[TowerTalk] Breaking up guy wires for nonresonance
K7GCO@aol.com
K7GCO@aol.com
Wed, 6 Jun 2001 18:06:44 EDT
The posts below show the questions that still exist on the need of
breaking up guy wires. N7WA got it right. Some time ago I gave info on TT
of Eznec patterns I ran of different length guy wires below beams. I
concluded from the Eznec and previous 2M test data using a scaled tower and
guy wires that--if the guy support was Phillistran for a 1/2 wave at the
lowest frequency of any beam--it could be any length of wire from there down.
Eznec showed with the current amplitude function wide open virtually NO
current on the guy wires 1/2 WL below. Many have since confirmed this to me
with actual tests on guy wires. There were ways we checked guy wire RF 50
years ago with flouresant lamps, neon lamps and light bulbs shunted across
the guy wires. Another test is--does the SWR change during rotation? I
found with test data that SWR change with beam rotation was often incorrectly
blamed on power lines and guy wires. The problem "went away" when a balun
was added to the coax feeding a balanced 50 ohm feedpoint. The "RF Spill
Over" on to the coax shield and to the tower and guy wires was "the source"
of the problem. I also had a 20' wood tower guyed with ropes. The power
lines 50' away were not the problem. I added RF anmmeters to a TH-4
feedpoint, measured the current balance and unbalance with and without the
balun and watched how the currents and SWR varied with rotation. This was
published 25 years ago. Self supporting metal towers and telephone poles
also solve a lot of problems.
A single insulator at the tower for all metal guys is a must and will
eliminate RF Spill Over transfer to the guys from the tower and coax shield.
There can still be RF coupling from the beam to a guy directly underneath
even with the insulator at the tower and a resonant length of a guy like an
inverted vee. I found minimum coupling with the inverted vees 10' or more
below the beam. The reasons are obvious as explained below and verifiable in
Eznec. I and others have run tests with a test dipoles a mile way and have
run patterns checking for pattern distortion and null fill in. The most
sensitive part of the pattern are the nulls which are easy to fill in if of
the same polarization. They will fill in by vertical polarization in all
directions (only if received signal is vertically polarized) with no balun
and even with a typical Gamma RF Spill Over on to the shield and tower. With
beams like the TA-33 without a balun, the TA-33 is mostly a "50 ohm matching
device for the coax to the tower and guys" (in particular with the metal guys
attached directly to the tower).
There is a very straight forward and logical justification for all of this I
realized over 50 years ago. This will "ring a bell" if you have ever spent
time trying to tune up a beam with a FS meter or just varied lengths in
Eznec.
Consider this: Assume a 3 element yagi on the tower and/or in Eznec. If you
add a director and vary it for length and spacing for optimum gain increase
the most you can get is about 1 dB and it will affect the DE Z some. In
order to do this it has to be:
1. An optimum spacing of around .1-.2 WL.
2. An optimum length within a percent (on each band).
3. Of the same polarization
4. In the same plane in front on the boom
5. A similar construction of tubing or wire
Only with all 5 of these "totally optimized" will you get a maximum of about
1 dB (even less per optimum element the more you add) and some change of
feedpoint Z. If anyone is not optimum you will get far less than 1 dB and or
"nothing" like if the element is not the same polarization and even of the
right length and spacing. So you tell me how a guy wire that is not of a
resonant or a certain length, not in the same plane out front and not of the
same polarization going to up set the beams pattern or SWR? If your beams DE
has a feed system with no RF Spill Over or a good balun is used, you are
going to go like hell to get a noticeable affect from guy wires unless
attached to the tower without an insulator and right under the beam.
With a 1/2 wave of Phillistran from the tower down to a metal guy wire, it's
totally out of any field that is of any concern. Scale this in Eznec, rotate
the beam (that will take awhile). and see for your self.
Further more beams have a vertical directive pattern that points straight
ahead. Therefore the pattern component pointing down at guy wires is greatly
attenuated and any reflected RF is attenuated again back toward the beam.
The higher the gain of the beam the more isolated the beam is from
surrounding objects to the side/back and below. It just doesn't see them.
Take a 2M 3 and then a 11 element beam and attach it to a MFJ SWR Analyzer.
Point the beam at metal objects and then put them on the side/back and below
the beam and observe what it takes and how close they have to be to affect
the SWR. You will get the "isolation message". 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. Any metal guy wire below that is "out of
the field of any concern." Use a 2M beam, scaled tower and guys and run all
kinds of tests if you still have any doubt. Also buy the Palomar RF Current
Meter to measure the RF on wires and tower legs. I did this 50 years ago for
my interest, demonstrated it at Convention and Club talks. Dr Don Reynolds
of the U of Washington EE Department did the same thing about 20 years ago.
