For much of what I can envision doing with the USRP and phased arrays,
it will be a narrow band phased array. This means that the bandwidth of
the signals of interest or bands of interest is small in comparison to
the carrier frequency. In those cases, and when the antennas are (say)
1/2 wave spaced, you easily phase the array by changing the phase of the
NCO.
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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