Home made dipole antenna - what length for airband?

[jimster99]

Hi everyone,

I am having a go at making a home made coax dipole antenna to pick up my local airport based on the following http://www.civilianaviation.co.uk/forum ... =71&t=2137 because: (i) it looks easy, (ii) it looks fun and (iii) it should significantly improve my airband reception over the current rubber duck I'm using on my uniden bearcat 92xlt.

So - my questions! First, I am trying to work out what length dipoles I should select for optimum airband reception (in the 108 to 137 mhz range). Could someone give me advice on the optimum lengths to use? And second, will this mini project make a big difference to my current handheld rubber duck set up?

Thanks!!!!

[2E1IIP]

Taking in to account the velocity factor of the coax, you will need 768mm per leg of the dipole which will put you bang in the middle of the band ~129MHz.

[jimster99]

Taking in to account the velocity factor of the coax, you will need 768mm per leg of the dipole which will put you bang in the middle of the band ~129MHz.

Thanks very much!!

[Tigersaw]

Taking in to account the velocity factor of the coax, you will need 768mm per leg of the dipole which will put you bang in the middle of the band ~129MHz.

why have you reduced a dipole leg by the velocity factor?
half wave dipole is approx 3 foot 10, each leg about 23 inches

[2E1IIP]

Taking in to account the velocity factor of the coax, you will need 768mm per leg of the dipole which will put you bang in the middle of the band ~129MHz.

why have you reduced a dipole leg by the velocity factor?
half wave dipole is approx 3 foot 10, each leg about 23 inches

Because the RF travels slower in PVC covered wire than bare copper.
Correction:
It should be 384mm per leg and 768mm overall length.

[bigpimp347]

velocity factor ???

you don't even know how long his coax is going to be,
you don't even know what coax he's using..!!
so how can you assume the velocity factor ??

try this link.. http://www.csgnetwork.com/antennaedcalc.html

find the frequency your most likely to listen to of one that's centre of the band..

then calculate it..
as it's for receive only you won't notice any difference if you were an inch or two out, as height would be more of a factor on reception..


velocity factors...lol...

[2E1IIP]

velocity factor ???

you don't even know how long his coax is going to be,
you don't even know what coax he's using..!!
so how can you assume the velocity factor ??

try this link.. http://www.csgnetwork.com/antennaedcalc.html

find the frequency your most likely to listen to of one that's centre of the band..

then calculate it..
as it's for receive only you won't notice any difference if you were an inch or two out, as height would be more of a factor on reception..


velocity factors...lol...

He is using coax to build the dipole, you need to account for this in the length - nothing "lol" about that.

[Tigersaw]

He is using coax to build the dipole, you need to account for this in the length - nothing "lol" about that.

He's using coax to feed the antenna, then using the braid and inner to make the legs.

[bigpimp347]

so he's using wire then for the elements,
regardless if it's inner conductor, outer braid bell wire, jump leads it's wire..
yes thickness can alter the SWR and bandwidth, but for receive just use the calculator and cut it and stuff it..

it'll work..

[kr0ne]

lol... velocity factors...

Unless I'm mistaken, the velocity factor of a transmission line is chiefly determined by the dielectric constant of the coax, ladderline, whatever - but the velocity factor of an antenna element is determined by the ratio of the wavelength in question to the diameter of the element...

Like the pimp says, eyeball it and stick it in the air and it will work. Well. Even if you screwed up the measurements a bit.

[2E1IIP]

I miss-read the OP's post, I thought he was making the antenna from coax.
You do need to account for the VF if using PVC coated wire be it coax or any other coated wire otherwise using the standard formula will make the antenna too long.

[kr0ne]

I miss-read the OP's post, I thought he was making the antenna from coax.

He is... but not with one leg inside the other........

[RogerD]

Cut it for 127.145 if you want the center freq to be the same ratio relative to 137 as it is to 118 MHz

It really won't be that fussy for receive. Just having a probe of some sort up high, with a decent coax feed down to the radio, is all you need to worry about to completely outdo the rubber duck indoors.

[scanhermit]

Probably a bit fussy for receive but won't a balun remove the influence of the coax?

[ChrisCSL]

Heyyyyyy... I know.

Why not try this: http://www.civilianaviation.co.uk/forum ... =71&t=2137

as in Post 1, and make the legs 23 inches long, as in Post 4?

As long as - if you are putting it outside - you just stick a big blob of waterproof 'goo' where the coax ends, sorted.