[OZAPRS] HF gateway antenna sugestions

[Ray Wells]

Tony,

An interesting question which is perhaps best answered after 
consultation with Grafex, the IPS prediction tool and taking into 
consideration that the distances involved (within VK/ZL) are not that 
great compared with working DX. Therefore the angle of radiation 
required would be reasonably high. A Grafex prediction on the 
Kempsey/Adelaide path (1200km) taken at 0800GMT shows that 40m is 
optimum, but just an hour earlier, 30m would have been optimum. The 
angles given for first hop are 15-19 degrees and for second hop 32-37 
degrees. These are the sort of figures one would expect from a dipole in 
both instances (assuming reasonable height - greater than a half 
wavelength) and a vertical in the first instance (assuming an effective 
ground plane).

I wouldn't be too fussed about polarisation differences with skywave 
communication because the wave can go through both polarisations. 
Polarisation shift can be responsible for some fading on HF.

In my opinion, the ideal 30m antenna would be two horizontal dipoles at 
90° to each other and mounted at least a half-wavelength above ground, 
and fed in quadrature (90° phase shift) to produce an omni-directional 
pattern - think crossed dipoles as in beacon work. I think I would be 
very unlikely to be attracted to a vertical.

There is another horizontal array whose name escapes me. It comprises 
two half-wavelength, end-fed dipoles at right angles to each other. 
Picture a full wave, centre fed dipole with each half of the dipole at 
right angles to the other half. If I recall correctly, the now defunct 
VNG used such an array to achieve omni-directional coverage.

Now we come to being practical. Ignore theory, and erect whatever your 
circumstances permit. Chances are you'll still have an effective, albeit 
maybe not optimum, antenna that will serve a useful purpose.

Ray vk2tv


Tony Hunt wrote:
> I was wondering what antennas everyone is using on 30m for the HF gates. So
> some discussion would be good. Considering services like RFDS and Outpost
> services are communicating with mobiles that usually use vertical whips one
> would think that a vertical would be the go at home because thats the
> approach they seem to take on their base setups with 5/8 vertical towers
> very often..  No doubt there are some arguments against this also. Ive seen
> in times past that the VKS737 network was using tower mounted helical
> dipoles horizontaly polarised so theres an exception.
>
> Currently I am using a Off centre fed wire (1.5wavelengths) on 30m an have
> no intention of putting a vertical up. Ive seen this argument/discussion a
> while back and still have no real idea of what the best answer is.
>
> Michael 5MW is looking at putting a HF gate in a Roxby in the North of SA so
> he has raised this point. I just decided to extend it to here for some
> input.
>
> Tony Hunt VK5AH
>
>
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