[] VHF Contest Antennas


by F1FRV

* Revision 0 september 2013 *

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This page is only about a new idea i had, to have very wide angle coverage (90°+0/-1 dB) with STATIC groupments of standard antennas not wide angle coverage alones. After a lot of simulations with 4NEC2 in 2012. 2 VHF test antennas groupments were installed for IARU VHF 2013 contest in JO10HE, with what was available, and also time to install before contest. One groupment is with 8 Flexayagi 4 elements directed to east, and the second groupment with only 4 VPA ( DK7ZB design) 4 elements directed to south. In TX, all groupments are used together, in RX, one groupment is selected by relays switches.

For all necessary equipments: antenna switches, cables, splitters, see the previous page "Contest antennas systems".

The basic antennas to be used, with the best quality/gain/price are DK7ZB design 4 elements VHF on a 1.25m reinforced boom. They can be installed with rear mount on the side of a tubular mast (up-to 60 mm (2") pipes dia.), no need for complicated fixations systems.


HOW TO ? That is the question ..

How to obtain a 90°+0/-1 dB pattern with unitary antennas given for +/-26° @ -3 dB ??. There is no need to use "Lourdes water" this is not a miracle !!!. It is only necessary to NOT ALIGN the antennas in the vertical plane. The angle between antennas is very critical and is depending on antennas used. Unfortunately, absolute simple rule can not be written, angles must be calculated for each antennas. Simulation results with 4NEC2 said that for VPA/DK7ZB 4 elements the optimum is +/-35°at 2.1 m stacking , and for (expansives) Flexayagi FX-205V optimum is +/-39° with 1.5 m stacking.

8 x VPA/DK7ZB stacked 2.1 m aligned

8 x VPA/DK7ZB stacked 2.1 m @ +/-35°

4 x VPA/DK7ZB stacked 2.1 m @ +/-35°

Obviously maximum gain is less in non aligned antennas than with aligned antennas (~-6 dB), but this is ONLY true in the direction of aligned antennas.

With ONLY 4 STATIC antennas groupments, it is possible to cover 360° @ -1dB. For information, the lowest antennas are at 9/10 m over ground.

In contests, it is more important to receive all around and select QUICKLY quadrant to be listened, than search SLOWLY with a rotable antenna, and NOT RECEIVE stations which are not in antenna exact direction. For information, a 12 elements DK7ZB yagi gain is 14.3 db, with only ~+/- 15°aperture @ -1dB, and ~-30 dB @ +/-45°.

Nota: EXACT orientation of the antennas is VERY important. It is MANDATORY to have a very accurate compensated compass during instalation. A +2 to +5° error on 35° shift will lead to UNACCEPTABLE gain losses. See simulation results. In case of doubt, due to not precise enough measurements, reduce shift angle, you will not have 90°@-1dB coverage, but, you will limit the losses.

For now, That's All Fox !!!

Tous droits réservés (Copyright) F1FRV

Download file with all informations: VHF Contest Antennas.zip

73's de Dominique.

f1frv@sfr.fr OU

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