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klehman(at)albedo.net
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 Posted: Thu Sep 07, 2006 6:28 am Post subject: Transponder antenna location |
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Has anyone tried mounting a transponder antenna on the bottom of the
rear fuselage? That location (about 2/3 of the length of the fuselage
back) slopes upward on a Rebel but it is the location least blanked by
large gear fairings, and metal radiator ducting. Unfortunately the worst
signal would likely be forward to where the ground station I'm trying to
reply to is likely to be during first contact. I am willing to mount the
transponder behind me to keep the coax within the 8.8 feet max specified
by Garmin.
The other option seems to be on the roof. That is apparently not
recommended and it would be near skylights and my head which I'm not
comfortable with. I could get it the minimum recommended 3 feet away
from the VHF antenna but the high wing might tend to blank ground
stations to the side. I suspect that roof mounting would provide a
better signal to other aircraft which is probably more important to me
than a signal to ground so I guess I could put the antenna on the roof
back near the tail. It seems silly to invest in a transponder though
unless it is likely to perform well with both ATC and also traffic
warning devices on other aircraft.
thanks for any comments
Ken
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nuckollsr(at)cox.net
Guest
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| Posted: Thu Sep 07, 2006 7:52 am Post subject: Transponder antenna location |
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At 10:28 AM 9/7/2006 -0400, you wrote:
| Quote: |
Has anyone tried mounting a transponder antenna on the bottom of the rear
fuselage? That location (about 2/3 of the length of the fuselage back)
slopes upward on a Rebel but it is the location least blanked by large
gear fairings, and metal radiator ducting. Unfortunately the worst signal
would likely be forward to where the ground station I'm trying to reply to
is likely to be during first contact. I am willing to mount the
transponder behind me to keep the coax within the 8.8 feet max specified
by Garmin.
The other option seems to be on the roof. That is apparently not
recommended and it would be near skylights and my head which I'm not
comfortable with. I could get it the minimum recommended 3 feet away from
the VHF antenna but the high wing might tend to blank ground stations to
the side. I suspect that roof mounting would provide a better signal to
other aircraft which is probably more important to me than a signal to
ground so I guess I could put the antenna on the roof back near the
tail. It seems silly to invest in a transponder though unless it is
likely to perform well with both ATC and also traffic warning devices on
other aircraft.
|
Any place on the bottom would be preferable to top mounted.
The aft fuselage location you cited would be fine.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
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gilles.tatry(at)wanadoo.f
Guest
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| Posted: Sat Sep 09, 2006 5:34 am Post subject: Transponder antenna location |
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|
Should the bottom of the front fuselage, 2 ft behind the engine, be a proper
location?
Is it a problem to be so close to the engine, isn't it far better at the
rear?
Thanks,
Gilles
<nuckollsr(at)cox.net>
At 10:28 AM 9/7/2006 -0400, you wrote:
>
>
>Has anyone tried mounting a transponder antenna on the bottom of the
rear
>fuselage? That location (about 2/3 of the length of the fuselage
back)
>slopes upward on a Rebel but it is the location least blanked by
large
>gear fairings, and metal radiator ducting. Unfortunately the worst
signal
>would likely be forward to where the ground station I'm trying to
reply to
>is likely to be during first contact. I am willing to mount the
>transponder behind me to keep the coax within the 8.8 feet max
specified
>by Garmin.
>
>The other option seems to be on the roof. That is apparently not
>recommended and it would be near skylights and my head which I'm not
>comfortable with. I could get it the minimum recommended 3 feet away
from
>the VHF antenna but the high wing might tend to blank ground stations
to
>the side. I suspect that roof mounting would provide a better signal
to
>other aircraft which is probably more important to me than a signal
to
>ground so I guess I could put the antenna on the roof back near the
>tail. It seems silly to invest in a transponder though unless it is
>likely to perform well with both ATC and also traffic warning devices
on
>other aircraft.
Any place on the bottom would be preferable to top mounted.
The aft fuselage location you cited would be fine.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
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FLYaDIVE(at)aol.com
Guest
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| Posted: Sun Sep 10, 2006 3:14 am Post subject: Transponder antenna location |
|
|
In a message dated 9/9/06 9:37:19 AM Eastern Daylight Time,
gilles.tatry(at)wanadoo.fr writes:
| Quote: | Should the bottom of the front fuselage, 2 ft behind the engine, be a
proper
|
| Quote: | location?
Is it a problem to be so close to the engine, isn't it far better at the
rear?
Thanks,
Gilles
==================
|
Gilles:
Keep it away from the exhaust gases. Keep it away from the breather tube.
Place it as perpendicular to the to the earth surface as possible and where it
sits in the center of at least 1 Sq. Ft of aluminum surface. And as close to
centerline as possible. ON THE BELLY!
Basic rule:
If the base station you are trying to communicate with is on the ground then
the antenna should be ON THE BELLY.
