Centre fed dipole Mounting

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Liamashley
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Centre fed dipole Mounting

Post by Liamashley » Wed May 06, 2020 7:14 pm

hoping for some help with this please if poss.
I have a 7mtr fibreglass pole and a centerfed Dipole. given that the fibreglass pole (acting as a mast) is non conductive, is there any reason why i cannot take the mounting arm off the dipole, and marry the two elements of it to the fibrepole so it is effectively operating like a vertically polarised wire dipole and tie it on with plastic ties for example? this would mean i have a very centred load on the mast...my instinct tells me i need to make sure the coax is kept as far from the elements of the dipole as possible and that the mounting arm has the added benefit of acting as a spacer between the radiating parts of the antenna and the feed line, but its a tempting idea to think that i may beable to put the the weight of the antenna that central to the mast. Otherwise i'll just put a J pole on it.

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teckniqs
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Re: Centre fed dipole Mounting

Post by teckniqs » Wed May 06, 2020 8:35 pm

it sounds like you are planning to turn your standard dipole into an end-fed dipole, which should work fine but you might have to make longer or shorter your elements to get the correct SWR compared to when using the standard cutting chart for a standard dipole.

....Here is a centre fed copper dipole without centre cap working like an end fed dipole that a pal of mine made about 15 years ago and it worked very well.

Image

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yellowbeard
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Re: Centre fed dipole Mounting

Post by yellowbeard » Wed May 06, 2020 8:47 pm

Do the j-pole man, or a slim jim. Running the coax past the lower element like that may induce currents in it causing it to radiate, RF interference in your gaff/audio gear and a high swr. Another solution that is end fed and low profile is a coaxial dipole:

Liamashley
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Re: Centre fed dipole Mounting

Post by Liamashley » Wed May 06, 2020 10:37 pm

teckniqs wrote:
Wed May 06, 2020 8:35 pm
it sounds like you are planning to turn your standard dipole into an end-fed dipole, which should work fine but you might have to make longer or shorter your elements to get the correct SWR compared to when using the standard cutting chart for a standard dipole.

....Here is a centre fed copper dipole without centre cap working like an end fed dipole that a pal of mine made about 15 years ago and it worked very well.

Image
Wicked! Yes mate that is exactly what im about to do. end fed dipole! getting on it tomorrow. pics look well good.

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radionortheast
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Re: Centre fed dipole Mounting

Post by radionortheast » Fri May 08, 2020 9:20 am

Yes there will be a difference in length, the other thing is were you coil the coax at the bottom to make the balun, makes it hard to tell were the aerial ends, in some ways the coax next to the bottom element will be partly acting as an aerial, until after the balun, if the coiling or length aren’t right more of the signal will go back down the coax. ;)

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Re: Centre fed dipole Mounting

Post by Legacy » Fri May 08, 2020 1:29 pm

IMG_20200508_131221.jpg
IMG_20200508_131221.jpg
IMG_20200508_123003.jpg
Ok, here are my modifications. I could be completely barking up the wrong tree with this but I've spaced the feedline slightly away....anything wrong with this?
You do not have the required permissions to view the files attached to this post.

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Re: Centre fed dipole Mounting

Post by Legacy » Sat May 16, 2020 10:29 am

teckniqs wrote:
Wed May 06, 2020 8:35 pm
it sounds like you are planning to turn your standard dipole into an end-fed dipole, which should work fine but you might have to make longer or shorter your elements to get the correct SWR compared to when using the standard cutting chart for a standard dipole.

....Here is a centre fed copper dipole without centre cap working like an end fed dipole that a pal of mine made about 15 years ago and it worked very well.

Image
Thanks Techniks, you say to shorten the elements which makes sense, I see though in your pic that the + element is alot shorter than the - element. Is there a formula for this? I imagine shortening one element like this makes it unbalanced to a certain Degree meaning I'll need a choke balun?

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teckniqs
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Re: Centre fed dipole Mounting

Post by teckniqs » Sat May 16, 2020 10:50 am

That one was not built by myself, it was built by someone in around 2005 after I'd explained to them what to do. I think it's just the camera angle which makes the top element appear a lot shorter than the bottom, although as far as I remember the bottom element length wasn't as critical as the top but the picture definitely makes it look a lot longer.

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Re: Centre fed dipole Mounting

Post by Legacy » Fri May 22, 2020 8:08 pm

yellowbeard wrote:
Wed May 06, 2020 8:47 pm
Do the j-pole man, or a slim jim. Running the coax past the lower element like that may induce currents in it causing it to radiate, RF interference in your gaff/audio gear and a high swr. Another solution that is end fed and low profile is a coaxial dipole:
Ello mate, good shout on the above, is this also known as a bazooka? i had a crack at it with some rg58, 85cm on each element for 87.5, braided sheath folded back over the feedline, taped on to a telescopic 7m flag pole, brilliant swr, hardly reading any.. but on the drive around i noticed a little bit less coverage and a bit of a drop in saturation in areas where i had it held down fairly close to the tx and then a better signal in other areas further away, so a bit more coverage but a bit less intensity if that's even possible?....

Albert H
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Re: Centre fed dipole Mounting

Post by Albert H » Sat May 23, 2020 1:41 am

I always found that the bazooka was identical to a correctly matched dipole in terms of coverage. It's a good emergency or temporary aerial, but you're better off with a carefully matched J-pole, or - even better - a "super-j".
"Why is my rig humming?"
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Re: Centre fed dipole Mounting

Post by Legacy » Sat May 23, 2020 10:39 am

Thanks Albert, whilst youre here;

is it possible to replicate a double stack in this bazooka/coax antenna configuration? as in the exposed conductor lengths are the same length as the 1/4 wave whips on a dipole, if i get the spacing right and do the same futher down the flag pole (representing the same distances as on a dipole array could i achieve a similar gain? also if you have a simple homebrew phase harness drawings and one for your J-poles that would be banging if you're happy to share :-)

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