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@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@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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