[eagle] Re: Phased Array Concepts
matt at ettus.com
Sat Oct 6 23:47:37 PDT 2007
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.
- The tap for getting the signal out may add capacitance of about the
same order of magnitude.
- The delay line needs to be long enough, or the caps all look like they
are in parallel.
- 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.
- You can probably reduce the reflections by putting attenuators in the
delay lines, but these would make the DC biasing of the varactors more
I have attached my simulation. It was done in qucs, which runs on
linux. See qucs.sf.net or get it from your distribution.
Tom Clark, K3IO wrote:
> I have (finally!) put the finishing touches on my magnum opus describing
> my ideas on phased arrays and shipped it off to be included in the
> Pittsburgh symposium proceedings. If you want to see it, it is available
> for viewing at
> http://mysite.verizon.net/w3iwi/electronic_scanning_antennas.pdf. This
> version has some figures in color, but the proceedings is in B&W (this
> format is available if you need it).
> This took me a lot longer than I anticipated. I had a couple of fairly
> major Gates-induced computer failures, some of which still plague me
> (like I'm using my notebook for all Email). I advise against the use of
> Office 2007.
> I am hoping that this week I will have time to test Dan's hardware so I
> can have some results to report by Pittsburgh.
> Matt -- this has the numbers I was trying to remember last weekend.
> For all -- your comments and critiques are solicited.
> 73, Tom
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