Stacked dipoles

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Rigrat
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Stacked dipoles

Post by Rigrat » Wed Apr 19, 2017 10:21 pm

Screenshot_20170419-220525[1099].png
Hi all just thought I would share my work on some stacked dipoles that I have made in the past iv included a drawing for all VSWR Around 1.2 across 87.7 /88.7 I used tv coax for matching network as that is what I had lying around but a better quality coax would be better regards rigrat
drawing.jpg
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sinus trouble
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Re: Stacked dipoles

Post by sinus trouble » Thu Apr 20, 2017 1:41 am

Greetings Mr Rat! Welcome to the forum! :)
Excellent job you've done!!! Very VERY neat, youd put many so called 'Pro' pirates to shame!!
Even the boom measurements are spot on!!
The only advice I could give is ditch the TV coax ofcourse and maybe drop the assembly by a 1/4 wave down the mast so that the top element is level with the mast?
Other than that? "Sweet as a nut" as us Londoners would say lol
wind your neck in son!

Rigrat
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Re: Stacked dipoles

Post by Rigrat » Thu Apr 20, 2017 7:20 am

Thank you sinus yes the antennas do work great I did wonder about the top dipole and if it should be lower also spacing between was a question I had also wondered about I have made a few different antennas over the years but fancied having a go at these the only draw back is physical size where as a dirty end feed vertical can also have good results I hope people find the drawings of use as info on the net is limited regards rigrat

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Analyser
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Re: Stacked dipoles

Post by Analyser » Thu Apr 20, 2017 8:38 am

You need to apply the velocity factor of the cable to the 75 ohm sections. A common value is 0.66 but varies with dielectric. For a frequency of 87.7MHz a quarter wavelength section would be 0.564m, not 0.804m.

Albert H
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Re: Stacked dipoles

Post by Albert H » Thu Apr 20, 2017 11:29 am

Multiplying by the Velocity Factor for the ¼-wave phasing line has been a topic of endless debate in the radio world! There is also some debate about the actual required length of the 75Ω feeders, and if each dipole needs a Pawsey stub or other balun to match properly.....

I did endless experiments with this configuration (at 1 GHz, to miniaturise the aerials), and with a three-stack, four-stack and even a six-stack! The twin gives you the best return on investment - almost exactly twice the ERP of a single dipole, and a flattened radiation characteristic. The Four-Stack will give a couple of more dB of gain, so the ERP is improved a bit, and the radiation characteristic flattens and even droops a bit, making it a good choice from a high site.

I experimented a lot with the element spacing, with the boom arm dimensions, with the matching lines and with the use of baluns. The best results were with the upper dipole boom coming off the very end of the support pole - the upper dipole is then above the mast, in free space. This changes the match slightly. Pawsey stubs (made from ¼-wave pieces of the 75Ω coax) improve the SWR significantly, and can get you another dB or so actually radiated.

The 75Ω feeders aren't as critical as you might imagine, it's the spacing between the dipoles that really matters. If you build the aerial, then clamp the lower dipole so that the tip of its upper element is ¼-wave from the top dipole, test the SWR (in Free Space - it's no good if you do it at ground level with walls, trees, hedges and so on around you!). Try moving the lower dipole up by about 3cm and test again. If the SWR worsens, move the lower dipole downwards a bit and test again. Eventually, you'll get the thing spot-on. Measure everything!!! Log the lengths of the coax, the length of the stubs, the dipole spacing and size and the boom lengths. You can now reproduce that aerial with minimum hassle!

If you have the time, and want to make the effort, it's useful to build the antenna for four frequencies in the band. I did 88, 93, 98 and 107MHz for my full-size experiments. You can then plot graphs of each dimension, so that you can make the antenna for any frequency in that range with confidence that it'll match and radiate effectively. It's hard work, but worth it in the end!
Is it meant to smoke like that? :shock:

radio-berlin
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Re: Stacked dipoles

Post by radio-berlin » Fri Apr 21, 2017 7:47 am

Funny enough I've been playing with baluns etc this week.
If you have an aerial analyser to hand and want to cut a length of coax to 1/2 wave length you can do this easily if you don't know the VF.
Put the coax into the analyser and cut slightly longer that 1/2 wavelength, strip it back a little and join the braid with the inner at the furthest end. Keep an eye on the reactance on the meter, keep trimming bits off and joining until x = 0. It will read higher than 0 initially and creep down every time you cut a piece off.
If you want 1/4 length you can leave the ends open ended.
If you analyser goes high enough you could double the analyser frequency and just trim ends off of the coax to save keep joining them.
Last edited by radio-berlin on Fri Apr 21, 2017 7:53 am, edited 1 time in total.

radio-berlin
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Re: Stacked dipoles

Post by radio-berlin » Fri Apr 21, 2017 7:52 am

Albert I've never noticed the swr changing when I adjust the gap between the dipoles unless they are to close. I will try this out this week though incase I'm wrong.
I usually tune each dipole up with a balun singly. I've always used wavelength x 0.8 for the spacing between boom arms.
Am I correct in saying that the distance between dipoles adjust radiation angle ? I think Closer together it becomes more narrow ??

Albert H
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Re: Stacked dipoles

Post by Albert H » Fri Apr 21, 2017 9:58 am

You're right. The shape of the radiation does change with aerial spacing. Tuning up the dipoles individually was the way I used to start, but I find that their characteristics change a bit when they're paired.
Incidentally, have you tried the "amazing" 0.64-wave vertical? I have one in use on a community station in East Anglia, and it seems to radiate like Stephen's half-wave job, but with a less critical match.
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radio-berlin
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Re: Stacked dipoles

Post by radio-berlin » Fri Apr 21, 2017 6:36 pm

Albert, when you make the 0.64 how do you match yours at the bottom and what size groundplanes do you use?

Yes I have tried making a couple, If memory serves me correctly it only ended up about 7 or 8cms longer than my usual 5/8's I make, is there much noticeable difference between two?
However I make mine using 1/4 ground planes and i angle them down until i hit 50ohm, I then have the a horizontal stub at the bottom to make up the length so electrically it appears as 3/4 long in total. im yet to make a coil version and tap it at 50ohm, i could use horizontal groundplanes then. Ive got a photo of my attempt somewhere, Ill post up shortly

radio-berlin
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Re: Stacked dipoles

Post by radio-berlin » Fri Apr 21, 2017 7:12 pm

Got a question for you boffins.....
Ive made a very nice J pole, however I wasnt happy with the balanced unbalanced coax problems a j poles throws at you, adding turns to the coax is a shit way of doing it. I want to be able to use a so239 socket on the j pole so it ruled me out from using a pawsey stub balun as this needs attaching to the feeder. So I opted for a 200ohm balun using a length of 1/2 wave coax, the balun where you join all the grounds and end up with centre cores going to each side of the j pole, this works brilliant and tunes up very nice, however my antenna is no longer dc grounded as the earths from the coax no longer connect to the antenna.
When using this type of balun on a folded dipole the earths join the the centre of the folded part of the dipole at 0 ohms.
The bottom of a j pole is 0 ohms so is it ok to add a link between my coax earths and the bottom of the j pole to ground it, becuase its now around 200-300 ohms it means the connection the the j pole is about 5 inches up from the bottom ( a lot higher than 50ohm point), ive used a piece of braid to link them together and it hasnt affected the swr or impedance readings but will this effect my radiation patten, im thinking it shouldnt as the bottom 1/4 of a jpole shouldnt be radiating much. Can any one with more knowledge shed any light on this...

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