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 SWR & Mast

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NoMad
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NoMad


Call Sign : 26-CT-4253
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QTH or Location : Cambridge, Cambridgeshire
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PostSubject: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 08, 2021 6:57 pm

Am I supposed to swr my vertical dipole when it's mounted to my 3meter aluminium pole , as 

I Have a swr of 1, UNTIL I fix it to my vertical mast, then its off the meter. 

Using internal swr on my ss6900, and the swr protection in the radio will kick in if Im using full power.

No shorts anywhere and heatshrunk and self-amalgum tape everywhere, so no water ingress.
I am not able to ground the mast.


So if someone could explain why this is happening so I can resovle it, I would be greatful.

Thanks.
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Northern Crusader
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 08, 2021 8:24 pm

No part of the vertical dipole wants to be anywhere near the mast, neither does the coax as in at least half a wavelength if possible, the more the better. If the mast and the coax are vertical near any part of the antenna it'll massively detune it. The coax needs bringing out horizontally several feet from the antenna and the antenna ideally wants to be on a boom 15-20ft long to get it as far away from the mast as possible or you need to use a non-metallic mast. Doesn't matter if the mast is grounded or not, it's going to be coupling to the antenna and altering it's tuning. I bet if you rotated that antenna 90 degrees so it was horizontal it would tune in just fine.

Because of the need to do the above vertical dipoles are generally a bad idea on anything other than for VHF, UHF and higher frequencies where it's feasible to have a boom coming out horizontally for a wavelength or more to keep the antenna a wavelength or more from the mast or tower it's bolted to.

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aleksandar
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeTue Nov 09, 2021 8:29 am

i have a vertical dipole and great results with it.I don't see a reason why he's a bad idea. If you're willing to do the job properly.
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeTue Nov 09, 2021 9:25 am

Y'know, "off the meter" (V:SWR) sounds very strange. I also have a vertical dipole that stands off a aluminium mast by about 10" with the coax going down to mast. I don't have any problems at all. That's what makes me suspect there is something else wrong. There's even a 50MHz halo near it's feedpoint Smile

Maybe you should try it horizontal to just see if things change?
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeWed Nov 10, 2021 8:00 am

After a little think, I wanted to add, that you probably want to ensure that the arm of the dipole fed by the centre of the coax is the one that points upwards and is sticking up above the top of the mast. I've not tried it the other way around but have doubts that it would work effectively Smile

Adding a 1:1 BalUn may or may not improve things
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NoMad
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeWed Nov 10, 2021 1:02 pm

Thanks for your replies::

@northern_crusader  "boom 15-20ft"  are your serious a boom longer than my mast  ?  Thats never going to happen here.  On a first floor flat, its impossible too. Seems Very Unrealistic.
The antenna isn't really ''coupled' with the mast, it is isolated from it conductivly.
@allexsandar ; that was my initial idea, you can't really go wrong with a dipole.

@Trunky, it comes of a 10 inch Stubby mount already. I have a 1:1BalUn up already. [it replaced the airchoke, which made no noticable difference]  [I guess this is why when we see ads for dipoles, they are mounted off a brick wall and not a metal mast.]

I checked and double checked I have the feed the correct way up , and even covered it all with hot-melt-glue to assure no movement. 

Should I use a non conductive mast, where would I buy one from ?  Do you mean fibreglass, or some 2x2 wood ?

tia.
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeWed Nov 10, 2021 1:23 pm

Have you tried rotating the dipole 90degrees as NC suggested? There would be practically no interaction with the mast and you could re-test the V:SWR.
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NoMad
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 15, 2021 10:00 am

well, at least today those nasty comments have been removed.
why do you have to be nasty in a reply anyway ?

the saying was, " If you've nothing good to say, then say nothing"

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Victor
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 15, 2021 10:47 am

Completely agree Andy.

The member responsible has 'left the building' and a lot of unnecessary comments removed by Admin.

Charlie Tango is and always should be a place welcoming everyone with all radio interests at every level or background.

....and simply being nice to each other. Smile SWR & Mast 1f44d 


All the best,
Victor

CT Admin.

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43CT016
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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 15, 2021 11:38 am

NoMad wrote:
Thanks for your replies::

@northern_crusader  "boom 15-20ft"  are your serious a boom longer than my mast  ?  Thats never going to happen here.  On a first floor flat, its impossible too. Seems Very Unrealistic.
The antenna isn't really ''coupled' with the mast, it is isolated from it conductivly.
@allexsandar ; that was my initial idea, you can't really go wrong with a dipole.

It will absolutely couple to the mast; capacitively.  It does not need to touch it.  If you have two conductors close enough, and run AC through one (Or RF, as in this case), it will induce a signal in the other.  In an antenna, this affect will detune it.

As to unrealistic….well, that’s why he said vertical dipoles are not a great idea on HF; unless you can hang them from one end, use a fibreglass mast, or find some other way to keep them away from metal for any part of their run.

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NoMad
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Equipment Used : CRT SS6900N, RTL-SDR Blog V3, Baofeng UV5R-3.

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PostSubject: Re: SWR & Mast   SWR & Mast Icon_minitimeMon Nov 15, 2021 12:10 pm

@ Trunky, not yet, but I might now wait a week or two to find a suitable wooden mast first. 

Perhaps it was because I srw'd it in whilst it was laying horizontal........
being mobile was easier.  I'm still learning about homebase.

 @victor, thankyou,  I am glad to hear that.

73's.

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