[eagle] Re: Phased Array Concepts

Tom Clark, K3IO K3IO at verizon.net
Sun Oct 7 21:30:07 PDT 2007


Matt Ettus wrote:
> I have the working simulation of the tapped delay line design.   I did
> all my sims with a delay line spacing of 30mm per element, or 0.3
> wavelengths at 300 MHz.  Some observations:
>
> - Once the varactor capacitance gets high enough, you start to get
> reflections which mess up the phasing.  It happens to the elements
> nearest to the generator (center of the array) first.  At 300 MHz, you
> get bad behavior at the center, first, and second elements if the
> capacitance gets over about 0.8 pF.  Going to a lower frequency will
> allow for more capacitance, but you will need longer delay lines, and
> will then end up with the same problem.
>   
Yes, at 300 MHz, 0.8 pf has a capacitive reactance of 660 ohms. That value will induce a return loss of about -11 dB (VSWR ~1.1:1). Part of the requirement is that the frequency be chosen such that the reflections don't add up badly. One idea that I plan to try is to choose the lengths such that each cable is an odd number of quarter wavelengths. This should allow a cancellation of the reflections.

> - The tap for getting the signal out may add capacitance of about the
> same order of magnitude.
The signal extraction tap in the test fixture is 10k, so its reflection
is negligible.
>  
>
> - The delay line needs to be long enough, or the caps all look like they
> are in parallel.
>   
Agree
> - The center element will be common to all 12 strings, so some method of
> splitting is necessary.  This splitting will introduce a phase offset
> which is not linear with the applied voltage, so it will need to be
> compensated for by changing the length of the delay lines between the
> center element and the first varactor.
>   
This was always in the plan, although I chose to omit the detail in the
paper for simplicity.
> - You can probably reduce the reflections by putting attenuators in the
> delay lines, but these would make the DC biasing of the varactors more
> difficult.
>   
My model has assumed ~1 dB of loss intrinsic in each coax line, and I
see you used lossless lines. It may also be  necessary to put a resistor
in series with the varactor to help with the reflections.
> I have attached my simulation.  It was done in qucs, which runs on
> linux.  See qucs.sf.net or get it from your distribution.
>   
Following your tip last weekend, I picked up QUCS from SourceForge.
Nifty program. I have had trouble doing a sim at a constant frequency
with a variable capacitor. Your example gave me some hints, but I still
have problems doing a sim under such conditions. I'm learning!

Dick also made these comments:
>
> You present a very interesting paper for the Symposium. What is your
> next step? When are you going to construct a test bread board of an
> array? To do this you need an amplifier concept as an element of the
> antenna. In the recent past (~2 years) we started such an amplifier
> examination but the effort withered, apparently, and not much came of
> it as I remember. I recognize that there are a number of closely
> related electronic, mechanical, and thermal issues for such an
> amplifier element, along with Dan’s delay line and other aspects.
>
Dick -- the task I took on was as an architect/physicist. I set my goal
as defining the concept, not doing the detailed design. As such, I have
tried to figure out how the critical components should work. My next
step at that level is to test the basic phasing concept to see if my
idea works. I had hoped to be at that point already, but I have problems
with round tuit procurement!
> As I see it, getting started in such an amplifier element, now that
> you have a concept for the beam steering, is just as important to the
> Eagle project as is such work that has been done recently on the U
> receiver by John and Juan. I realize that the electronic aspects of
> such hardware is somewhat of a moving target (the pace of developments
> in that field), but somewhere you will have to put a stake in the
> ground and say that this is where we shall start. To be able to deal
> with the mechanical and thermal aspects, which are not trivial at all,
> I would need you to put the stake in the ground so that I would have a
> starting point for such an effort.
No disagreement. I hope that this paper will serve as a probe for the
real RF designers. The big change from our earlier discussions on
amplifiers is that we seem to have reached agreement that the microwave
PAs can be hard limiting which will have a BIG impact on power
consumption and heat loading.

However, until we have a real schedule that is focussed on a real launch
opportunity, I believe the volunteers (including me) devote less than
the necessary 110% effort to the problem. I'm really hoping that Rick
will be able to make such an announcement at Pittsburgh. Then things
will really start moving.

For all -- Dan asked me to reformat the paper's margins for the
proceedings. As a result, the "final" version posted at
http://mysite.verizon.net/w3iwi/electronic_scanning_antennas.pdf (those
are _ characters separating the words) has grown from 17 to 20 pages in
length, with no change in content except for the repair of some minor typos.

73, Tom






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