Ham Radio Forum

I need to make a UHF (435MHz, 70cm band) aerial which has good gain, omni driectional and does NOT use radials; which can be taken out on field trips with our local Air Training Corps cadets (13-18 yrs). A J-pole (or a Slim Jim) does not have the gain we need.

I've tried (without success so far) to make one which features several (electrical) half-wave lengths of coax (RG58, UR43), but I cannot get it to resonate at the right frequency (it comes in at 1.2:1 at 442MHz). It did not matter whether the thing was in a plastic or fibre-glass tube or hanging in my (wooden) shed, or whether there were an odd or even number of half-wave elements.

I'm using a Kuranishi BR400 RLR meter for testing it.

Can someone please recommend a good, working design ? Or maybe tell me where I'm going wrong?

73
M0UXB

PS. I'm glad I found this site!

I assume that you are alternating the polarities of coax cable connections at half-wave intervals up the length of the antenna to the keep RF currents in phase. There is a fundamental problem with that design because the RF propagation velocity inside coax is slower than outside. As a result, if coax elements are cut to be half-wave resonant on the outside the current phasing between them will be wrong (they will not all radiate in phase) and if the coax elements are cut to be resonant internally the external radiating elements (the coax sheaths) will not be resonant.

The only thing that can be down about that is to use coax cable that has the highest possible internal propagation velocity. That, of course, is air insulated "hard-line" which won't be as convenient as flexible cable to transport on field trips. The next best option is foam insulated line like RG6U. RG58 is especially bad for use with that type of antenna, because of its unusually slow propagation velocity.

Another problem with that antenna design is that there is a considerable amount of RF coupling between the half-wave radiating antenna elements and the feed-line below if some kind of decoupling, such as ground radials or a tuned coax trap, is not used between them. That coupling induces RF currents in the feed-line that affect both antenna resonance and the radiation pattern.

Thanks, skywave, for a speedy, comprehensive and illuminating reply. I haven't a clue where I can get some RG6 foam cable, but I'll enquire. There may be something of like kind available in the UK.

I can confirm that the elements are connected alternately. The whole thing is fed with a Pawsey stub balun, and a quarter-wave matching before the first element. There's another quarter-wave before the wire top element (electrical, not free-space lengths).

Alternatively, can you suggest an alternative type of construction for a 70cm aerial using no radials?

Many thanks

Tom, K7ITM, posted some notes here http://members.aol.com/K7ITM/ about coaxial collinear antennas that you may find helpful.

Kendall