On Thu, Apr 05, 2001 at 02:33:17PM +0100, Bill Tippett wrote:
> W8JI:
> >While that system probably works, with proper selection of feedline
> >lengths or use of baluns there would be no need to switch the
> >shield.
>
> If I recall correctly, the feedlines must be open-circuit (OC)
> because ~3/8 wavelength OC adds inductance to make the undriven elements
> look like parasitic reflectors in the system. I guess they could be
> made ~5/8 wavelengths and shorted if you didn't mind the cost of another
> 1.25 Vf wavelengths of coax (5 X .25 wavelengths X Vf). I would still
> worry about what is happening in the system if the shield is grounded
> to the tower and all other shields (driven and undriven). The modeling
> probably only considers the undriven dipoles as reflectors and not part
> of a complex system with shields of everything tied together.
I have used several systems similar to this on 80 and 160. While the
above might be true it is something that I have never really
worried about too much since the antennas seemed to work quite well.
At N5AU we put up a system that was sorta based on this - we even
cut the feedlines so they would be 3/8 wave long. We also used
a switchbox that did not ground the center conductor of the feedlines.
We had a 190' 25g tower guyed six times with unbroken steel guy wire.
We used three sloping dipoles from the 180' level - each sloper
bisected the guy angles so we had antennas sloping NE, SE and W.
Here is a photo of that tower:
http://www.kkn.net/~k5tr/photo/pcd2817/n5au-stacked-40m-two-9.3.jpg
The yagis are Cushcraft 40 meter yagis.
This antenna is the most impressive antenna we had at N5AU - it
had great front to back, it heard well and we had no trouble
working anything.
Here is a recording I made of it when I was operating at HR6A.
http://www.kkn.net/~k5tr/various-rec.html
When I was designing the W5KFT station I decided that we would do something
similar on 80 meters - we have a 150 tower with steel guys on it and
I put up slopers NE, NW and South. These antennas have good pattern and
once again they work very well - while putting up a 4 sq or even using
4 slopers off ther tower to make a 4 SQ might work a bit better it
sure is hard to beat these simple slopers for performace/cost. In the
80 meter system I did not bother to make the feedlines any lenght - I just
used enough to get back to the switchbox at the base of the tower.
--
George Fremin III
Johnson City, Texas "Experiment trumps theory."
K5TR (ex.WB5VZL) -- Dave Leeson W6NL
geoiii@kkn.net
830-868-2510
http://www.kkn.net/~k5tr
List Sponsor: ChampionRadio.com - Trylon self-supporting towers, safety
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