A Portable and Collapsable 2m Contest Yagi
When contesting and quick casual trips to the top of a hill, one wants something that is small and takes up little space. With a 2m antenna, loading into the back of the car can be a tricky business where you end up bending elements and such like, possibly causing damage. With a 2m beam, it is starting to get on the edge of unwieldiness.
Harry ZL1BK came up with a load of old cordless telephone whips and got enough for us both. He came up with the idea of making a collapsable antenna. The idea being that the telescopic elements can be pulled out to the needed length and then cut to size once the alignment for SWR and bandwidth was good. He fed his system by connecting a BNC to one side of the Driven Element (DE) with the plan that the other side will be the counterpoise so to speak.
I wanted to improve on the design and go for a more balanced approach and set about making a feedpoint system that would do the job.
Criteria were as follows:-
Get a balanced system that you could still fold up. Get the type of telescopic elements that pivot at the mount point.
Have a DC path to ground to bleed off any rain static ( you can bet on that it will be raining where you are!) I have watched the noise level rise up on the S Meter even with Gamma Matched antennas where there is no ground return.
Simple to get going.
Parts You Will Need
A 2.5m 19mm box section boom.
A small pice of polythene rod or similar insulating material with an OD of about 12mm minimum. I needed to cut mine to 40mm long to pass through the boom and a bit more.
A number of telescopic whip elements that have a pivoting base approx 550mm long as you will need to cut some off.
A pice of 3mm copper tubing or 3mm copper wire.
Some scrap copper sheet.
Some RG58 for the Balun.
M4 (preferably stainless)
How I Went about it....
The first thing you need to do is to create a design and how big you want to go. I used Quick Yagi from an old friend, Chuck Smith WA7RAI. He wrote the program and has the popular "RAIBEAM" designs. Google it to find it or it is at www.raibeam.com .
Then I worked out a feeding system that uses a "Hairpin Match" and a 4:1 Balun with a 1/2 wave section of RG58. For reference, RG58 has a velocity factor of 0.659. The hairpin creates an inductive reactance that needs to be cancelled out. This is done by shortening the DE so it has a capacitive reactance that will cancel out the inductance.
The program to work this out is on the latest HAMCALC suite of programs. Download this and run it after using Quick Yagi to get the feedpoint impedance. The program in question is hairpin.bas, you will alo need to get the gwbasic runtme as it needs it to run it. This all comes in the zip archive. I think it is up to version 10 now, but I see ver 7.4 has it. A google search will find it.
After being armed with numbers, I then went about building the antenna. I used 19mm Box Section ali tubing.
The aim of the game is to get the telescopic elements above one another so that they lie in the same vertical plane.
The elements I got had a flange that needed to be cut off and then a hole was drilled in the end to take an M4 Bolt. Drill a hole in each filed off end with a 3.5mm drill. Tap all the holes to take the M4 Bolt.
When you prepare the boom for the elements, you will need to work out the diameter of the whip mounting points and verify that you can in fact, get two of them inside the boom.
If not, you will need to go up a boom size perhaps. Mine were 8mm diameter so I could fit 2 inside. Just make sure that the outer edges just clear the inside of the boom wall.
You will be drilling alternate 4mm and 8mm holes to take the elements. So on one side of the boom, you will have an 8mm hole where the whip base passes through and it will hit the other side of the boom wall, on this side, you will have a 4mm hole that mounts the whip element to the boom. Repeat in an alternating fashion for all but the DE as we will have a 12mm hole here for the DE mounting insulator. See the photos above to get the idea.
The Element Lengths.
Element Lengths Divided by 2 from the centre of the boom (Install all boom elements and cut off each side to the corresponding length)
515 DE 474 D1 460 D2 432 D3 468 D4 445 D5
The Element Spacings.
REF - DE
357mm DE - D1 300mm D1 - D2 357mm D2 - D3 429mm D3 - D4 521mm D4 - D5 639mm
Here we will cut a length of Plastic rod approximatey 40mm long to pass through the 122m boom hole for the DE. I found that putting the rod in the drill press chuck and rotating the work while using a pilot drill to get the hole centred first off works fine. We don't want the hole to go right through for the 8mm hole but the smaller pilot drill is fine to go right through. Once you are happy the pilot hole through the tube is centred at both ends (pull it out of the chuck and check, don't hope that it is!) If all is Ok, the prepare to drill the hole to fit the antenna bases. Mine was 8mm but check yours. You want a snug fit here.
Drill the bigger holes so thet they are not going right through, I left about 6mm or so between the ends of the holes in the middle of the rod. This will stop the DE sections touching as we need to keep them insulated and apart from each other.
Araldite the bush in the boom once happy that all parts fit snugly together. Make sure it pokes out the sides evenly and that it is straight an square with the boom.
While the glue is drying, cut some thin strips of copper about 4mm wide. File off the chrome coating to reveal brass at the point where you will solder the two connections to. The connections will wrap around the element just where it comes out of the rod insulator you have just glued in. Wrap the copper strap around the element mount and then sweat solder the strap to the mount with a tag of strap pointing up to where the feedpoint will be. This should be evident from the photos below.
Cut a pice of G10 Fibreglass PCB to make a mount and join you balun and feeder as shown on the photos above.
The balun is 681mm long from braid end to braid end for half wave of RG58 Coax. You can check it is good by putting a 200 ohm resistor on the open ends and check SWR at the feedline. Going from the bottom of the band to the top plus any outer extremities you can TX on should show a dip in the SWR that should be basically nothing.
The hairpin dimensions are 45mm centre to centre and 67mm long.
Once constrcted , try it out and see how it goes. Mine had a flat SWR from 140 through to 148 where it rose to 1.6:1 past the NZ band edges.
Front to back testing using the Marconi power meter showed about 25-28 dB but our testing was probably not optimal as the thoerectical should have been 38dB. Forward gain is 11dBi. The array length is 2.43m long.
The construction details I have given maybe a bit pathcy but anyone with a practical hand shouldn't have too much trouble.
Harry with a raft of antennas to test The signal source, still it was too loud so we added 30dB attenuatoer at the radio end.
Of course - it was raining - what else does it do when preparing for contests!!!
The "Macaroni" power meter
73's and happy DXing ,