Dipole help please

[Banus_radio]

Hi all, I'm a new bee here, great forum.

Could I try and get some help

I make my own dipole centre pieces using resin and ally tubes but I can never ever seem to find any info on the correct gaps between elements.

I'm fully aware of how to tune a dipole and I use an Mfj259, I get swr right down each and every time and I use the old steve moss formula to first calculate the lengths to get me in the correct ball park.

But I can never figure out how big the gap should be between elements, the standard off the shelf centre caps have about 20mm spacing. Is there a theory/formula for this? At the moment use a 20mm gap and use solder eyelets riveted to the ally tube.

My next question is, using an off the shelf centre cap like the dpc ones everybody is using in london I've noticed the boom arm mounts between the two elements (obviously insulated by plastic centre cap) with the fixing bolt right through the centre of the radiators, now surely if the boom arm is metal and earthed to mounting pole than this would affect the aerial impedance or other factor? Would a plastic boom be better?

Any help would be great' thanks in advance

[teckniqs]

It doesnt really matter how far apart they are aslong as they're obviously not touching or miles apart you'll be fine, it's the distance between the two tips which is the important bit...

....The boom arm doesn't affect it because of the way it's fixed at a right (L) angle of the elements, as long as it's properly insulated from both of the driven elements.

[Albert H]

My old dipole centres allowed the elements to move closer or further apart by about 25mm each side. This allowed small changes to the overall length of the dipole.

However - remember that the length of the dipole isn't all that critical. It's the match that makes the aerial work or not! It is essential to use some kind of balun. This will bring the SWR down from 1.5 : 1 to close to 1 : 1. More importantly, it will prevent the outer braid of the feeder from becoming part of the aerial and radiating where it shouldn't.

The number of times I've heard idiots complaining that the length of their antenna feeder is really critical... What they don't realise is that the "aerial" on the end of the feeder is just providing a terminating load (probably mismatched, too) and it's actually the feeder that's doing the (inefficient) radiating!

I had a field-strength shoot-out with a so-called "rig doctor" who claimed that he could get his dipoles to match with "a couple of turns" of the feeder.... We used the same 5 Watt exciter and tried his dipole against mine. My field strength was almost three times his at 100m....

The best balun is the Pawsey Stub. The old NRG web page gave a good description of how to do it.

[Banus_radio]

Thanks for replies, so I'll stick with the 20mm I normally use them.
I use a pawsey stub already from the old nrg pages, again I have another question for that. I use Rg8 as my feeder and I use a length of rg8 to make the stub, I normal strap the stub along the feeder and solder it onto the feeder braid, however is there anything stopping me looping the stub back into the centre cap and soldering it to the braid there? Hard to explain, but it would be a loop of coax across the dipole terminals if that makes any sense

Thanks for your help

[sinus trouble]

Hello Mr Banus!
I could be wrong? but i dont think your loop balun would work?
It is a 1:1 balun which needs to be 1/4 length along the feed coax and the balun itself

[sinus trouble]

Could try this version?

http://pira.cz/adipol.htm

[Banus_radio]

Thanks sinus, yes I looked at that, that pira one is for a 300ohm folded dipole but there are other various formulas on google, I'll try it tomorrow and see how the mfj reads.

I only asked about the pawsey stub because I'd like to use a length of flat aluminium folded into a u shape across the dipole element fixing holes, this way I could make it adjustable. When I make the pawsey stub with coax across the fixing holes (instead of soldering to the feeder cable) it reads ok on the mfj but that doesn't mean it's correct! The mfj so a bit funny like that.

Thanks man