Steve: You are absolutely right on your statements below. I addressed only
the one beam on top of the tower. My 5 conditions zero'd in on it although
it applies to other beams at a distance for reflections--not detuning the
beam--they are too far away. My post mainly addressed the free space pattern
before ground and other reflections. If I had multiple beams on a tower I'd
have Phillistran all the way to about 33' of the ground (a 40M ground plane).
Other beams on other towers and their free space patterns can be reflected
by the guys on other towers but they are too far away to affect the SWR. So
SWR change is not a complete test.
It's easy enough to scale a 2M tower with 2M beams on top, guys and feed 100W
into it and measure the RF on the guys and other guyed towers. I found out
what I wanted to know doing this over 40 years ago and verified it in Eznec.
In the interest of saving money with a single beam on top the idea of having
Phillistran for a 1/2 wave at least and then a 1/4 or 1/2 wave vertical
eliminates a lot of grips and insulators and makes use of the guy wires as a
very effective vertical with some directivity affect from the tower. There
are patterns of this in one of the QST Handbooks. A DX station about 25
years ago sent me data on this also. It may not be as directive as a 4
Square but I'm working on that. It you have space only for 1 tower it beats
the hell out of a 4 square you don't have.
Honda has a 4 wheel dune buggy for the back woods that has some interesting
accessories. It has a snow plow which I will need in SD, a ground plow and
rotatiller for a big garden I plan (fresh health food you know) and some
other aids. A single blade from the rotatiller would be ideal for digging
radial trenches. It's costing me $600/Mo to get my lawn mowed there again
this spring and I'm looking at lawn mowers I can pull behind the rig also.
K7GCO
In a message dated 6/7/01 8:10:02 AM Pacific Daylight Time,
millersg@dmapub.dma.org writes:
<< K7GCO wrote:
<snip>
> RF can bounce off metal objects but unless it bounces back at the "right
everything in the right plane", the beams pattern and SWR are virtually
unaffected. When you run these tests with say a 2M beam and scaled guy wires
and a tower or in Eznec you will see there has been too much concern for guy
wires affect except for what is right under the beam for a 1/2 wave. The
multiple-insulator
> installation all the way to the ground is mostly just another "TT Band
Aid"
> for a problem not properly addressed like poor feed systems used in beams.
> Fix the source of the problem first. Unless Phillistran is used for a 1/2
> wave from the tower I'd suggest an insulator at the tower, one at 5', one
at
> 15' and one at 30' in metal guys. <snip>
In some cases, K7GCO is right on the mark. However depending on the
installation, there are valid reasons (not "band aids") to use non-
conductive guy wires (or EHS broken up with insulators) for distances
beyond 1/2 wavelength from the tower.
Case 1: Sidemounted antennas
VSWR and patterns of sidemounted yagis below the top set of guys can be
altered even with the first 1/2 wavelength being nonconductive.
Case 2: Multi-tower installations
A neighbor had a 18 MHz yagi on a 56 foot freestanding tower that would
change VSWR when rotated with |Zin| varying up to 25%! The maximum change
occured when pointing at his big tower located about 100 feet away. After
replacing the big tower's EHS guys with Phillystran, the beam's VSWR no
longer varied as it was rotated. Guy wires located about 2 wavelengths
away were causing significant interaction. I believe there is an upcoming
article (perhaps in QEX) on the specifics....
Personally, I opted to use fiberglass rod guys (down to ~10 feet above
ground level) since I will be sidemounting several antennas. By removing
conductors that are not part of the antenna, any potential guy interaction
and reradiation problems are completely avoided. --
Steve Miller N8SM millersg@dma.org http://www.dma.org/~millersg
>>
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A
HREF="http://www.ChampionRadio.com";>
www.ChampionRadio.com</A>
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|