"A small granule of truth." A factoid is to insight, as a small hard
turd is to a bowel movement. ,.....................

Sounds like one of those green lizard geico commercials... I can just
imagine the sound of that statement with that little green lizard
aussie
voice he has...
MK

The hour is growing late and the urge to tell a story is upon me. Note,
that unlike others, I never imbibe alcoholic beverages. This is a true
story.

A large, expensive, rotatable LPDA was erected on a tall tower and
tested in an informal way with a receiver and short-wave broadcast
transmitters as signal sources. Nulls were where they were supposed to be.
F/B was roughly what was expected. Front lobe seemed about right.

A month went by and it was time to perform more careful measurements. A
180 swing while listening to a distant carrier suggested that there was more
gain off of the back than off of the front! Wise men pondered that this
must be a measurement error.

Impedances were very close to expected. The scheme that was used to
determine that the antenna needed to be brought back to earth were these:

(Keep in mind that this is a LPDA and not a Yagi.) A 6db pad was put in
the line to ensure that the nonexistent impedance changes with no change in
frequency would not affect things. An accurate, step attenuator was then
added in the line. (Should sound familiar.) Antenna was pointed towards a
known very high power broadcast station with a known location such that the
expected receiving take-off angle was about 6 degrees. Bandwidth was made
very narrow and centered on the carrier. Enough attenuation was added to
get the S-meter to average either side of a mark. The attenuation was
varied by one db up and down with several minutes at each level. Then the
antenna was rotated 180 degrees and the change in attenuation to return to
the same indication was noted. Ouch. Repeated several times it became
clear that the antenna had reversed direction and (are you listening?) a
number for the difference in gain (at the angle involved) and the
uncertainty in that number was crafted. Lord K. should have said - and may
have said - that all measurements comprise at least two numbers: an
estimate expressed as a number and an estimate of the uncertainty in the
first number expressed as a number.

The evidence was conclusive that something had happened. A search for
causality took place with the assistance of NEC. With a good deal of
speculation and noting how the largest elements were constructed, it was
speculated that one of the rearmost elements (or a part of one of the
rearmost elements) may have become detached electrically. An NEC simulation
was produced that fit the data. No, this is not proof, but it did furnish
additional support for the need to lower the antenna. More support came
from measurements at the high end of the frequency range where no anomalies
were noted and where the long elements would not be expected to play much of
a role.

At significant cost and effort, the antenna was lowered and it was found
that one of the mechanical connections of a sleeve with normal screw had not
been tightened in a rear element. The conclusive proof was that the screw
had not "cut." Apparently, a month of very sight corrosion had isolated
part of an element.

So, be my students and find the lessons in this tale. Make a list.

73 and I am off to bed, perchance to dream of antennas, Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"

J. Mc Laughlin wrote:
The hour is growing late and the urge to tell a story is upon me. Note,
that unlike others, I never imbibe alcoholic beverages. This is a true
story.

Despite his enjoying wine, I believe Reg's stories to be true also! 8^)

A large, expensive, rotatable LPDA was erected on a tall tower and
tested in an informal way with a receiver and short-wave broadcast
transmitters as signal sources. Nulls were where they were supposed to be.
F/B was roughly what was expected. Front lobe seemed about right.

A month went by and it was time to perform more careful measurements. A
180 swing while listening to a distant carrier suggested that there was more
gain off of the back than off of the front! Wise men pondered that this
must be a measurement error.

Impedances were very close to expected. The scheme that was used to
determine that the antenna needed to be brought back to earth were these:

(Keep in mind that this is a LPDA and not a Yagi.) A 6db pad was put in
the line to ensure that the nonexistent impedance changes with no change in
frequency would not affect things. An accurate, step attenuator was then
added in the line. (Should sound familiar.) Antenna was pointed towards a
known very high power broadcast station with a known location such that the
expected receiving take-off angle was about 6 degrees. Bandwidth was made
very narrow and centered on the carrier. Enough attenuation was added to
get the S-meter to average either side of a mark. The attenuation was
varied by one db up and down with several minutes at each level. Then the
antenna was rotated 180 degrees and the change in attenuation to return to
the same indication was noted. Ouch. Repeated several times it became
clear that the antenna had reversed direction and (are you listening?) a
number for the difference in gain (at the angle involved) and the
uncertainty in that number was crafted. Lord K. should have said - and may
have said - that all measurements comprise at least two numbers: an
estimate expressed as a number and an estimate of the uncertainty in the
first number expressed as a number.

