3 element antenna for 2m VHF

[raynerd]

After passing my Foundation Licence only a few weeks ago, I wanted to work SSB on 2m and my roof mounted vertically polorised colinear X200 clearly wasn`t up to the job. I decided to try a homebrew 3 element Yagi, the dimensions and details were all provided by Neil, many thanks:

1. Reflector - 1040.6mm long and fixed at the end of the boom.
2. Diven element - 2 lengths of 475.3 - 10mm gap between them. each individual part of the driven element is insulated from the boom and each other.
3. Director - 842.8mm long and fixed to the boom 737mm from the reflector.

I started of with the parts: 12mm steel rod - 2m length, 12mm steel rod - 1m length, 20mm square ally tubing - 1m length, 20mm dia clear acrylic bar.



I then started on the driven element, making my acrylic insert that would slide into the 12mm tubing and provide a 10mm insulated gap between the two. They needed to be a tight fit so that the driven element would remain rigid as one despite the gap.
 




The insert in one of the sides of the driven element:




Both connected, nice and tight fit and they felt pretty rigid:



Then the driven element was mounted in a box to protect the cables and insulate from the boom:



I then needed to produce some sort of insulating mounts to hold the elements correctly on to the boom. I had some plastic mounting blocks knocking around, so using a 12mm drill I drilled a half circle into a couple of these:





I bolted the elements onto the boom and there it finished - a 3 element Yagi for use on 2m. A brief test portable showed it to work really well but I`ll post more when I have tested again.

[HS93]

I used a similar design to use it for fox hunts some years ago they work well,

congatulation on the pass by the way

Peter

G0PAH

[Brass_Machine]

Heya Chris,

Fixed some tags in your post so all the pictures would show. Congrats on the pass!

Never got into radios... did help a guy put up a 20' ft antenna in his yard though!

Eric

[shoey51]

Congrats on the pass mate.
what did you use as a balun mate
job looks good
If you get yourself a copy of the RSGB book it has some great tips there for making antenas

73's Graham

[andyf]

I then needed to produce some sort of insulating mounts to hold the elements correctly on to the boom. I had some plastic mounting blocks knocking around, so using a 12mm drill I drilled a half circle into a couple of these:

I'm not sure it was necessary to insulate the director and the reflector from the boom, Chris. It's 30 years since I passed the RAE, but I seem to recall that there is a current "null" in their centres.

Andy

PS Have just looked at an old TV Yagi. The directors are connected (both physically and electrically) to the boom by self-tapping screws.

PPS Why are those furniture connectors called "modesty blocks"?

[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 ..   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]

I checked SWR and is acceptable but not ideal. Need to find what to change to drop it down a little.

Chris

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

[scrapman]

Hi chris congratulations on the pass, and the progress on the antenna.

Ray, 73's  MW0COE