Robert,
As you have identified, this is a risky situation that requires careful
thought and planning.
Some references to study:
Physical Design of Yagi Antennas by Dave Leeson W6QHS (now W6NL). A
wealth of information on masts and tower stresses as well as the Yagi
topics suggested by the title. This book, though out of print, is
often available second hand at alibris.com or similar. Every Club
should have a copy!
This will help you decide, amongst other things, which material to use
for the longer mast. It's likely that scaffold tube won't be strong
enough.
Mast strength calculator at
http://thebont.com/spreadsheets/AnalysisOfAntennaMastStrength.htm See
link near top of the page for the calculator.
Stresses when luffing a tiltover tower. G4HUP gave a paper on this at
EME2012 and the 2012 RSGB Convention. The EME paper and a calculator in
Excel are at http://g4hup.com/Biblio.htm This will help you decide if
your luffing cable is up to the job. You may need to go to a larger
cable, and avoid stainless steel, for the job you have in mind.
Avoiding extending the tower when luffed over. I don't know your exact
situation, but a tilt plate is a safe way to work on antennas when the
tower is luffed over. See www.nn4zz.com/tiltplate.htm and links therein
for pictures/videos.
HTH
73 Dave G3WGN
----------------------------------------------------------------------
Message: 1
Date: Sat, 27 Apr 2013 17:35:01 +0100
From: Robert M0RCX <m6bfd@yahoo.com>
To: "towertalk@contesting.com" <towertalk@contesting.com>
Cc: Conrad Farlow <conrad@g0ruz.com>
Subject: [TowerTalk] Stub mast loading and forces
Message-ID: <C5B2EBD0-7A35-4450-B2FE-32F5DEC2F8F0@yahoo.com>
Content-Type: text/plain; charset=us-ascii
Hello I am new to the forum and thank you for your acceptance into the
group.
There have been many discussions on wind loading to mast supported
antennas but I was wondering if anyone has calculated forces applied to
stub masts and how they vary with extra length.
We have an hf A3S with 40m add on at 60 foot max elevation. Immediately
above it we have a 6 element dual band VHF beam for 70 and 50mhz.
We wish to raise our stub to increase the distance between the two and
help reduce SWR on 70mhz which is currently resonant at approx 70.600
and as such is out of band Ideally for us resonance should be about
70.300 ish.
We feel the hf beam is almost the cause of this.
The setup tolerances of the antenna are very tight and spot on for
manufacturer recommendations.
We use a 2 inch diameter stub of approximately 6 foot. 2 foot approx are
in rotator cage and there is three feet between beams.
We would like to extend by another 10 feet giving us 14 foot of
available stub.
Our rotator can cope with this but when tower is over we need to wind
out to facilitate ground working without catching trees etc This
extra length applies more forces when cranking the tower back to
vertical position.
We feel out stainless winch cable should support it but this subject has
opened a whole network of interesting questions and the theories and
was wondering if any of you guys has such a formula or has any
experience on such matters.
In particular
A. Strain to cabling
B. additional force added by increasing stub length
Any experience is most welcome
Robert Rawson
M0RCX
North Wakefield Radio Club
www.g4nok.org
------------------------------
Message: 2
Date: Sat, 27 Apr 2013 21:05:03 +0100
From: "John Lemay" <john@carltonhouse.eclipse.co.uk>
To: "'Robert M0RCX'" <m6bfd@yahoo.com>, <towertalk@contesting.com>
Cc: 'Conrad Farlow' <conrad@g0ruz.com>
Subject: Re: [TowerTalk] Stub mast loading and forces
Message-ID: <3EAC4573B0934536BF6CDE978AC9CE04@G4ZTR>
Content-Type: text/plain; charset="us-ascii"
Robert
The principal forces on your stub mast are bending and torque. If you
place an aerial on a stub mast which is twice the length of your current
one, the bending moment at the rotator will be twice as much.
Regarding interaction between beams for different bands on the same
pole, take a look at the excellent web pages of GM3SEK. Almost always,
it will be the beam for the higher frequency which is affected most.
Intuition tells me that the separation that you have at present is not
sufficient.
I think it would be best if you could erect the 4m/6m beam somewhere
where it is not affected by other nearby aerials for a test and see if
you can get a good match on both bands. Then you can think about a
longer stub mast.
Regarding winding out the mast sections when the mast is luffed over,
this is Very Bad Practice. I am sure that neither the mast nor the
foundation were designed for this sort of abuse. The winch and cables
will also be over stressed. By considering a longer stub mast you will
make a bad situation worse.
John G4ZTR
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