tower-fed slopers depend on pure blind luck to work.
This is true given the way most amateurs approach the design of a
tower fed sloper. But it is not _necessarily_ the case.
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.
But Tom is correct to point out that every sloper is attached to
a completely unique tower system. And the exact nature of that
system determines how the sloper should be configured.
>Top fed slopers are totally dependent on tower attachment point
>impedance for the other half of the antenna. What you do with a
>little "extra ground" at the bottom of the tower makes almost no
>difference at all in the tower's impedance up the tower where the
>sloper is attached. That point is many feet away from the earth,
>and can be a very odd impedance. That is almost certainly what is
>going on for you, especially since you have no guy lines or
>anything else near the sloper of any large electrical size that
>will work as a counterpoise.
This is absolutely true. But it is dangerous to think about the
tower and its attachments as a counterpoise for the sloper. My
post disaster analysis of several slopers revealed that the ones
that were very poor antennas all turned out to be doing exactly
that. They were attached to the tower at a point that forced the
tower currents to result in cancellation of nearly all far field
vertical radiation from the tower. This forced the sloper to be
the sole radiator in the system. This configuration results in
the worst of both worlds. You still have all of the near field
losses associated with the tower currents but you get no
radiation for it and you are left with a very low horizontal
radiator for the antenna.
The successful slopers turned out to be ones that were
effectively driving the tower as a vertical radiator. In these
installations, the tower was doing the vast majority of the
radiating and the sloper was acting more like the feed mechanism
than as a part of the antenna. In these cases, the sloper did
indeed contribute a small horizontal component to the total
pattern but it was a very minor contribution. Understandably,
these systems were very much better DX antennas than the other
type.
>
>Think of the antennas as a dipole, with one leg bent vertical and
>stuck in the mud. Near the feedpoint of that bent dipole, you
>mount a big tribander. Remember the shield attachment end of a
>dipole affects resonant frequency and SWR just like the center
>conductor attachment end.
>
>With that picture in mind, its easy to see if the antenna works
>well and has a low SWR it is a miracle rather than a result of
>good design work.
Unless it was arived at by virtue of good design work.
>The key to making your antenna work is to make the attachment
>point on the tower have a low impedance at the desired operating
>frequencies. The only solution is to hang counterpoise wires
>(attached to the tower) at the feedpoint of the slopper, with
>counterpoise wires cut to the band(s) you want to operate. They
>need to be pulled out away from the tower, and insulated at the
>open ends.
This is far more work than modeling to find the proper feedpoint
and will not solve the low horizontal antenna problem that will
result.
In fact, finding the point on the tower that provides the
absolute lowest feedpoint impedance frequently lands the
feedpoint at exactly the worst possible position. The feed
impedance should indeed be relatively low. But finding the
minimum is a recipe for disaster.
>The result is really just a seriously bent dipole, with one leg
>grounded to the tower.
>
>IMO, you'd be better off with a regular inverted Vee dipole with
>the ends of the legs as far above ground as possible. If you want
>multi-bands, use a trap dipole or open wire line and a
>tuner. Tower-fed slopers should really be called "sloppers",
>because that is a more accurate description of what they really
>are. Don't expect them to work properly unless you are very
>lucky.
>
>73 Tom
I'm also a fan of open wire feed for a multiband dipole of some
configuration or other. I don't see how anyone can get along
without one at his station.
But a properly functioning vertically polarized radiator on the
lower bands is hard to beat for longer range work. And a sloper
is a very nice way to drive a tower for this purpose. It is
frequently the best alternative for a restricted space
residential station where a tower with top loading from higher
band antennas already exists.
The sloper can be far easier to adjust and maintain than a gamma
match and it uses far cheaper and easier to get components (only
need a nearby ACE hardware store).
However, Tom is right to point out that putting one on the air
should not be approached as a matter of luck. Besides modeling
to determine a useable attachment point, there are a number of
other issues that need to be carefully considered. Issues such
as lead dress for both the sloper feedline and the HF antenna's
feedlines, the rotor control lines, bonding integrity of the
tower sections, sloper proximity to nearby objects (particularly
near the ends), etc. Leaving any or all of these to chance
reduce the probability of getting a good working installation.
73, Eric N7CL
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