The higher band in the open sleeve cell is inherently much narrower in
bandwidht than a free 1/4 Wl radiator.
I'm not sure the open sleeve is a good idea in this case where the bandwith
has to be
consistent.
I've a structure that's reasonant @ 3.5 MHz and I use that vertical on 80
and 40m.
I fed it with two separate gamma matches and the solution looks easy and
effective.
73,
Mauri I4JMY
----- Original Message -----
From: "Ford Peterson" <ford@cmgate.com>
To: <towertalk@contesting.com>
Sent: Saturday, July 07, 2001 9:31 PM
Subject: [TowerTalk] Open Sleeve 80/40 Vertical
>
> My goal is to build an 80/40 vertical antenna this summer. I've been
> modeling the antenna and have some questions for those "in-the-know" on
> Towertalk...
>
> There is an article on the design on pp 7-16 of the 19th Ed of the Antenna
> Book (ARRL) by Roger Cox-WB0DGF. The article describes the function of
the
> "Open Sleeve" antenna but no real design dimensions. Modeling the antenna
> was the only reasonable approach.
>
> I have access to substantial quantities of 20' lengths of used well
casing.
> It is 2-3/8" galvanized (thick wall) steel pipe. The main element ( 60'
> with some top loading) is insulated from ground. Two grounded side
elements
> of ~ 1/4 wavelength (on 40 m)form the transmission line for the 40 meter
> element. On 80 meters, they are almost transparent. For 80 meter
> operation, the whole mess loads together.
>
> After screwing around with AO and NEC2 for some time, I have the
dimensions
> for the sleeves. Using 1.315" chain link fence top rail material for the
> sleeves dictates a modeled dimension of 32.86'. The spacing on the
sleeves
> is 2.45' on each side of the main element. The feedpoint impedance
dictates
> a series capacitor of about 1000 pf to resonate at close to 50 ohms on
both
> 80 (3.550 mHz) and 40 (7.150 mHz). Both AO and NEC2 confirm similar
> results. Using 2, 3 or 4 sleeves changes all the dimensions slightly to
> obtain resonance. I see no benefit to using more than two sleeves.
> Although I didn't model it, one sleeve may be sufficient.
>
> The hat in my model is a pair of perpendicular extensions extending 3.88'
> off the main element and the tips are connected with 14 awg wire. This
> forms a square hat roughly 5.5' on a side with the corners supported by
the
> cross members attached to the top of the main element.
>
> The model shows a good omni pattern (expected) and fair bandwidth. < 2:1
up
> to 3.85 mHz or so and the whole 40 meter band. I can't wait to get my
hands
> on it this winter when the bands are quiet!
>
> I have questions though. Perhaps some have had experience with this type
of
> antenna and can shed some light.
>
> 1) The model shows that the sleeve spacing is quite critical. Do I need
to
> be able to adjust this distance in place or can I simply fabricate a fixed
> element bracket--slam it into the air and forget it? More to the point:
Are
> open sleeve designs a bear to resonate?
>
> 2) The main structure will be 60' with some top hat (see above). The well
> casing has quite a thick wall but the couplings are threaded. Should I
weld
> these couplings solid first before hoisting it up?
>
> 3) I am hoping to get by with a two tiered guy system. Perhaps 50' and
25',
> spaced 120 degrees. Adequate?
>
> 4) What distance should I place the guys from the tower? 35' OK?
>
> 5) I want to hoist the antenna using a derrick arrangement (well casing
> again). Using a 6" x 6" post for center support (cemented in of course).
I
> will drill a large hole in the main element and run a large diameter bolt
> (perhaps 1/2" solid rod threaded on the ends) through the main element and
> the post. With the assembled antenna laying on the ground, a 35' length
of
> well casing will be temporarily attached to the post using a similar solid
> rod arrangement. The guys will be attached all the way around. The
derrick
> will be vertical with the two guys attached to the end (top). Using the
> pickup, I'll attach to the derrick and pull. The top of the derrick will
> have ropes attached that run to either side of the tower to prevent it
from
> tipping over during the lift. Does this sound reasonable? The well
casing
> must weigh in at 60 - 80 lb per 20' length. Add the top load and the
> sleeves -- it will be quite heavy. I want to succeed in building this
> antenna. I don't want to kill anybody in the process!
>
> Any and all comments, including "you're f....ing nuts!" are greatly
> appreciated. I'll let you guys know my progress went there is some
> progress.
>
> Thanks for the bandwidth...
>
> Ford-N0FP
> ford@cmgate.com
>
>
>
>
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