The search for the Holy Grail Antenna…

Preface:

The author is since 1990 committed to repeater system design.  In the quest for the ultimate coverage, he is always looking for better sites, better cable and the best antennas.

When installing his latest multiband repeater system on a TV transmitting tower of 300 meter, he discovered that the common HAM Japanese multiband antenna is not always the best choice, especially not when you have some oversized towers at your disposal…

The problems started with the installation of the 1298,250 MHz Machine at 117 meter asl.  The repeater previously was located on a hilltop 50 m asl, on a moderate 26 m tower.  Performance was average, using a Diamond X5000 antenna.  When moved to the higher locations signals were far below expected values, especially when compared to a similar system in Antwerp.

When installing an additional machine on 438,650 MHz similar findings were observed.  Apparently the antenna location was too high so the main lobe was overshooting the target area.

Previous articles by some fellow Danish hams showed that some of the Japan- and Taiwan-made (not to mention the Chinese ones..) have quite good matching specifications but show main lobes heavily tilting upwards.  Commercial UHF gain antennas as used in Tetra applications use intentional downtilting by shifting phase between 4 or more stacked dipoles.  Another article is being prepared about dissection of two of these high-tec UHF gain collineairs.

We also know that it is possible to obtain downtilting by using a gain collineair below the design frequency, with a VSWR mismatch as a penalty.

Because the author doesn’t like changing antennas every week, he decided to do some controlled test setup to see how above theory is behaving in practice.  So one windy Friday night after work at 16:00 he mounted on one of the company towers with a radio and a bunch of commercial antennas on his back.  The day before he had prepared at test signal originating from his home QTH some 40 km away.  The test included absolute measurements of this known signal as well as receive signal strength measurements of some known repeaters, both in the upper and lower part of the 430-440 MHz band to distinguish the possible difference in performance in relation to the frequency.

Enclosed is an .XLS file with the measured results, the used equipment and the path loss calculations.

Also included a .pdf with the VSWR plots of most of the antennas to try to analyse the relation between the matching bandwidth of the antenna and the optimum receive performance frequency.

The results ?

It is very astonishing and not exactly what I was expecting, but the Diamond V2000 stood up very firm against the dedicated 5-fold more expensive Kathrein and Procom monoband antennas.  Very nice performing was also the Comet CA712EF coax collinear but here the frequency bandwith limitation is extreme and the 90 km/h wind velocity at the time of the test also showed unacceptable bending of this antenna combined with severe signal strength fading.  Definitely your favourite California sunshine antenna, but worthless for EU-applications.

I hope this small finger exercise of mine summarised in this report is of any value to colleague repeater builders, or just the average FM rag chewer.

I often laugh at those HF freaks by saying that the difference between the best and the worst 3 band beam with similar boomlength is less than 1 dB.  Well, this is definitely not the case with UHF antennas.  Who said HF was for pussies….

Hallaar, January 15^th 2008

Pedro M.J. Wyns

ON7WP – AA9HX

technicalpart/UHF Monoband comparison.pdf