[TowerTalk] Multiple Antennas on One Tower

[Ian White, G3SEK]

Charley Schwartz wrote:
>
>Hello ..
>
>I am planning an installation of a Force 12 C4XL and a Force 12 EF706 6
>meter antenna on a 70 foot Rohn 45G tower. I will be using a 3/8 wall 2"
>
>steel mast which is 24 feet long. At the present location I have
>separate towers for HF and VHF. At a new location I am planning on only
>a single 45G tower this year. My questions are:
>
>1 - How much mast should I allow to extend from the top of the tower ?
>
>2 - How much spacing should I allow between the antennas ?
>
>3 - Which antenna should be on the top?
>
>My main goal is good performance on HF and 6 Meters. I have equipment
>for VHF/UHF and would like something on the tower for these bands but do

There is information on my web site about stacking VHF/UHF antennas, and
basically the same principles apply to HF/VHF. You're unliekly to see
much effect from the VHF antennas on the C4XL; but if you stack the 6m
yagi too close its performance will probably degrade.

Maybe you should ask the questions in a different order:

1. How much spacing do I need? One-half of the manufacturer's
recommended stacking distance for a pair of the 6m beams is a good
guideline. This is based on the idea that a pair of beams should be
stacked so that their capture areas just touch, so stacking an HF beam
at one-half that distance puts the HF beam just outside the capture area
of the single 6m beam.

2. Which antenna at the top? From the electrical point of view, the HF
beam on top, to maximize the height in wavelengths. From the mechanical
point of view, the smaller beam on top.

3. How much mast do I need? 

4. How much mast height do I have, and what compromises am I prepared to
make?

All this can only be guidelines, because capture area is a fuzzy concept
and there is no substitute for detailed modeling at various stacking
distances. Force 12 should be able to advise you on that basis.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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