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blackoaks(at)gmail.com
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 Posted: Tue Jul 11, 2006 3:53 pm Post subject: Grounding lugs and transponder radiation |
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Bob,
In your writings you suggest a grounding or through firewall brass bolt of 5/16" diameter.
In your article on local grounding, the notation on the photo suggests a 1/4" or 5/16" bolt. Lancair used 1/4" studs through the firewall on my ES. Are these sufficient or should I change them to 5/16"? I'm using #2 welding cable for my big wires.
Second question regards the transmission of the transponder signal. Does the signal rediate from the shaft and the ball or only the ball at the tip?
I have a suggestion for future revisions. The military flight manuals mark the revised material in each change with a dark bar in the margin. I don't know it would be possible for you do do something similar with your word processor to make finding the changes in your text revisions.
Thanks for all your information and insights, also your PATIENCE. We got a lot out of and very much enjoyed the Prescott seminar.
John McMahon
Lancair Super ES
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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| Posted: Tue Jul 11, 2006 6:00 pm Post subject: Grounding lugs and transponder radiation |
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On Jul 11, 2006, at 7:36 PM, John McMahon wrote:
| Quote: | Second question regards the transmission of the transponder
signal. Does the signal rediate from the shaft and the ball or
only the ball at the tip?
|
The signal is radiated only from the shaft of the antenna. The ball
is just there mainly to keep people from getting hurt when they bump
into it.
Brian Lloyd 361 Catterline Way
brian-yak AT lloyd DOT com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
— Antoine de Saint-Exupéry
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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FLYaDIVE(at)aol.com
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| Posted: Wed Jul 12, 2006 12:21 am Post subject: Grounding lugs and transponder radiation |
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In a message dated 7/11/06 10:03:23 PM Eastern Daylight Time,
brian-yak(at)lloyd.com writes:
| Quote: | > Second question regards the transmission of the transponder
> signal. Does the signal rediate from the shaft and the ball or
> only the ball at the tip?
The signal is radiated only from the shaft of the antenna. The ball
is just there mainly to keep people from getting hurt when they bump
into it.
===========================================================
|
Brian - Group:
A bit of clarification:
If it is metal and it is attached to the antenna (straight portion of the
shaft) it is radiating. There are two reasons for the BALL:
1 - To prevent Corona Discharge - Which is a static discharge that occurs at
antenna ends when there is a SHARP mechanical end to an antenna.
2 - As Brian described - It helps prevent injuries when working around the
antenna.
The other part of the antenna which many tend to forget is the Metal mounting
surface - It functions as a Reflector aka Ground Plane (Plane as in an axis
.. X, Y or Z. In this case an X axis.) Without it the radiation pattern
would be very poor and your transmit & receive signal would be very weak. You
would also have a high SWR (Standing Wave Ratio) and that would reduce your
signal and shorten the life of the Xponder output circuit.
Now, your next question should be: What about planes that are not made out of
metal, such as the Long EZ type? Well, they install a [or should install]
small metal reflective surface. This can be thin sheet metal or even peel and
stick copper tape. It can be mounted directly on the outside of the plane,
imbedded into the composite lay-up or on the inside. Only thing that matters is
that it is mechanical and electrically connected to the antenna.
When doing the math for the construction of the antenna [because all
electrical wavelengths are converted to a mechanical wave length or part there of] the
Ball must be taken into account for the overall length. This is especially
true at the higher frequencies VHF, UHF and Microwave. Transponders are
Microwave.
Barry
"Chop'd Liver"
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europa flugzeug fabrik
Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA
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| Posted: Wed Jul 12, 2006 3:31 pm Post subject: Re: Grounding lugs and transponder radiation |
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| brian wrote: | | The signal is radiated only from the shaft of the antenna. |
A couple theoretical texts I have suggest it radiates only from the base of the antenna. Has to do with 90-deg phase lag, current and voltage, I think. The vertical element is there merely to resonate.
Dunno also, but the ball has been alleged to be either a capacity hat, or to have proverse effect on radiating pattern. One such allegation came from a retired, aircraft antenna engineer I know. But he thinks my questions are often inane! 
Fred F.
