[TowerTalk] Active phased arrays.

[Jim Lux]

----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
To: "Jim Lux" <jimlux at earthlink.net>; "Dudley Chapman" <chief at thechief.com>;
<towertalk at contesting.com>
Sent: Monday, March 07, 2005 2:51 AM
Subject: Re: [TowerTalk] Active phased arrays.


> >      Actually, there is a difference between this and a
> single MFJ-1025.  Each element's signal is being phased and
> amplitude adjusted in the DSP as a complex signal before you
> do the combining.
>
> That's exactly what the 1025 does.
>
> >This is a key point that all the computing is done with
> both the I and Q part of the signal from each element.  It
> gives you more options when you are beamforming nulls.
>
> I think what you are missing is the fact the signal can't be
> separated from the noise and phased differently. The
> combining of multiple phased antennas produces a pattern
> that can be duplicated by any phasing system with 0-360
> degree phase range and amplitude control. That pattern is
> restricted by element spacing and phasing.


The difference between a 1025 and a good DSP implementation is:
- you can use more than 2 antennas, which greatly increases the options as
far as combining (although there are some people who have ganged up multiple
1025s or equivalent)
- the dsp can be adaptive and can (automatically) track both noise and
signal as the angle of arrival(s) and spectral characteristics changes
- the dsp has "memory" so you can combine both spatially and over time (in
the radar world, this is called STAP).  This means that, on a very fine
scale, one can have a noise and signal filters that have varying spectral
characteristics.

>
> I really wish there was a good way to do something magical
> with noise floor conventional signals at HF, or some magic
> with transmitting antenna efficiency.

If you're dealing with additive white gaussian noise and constant tone
signals, then the DSP doesn't buy you much over a conventional filter, but
if the signal is moving, and the noise/interference isn't white and flat,
then the DSP can buy you a whole lot.  I think it's safe to say that on low
bands, the dominant noise source is not receiver noise, but comes from
"outside", and the interference and signal are hardly flat and stationary,
so there's great potential for DSP.

On transmit, though, as Tom says, there's no magic available to overcome
ground losses, although there are potentially some designs that might reduce
"implementation" losses.

Jim, W6RMK