The exception to the rule is COM#2, there the antenna should be ON THE TOP.
The reason for that is, COM#2 is used to communicate while on the GROUND and
the base station antennas are UP above the plane. You don't want to transmit
into the ground.
Examples (ON THE BELLY):
COM#1
XPONDER
ADF
LOC/GS
RNAV
and VOR, yet 99% of VOR antennas are mounted on the rudder. [Not on my RV-6
.. On the Belly at the tail]
Examples (ON THE TOP)
COM#2
GPS [Gee, I wonder why?]
ELT [Gee, I wonder why?]
Hope this helps.
Barry
"Chop'd Liver"
"Show them the first time, correct them the second time, kick them the third
time."
Yamashiada
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nuckollsr(at)cox.net
Guest
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| Posted: Sun Sep 10, 2006 7:46 am Post subject: Transponder antenna location |
|
|
At 03:31 PM 9/9/2006 +0200, you wrote:
| Quote: |
<gilles.tatry(at)wanadoo.fr>
Should the bottom of the front fuselage, 2 ft behind the engine, be a
proper location?
Is it a problem to be so close to the engine, isn't it far better at the rear?
Thanks,
Gilles
|
I guess I don't know how to quantify "proper". Everything in
the choice of system architectures is a trade off. As an antenna
guy I could demand of my fellow airplane builders that they provide
me with installation conditions aboard the airplane that approach
the idealized antennas I can build and fully test in the lab. By
the same token, gurus of other disciplines could impose similar
constraints that make their installations look good too. The
end result is a product that performs poorly and looks so bizarre
that nobody would want to be seen flying it.
It's not in the spirit of compromise that we work together to
craft a product that performs adequately -AND- attracts customers.
It a goal of "optimized" performance given a relatively inflexible
platform. Once optimization is achieved, then performance needs to
be evaluated for adequacy to the task.
In the case of antennas, adequacy to the task is dependent
on the user's mission requirements. I've seen relatively
mediocre antennas slammed by pilots who routinely needed
to talk to RCO's on the horizon while others who only talked
to other airplanes and controllers in the airport control area
thought it worked "great".
In the case of transponder antennas, the manufacturer will suggest
coax length limits based on perceived reductions in the optimized
performance of his product (usually the receiver side) to "hear"
that very strong interrogation pulse shot out to the aircraft from
the ground based radar afar.
In the instance were considering, moving the antenna say 5' further
aft might reduce receive sensitivity by some factor we can deduce
by use of a coax loss calculator found at:
http://www.timesmicrowave.com/cgi-bin/calculate.pl
Plug in RG-400, 1050 Mhz and and a RUN of 6'. We find
that a typical installation has an attenuation of about 0.9 db.
Now, plug in 11' for an installation further aft and we get
1.7 db or an ADDITIONAL 0.8 db of attenuation.
If one chooses to worry about this, we could go to
one of the super-sexy coaxes like LMR-400 for a 6 foot
loss of 0.3 db and 11 foot loss of 0.5 db . . . still better
than 6' of RG-400.
Now, suppose we went flying and did some testing of our
transponder's ability to be "read" by ground stations
and could switch back and forth between an idealized
installation per the book: "right under the pilot's
seat antenna and 6' of LMR-400" and the "compromised
location and 11' of RG-400".
We're talking about such small changes in the grand
scheme of things as to make differences detectable in anything
less than a precision antenna lab environment. The guy
on the ground and the guy in the air will have zero
probability of telling which is the "worse" antenna
buy observing behavior of the electro-whizzies in front
of them.
Bob . . .
| Quote: |
<nuckollsr(at)cox.net>
At 10:28 AM 9/7/2006 -0400, you wrote:
>
>
>Has anyone tried mounting a transponder antenna on the bottom of
the rear
>fuselage? That location (about 2/3 of the length of the fuselage back)
>slopes upward on a Rebel but it is the location least blanked by large
>gear fairings, and metal radiator ducting. Unfortunately the worst
signal
>would likely be forward to where the ground station I'm trying to
reply to
>is likely to be during first contact. I am willing to mount the
>transponder behind me to keep the coax within the 8.8 feet max
specified
>by Garmin.
>
>The other option seems to be on the roof. That is apparently not
>recommended and it would be near skylights and my head which I'm not
>comfortable with. I could get it the minimum recommended 3 feet
away from
>the VHF antenna but the high wing might tend to blank ground
stations to
>the side. I suspect that roof mounting would provide a better signal to
>other aircraft which is probably more important to me than a signal to
>ground so I guess I could put the antenna on the roof back near the
>tail. It seems silly to invest in a transponder though unless it is
>likely to perform well with both ATC and also traffic warning
devices on
>other aircraft.
Any place on the bottom would be preferable to top mounted.
The aft fuselage location you cited would be fine.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
|
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
| | - 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:
http://www.matronics.com/Navigator?AeroElectric-List |
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