Active receive 4 square and others
 

Denny wrote in news:1194979491.757106.117910
@o80g2000hse.googlegroups.com:

In doing some study of active receiving antennas I see in a couple of
white papers on the subject that the author determined he needed a
specific phase shift between one or more of the elements and the
combiner, such as 110 degrees, so he proceeded to cut a specific
length of coax determined by it's velocity factor X the desired
fraction of a wave...
This is one of those things that you always accept at face value in a
published work, but this time my doubting side kicked in... We know
that simply inserting a nominal electrical length of coax - such as 90
deg or 180 deg - into phased transmit arrays does not work as planned
most of the time due to mutual coupling... I have read the
contributions of Roy and Al Christman, et. al. on this not
intimating that I understand it...
Anyway, in shortened, loaded, active amplifier, receiving arrays can
one simply insert a nominal number of electrical degrees of phasing
line as the mathematical model calls for, or is their more to it...
Hoping that Roy, AL, or Walt, or others will chime in here...

I would tend to think that simple delay lines would work here, as there
is no real appreciable coupling between the antennas, due to their non-
resonant condition and small size.


--
Dave Oldridge+
ICQ 1800667

Active receive 4 square and others
 

Allodoxaphobia wrote:
On Wed, 14 Nov 2007 08:51:10 -0800, Jim Lux wrote:

Roy Lewallen wrote:

I'm glad my harping on this topic (mostly via Chapter 8 in the _ARRL
Antenna Book_) is having an effect, and that some folks are catching on.

The receiving antenna trick is to begin with elements that are either
very short or very lossy or both. This reduces the effect of mutual
coupling to a negligible value, so all elements will have the same
source impedance. Then the transmission lines are all terminated in
their characteristic impedances at the receiver end. This is the
condition under which the delay in the lines equals their electrical
lengths. Under those conditions, the scheme works nicely.

Or, as the OP mentioned, if you have an *active* array, with buffer
amplifiers at the elements, the amplifier provides the needed impedance
characteristics.


If you're speaking of phasing together two or more *active* antennas,
you'd best employ identical brand and models of the devices. Elsewise,
knowing the phase change _through_ the active antennas would be a crap
shoot.

Indeed.. one would have to calibrate this. Mutual Z between antennas
(unless they are very tiny and loaded heavily) also changes the phase
(relative to the free space phase at that point).

Some amplifiers are more phase stable than others, too.



Jonesy