The evidence was conclusive that something had happened. A search for
causality took place with the assistance of NEC. With a good deal of
speculation and noting how the largest elements were constructed, it was
speculated that one of the rearmost elements (or a part of one of the
rearmost elements) may have become detached electrically. An NEC simulation
was produced that fit the data. No, this is not proof, but it did furnish
additional support for the need to lower the antenna. More support came
from measurements at the high end of the frequency range where no anomalies
were noted and where the long elements would not be expected to play much of
a role.

At significant cost and effort, the antenna was lowered and it was found
that one of the mechanical connections of a sleeve with normal screw had not
been tightened in a rear element. The conclusive proof was that the screw
had not "cut." Apparently, a month of very sight corrosion had isolated
part of an element.

So, be my students and find the lessons in this tale. Make a list.


Lesson #1 - Put the thing together correctly. Check the connections.
Check it again.


Lesson #2 -some people needed to hone their troubleshooting skills. If
the antenna had performed well during the initial tests, using an
analysis program is probably the last thing to do. The time to do
analysis was long past.

I don't know much about antennas, (which is why I'm here) but I do know
troubleshooting. All the numbers that popped up on the tests and the
simulations were just blind alleys for y'all to go down and get beat up
in. Something changed in that month. Is it likely that the laws of
physics changed? Much more likely something in the installation was at
fault. The antenna should have been lowered when the second tests were
so different than the first ones. Of course following rule #1 would have
obviated the need for that!




73 and I am off to bed, perchance to dream of antennas, Mac N8TT

8^) -73 de Mike KB3EIA -

ORIGINAL MESSAGE:

Mike Coslo wrote:

So, be my students and find the lessons in this tale. Make a list.



*********** REPLY SEPARATOR ***********

My very first thought was you were receiving an LP signal instead of
SP.

Shows how a DXer thinks. :-)

73, Bill W6WRT

On Mon, 06 Mar 2006 18:43:22 -0800, Roy Lewallen
wrote:

Gary Schafer wrote:
. . .
The calibration points that Mike did on his receiver should be valid
for any band for his antenna comparisons. An actual signal strength
measurement is not required nor would it be valid between bands. All
that is really needed is the difference measurements between the two
antennas so his calibration between points on the meter scale will be
valid on any band.

I'm not sure I fully understand this. The difference from one S meter
division to another *is* likely to be different on different bands,
since it depends on the gain-vs-voltage characteristics of the
controlled stages which can vary with frequency. But I do agree that he
can make good comparative antenna measurements without good S meter
calibration, because he has a step attenuator. By simply setting the
attenuator so he gets the same S-meter reading on both antennas, S-meter
calibration is completely irrelevant -- the antenna gain difference is
the attenuator setting.

I find it useful, however, to be able to see the difference with
reasonable accuracy just by looking at my S meter. But that does require
calibration for the band in use.

. . .

Roy Lewallen, W7EL

Hi Roy,

I have been away for awhile so haven't had a chance to reply.

My point of the S meter being the same relative difference between S
units on all bands comes from the assumption that like most modern
radios, there is no AGC controlling anything in the front end of the
radio. All the gain control is done in the IF so it is impartial to
frequency.

So even if there is some difference in the gain of the first mixer, or
preamp if there is one, the gain controlled IF sees the same relative
signal level regardless of band.

I did a quick check on my old Kenwood TS430 using a Wavetek 3001
signal generator that has a step attenuator in it and got the
following results:

+60= -10dbm
+50= -20dbm
+40= -30dbm
+30= -40dbm
+20= -50dbm
+10= -60dbm
S9= -70dbm
S7= -80dbm
S4= -90dbm
S1.5= -100dbm

Test was done on 80, 20 and 10 meters with the readings the same on
all bands as close as I could determine the meter reading. I would
suspect that the signal generator leveling accuracy may be no better
than what error can be read on the S meter.

73
Gary K4FMX