I used the antenna 'calculator' see here.
I put in 145MHz and all the dimensions are calculated. I used rugged thick copper pipe. It is not very beautiful, but it works 100%, the pipe is easy to get, and easy to saw. And also easy to solder with a hobby gas burner. The 'H' of the antenna is 12mm and the match is 10mm. After a hour of measuring, sawing and soldering, the antenna was ready. Soldering the connector to the corner of the long part of the H was a bit of a struggle. It needs pretty much heat, but finally I managed to get it done.
I started with the hot end of the coax directly connected to the corner of the match part. The cold side of the coax was connected to the corner of the H via the connector.
I connected a piece of coax to it and an the end of the coax a VNE ( the one of 50 bucks in China,....).
It worked nice. You can show the Smith diagram and the SWR diagrams very easily.
But the resonance frequency was 140MHz,.......I wanted 145Mhz. So de-solder the tips of the long radiator and cut of 1 cm. Solder the tips again.
That helps,.....140MHz went up to 143MHz. OK, de-solder the tips of the short radiator (reflector) and cut of 1 cm. Tips on again.
OK, 144.6 MHz. Close,.........a few mm's of the long radiator and it was exactly on 145 MHz.
But,........the impedance was not 50 Ohm,.....the best I could get was 1:1.9 SWR. That should be better.
So I took the advice of the designer,...and soldered a 15 pF variable capacitor between the hot coax wire and the corner of the match pipe. The result was that I could get the SWR down to 1:1 by turning the variable C. But now the resonance frequency was down to 143MHz. So, I repeated the shortening procedure of both radiators and had to cut of at least 1 cm again.
After 5-6 try outs, cutting, measuring, cutting, measuring, etc, the resonance freq was 145MHz and the SWR was 1:1.
At the ends of the band (+1 and -1 MHz), the SWR was still 1.7, which is very reasonable I would say.
Important to realize is that if you put the antenna on a mast, in the free air, it will change the resonance frequency and the SWR, so fine tuning could be possible if you want to use the antenna for transmitting. I use it mainly for measuring QRM, so receiving. Then it is less critical. But still fun to get it exactly on the freq you want with 1:1 SWR.
What I also measured was a very steep SWR dip on the resonance freq when I put the VNE very close to the antenna. Coax less then a meter. If I connect a coax cable of 10 meters, the dip was less steep. At non resonance frequencies i.e. 143MHz, the measured SWR was still 1:2. It looked like the coax was deteriorating the high Q of the antenna.
My explanation for this phenomenon is that the coax is indeed attenuating the signal of the VNE and less energy comes back,....so the antenna is looking like having a lower SWR, but that is not true of course,.....
An endless or very long coax cable has always SWR 1:1,.....
To make the antenna 'waterproof', I took a plastic milk bottle, cut it in half and shaped the bottom a bit, so I could put it over the variable capacitor and the connector. I would not recommend putting the antenna on your roof this way, but for measuring QRM it is good enough.