PDA

View Full Version : 2M Beam Conversion



MI6KAK
02-03-2012, 10:27 AM
Hi folks, I recently bought a 88 - 108 Mhz 4 element Yagi very cheap. I wish to convert this to a 2m beam. The elements are half inch alu tubing. The driven element is a folded dipole. Im thinking of making this a normal dipole. Would this be the easiest way to match and if so what is the best way to match it? I have seen a hairpin with 4:1 coax stub, any advice welcome.

some pics below

8507085071

KD8DEY
02-03-2012, 01:56 PM
88+108=196
196/2=98

144+148=292
292/2=146

98/146=.6712

measure each element and multiply by .6712 ??

1/4 wave spacing (free space) between elements ?
492/146=1.684 feet

use a capacitor sealed in a plastic enclosure mounted to a gamma rod?
I'm just guessing

I know nothing and I can prove it
(After all I'm just a General Lite)

Yagi for windows?

WA8ZYT
02-04-2012, 01:18 PM
Go to Justin's site, http://www.g0ksc.uk.co and you will find some dimensions for yagis there.

MI6KAK
02-04-2012, 07:45 PM
Thanks for info so far guys!

KO6WB
02-05-2012, 03:55 AM
The measurements for the elements will have to be calculated since you really have no idea where the 88-108 yagi is actually tuned for. Most likely it will be the middle of the band. There are a lot of sites on the internet that have information you could use. You could also use Quick Yagi (QY4) to come up with the required dimensions. You can get QY4 at http://www.raibeam.com/wa7rai.html.
You could also look at http://www.cu2ara.com/files/144Yagi.pdf. More information available if you search the net by typing in "2 meter yagi antenna".
Hope this helps
73
Gary

W6DPS
02-05-2012, 04:52 AM
Remember that you will want to adjust the spacing of the elements, also. And remember that it makes a difference in length and spacing if you mount them straight on a conductive boom, or if they are isolated.

You might want to look at converting the driven element to something like Kent Britain's "Cheap Yagi" (http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf) design. It makes them easy to match to coax.

Most of these I have tried have basically wound up with me completely rebuilding the antenna, beyond just cutting down the elements. It does provide an advantage when you have the elements and mountings.

Good luck, and have fun!

Dave, W6DPS

MI6KAK
02-05-2012, 11:13 PM
Thanks for the input guys, the antenna elements will be cut and spaced to the2m band. I have been round the net looking at matching beams for 2 weeks and there is a lot of different versions of achieving this. One I was looking at is the driven element is a dipole then add a hairpin to make it 200 Ohm then feed with a 4:1 coax stub to get 50 Ohm or is this OTT?

I have seen a lot just fed to the dipole direct with no matching which seems just not right. Any opinions welcome?

Suppliers are getting rid of these aerials in the UK as DAB will be taking over the 88-108 FM broadcast band so we might as well take advantage of the redundant hardware :cool:

WA9SVD
02-06-2012, 04:32 PM
Thanks for the input guys, the antenna elements will be cut and spaced to the2m band. I have been round the net looking at matching beams for 2 weeks and there is a lot of different versions of achieving this. One I was looking at is the driven element is a dipole then add a hairpin to make it 200 Ohm then feed with a 4:1 coax stub to get 50 Ohm or is this OTT?

I have seen a lot just fed to the dipole direct with no matching which seems just not right. Any opinions welcome?




Suppliers are getting rid of these aerials in the UK as DAB will be taking over the 88-108 FM broadcast band so we might as well take advantage of the redundant hardware :cool:



parts is parts...

But one qualification is that the original driven element (and subsequent elements) will be LONGER than needed for the 2 M Amateur allocation. Thus, they need to be shortened. The spacing between elements will not be optimal.

If you can use the hairpin match accurately, (requires both sides of the dipole to be insulated from the main antenna boom) that is a reasonable feed system; if the driven element HAS to be or can be insulated from the boom, then "T" match is a distinct possibility, despite of need for a balun.
otherwise, a Gamma match may be workable at 2 Meters, and provide a good (unbalanced) match to 50 Ohms.

