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
_____
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Robert M0RCX
Sent: 27 April 2013 17:35
To: towertalk@contesting.com
Cc: Conrad Farlow
Subject: [TowerTalk] Stub mast loading and forces
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
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