5 element 144MHz Yagi (50ohm) DK7ZB GW4RWR

Introduction
I had erected a 9 ele portable Tonna for the 2010 Es season, and that antenna was still in place almost two years later. I dreaded to think what condition it was in. A more permanent antenna for 2m was indicated.

I had pulled lengths of 1" PVC piping from a skip. These had formed an industrial smoke detector, whereby the pipe carried air from the test area back to a photocell which detected smoke impurities as a change in laser intensity, or perhaps the light was diffracted. Anyway, I had a few 2m lengths and thought it could find use as a short antenna boom.

Some 6m of LDF-450A in my 'junk room' was also too valuable to discard, but too short to be useful as a feeder. I removed the outer and the dielectric. The centre conductor is 5mm diameter copper, and 1m lengths of it do not show any sag. This is used for the elements, which are a push-fit into the boom, drilled just a bit narrower than 5mm. Elements are secured with a heated glue gun. The boom is sawed open, allowing the driven element is centre fed, both halves kept spaced a few mm apart using a narrow PVC tube (arrow shaft) that just accommodates their tips. The driven element is wide enough to be drilled and self tapping screws hold the solder lugs in place.

The stub mast to boom clamp is made of the thickest plastic I could find in the garage. I think it was the corner of an old paddling pool.

I chose to make the antenna in 50ohms since I hadn't any decent 75ohm cable to match a 28ohm antenna.

Results?
This is the lightest antenna that I've used. Gain without weight is the holy grail of VHF DX.

DK7ZB calculated a gain of 8.4dBd. I measured a return loss of 17dB.


The beamwidth is noticeably wider than the 9el Tonna, and having listened to the beacons, the Tonna has a little more gain, as expected.

I'm not sure how long this will last, but given that it cost nothing, I won't be upset when it snaps, unless I'm playing radio at the time. The driven element connection is the weakest link. A box at this point, perhaps open at the earthward side, to brace the elements about 3cm each side of the boom would make for a much stronger construction. I had sufficient copper for a 6th element, but the boom was a bit prone to shake at that length. I reckoned that a rigid 5 element was better than a flexible 6 element yagi. If I found an extra bit of scrap 1/2" cable, I'd be keen to make a 2x5 and compare its performance against my 9el tonna, which survived the weather. Its elements showed a little oxidation, but none of the clamps or screws refused to open.

During the summer of 2013, I took the antenna down and rebuilt it as a 6 element on 2.5m boom. This boom length of plastic needs a support from above - I'm using strimmer 'chord'. I can detect a sharper pattern; very happy with this.