This is quite true although some patterns will be closer to theory than
others. In a commercial phasor as is used for broadcast there are
several elements that must be taken into account. The first is the
matching at each vertical element. It must take the actual drive
impedance of the elements and match to the transmission line (usually 50
ohms). These networks all have phase shift which must be taken into
account as a part of the total system design. The next important element
is the power divider. There are various designs but in the end they all
permit the adjustment of power to each element so the currents are
correct. The ARRL patterns are idealized examples where the current in
both elements are equal but in the real world this is not often the
case. A power divider also has phase shift. The final element is the
means of making the phases of each tower the desired value while taking
into account the fixed phase shift of the matching network and the power
divider. There are two common methods used. The first is a lumped
constant network which adds or subtracts phase to achieve the correct
values after transmission line phase delay is added into the system. The
other method is to use the transmission line to achieve the proper
phase relationships. Even when transmission lines are used it is
necessary to have a small lumped constant network to trim for minor
variations in phase. It is apparent that such a system is very sensitive
to a change in frequency.
Adjusting a typical phased array requires a means of measuring impedance
and typically there is a phase monitor which shows the phase and current
ratio of each tower. This is all verified by field measurements that
require a calibrated field intensity meter and a lot of paperwork to
plot the pattern or a more complex (and expensive) GPS based measurement
system.
It should be obvious at this point that no amateur phased arrays are
built as described above and do not have the same requirements.
Commercial arrays are usually designed to provide protection to other
stations on the same or adjacent frequencies. Amateur arrays are
generally intended for gain in a desired direction and must have at
least a small amount frequency agility. While the ARRL patterns are very
much theoretical a few can be implemented for practical amateur use. The
broadside two tower array spaced 180 degrees is probably the best if
space is available. One drawback is the figure 8 pattern has zero db
front to back ratio. This could be handled with a parasitic reflector
but at the expense of more complexity and space. The bottom line is that
practical amateur arrays will not achieve performance close to theoretical.
On 11/24/11 10:59 AM, Rik van Riel wrote:
> On 11/22/2011 08:06 PM, Gerry Treas, K8GT wrote:
>> Hi Dale,
>>
>> I'm no antenna expert, but certainly read as much of the experts
>> publications as I can get my hands on, but having a Teflon brain, it doesn't
>> stick very well.
>>
>> That said, the ARRL Antenna Book has a page that shows the patterns of
>> various spacings and phasings of vertical antennas, which I found very
>> enlightening.
>>
> Enlightening, but also somewhat misleading...
>
> The patterns in the ARRL Antenna Book are correct if the
> current in both elements is the same.
>
> However, if you feed an array of antennas with delay lines,
> those delay lines will act as impedance transformers for
> the antenna impedance of each element (like all feedlines do).
>
> This can result in each element getting different currents,
> and the pattern no longer being what it was.
>
> This makes feeding a phased array with delay lines much
> trickier than one would imagine at first glance.
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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