I have suggested making 1/4 wave verticals using the guys from the ground up
with radial systems under each guy wire at the ground attachment point up to
within a 1/2 wave of the tower at the beams lowest frequency. Since they
slant toward the tower and are about the right spacing, there can be a "fair
reflector affect" from the tower. Now you have a directive 3 or 4 antenna
system for say 80&40M with no affect to a beam. If the tower is high enough
I'd suggest 3 or 4 1/2 wave 40M verticals center fed--no radials needed. Or
they can be fed with an L network at the base and no radial system is
needed--just a ground rod to cool the coax shield. It's a great system that
I and other have used. I've also phased the 2 guy antennas at right angles
to the main driven antenna to obtain a directive pattern. One of my next
long winter projects is to phase in the back antenna somewhat like a "4
Square" and see if I can minimize even more the affect of the tower in the
center. I've already done it using 3 and/or4--l/4&1/2 waves and will
finalize the results. The guy wire verticals can be trapped for 80/40M also.
I've added another 80M trap at the top and a 160M pigtail back away from the
tower for resonance at right angles to the guy with great results on 160M.
It's a way of maximizing everything you have a for all bands if restricted to
one tower. Guy wires as antennas has been greatly ignored in ham use
unjustifiably. Fortunately no one told me I couldn't use them. Perhaps this
will spur some interest in that direction. K7GCO
In a message dated 6/6/01 9:16:23 AM Pacific Daylight Time,
michael.dinkelman@physio-control.com writes:<<
< Then, I would consider only using Philly for upper guys or near
antennas depending on how you are going to load the tower. Once you
get out and/or down a ways, use steel - who says the entire guy
has to be Philly? (It should be partially steel anyway for
safety) It really depends on your individual setup.
Philly and steel guyed tower owner
dink, n7wa
***Right on!! >>
In a message dated 6/5/01 4:44:11 AM Pacific Daylight Time, n4kg@juno.com
writes:<<
Guy Insulator Placement de N4KG (Feb 2000)
The following lengths between insulators have
resonances between the conventional bands
(which places their resonancesin the WARC bands :-)
- 27, 40, 58, and 76 ft. (per ARRL Antenna Book)
You may want to make your wires slightly shorter
to compensate for the capacitive end loading of the
loops through the insulators.
You need to place the first insulator as close to the
tower as possible to prevent coupling to continuous
wire from one insulator, through the tower, to another
insulator.
For the first insulated section, I like to use a short
piece of 10 to 12 ft between insulators. This length
is substantially less than 1/2 wavelength (WL) on
10M and will therefore be nearly invisible on all frequencies
below 28 MHz. For even better isolation, use two 10-12
ft sections before going to longer spans.
de Tom N4KG
On Fri, 04 Feb 2000 02:34:09 +0000 Peter Larsen <ve6yc@home.com> writes:
> Hi All:
> Could some one please send me the recommended lengths
> of guy wires to break up resonance.
> I know this has been discussed here before, but I just get lost
> in the archives.
>
> Finally the 100 foot guyed tower is going to go up!
> Peter J. Larsen
> VE6YC DO21wc
In a message dated 6/5/01 6:03:19 PM Pacific Daylight Time, w5kp@swbell.net
writes:
<< Thanks for all the helpful replies on my previous question on this
subject.
One Talkian pointed out that Phillystran isn't that much more expensive than
steel, and of course does away with all the guy wire resonance problems. I
didn't believe him at first, but I did some some arithmetic, and danged if
doing this 74-foot 45G with Phillystran doesn't come out only about $150
more than using steel EHS, and I wouldn't have to get carpal tunnel
installing a bazillion insulators. It's amazing how things add up when you
are buying upwards of 25 insulators, 50+ preforms, several hundred feet of
EHS, a whole bunch of thimbles, etc. etc.
So now, whilst waiting for the local power company to mark their buried
lines near the tower site, I'm rethinking my position and looking to
possibly go to Phillystran. Is Texas Towers the only game in town for this
stuff and it's associated accessories like grips? Nothing against TT, they
have always treated me well, just wondered if they have a lock on the
market. Are there any pitfalls or horror stories about Phillystran I should
hear?
Thanks,
Jerry W5KP
>>
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