[TowerTalk] Phasing verticals and stubs
Ian White GM3SEK
gm3sek at ifwtech.co.uk
Wed Apr 5 16:44:30 EDT 2006
Don Havlicek wrote:
>Comments inserted below:
>
>Ian White GM3SEK wrote:
>> Don Havlicek wrote:
>>
>>>The 'right triangle' system utilizes two verticals per selected
>>>direction with the third 'floating'. The 'equilateral triangle'
>>>system uses all three verticals simultaneously,
>> Any triangle system can be used with either two or three elements
>>energized - check with ON4UN's book for details. In either case, the
>>most practical and versatile system for beam switching is the
>>equilateral triangle.
>
>Most practical? It requires separate control line for .2db additional
>in any one direction, but narrows the beamwidth [with reference to
>1/2-power points] to do this, making signals +/- 60 degrees much lower
>in intensity than the right-angle system.
>
>Versatile only in respect to that .2 db gain and beam 'pointing'.
>
When using pairs of elements with the same spacing and phasing, and the
third element disconnected, an equilateral triangle array and a right
triangle array will have exactly the same gain and beamwidth. At the
expense of an extra control line, the equilateral array will have the
same gain and beamwidth performance in six directions, while the right
triangle with AC/DC switching through the coax can only manage four.
As well as gain, it is at least as important to count the number of
different switchable nulls, which work for you on RX.
>> When only two elements at a time are used (the third being
>>disconnected), an equilateral triangle gives three directions along
>>the sides. With three identical elements and ground-planes, you have
>>easy reversal for a total of six directions.
>
>Easy reversal - with a separate control line and more phasing lines.
>
Separate control line, yes (or more complex electronics at both ends of
the control link). However, more phasing lines are *not* required, only
a couple more relays.
>> When all three elements are used, usually one is driven with 100%
>>current, and the other two are driven at the same phase with about 50%
>>current each. The beam directions are off the top or the bottom of the
>>triangle (so they are moved around 60deg compared with the two-element
>>case). The gain is higher using all three elements, but beam switching
>>is much less simple so you may be practically restricted to only three
>>
>
>Yes ... much less simple.
>
>> Any right-angled triangle would be less versatile than either of
>>these equilateral cases.
>
>Absolutely NOT less versatile. Look at the beam pattern for a
>two-element array fed 90 degrees out of phase and spaced 1/4
>wavelength. Then place FOUR of these patterns around the origin of the
>plot at 90 degree rotation. Notice that all directions are covered
>with less than .5db reduction from the center of the beam pattern.
>
>Being able to do all the switching over the coax with ONE phasing line
>makes for a very simple array.
I was saying "versatile", in terms of the numbers of different choices
of beam and null directions. "Simple" is a different criterion, which
may be more important to some users.
The optimum configuration will also depend on what part of the world
you're in, and the numbers of different beam and null directions you
require.
--
73 from Ian GM3SEK
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