Monday, September 17, 2012

Building a 2 Meter Yagi

I managed to purchase a Kenwood TH-215 2 meter handheld off EBay for the princely sum of £11.50. 
The radio works ok off the mains but the two batteries are not holding their charge, no real surprise as the handheld is an old bit of kit. So I'm on the lookout now for a spare battery, whether I find one could prove interesting. I rarely use 2 meters but to be honest there is a good morning net that I'm missing out on, plus I can keep in touch with all the local operators.

Anyway now that I have the rig I needed an antenna and although somewhere in my shed I have an old 2m vertical I quite fancied the idea of building a Yagi. Nothing too elaborate just a small 3 element to boost things up. So I had a search on the web and found details of a fairly simple yagi build  which seemed to promote a good db gain.
According to the details the maximum forward gain was around 8db (6 dbd) on 145 Mhz. So doing the math, with 6 db (6 dbd = 8,15 dbi) gain, this antenna offering an “effective radiation power” 4 times greater of the transceiver output (without Coaxial-Loss), i.e with my little handheld with an output power of 5 watts, my ERP will be multiplied 4 times = 20 Watts (in the forward gain). 
Reading further it went on to say this antenna was compact enough (78 cm boom between Reflector-director) with excellent F/B ratio (20 db). The Impedance on center frequency (145,000 MHZ) is 65 Ohms, so in practice the antenna needs a "matching system" for a 50 Ohms coax and to get the SWR down to 1:1, the website recommended a "Hairpin" system which is very simple and effective "matching method"

So I gave it ago and had a prototype antenna built within a day, but without the hairpin method and some slight alterations in measurements which proved costly as my SWR was totally out. 

Build number two I followed the plans regarding the measurements and managed to find some thick copper wire for the hairpin. After checking everything was tight and correct I did a check with the MFJ 259 and Bingo I had an SWR of 1:1 at 145 Mhz. Just goes to show, follow the plans and you can’t go wrong!

Build Number two, it's pretty basic, held together with pressure clips and tape. The elements are made from the old aluminium tubes I had on the Moxon and the boom is an old TV boom I found in a skip.

The Hairpin fitted in place and taped up to make it weather proof, luckily I had some reasonably sized copper wire which I had originally used when constructing the Cobweb. Again fairly basic but as long as it does the job.

The final product slotted in to the rotator and mounted on a 20 ft scaffold pole. Its just enough height to clear the nearby tree and first impressions look good with a 1:1 swr at 145.00. I have already heard some chatter from pointing north up the Severn valley towards Birmingham and this morning before going to work heard the local Cheltenham net having a chat. Certainly when rotating the signal fades in and out so all seems to be working ok. Just got to do a transmit to make sure I'm getting out!

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