Oh my goodness! No bias! Wow.
Both Jim's and Richard's observations are the sort of things that need to be
told to budding engineers.
I tell quite a few stories in class. After all, engineers kill people
with their mistakes in wholesale lots. Stories, since well before writing,
have been the way to communicate the important "stuff." The black box story
needs to be told. I effect some of Jim's story with the (apparently) simple
lab job of connecting N CMOS inverters in a ring - and then "playing."
(Fortunately, we have very high bandwidth scopes in each workstation so that
clues to what is going on can be seen. Thanks to a generous grant from a
company formally known as HP, we can see spectrum too.)

Many thanks for your contributions.
73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Richard Clark" wrote in message
...
On Fri, 24 Sep 2004 14:42:54 -0400, "JLB"
wrote:

As for digital signal processing is concerned, just because you have
digitized the signal doesn't mean that you have eliminated all of the
analog
problems.

Hi Guys,

This reminds me of the problems I had teaching the digital types
Shannon's law for BER.

When I designed the black box for the 757/767, all of my digitized
readings (taken from 600 leads) was passed over to a specialized tape
recorder (25 hour capacity).

Problem was that this digital signal was fed into the recording head
without any bias. Many may be unaware of the advances in audio tape
recording BW that came by the addition of bias, and more, that it
reduced the head's tendency to erase its own stored signal. In
essence, with no bias, you were recording data with two strikes
against you. This was not the hallmark design for a data sensitive
product. Worse yet, was this digital mentality had recorder
specifications that allowed for a S+N/N of 2.

What were the comments I heard in response? "This is not HiFi, it's
digital data, on/off."

Within weeks I was drawn into their simulator lab to view a simulated
cockpit of an KLM aircraft that was being used to display flight
recorder data that was rather -um- noisy (much too much for KLM's
engineers to make sense of it). I watched that plane hit the ground
several times as they struggled to recover digital bits lost in analog
noise.

Several (many) years later I was called to consult on the TWA flight
800 data - more noise that lead to hints of missile strikes. The
panel's best spin on the topic was that it was old data printed
through the new data (even though the difference in time would not
correlate to the two data sets overlapping). We reported it as
exploding gas tanks, Tehran reported it was revenge for our shooting
down one of their civil aircraft during the first Gulf War. Like the
first attack on the Twin Towers, government and the media shrugged off
correlations.

73's
Richard Clark, KB7QHC

"JLB" wrote in message
...



"Fractenna" wrote in message
...

I am confused: are you saying that my comments contain errors? If so,
what
is
incorrect?:-)

Yes; OSU masters students in antennas are very good. I have one working
for me
right now.

73,
Chip N1IR


No, Chip.

I do not see any errors in your comments.

What I was saying was that building such a system as originally described
is
a daunting task. There are many problems to overcome, one of which is
getting an antenna to work over a 25: 1 bandwidth with reasonably constant
performance.


I regularly use an active 41" monopole to accurately measure electric field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1, and
that is 25-year old technology.

--
Ed
WB6WSN

I regularly use an active 41" monopole to accurately measure electric field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1, and
that is 25-year old technology.

--
Ed
WB6WSN




And your variation of gain, excluding mismatch, is...?

73,
Chip N1IR

"Fractenna" wrote in message
...
I regularly use an active 41" monopole to accurately measure electric
field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1,
and
that is 25-year old technology.

--
Ed
WB6WSN




And your variation of gain, excluding mismatch, is...?

73,
Chip N1IR



And that should be complex gain, since in the stated application you are
concerned about phase and magnitude.

Also, don't forget about the mutual coupling between elements. What happens
to it over a 3000 to one bandwidth? Remember we are talking about an array
of antennas, not a single isolated one!

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net

Ed's one meter vertical over a large conductive ground plane has an
effective height of close to 0.5 meters up to something like 20 MHz. With
an amplifier at the base of the vertical that has high input impedance and
some tailored feedback, one can have a system that can be used to measure
field strength with very little frequency dependence.
I use a miniaturized version of such an antenna as a probe in a TEM
cell.

I have seen the use of resistively loaded (short) dipoles connected to
resistively loaded transmission lines used by the NBS (as it was then
called) to measure field strength with minimum disturbance to the field.

These are all receiving antennas with essentially uniform performance
over very large frequency spans.

My feeling is that to have a small variation in transmitting gain over
more than something like five to one will require an adaptive antenna
system. (I am assuming antennas with an "average" gain that is close to
one - no resistive loading.) 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Fractenna" wrote in message
...
I regularly use an active 41" monopole to accurately measure electric
field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1,
and
that is 25-year old technology.

--
Ed
WB6WSN


And your variation of gain, excluding mismatch, is...?

73,
Chip N1IR

Ed's one meter vertical over a large conductive ground plane has an
effective height of close to 0.5 meters up to something like 20 MHz. With
an amplifier at the base of the vertical that has high input impedance and
some tailored feedback, one can have a system that can be used to measure
field strength with very little frequency dependence.
I use a miniaturized version of such an antenna as a probe in a TEM
cell.

I have seen the use of resistively loaded (short) dipoles connected to
resistively loaded transmission lines used by the NBS (as it was then
called) to measure field strength with minimum disturbance to the field.

These are all receiving antennas with essentially uniform performance
over very large frequency spans.

My feeling is that to have a small variation in transmitting gain over
more than something like five to one will require an adaptive antenna
system. (I am assuming antennas with an "average" gain that is close to
one - no resistive loading.) 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Fractenna" wrote in message
...
I regularly use an active 41" monopole to accurately measure electric
field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1,
and
that is 25-year old technology.

--
Ed
WB6WSN


And your variation of gain, excluding mismatch, is...?

73,
Chip N1IR


The array is RX only, and G/T is important for each individual element, over a
broad range. You need a truly compact element that is wideband. Wideband means
about the same gains and impedances across a very wide range.

Mediocre broadband RX antennas have been around since WWII.

The discussion on TX, albeit interesting, does not apply.

Also, mismatch produces dramatic signal drops without an ATU in conventional
designs, such as inverted V's. The assumption has been 'no ATU' in this
project, because the costs are prohibitive.

The one mitigating issue is that the sky temperature --and this system is
designed to synthesize a measurement sky tempertaure with high angular
resolution--increases with lower frequency (below about 1GHz) because the
emission is non-thermal. Thus the signal to noise gets better in VHF.

73,
Chip N1IR

"Fractenna" wrote in message
...
I regularly use an active 41" monopole to accurately measure electric
field
strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1, and
that is 25-year old technology.

--
Ed
WB6WSN




And your variation of gain, excluding mismatch, is...?

73,
Chip N1IR



Maybe I gave you a trick answer, because the antenna I cited is a
receive-only device. It is concerned only with measuring the strength of the
electric field, and derives no phase information. I'm sure its efficiency is
horrible, but its bandwidth is huge and its gain is about +/- 0.5 dB.

FYI, here's an example of this style of antenna:
http://www.ets-lindgren.com/productp...ttype=Antennas

Even if it did not face into an active pre-amplifier, it would still be a
poor emitter anywhere below a few MHz. Electrically short is something that
hasn't been finessed yet.

Ed
wb6wsn