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From: | Robert McGwier |
Subject: | Re: [Discuss-gnuradio] software controlled antenna |
Date: | Tue, 20 Dec 2005 16:29:06 -0500 |
User-agent: | Mozilla Thunderbird 1.0.2 (Windows/20050317) |
If Bl is the left band edge and Br is the right band edge then we define a narrow band phased array is typically one in which
(Br - Bl)*2/(Bl+Br) < 0.00N Let's say N is 5 and (Bl+Br)/2 is 100 MHz. Then the effective bandwidth of our NCO only tuned phased array would be 500 kHz !TDL's whether they are "have to be calibrated at each and every frequency" varactor based or from sampling at several hundred MHz seem not to be needed. I agree that a drawback is that in order to get decent noise performance (low NF) the best solution is to put a preamp on each element. The other aspect of phased arrays that radio astronomers make sure is grating lobes in their interferometers. For much of what we are likely to need to do, we do not want grating lobes and this limits us to spacings between elements < 1 lambda and probably lambda/2 is required for most apps. Otherwise your lobes are well down from a large single lobe and they are well out into the "element pattern" roll off irrespective of what the array pattern is. This requires one to recall that a phased array has two patterns that impact its final result. The array pattern (assuming isotropic radiators) and then multiplied times the element pattern (assuming a single element pattern for each element).
Eric wants to do bistatic radar, and mapping ionospheric E clouds, aurora, etc. using fixed known emitters, I want to actual phased arrays for the purpose of building a C band antenna for a space craft. In this case, it is a large plus that AMSAT wants to do the amplifier thing on each antenna as it spreads out the heat and allows me to use smaller more efficient devices rather than one large one. The USRP is a useful widget and so far, we have not pushed it to its potential at all.
Bob Marcus Leech wrote:
Angilberto Muniz Sb wrote:Chuck, I'm pretty sure you are aware of it -- that's the bases of some Phased-Array antennas -- I'm woking on this subject albeit its not related to USRP but your experiment could be the starting point for this kind of venture... Good job, Angilberto.Paul Shuch was working on a phased-array panel antenna for SETI work. I don't know how many elements, but it was steered using varactors. For some environments, this would be really nice--no need to build mechanicals to swing a dish around. On the down side, a phased-array patch antenna is potentially a lossy thing, so useless for weak-signal work like EME and radio astronomy. Unless you have an LNA at every element in the array. Which maps into $$$.
-- AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats Laziness is the number one inspiration for ingenuity. Guilty as charged!
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