[TowerTalk] Stub mast loading and forces

[George Dubovsky]

You might also consider rotating one of the antennas 90 degrees for a
coupling test "in place". If the interaction goes away, you might want to
make the 90 degree offset permanent. Either keep track of the directions
with a mental "shift" or get one of the smart rotator controllers, like
Green Heron, to do it automagically.

73,

geo - n4ua


On Sat, Apr 27, 2013 at 4:35 PM, MORCX <m6bfd at yahoo.com> wrote:

>
> Hello John,
>
> Thanks for your reply.
>
> I can confirm when the tower is luffed over it is at minimum height plus
> approx 6 foot and no more. This is what has caused us to call a halt to the
> work and conduct more research. We are also looking into some tree surgery
> too to assist the issue.
>
> You like Kelly offer a very valid point in that the VHF beam could be set
> up on a temporary location purely for testing or remove the HF either way
> would do it
>
> I also thank you for the link to GM3SEK site of which I shall view
> immediately.
>
> Thank You
> > Robert Rawson
> > North Wakefield Radio Club
> >
> > www.g4nok.org
>
>
> On 27 Apr 2013, at 21:05, "John Lemay" <john at carltonhouse.eclipse.co.uk>
> wrote:
>
> > 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 at contesting.com] On Behalf Of
> Robert M0RCX
> > Sent: 27 April 2013 17:35
> > To: towertalk at 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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