OBSERVATIONS FROM NORWAY.   050320101315Z.

A HALF RHOMBIC ??.

Nah !! ... it's a V Beam.

Used primarily by the military in the 1960's for long distance point to point communications on VLF and HF.

There is no reason why HF aerial principles can't be used  on VHF, UHF or even MICROWAVES. The 'aerial physics' principles are the same - regardless of the frequency used.

Only the implementation and materials differ.

A half rhombic or 2 element V beam - (call it what you will) - can be used at VHF just as well as at HF. If carefully constructed - with a spacing of 0.25 wavelength and an aperture of 90 degrees, it can be made to perform as well as a 3 element yagi - but take up less space and it will literally knock the socks! off an inefficient helically wound S.V.V.G. aerial.
(Short Vertical Vulcanised Goose) ... rubber duck, to you, Duckie. In most cases, a resonant wet shoelace will outperform most commercial rubber ducks. The 'standard' SWR being  3:1. I know certain people don't bother about the SWR and claim it belongs to the 'chicken' band but SWR is the ratio of the power of the wave travelling from the load back to the source so while a lot of people claim it it is *NOT* loss - it's definitely not performing any useful work and is therefore only contributing to heating up the transmission line - which *IS* dissipating energy ... err ... loss!.

A 'rhombic' aerial is generally accepted to have at least 3 x half-waves on each leg and provides 'gain' due to the fact of the current and voltage nodes adding in the first and third half-waves and cancelling each other in the second ... providing directivity and a good front to back ratio - compared to a half-wave dipole.

Since the aerial I describe only has a quarter wave on each leg it's more suitably referred  to as a 2 element V beam. The gain is realised slightly differently from the rhombic. In the 2 element beam, the principle is the same as that of a Yagi, it realises forward gain by passive phasing since the reflector is a quarter-wavelength behind the director.

Different schools of thought provide different gain figures for Rhombics and V beams but it's safe to assume that a rhombic with 3 x half-wavelengths on each leg will provide 6dBd over a dipole whereas the 2 element V beam with a quarter-wavelength on each leg will give a forward gain of about 4dBd and a front to back ratio of somewhere in the region of 10 to 12 dB.

Some aerial manufacturers quote some truly amazing! gain figures for their products and it must be remembered that these are almost always produced in a 'lab' using 100Mhz models on a computer under perfect conditions. They will invariable *NOT* perform to the same specifications at your QTH unless you live in the middle of a bog on an island surrounded by sea water.

(For 7 years I did ... Gossen Island. IOTA EU 36. I currently live on Frei island. EU-56).

The vertical radiation angle for all aerials depends primarily on the height over effective ground - (which isn't necessarily the surface of the earth) - it depends on the conductivity of the surface in question. An amp-meter measurement of the conductivity of the ground  is beyond the scope of this article.
 

A shoulder mounted V beam.

The photos should be self explanatory.

Parts

Portable Shoulder-Mounted V-Beam system. 

Yes, it's versatile!  Window Mounted.

Carrying bag.

The 'math' - (formulae to you) - can be found in any aerial book or on the internet.  As previously mentioned - measurements from one location will almost invariably never repeat at a different location - (physics + Murphy's law) - so 'tweak' for your own QTH.

I used 10 x 3mm aluminium bar for the shoulder bracket, boom, reflector and fittings, 2 x surplus Watson FC-130 frequency counter telescoping whip aerials and a couple of Junque-box BNC's. The reflector is split into 3 pieces to conserve space when the aerial is collapsed and you're gadding about on a bus.

The feed-line is a half-wavelength of RG-58 to an IC - E92D in the shoulder bag.

Adjust the V for a perfect 1:1 match.   This aerial will kick your rubber ducky's bum.

While not quite as effective an an 'Arrow' clone this aerial *WILL* save you thousands of Pounds, Dollars, Bath, Rubles, sheep (!) in compensation for poking people's eyes out.

Mounted on my shoulder the aerial is about 3/4 wavelength over the ground and so has useful lobes for satellite operations ... and err ... you can always rotate about your own axis :-)

An earlier Observation describes the SATme tracking software on a mobile phone.

NOTES.

a) For portable satellite operations, if like me, you are (reasonably) concerned about  poking somebody's eyes out (The 'arrow' dance syndrome) you can always use a smaller aerial.

b) Those that are old enough will recall that it was St Vitus who originally invented the Arrow 'dance'.

(Methinks he just had fleas!..... GM1SXX)

73 John.   LA2QAA.

<la2qaa@amsat.org>