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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| Posted: Wed Jul 12, 2006 5:09 pm Post subject: Grounding lugs and transponder radiation |
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On Jul 12, 2006, at 7:31 PM, europa flugzeug fabrik wrote:
| Quote: |
<n3eu(at)comcast.net>
brian wrote:
> The signal is radiated only from the shaft of the antenna.
A couple theoretical texts I have suggest it radiates only from the
base of the antenna.
|
The entire antenna radiates but most of the radiation comes from the
portion of the conductor with the greatest current. Since the current
node occurs at the feed point of a 1/4 wave antenna, you are correct
in that the portion of the conductor nearer the base radiates more of
the signal. Regardless, the entire radiator radiates to one extent or
another.
If you want you can make a 1/2 wave radiator and feed it at the
voltage node (the end). Now you don't need much of a counterpoise
(ground) at all because the current is very low at the feedpoint. In
that case radiation is primarily from the current node which is now
at the center of the radiator. This is commonly used where a ground
is not available, such as on plastic cars and boats.
| Quote: | Dunno also, but the ball has been alleged to be either a capacity
hat, or to have proverse effect on radiating pattern.
|
It doesn't really have enough area to be a capacity hat but even so,
all that would do is to allow you to reduce the length of the
radiator and still be in resonance. It could be a way to reduce
corona discharge but the power levels used by transponders is low
enough that it really isn't an issue.
I don't have any of my antenna books with me but, as I recall, the
effective impedance of an end-fed 1/2 wave antenna is about 2000
ohms. Since the voltage node of a 1/4 wave antenna (the free end)
should be about the same, I should be able to calculate the voltage
at the end of 1/4 wave antenna.
Assuming that the 1/4 wave antenna is resonant, the impedance should
be 100% resistive so I can use the standard ohms law stuff to
calculate voltage. For resistive loads it is:
P = V^2/R
(power equals voltage squared divide by resistance)
If your transponder puts out a peak power of 200W we have:
200 = V^2 / 2000
If we do the algebra we get:
V = sqrt (200 * 2000) or 632V
I don't think we are going to get a corona discharge from 632V. It
could produce a pretty good RF burn if you touch it tho'.
(BTW, I could be wrong about the impedance at the voltage node and
that would skew things a bit but I am pretty sure I am in the ballpark.)
Therefore I hold that the ball is there to keep you from poking holes
in yourself should you rise up rapidly whilst cleaning exhaust stains
off the belly of your airplane prior to judging at OSH.
Brian Lloyd 361 Catterline Way
brian-yak AT lloyd DOT com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
— Antoine de Saint-Exupéry
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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europa flugzeug fabrik
Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA
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| Posted: Thu Jul 13, 2006 2:50 pm Post subject: Re: Grounding lugs and transponder radiation |
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| brian wrote: | ...If your transponder puts out a peak power of 200W we have:
200 = V^2 / 2000
If we do the algebra we get:
V = sqrt (200 * 2000) or 632V....
|
That could be for what I know, but I thought we worked on 50 ohms by fiat. I have a musty, ancient xponder service manual I eBay’d cheap for maybe giggles. To measure power, they had you kludge a commercial “voltage detector†together with a special diode, IN23B. Then you observed reply pulse trains on any whatever “scope†and counted P-P volts by graticule divisions. Volts squared, divided by 50 (into a dummy 50-ohm load) and them’s peak watts. So only 100V P-P would work out to 200W.
That’s the purist’s approach, and funny that around 1990, I had my xponder check done by a crotchety old-timer. He’d “ramp†check also Mode C by observing similarly kludged, detected xponder output on what looked like his Dad’s old scope -- on wheels. For his selected altitude test points per Part 43, he had sketched the bit pattern on little index cards. Head cocked skyward to use his bifocals, holding said card up to the scope with round display. Pump vacuum at the plane, then run to the scope to check. He had to work fast, as permissible leak-down in the static sys at higher alts can cause up to multiple bits to flip Grey-code style (he seemed to rely on an observed time lag). Who needs very expensive equipment, when all you need are FAA-approved index cards? Worked for him and me, a priceless observance of an actual FAR Part 145 repair facility at work, worth every penny for the log entry. Purists impress me.... Had the temptation to hit the IDENT button whilst he was straining through bifocals, but dared not!
