Dipole for airband which way to mount

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Dipole for airband which way to mount

Postby 10red80 » Sun Jan 27, 2019 2:27 pm

Newbie question, I’m using a dipole made out of coax for scanning airband. What would be the best way to mount it? Horizontal or vertical or does it not matter maybe?
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Re: Dipole for airband which way to mount

Postby m0lsx » Sun Jan 27, 2019 2:36 pm

Vertical, as that is what is used on the ground stations & using horizontal whilst they are vertical would loose you 3dB or 50% of the ground stations signal. The aircraft's are so much higher than the ground station & thus their range is much greater.
I am not 100% sure what polarisation the aircraft use. But I would assume that it can vary depending upon exactly how & where the antennas are mounted any how.
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Re: Dipole for airband which way to mount

Postby 10red80 » Sun Jan 27, 2019 4:50 pm

Thank you, thats how I’ve been using it.
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Re: Dipole for airband which way to mount

Postby radiostationx » Sun Jan 27, 2019 9:56 pm

Coaxial dipole , Nice choice for airband 10red80 !
Did you make it yourself ? If so congrats, you saved yourself a few quids ! :wink:

You could convert it to a double bazooka airband dipole,heres the plan for one I made recently.
I wanted a quick & relatively cheap to make civil airband antenna to mount in a loft space.
Thick coax like rg-213 is good for the elements, this makes the antenna resonant over a wider part of the airband than thinner coaxials.
Double shielded braid/foil types and foam dielectrics are not a good choice.
Coaxial with a velocity factor of 0.66 with a milky transparent dielectric is best for the job of making the actual antenna elements, run back to your radio with any 50/75 ohm coaxial you like, foil foam the lot.
Ill do a write up on my blog soon.
I could have gone out and bought a diamond 777 but they are a lot bigger (cumbersome in a loft space I wager) and a wad more expensive plus theres more fun in making your own stuff anyway. :smiles:

From the excellent program "coaxdpl" by VE3SQB which works out lengths for you , all you have to do is insert freq and coaxial of your choice. The program does all the number crunching with velocity factors for you.
http://www.ve3sqb.com/

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Option 1: Rough plan using a 3 way Tee box

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Option 2 : Materials from screwfix/toolstation btw (I used 20mm a 2 way PVC besa box/aka through box for this one) plus some more details.

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The finished article less the rope for hanging it up

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Total cost about £7 for the rg-213, so-239 and pvc bits.
Works very well indeed !
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Re: Dipole for airband which way to mount

Postby Chris P » Sun Jan 27, 2019 11:14 pm

white PVC would be more suitable as the black often has a carbon filler which can detune the antenna from its expected frequency response(if the calculator is used to determine the dimensions)


PS I use a slim jim made to dimensions to suit the airband in my loft it is almost as good as my D777 which is fitted on the gable end of my house some 3m higher than the Slim jim .

I might try the bazooka just for comparison
Regards Chris aka G8FFF nipper or tazmin88
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Re: Dipole for airband which way to mount

Postby radiostationx » Mon Jan 28, 2019 12:25 am

Hi Chris,
A quite few years ago now carbon was taken out of the black pvc extrusions.
Presumably down to the eco police or dare I say it, the EU specifications police. :huh:
I see it day in day out EN this and EN that ..its all regurgitated BS.. forgive the pun !
We pay EU officials obscene wads of cash to invent specs, so they have to do something for the tree huggers now and again.

White has Titanium dioxide TiO2 which is used in pigment/colouring of the white PVC, it is a conductor although not a very good one.

One thing is the ever shrinking wall thickness of pvc tube,its getting to be a bit of a joke now, a bit like your favorite chocolate bar..unrecognisable from the one in the shops 10 years ago.
When you do get hold of thicker walled stuff, its as soft as cheese nowerdays and easy bends out of shape if left out in the elements.



I have superyagi airband j-pole ( a great job by Kevin btw..nice build) and the now the bazooka, one thing you do notice is the different angle of attack. J-pole great for low alt/gnd stns.
The j-pole is a goodun over 5 mhz either side of the tuned freq or so of the band then the performance drops off until a point where the dipole outperforms it..
The coaxial dipole is more forgiving, a good all rounder in so much as it will be a "jack of all of 18 mhz " (master at none) in the airband with reasonable performance in the entire 118-136 spectrum. Coaxial dipole seems not to pick up as much local interference as j-pole when both are mounted in loft, same coax type feeders.
Civil air 128mhz Coax double bazooka dipole will do okay on 300mhz mil air also, 128 mhz j-pole nowhere near as good.
Horses for courses I suppose.

Interested to know about your findings on d-777 and j-pole comparison Chris.
The 777 is a bit of a monster white stick at 6ft long or so. 5/8 co-linear isnt it ?
Always liked Diamond build quality, had a 2/70 up on roof of my old house and one of their wire dipole centres which I used to bodge together a fan dipole for HF, worked faultless for years.
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Re: Dipole for airband which way to mount

Postby Chris P » Mon Jan 28, 2019 10:01 am

Thanks for the update re the carbon content ,I will see if I can get further info from the manufacturers there must be an EU ruling stating that they must disclose the chemical content . I agree fully with your findings re the PVC wall thickness even Bovril is much thinner these days . I recently built a support for a Moxon FM antenna and found the 20mm tube too bendy so ended up using 25mm pipe . I have some black conduit in the garage which may not contain the carbon as it was bought fairly recently but have avoided it for antenna construction perhaps another experiment in the pipeline (pun intended)
Regards Chris aka G8FFF nipper or tazmin88
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Re: Dipole for airband which way to mount

Postby m0lsx » Mon Jan 28, 2019 10:29 am

One easy way to see if PVC pipe contains Carbon or not is to put a small piece into a Microwave Oven for a short period. Not forgetting of course to add a small cup of water too.
If the PVC gets warm, then it's no good with RF.
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Re: Dipole for airband which way to mount

Postby 10red80 » Mon Jan 28, 2019 8:32 pm

Confession; I didn’t make it myself!! £8 ready made, picking up what I believe is the reprater from Clee Hill and I’m not far from BHX myself :)
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Re: Dipole for airband which way to mount

Postby G4RMT » Tue Jan 29, 2019 10:10 am

I made some verticals from white and grey tube from Screwfix. The white didn't;t impact the antenna spec on the analyser, while the grey did lower the frequency a small amount - but well within what I needed (157MHz) However - after just 6 months, the white one was turning banana shape, but the grey one wasn't!
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