[TowerTalk] Smart antennas

[Jim Lux]

This is an area of great interest to me, both professionally and as an 
amateur in a fairly rigidly controlled HOA.  In fact, I think that adaptive 
antennae and phased arrays are a reasonably practical way to deal with HF 
communications in "tough" environments.

While you're not necessarily going to be able to beat the huge antenna on 
the huge tower, an adaptive phased array can do the following:
1) Form nulls on intefering signals
2) Form the main lobe on the signal of interest, and, with sufficient 
sophistication, actually coherently combine the skywave from more than one 
angle and polarization.
3) Move the pattern in the "blink of an eye"

If you are cursed by geography, nothing's going to help (say your QTH was 
at the bottom of a big pit... no amount of gain or Tx power will help you 
get sufficient signal out at low angles)


Interestingly, on receive, because you're normally atmospheric and 
environmental noise limited, you can tolerate a low gain (in absolute 
terms) antenna, as long as you have directivity, which is easy to come by 
in a phased array.

On transmit, it's a bit tougher, because you have the 1500W limitation on 
transmitted power, and a physically low and small antenna is going to put 
more of that power into the dirt or the clouds overhead.  In pretty much 
the rest of the radio world the licensing and power limits is in terms of 
ERP.  Maybe we should push for the FCC to do the same for hams?  It's not 
that different than how RF exposure is handled, or the limits on the 5 MHz 
band.  You can either guarantee compliance by an antenna of known gain 
(i.e. a dipole) and limited transmit power, or by analysis/measurement of 
actual field strength.

In reality, the rule requires use of the least power to communicate, and 
from a theoretical standpoint, that's the radiated far field.  If you use a 
1000W transmitter and a dipole or a 100W transmitter and a 12 dBi directive 
antenna, the field strength is the same.  Philosophically, I should be able 
to allocate my resources how I see fit, subject to the radiated power 
limit.. If I want to invest in big high gain antennas and small 
transmitters or small antennas and big transmitters, that should be my 
choice (subject to safety rules, etc.)

Of course, there are those who run big antennas AND big transmitters who 
may not agree.

Jim, W6RMK
(http://home.earthlink.net/~w6rmk/antenna/phased/index.htm)

At 03:19 PM 9/2/2003 -0400, VeeAthreePL wrote:
>There is interesting article by Martin Cooper in July 2003 issue of
>Scientific American pages 49 to 55. He is talking about Smart and
>Adaptive antennas. This basically is aimed at cell phones but who knows.
>Worth the reading.
>73 de Andy - VA3PL
>
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