How 50-ohm impedance became the norm, where impedance of the atmosphere is etched in stone by a higher power, and a 1/4-wave straight element is natural at 37, well I beg off. Perhaps ‘cuz it all works, or perhaps you know.... 75 works for video hi-def TV with 2-way for nefarious purpose. The history of the “decibel†may be similar whimsy? Not the convenient, additive/subtractive nondimensional math, but Dr. Bell didn't get recognized as did Watt, Ohm, etc., and his employer allegedly thought dividing by 10 (deci) was more practical. Purists like me write "dB,"not "db."  It's like in very current cell phone technology, where despite awesome cleverness to me at least, nobody's been memorialized for nuthin'. 
Fred F.
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
|
| Posted: Fri Jul 14, 2006 6:13 am Post subject: Grounding lugs and transponder radiation |
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On Jul 13, 2006, at 6:50 PM, europa flugzeug fabrik wrote:
| Quote: |
<n3eu(at)comcast.net>
brian wrote:
> ...If your transponder puts out a peak power of 200W we have:
>
> 200 = V^2 / 2000
>
> If we do the algebra we get:
>
> V = sqrt (200 * 2000) or 632V....
That could be for what I know, but I thought we worked on 50 ohms
by fiat. I have a musty, ancient xponder service manual I eBay’d
cheap for maybe giggles. To measure power, they had you kludge a
commercial “voltage detector” together with a special diode, IN23B.
Then you observed reply pulse trains on any whatever “scope” and
counted P-P volts by graticule divisions. Volts squared, divided by
50 (into a dummy 50-ohm load) and them’s peak watts. So only 100V
P-P would work out to 200W.
|
You are correct for measuring the power at the output of a
transponder as the impedance there is 50 ohms. But we were not
talking about the impedance at the output of the transponder or of
the feedline. We were talking about the voltage at the tip of the
antenna.
The impedance of an antenna changes depending on where you feed it.
In a quarter-wave monopole (a "whip" antenna) the feedpoint impedance
at the base (the part nearest the ground plane) is something close to
50 ohms. (It is actually 36 ohms but that is close enough for
government work.) The impedance at the other end, where the little
ball is, is around 2000 ohms. The current in the conductor is at a
maximum near the base and a minimum at the end. Conversely the
voltage is at a minimum near the base and at a maximum near the free
end. If you consider the impedance to be the ratio of voltage to
current you can see that the impedance is constantly changing along
the shaft of the antenna. (This RF stuff is *WAY* different from the
DC stuff as voltage and current vary along a conductor or antenna
unlike DC were they stay the same.)
BTW, your comment about peak voltage caught a mistake of mine in that
I did my calculations in RMS volts when I should have done the
calculations in peak volts (RMS volts X 1.414 for a sine wave). So
the peak voltage at the tip of our transponder antenna with 200W
input is quite near 900V.
| Quote: | That’s the purist’s approach, and funny that around 1990, I had my
xponder check done by a crotchety old-timer. He’d “ramp” check
also Mode C by observing similarly kludged, detected xponder output
on what looked like his Dad’s old scope -- on wheels. For his
selected altitude test points per Part 43, he had sketched the bit
pattern on little index cards. Head cocked skyward to use his
bifocals, holding said card up to the scope with round display.
Pump vacuum at the plane, then run to the scope to check. He had
to work fast, as permissible leak-down in the static sys at higher
alts can cause up to multiple bits to flip Grey-code style (he
seemed to rely on an observed time lag). Who needs very expensive
equipment, when all you need are FAA-approved index cards? Worked
for him and me, a priceless observance of an actual FAR Part 145
repair facility at work, worth every penny for the log entry.
Purists impress me.... Had the temptation to hit the IDENT butt!
on whilst he was straining through bifocals, but dared not!
|
It is pretty cool that you can do it that way. You are just looking
at the pulses (RF on/off) and reading the code. I have done that as a
quick-and-dirty test with a cheap scope, a wire probe, and a crystal
diode configured as a shunt detector. You can get a pretty good
feeling for whether your transponder is working or not (assuming
there is an ATC RADAR close enough to interrogate the transponder
into a reply while on the ground).
| Quote: | How 50-ohm impedance became the norm, where impedance of the
atmosphere is etched in stone by a higher power, and a 1/4-wave
straight element is natural at 37, well I beg off. Perhaps ‘cuz it
all works, or perhaps you know....
|
Ah, you have discovered the difference between physics and
philosophy. Physics just tells us that the feedpoint impedance at the
center of a dipole is 72 ohms. (A monopole being half a dipole is why
its impedance is 36 ohms.) It doesn't tell us *why* it is 72 ohms.