WB2WIK
02-06-2012, 07:03 PM
I just saw this and the antenna looks kind of like an Antennes Tonna F9FT product.

If so, they also make 2m beams; and if so, I'd probably just use those materials to copy their 2m beam design.

Frankly, I'd stick with the folded dipole driven element and just make it shorter by using a tubing cutter at four places and then splicing it back together using slightly larger diameter, short sections of aluminum tubing overlapping the splices and held in place with sheet metal screws. A folded dipole has some neat advantages, including less detuning by rain and snow or ice, and somewhat broader bandwidth, than a "flat" linear dipole.

MI6KAK
02-06-2012, 07:20 PM
parts is parts...

But one qualification is that the original driven element (and subsequent elements) will be LONGER than needed for the 2 M Amateur allocation. Thus, they need to be shortened. The spacing between elements will not be optimal.

If you can use the hairpin match accurately, (requires both sides of the dipole to be insulated from the main antenna boom) that is a reasonable feed system; if the driven element HAS to be or can be insulated from the boom, then "T" match is a distinct possibility, despite of need for a balun.
otherwise, a Gamma match may be workable at 2 Meters, and provide a good (unbalanced) match to 50 Ohms.

The antenna is vertually ready made apart from the cutting the elements to right size and matching issue to get around. I have found a 3 element yagi design on line with the correct spacing and element lengths and the dimensions of the hairpin match.

From what I understand the driven element if cut into a dipole (which is insulated from the boom) will give an impedence of around 28ish Ohms with the director and reflector. In order to bring it to 50 Ohms the elements then require to be slightly shorter and the hairpin added and adjusted to lower the SWR. A choke balun will then follow.

Feel free to correct me:rolleyes:

MI6KAK
02-06-2012, 07:36 PM
I just saw this and the antenna looks kind of like an Antennes Tonna F9FT product.

If so, they also make 2m beams; and if so, I'd probably just use those materials to copy their 2m beam design.

Frankly, I'd stick with the folded dipole driven element and just make it shorter by using a tubing cutter at four places and then splicing it back together using slightly larger diameter, short sections of aluminum tubing overlapping the splices and held in place with sheet metal screws. A folded dipole has some neat advantages, including less detuning by rain and snow or ice, and somewhat broader bandwidth, than a "flat" linear dipole.

Thanks for the input, I would assume that if I go this way I would need to use a 4:1 balun to match the folded element?

The antenna is a receiving 4 element yagi by Antiference for the commercial radio band 88 - 108Mhz,

WB2WIK
02-06-2012, 09:33 PM
Thanks for the input, I would assume that if I go this way I would need to use a 4:1 balun to match the folded element?

The antenna is a receiving 4 element yagi by Antiference for the commercial radio band 88 - 108Mhz,

I don't know who "Antiference" is, but the construction looks the same as the many antennas I've seen from Antennes Tonna: http://www.f9ft.com/indexe.html

Tonna also uses a square aluminum boom for most antennas, and uses a folded dipole for many models.

Yes, with the folded dipole a 4:1 balun would be pretty normal.

MI6KAK
02-16-2012, 03:11 PM
Hi folks thanks for the input I thought I would post the outcome. I decided to go with the hairpin match with a standard dipole insulated from the boom and use 3 elements instead of the 4

I followed a plan on the net for the lengths and spacings and added the hairpin and adjusted using a reflectometer/SWR meter and the results are a perfect (well as close to) match!

The highest reading near 146 is 1:3. will add a balun at some stage.

here are some pics, one thing I have learned in VHF is even the coax spacing in the centre of the dipole has to be counted in the length unless the spacing is very small.

8635686357

K8ERV
02-16-2012, 03:41 PM
Why not go whole hog and convert it to the new 400KHz band?

TOM K8ERV Montrose Colo

MI6KAK
02-16-2012, 04:37 PM
Why not go whole hog and convert it to the new 400KHz band?

TOM K8ERV Montrose Colo

Hmm...... might have to extend the elements a bit!

ad: N3WG-1