[TowerTalk] Stub mast loading and forces

[MORCX]

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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