[TowerTalk] Circularly Polarized Receive Antenna

[Jim Lux]

More to the point, a 2 input phasing box can only null one source. 
I suppose one could get a whole stack of boxes and connect them. 
Then the problem becomes "adjusting the knobs", which is a solvable one (at least in terms of algorithms) - it's the same as you'd do in DSP.

And, early work on adaptive arrays did essentially that: servo motors driving adjustable reactive components - I'll have to look for the picture of the array they built at JPL in the 1960s - A whole rack full of gears, motors, and Ls and Cs.

Other schemes use a vector modulator (which works quite nicely today, because you can calibrate the system, as long as you've got enough bits in the DAC) - I used this technique to cancel the Transmit signal into a Receiver in a CWFM radar at 3 GHz, using a pretty grungy IQ mixer driven from a Teensy.

Even some Adaptive cancellers used at HF in mobile applications in the 90s still used electromechanical mechanisms - If you've got more than one HF 100W transmitter on a vehicle, and you don't want to blow up the receivers, you need something. 

Check out something like patent #5152010 to Ashok Talwar at American Nucleonics Corp (they used to be down the street from where I used to live, and a friend worked there)  HIGHLY DIRECTIVE RADIO RECEIVER EMPLOYING RELATIVELY SMALL ANTENNAS

Or 4952193, which shows 4 antennas and 4 cancellation paths. INTERFERENCE CANCELLING SYSTEM AND МЕТHOD

Or even better 4466131 from 1984 - Automatic Separation System
"The present invention is a method and apparatus for separating a plurality of radio frequency signals, comprising N number of signal collectors, each signal collector receiving the plurality of signals from a location different than the other signal collectors, and N being an integer from 2 to infinity; N number of transmission means, each transmission means conducting the signals received from one signal collector; and, N (N-1) number of signal cancellation means, each signal cancellation means having reference, error input and output lines, the reference line coupled to one transmission means and the error and output lines coupled to another transmission means; the signal cancellation means and transmission means being interconnected so that each transmission means is coupled to the output and error lines of N-1 signal cancellation means and the reference line of N-1 other signal cancellation means, and no signal cancellation means having the same transmission means coupled to their output and reference line; each signal cancellation means substantially nulling in the transmission means couled to its output line a predominant signal appearing in its reference line, thus defining a different predominant signal in the transmission means coupled to the output line of the signal cancellation means; thus, the signal cancellation means substantially null out N-1 signals from each transmission means."

Going back a bit further, the above patent cites a patent from 1972
"U.S. Pat. No. 3,648,176 discloses a signal controller to cancel out unwanted signals by employing an RF potientiometer directly coupled to a drive motor and a velocity generator whereby corrections in potentiometer ratios may be made automatically by driving the motor via a feedback loop."

That patent has cool drawings showing the motor operated pots, and this delightful thing (something I contemplated building at one time):
Typical delays of 2.4 to 430 nanoseconds  are achieved by respective lengths of coaxial cable for each of the switches as tabulated below:
switch 41 300' RG 55B/U coaxial cable 42
switch 42 100' RG
switch 43  50' RG 55B/U coaxial cable
switch 44 30
           45 1.7'


Clearly those folks at American Nucleonics have been doing this kind of thing for a while..



On Sun, 17 Aug 2025 16:49:10 -0700, Jim Brown <jim at audiosystemsgroup.com> wrote:

On 8/17/2025 2:17 PM, David Gilbert wrote:
> I believe those are passive boxes, not DSP

> (somebody please correct me
> if I am wrong).  They work fine, but are pretty expensive.
You are correct on both counts, although there are provisions to add
preamp modules to the inputs. And they are not cheap. And what Jim Lux
is saying is that there is some time offset between the two DSP
receivers. We can pretty take what Jim says to the bank -- his day gig
is (was?) as part of the teams that put us on Mars and other cool
places. Jim and I are both contributors to the ARRL Handbook.

73, Jim K9YC


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