Frank wrote:
How much current will actually flow in the center conductor of the shorted
coaxial feedline?

Good question. In this case, the more difference in the two
currents, the greater the radiation from the horizontal section.

I never liked the G5RV antenna since it just seemed like a high loss method
of feeding a 102 ft piece of wire.

Actually, if the purpose of the matching section is to lower
the SWR (and thus the losses) on the coax, it does a reasonable
job on 80m, 40m, 20m, and 12m. But forget the other four bands
unless you change the length of the matching section.
--
73, Cecil http://www.qsl.net/w5dxp

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On Wednesday, 11 May 2005 09:06:36 -500, "Asimov"
wrote:

Anyways my question is, in your opinion, what is the purpose of the
pseudo random arrangement of this "apperture mask" antenna?

Hi Isaac,

The random code of the "coded aperture mask" can be correlated to the
grid pattern of detectors to make a virtual lens.

I haven't tried adding a signal to the
noise in the simulation yet though...

You probably won't unless you have deconvolution math behind it.

73's
Richard Clark, KB7QHC

Bob Miller wrote:

One question, if shorting the leads to feed the g5rv "marconi style",
would it be better to connect the single wire to the antenna tuner's
random wire connection, or to the coax connection, or does it matter?

It's hard to say, because too many variables are involved. What I'd do
is to model the antenna with EZNEC and if the impedance is high use the
random wire connection and if low the coax connection.

Roy Lewallen, W7EL

Asimov,

I believe you have misinterpreted the geometry. The detector array is
some distance behind the aperture array, perhaps one meter or more. The
concept is that of a simple shadow mask. There are 52000 shadows on
32000 detectors, so the details of the computation are probably a bit
complex.

I recommend a paper by Scott Barthelmy, which is available at

http://swift.gsfc.nasa.gov/docs/swif...SBarthelmy.pdf


73,
Gene
W4SZ

Asimov wrote:
Hi,

Say, I've got a question about something I've read about the Swift
gamma ray detection satellite. It's detector consisted of an antenna
composed of 52,000 lead blocks arranged by computer generated
pseudo-random locations and glued to the back of a sheet of 4x8
plywood. The actual detectors where located in the spaces between the
blocks. It wasn't mentioned but I think these were probably crystals
which give off a pulse of electricity every time they are struck by a
gamma ray.

Anyways my question is, in your opinion, what is the purpose of the
pseudo random arrangement of this "apperture mask" antenna? I suspect
it has something statistical related to the nature of ambient noise
signals. I tried making a pc simulation of this but I get the same
answer whether the array is in rows or pseudo-random. I tried with
noise but I saw no reduction. I haven't tried adding a signal to the
noise in the simulation yet though...

A*s*i*m*o*v

... Three types of lies: lies, damn lies, and statistics - Mark Twain.

"Gene Fuller" bravely wrote to "All" (11 May 05 21:02:18)
--- on the heady topic of " Apperture Mask Antenna"

GF From: Gene Fuller
GF Xref: aeinews rec.radio.amateur.antenna:30137

GF Asimov,

GF I believe you have misinterpreted the geometry. The detector array is
GF some distance behind the aperture array, perhaps one meter or more.

The original release mentions that the gamma rays can only penetrate
inbetween the lead blocks but doesn't mention how far back the
detectors were located or how they were arranged.


GF The concept is that of a simple shadow mask. There are 52000 shadows
GF on 32000 detectors, so the details of the computation are probably a
GF bit complex.

"The mask together with a set of gamma-ray detectors, totaling 32,768
pieces of cadmium-zinc-telluride each measuring four square
millimeters are the main components of Swift's Burst Alert Telescope
(BAT). BAT will locate hundreds of bursts to better than 4-arcminute
accuracy and provide enhanced sensitivity to faint bursts that earlier
detectors have missed."

Maybe it is a little less complex than we might assume...


GF I recommend a paper by Scott Barthelmy, which is available at

GF http://swift.gsfc.nasa.gov/docs/swif..._SBarthelmy.pd
GF f

"For an image of the coded aperture mask, refer to:
http://swift.gsfc.nasa.gov/news/coded_app_mask.html."

That's all I have...

A*s*i*m*o*v

.... "Beware of all enterprises that require new clothes." -- THOREAU

"Richard Clark" bravely wrote to "All" (11 May 05 12:05:28)
--- on the heady topic of " Apperture Mask Antenna"

RC The random code of the "coded aperture mask" can be correlated to the
RC grid pattern of detectors to make a virtual lens.

I haven't tried adding a signal to the
noise in the simulation yet though...