Philosophy is supposed to tell us why. The easiest answer is,
"because God wanted it that way," but that is too hard to get my tiny
brain around. I will just settle for trying to understand a few of
the relationships between the how, what, and where.
| Quote: | 75 works for video hi-def TV with 2-way for nefarious purpose.
|
The answer is, "because the feedpoint impedance at the center of a
dipole is 72 ohms." Once you make a wire intended to feed that
impedance you may as well use it for something else too. If you have
a coax with a characteristic impedance of 75 ohms you have to
terminate it at 75 ohms to minimize reflections. Everyone else may as
well use that impedance as well. And then people started playing with
monopole antennas and found 50 ohms to be more convenient so we got
another kind of coax we could use with lots of different things.
| Quote: | The history of the “decibel” may be similar whimsy? Not the
convenient, additive/subtractive nondimensional math, but Dr. Bell
didn't get recognized as did Watt, Ohm, etc., and his employer
allegedly thought dividing by 10 (deci) was more practical.
|
No, the official unit is the Bell, just like the other official units
are things like the watt, volt, ampere, coulomb, etc. It is just that
sometimes it is easier to measure things with multipliers in the
units like micro-volt, milliampere, kilowatt, and decibel.
| Quote: | Purists like me write "dB,"not "db."
|
Well, that is the way it is supposed to be written. Just like Hertz
is Hz, not hz. Mega-Hertz is MHz, not mhz.
| Quote: | It's like in very current cell phone technology, where despite
awesome cleverness to me at least, nobody's been memorialized for
nuthin'.
|
There hasn't been a lot that is new in radio theory. The "new" stuff
is just clever application of what we have known for a long time. It
is now with digital signal processing we can do some things that we
knew could be done but couldn't do effectively using the technology
at hand back when the math showed the possibility.
Brian Lloyd 361 Catterline Way
brian-yak AT lloyd DOT com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
— Antoine de Saint-Exupéry
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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FLYaDIVE(at)aol.com
Guest
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| Posted: Sat Jul 15, 2006 1:20 am Post subject: Grounding lugs and transponder radiation |
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In a message dated 7/12/06 7:45:08 PM Eastern Daylight Time, n3eu(at)comcast.net
writes:
| Quote: | > The signal is radiated only from the shaft of the antenna.
A couple theoretical texts I have suggest it radiates only from the base
of
|
| Quote: | the antenna. Has to do with 90-deg phase lag, current and voltage, I
think.
|
| Quote: | The vertical element is there merely to resonate.
Dunno also, but the ball has been alleged to be either a capacity hat, or
to
|
| Quote: | have proverse effect on radiating pattern. One such allegation came from
a
|
| Quote: | retired, aircraft antenna engineer I know. But he thinks my questions are
often inane! [Wink]
Fred F.
==================================
|
Fred - Group:
If people think that the ball does not radiate cut it off. Better yet TOUCH
it and see if you get an RF Burn! Want to get a bit more scientific? Make up
or obtain a second antennas and check the SWR with and without the ball. SWR
will be higher without the ball.
As I stated before ... If it is metal and it is attached to the antenna it is
part of the radiating medium of the antenna.
As far as Lead Vs Lag ... Current Vs Voltage ... ELI the ICE man ... [The
previous are all theories taught in electronics and physics.] Who gives a
Dittlie-Womp. That is all theory in Capacitive Reactance - Inductive Reactance and
Wave Length. It only matters when obtaining the correct impedance for
matching the antenna to the transmission line and the transmission line to the
transponder (in this case). Bottom Line ... IT RADIATES. Check out SMITH Charts
and that other guy's charts I can't remember right now.
It is NOT a capacitive hat ... It is NOT a loading coil ... Again as I stated
it is for eliminating Corona Effect and the dissipation of static.
Barry
"Chop'd Liver"
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