[TowerTalk] Portable Phased Arrays!

[Jim Lux]

At 10:26 PM 6/27/2006, Paul Playford wrote:
>You may want to consider parasitic feed.  That could simplify phasing a bunch.

Phasing is about the same regardless: you need an LC network for each 
element, and having active feed to all elements allows a bit more freedom, 
because you don't have to depend only on mutual coupling to excite the 
parasitic radiators.  This is especially important when you have to 
tolerate arbitrary arrangements of the elements.

 From a deploy and setup standpoint, you already have to have a cable 
running to each element anyway, so you can do the tuning and/or switch 
things for calibration.  So, overall, no real savings from a parasitic 
strategy.

I do note that if you have a fixed arrangement of single band antennas, and 
they're reasonably close together (< 3/8 wavelength) tuned parasitic 
elements can be very effective.  There's been quite a lot of traffic about 
just such schemes for the WLAN frequencies in IEEE Trans on Ant and Prop in 
the last couple years.  Nothing there would surprise a ham who's been 
following the various "use tuned slopers off a vertical" schemes over the 
years.


>And you may find that vertical dipoles are quicker to deploy (depending on 
>the frequency)

I would imagine that the scheme, when I get it working, would "work" with 
any sort of radiator.  I've got my hands full with just the control and 
metrology, so I didn't want to also have to design and build antennas, so I 
chose something off the shelf and inexpensive to fool with. I'm using 
trapped verticals for now (and, they're useful for other things in the mean 
time).  I have looked at other radiator ideas.. a compact loop is 
attractive, but band switching is slow with most off-the-shelf 
implementations.