3 element antenna for 2m VHF

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Bluechip:
'Modesty Blocks' ?

I know them as KD (knock down) blocks (fittings).

Presumably because one use is to attach the 'Modesty Panel' to a desk.

ie, the bit at the back between the desk pedestals, to prevent Panty Oggling.

Shoey51 is spot on. You should get the RSGB Communications Handbook, no end of stuff in it about aerials.
Mine is 1990-ish, ISBN 0 900612 58 4, if you want to shufti one in the Library.

Did aerial theory in the RAF. A bit abstruse IIRC .... directors, reflectors, folded dipoles, SWR, parasitic elements, wave guides ..... AAAAaaaaaaaargh !!

Not going back there .. :D My brain hurts

Dave BC

raynerd:
Sorry for not replying earlier, I`m currently changing t'web providers over to Sky so I don`t have connectivity for the next week or so.

Andy - you ask a really interesting question and one that I kept pondering and stopped me for a few days in starting this project. I didn`t have a TV Yagi to hand and went to a tv repair in town and just asked if there was continuity. Like you said, he tested it for me and showed an electrical contact between director and reflector elements and boom. Yet here, which seems to be a very highly regarded site in the HAM homebrew antenna world: http://www.qsl.net/dk7zb/start1.htm he documents many many methods of insulating the boom from the elements. Not only that, what really got me was that he then proceeds to screw the element onto the holder and boom and hence making a contact!!

The message I got was as follows:

"The insulation that is proposed in the DK7ZB design is just to ensure a point contact (in this case the width of the fixing bolt) between element and boom. In a 1/2 dipole (which we can consider all the elements of a yagi to be) the current minima is at the dead centre so a fixing here will not affect the electrical performance. As soon as we create an electrical connection at any point away from the centre the element length needs to be adjusted. This is not a problem and a common formula proposed by DL6WU is used. At 144Mhz this is perhaps of academic interest only and much time and effort can be expended to no practical benefit. The adjustment at 144 mhz for a 20mm wide boom with the element directly fixed across its full width is only 4.5mm. "

I have to be honesty and say that I`m still not clear but I went ahead and followed what others have proposed and worked!! I`ll find an answer I`m sure.

Dave - yes a bit of a mine field. I checked SWR and is acceptable but not ideal. Need to find what to change to drop it down a little.

Chris

kwackers:
Simples.

The centre point of the directors is electrically inert, its like the handle of a tuning fork.

Imagine your boom has a row of tuning forks each held by the handle, the forks will all resonate no prob.
Imagine instead you bolt the boom to one of the tuning fork arms instead, now they don't resonate and your aerial is rubbish.

The insulator is used to keep the contact point small - since if you start to get contact away from the centre it starts to effect the resonance - the analogy still works, it's a bit like holding a tuning fork in your fist rather than with two fingers.

Whether it really makes much difference at 2m is debatable. I suspect you could simply screw the directors directly to the boom, TV aerials manage it and at much higher frequencies (which means tolerances are smaller).

SKIPRAT:
Hi Chris

Congrats on the pass i hope you get as much enjoyment out of amateur radio as i do there does not seem to be as much activity on 2 metres SSB as there used to be a few years ago mind you a lot of stations do not have a beam nowadays and use vertical polarisation but i find that a horizointally polarized yagi beats a vertical anyday you never know i might meet up with you on the air or via the repeater network through echolink or irlp about your antenna its a long while since i have seen an engineered antenna they usually are slung together thees days .I have a radio project on the go at the moment i am making a kite reel ,i went up to a high spot in the lincolnshire wolds on tuesday with a mate and had a demo of fliyng an antenna on a kite imagine 300 feet of alloy welding wire suspended under a 10ft wingspan kite great fun as long as you arrange to discharge the wind static that builds up on the wire to ground .
have fun

Cheers Paul 73's GD DX from G6FOW

75Plus:

--- Quote from: craynerd on April 30, 2010, 10:23:12 AM --- I checked SWR and is acceptable but not ideal. Need to find what to change to drop it down a little.

Chris

--- End quote ---

The first thing to do is find where on the band that the SWR is best. If at the lowest frequency it is better than the highest then the driven element is too long. If it is best at the top end of the band then it is too short. If it appears that the DE is too short make a couple of plugs, from metal, drill and tap them then insert one in the ends of the DE. Now you can use screws to add just the right amount of length. If it is too long just shorten it a bit.

Here is the method I have used when tuning VHF and UHF Yagi's for the past 25 years.

Clamp the antenna to a piece of wood or plastic water pipe, non conductive, and point the antenna straight up. This way you are sure that you are only measuring the antenna at hand and not some outside influence such as a reflection off some object or some other, resonant, article.

Using this method should allow a near 1 to 1 SWR or the antenna will be "flat" as the saying goes.

Joe

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