RC You probably won't unless you have deconvolution math behind it.

Would you please, explain?

A*s*i*m*o*v

.... Essay: Describe the universe in 500 words or less & name 2 examples.

"Asimov" wrote in message
...
Hi,

Say, I've got a question about something I've read about the Swift
gamma ray detection satellite. It's detector consisted of an antenna
composed of 52,000 lead blocks arranged by computer generated
pseudo-random locations and glued to the back of a sheet of 4x8
plywood. The actual detectors where located in the spaces between the
blocks. It wasn't mentioned but I think these were probably crystals
which give off a pulse of electricity every time they are struck by a
gamma ray.

Anyways my question is, in your opinion, what is the purpose of the
pseudo random arrangement of this "apperture mask" antenna? I suspect
it has something statistical related to the nature of ambient noise
signals. I tried making a pc simulation of this but I get the same
answer whether the array is in rows or pseudo-random. I tried with
noise but I saw no reduction. I haven't tried adding a signal to the
noise in the simulation yet though...

A*s*i*m*o*v

.... Three types of lies: lies, damn lies, and statistics - Mark Twain.

Plywood on a satellite?

73 Roger ZR3RC

On Thursday, 12 May 2005 00:09:36 -500, "Asimov"
wrote:
RC You probably won't unless you have deconvolution math behind it.
Would you please, explain?

Hi Isaac,

The technique is one of a suite of tools derived from Fourier
analysis. The background signal contains a random response that is
largely immutable. That signal can be used as a key (if we were to
use the parlance of cryptography). The signal of interest still
appears to be just as random - in isolation. However, when both are
"mixed" (if we were to use the idiom of electronics), then the signal
of interest rises out of the noise.

It is not a simple mixing process, however, because in that idiom we
don't know the center frequency - so to speak (and instead of getting
clear speech in the typical SSB application we get something that
sounds like Donald Duck at best).

However, in Fourier analysis, you can slip the two signals past each
other (akin to matching the key of the "Gold Code" of a spread
spectrum signal to the incoming signal) in what is called cross
correlation and the deconvolved signal will spring out as a result
(when done with what is called confidence math, also found in the
Fourier repertoire). To return to the idiom of the SSB application,
the background noise becomes the center frequency.

The same application has been done to map the ocean floor using
natural wave and wildlife noise.

73's
Richard Clark, KB7QHC

Roger Conroy wrote:

"Asimov" wrote in message
...
Hi,

Say, I've got a question about something I've read about the Swift
gamma ray detection satellite. It's detector consisted of an antenna
composed of 52,000 lead blocks arranged by computer generated
pseudo-random locations and glued to the back of a sheet of 4x8
plywood. The actual detectors where located in the spaces between the
blocks. It wasn't mentioned but I think these were probably crystals
which give off a pulse of electricity every time they are struck by a
gamma ray.

Anyways my question is, in your opinion, what is the purpose of the
pseudo random arrangement of this "apperture mask" antenna? I suspect
it has something statistical related to the nature of ambient noise
signals. I tried making a pc simulation of this but I get the same
answer whether the array is in rows or pseudo-random. I tried with
noise but I saw no reduction. I haven't tried adding a signal to the
noise in the simulation yet though...

A*s*i*m*o*v

... Three types of lies: lies, damn lies, and statistics - Mark Twain.

Plywood on a satellite?

73 Roger ZR3RC

Honeycomb composite, about 2" thick and about 1/4 the area of a sheet of
plywood.

http://swift.gsfc.nasa.gov/docs/swif.../bat_desc.html

According to the web page, the answer to the original question is the
geometry of the mask elements allows the telescope to maintain its field
of view and angular resolution in the event of a wide variety of
different sensor failures.

ac6xg

"Richard Clark" bravely wrote to "All" (12 May 05 08:23:37)
--- on the heady topic of " Apperture Mask Antenna"

RC From: Richard Clark
RC Xref: aeinews rec.radio.amateur.antenna:30190

RC On Thursday, 12 May 2005 00:09:36 -500, "Asimov"
RC wrote:
RC You probably won't unless you have deconvolution math behind it.
Would you please, explain?

RC Hi Isaac,

RC The technique is one of a suite of tools derived from Fourier
RC analysis.
[,,,]
RC The same application has been done to map the ocean floor using
RC natural wave and wildlife noise.


Okay, thanks but not Isaac rather Mike...
Is this something like the noise forming a "bell curve" and the
desired signal appearing on the outside?

A*s*i*m*o*v

.... Isn't Fourier and it's applications a bitch!