On 7/14/2011 11:20 PM, Richard Clark wrote:
On Thu, 14 Jul 2011 19:31:17 -0500, wrote:

Sounds like you didn't look at the site so "what is left unsaid."

Hi Mike,

As I said, nothing there but a vague suggestion of the topic heavily
cloaked in an admonition.

Yes, I'm looking a steerable receive only arrays for the AM broadcast
band. Myself, I want one that will fit on a 150ft lot.

Is that 150' on a side? 150' long?

There are any number of ways to construct variable delay lines. A
little bit of computer control will pull their outputs together to
form beam steering.


For receive only, and moderate bandwidth, then digitizing the multiple
antennas and doing the phasing in software is probably the easiest
approach. Essentially infinite variability, you get "true time delay"
as opposed to phasing (the latter is inherently narrower band, although
for AM BC band, it might not make any difference.. 150 ft is a tiny
fraction of a wavelength).




Planting the antennas is simple and allows for testing of any of those
many ways to construct variable delay lines. Those designs themselves
have various trade-offs for issues you need to prioritize and
pronounce. The antennas don't need to be in any particular, regular
pattern as that would be accommodated by computer alignment.

If you choose to go manual steering, with analog variable delay line
designs; then, yes, having a regular pattern of element layout would
reduce complexity.

How many antenna elements? At least three, I would suspect. From
there on it is a matter of how well you can resolve the various delay
angles for steering a broad beam angle. Increase the number of
elements and the beam angle gets progressively tighter. You don't say
how much directionality you want, but it will be intimately correlated
with this count.

In general, you can place N-1 nulls with an array of N antennas. Broad
nulls (in angular extent) are done by stacking multiple narrow nulls.
For AM BC band (which is like 160m ham band), gain isn't a big deal,
you'll be interference and atmospheric noise limited.



The delay lines will have to be tolerant of mutual coupling - better
yet, they should not perturb the existing mutual coupling at all. Good
isolation amplifiers between each element and its delay line before
going to a summing amplifier would seem a solution. Tracking between
delay lines will affect beam width. More elements will require
tighter tracking.


The usual scheme is to use a lossy short untuned antenna and a FET
pre-amp. There are several nice designs out there, or you can buy them
off the shelf.
http://www.amrad.org/projects/lf/actant/ is an example

In the end, they all comprise the same fundamentals. If you cannot
support a symmetrical design, that is your primary limiting factor. If
you can digitize the delay lines, then symmetry can be synthesized and
the antennas can be placed physically anywhere limited only by the
resolution of digital sampling rate.

And the dynamic range of the amplifier/digitizer.





73's
Richard Clark, KB7QHC