As my 2m Slim Jim is no good for 70cm I was looking for an alternative antenna to use, I found the PA0FBK design (that is mention in another reflector thread) and having carefully cut one from some URM-76 that was lying around the shack found it was resonant around 139MHz and no good at all on 145MHz - my Analyser does not cover 70cm so cannot test it so easily on that band.
Having repeated the exercise using RG58 I have a similar result - anyone got any idea what I’m doing wrong (I have checked and re-checked the dimensions).
Off the top of my head, the velocity factor being used is wrong. i.e. the Vf for your pieces of cable is not the same as the Vf used by the original designer.
Have a good Google for the design you are building to see if anyone has had similar problems, they may have posted a fix. I had the same issue when I made my 1st 2m 300Ohm ribbon cable J-pole. The design dimension I copied were simply wrong and further Googling showed many designs which all used the same dimensions different to the one I copied. Those designs worked.
This looks quite similar to the “Flower Pot” antenna designed by VK2ZOI, however note that people have had troubles with getting that to resonate in the right place too, the best article seems to be:
I see on what appears to be the original PA0EBF article the comment:
The length of d and e is rather critical, by vary this, you can improve the swr, if necessary.
Having played around with the length of shorter part of the impedance transformer (e) starting from something a little longer than specified (35mm) of about 40mm I got it to work, at least until I started to seal it up when I managed to break the connection between the impedance transformer and the connector.
In VK the most used 2 m FM simplex frequency is 146.500 MHz with frequent use of channels in 25 kHz increments up and down from there. 439.00 MHz is the corresponding frequency for 70 cm.
At the 3rd Attempt I seem to have managed to get something that works, but the inductive matching stub is significantly shorted than the original design suggests.
At least all the local repeaters on both 2m and 70cm are accessible with the antenna on 2 sections of a roach pole in the shack with the Handy, will have to try it out on a summit next.