On 10/02/2010 07:12 PM, Jim Lux wrote:
On Oct 2, 4:28 am, Alejandro Lieberalejan...@Use-Author-Supplied-
Address.invalid wrote:
On 10/01/2010 06:13 PM, Jim Lux wrote:

Owen wrote:
On 01/10/10 07:44, Jim Lux wrote:

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..

How come ?
Can you elaborate how can these differences happen ?


it's the difference between the effect on a peak vs effect on a null.

consider a simple 2 element array.. for sake of argument, say it's 1/4
wavelength apart and phased 90 degrees, so it has a cardioid
pattern.... a gain of 2 in one direction (where the signals from the
two antennas align), and a gain of zero in the opposite direction.
The gain is 1+cos(phi - spacing*cos(theta)) where phi is the feed
phasing, and theta is the direction.. in the preferred direction
1+cos(90 - 90*cos(0)) = 1+cos(0) = 2
in the 45 degree direction: 1+cos(90-90*cos(45)) = 1+cos(90-90*.707) =
1.895
in the 90 degree direction: 1+cos(90-90*cos(90)) = 1+cos(90) = 1
in the 180 degree direction: 1+cos(90-90*cos(180)) = 1+cos(90-90*-1) =
1+cos(180) = 0

Now spoil the feed phase (phi) by 10 degrees... (80
on boresight: 1+cos(80-90*cos(0)) = 1+cos(-10) = 1.984
on 45: 1+cos(80-90*cos(45)) = 1.959
on 90: 1+cos(80-90*cos(90)) = 1.174
at 180: 1+cos(80-90*cos(180)) = 1+cos(80+90) = 1.52E-2

The gain on boresight didn't change much... from 2 to 1.984 (0.03dB)
But the null in the back came up from zero to 1.5E-2.. (instead of -
infinity, it's now -18dB)

Change the phase error to 45 degrees...)
@theta=0: 1+cos(45-90*cos(0)) = 1.707
@theta=180: 1+cos(45-90*cos(180)) = .292

So, from the 10 degree error case, the forward gain went from 1.984 to
1.707, about 0.6dB...
but the null went from 1.52E-2 to .292 (from -17dB to -5 dB)..


The thing to remember on any gain antenna is that it takes very little
power to disrupt a null (after all, a -30dB null means that if you're
radiating 1kW in the forward direction, you're radiating 1 W in the
null.. so just another watt will double the energy in the null,
turning it from -30dB to -27dB...)

(And, you can see why making antennas with sidelobes-60dB is VERY
challenging... )


Now, change the phasing to, say, 80 degrees.. in the preferred
direction, the gain is now 1+cos(10degrees)

Thank you Jim for the explanation. Sorry I wasn't more specific.
I was refering to the difference between receiving and transmiting gain.
--
Alejandro Lieber LU1FCR
Rosario Argentina

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