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Mark Phillips in TN
Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN
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 Posted: Sun Jan 18, 2009 3:49 pm Post subject: Feedline Radiation in Composite Airplane |
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I'm trying to help a builder kill the dragon in the subject line-
Specifics: Garmin SL-30/40 with RG-400 to dipole on vertical stab spar
immediately in front of rudder, terminated with ring
connectors/screws/nuts. Coax terminated at radio using the supplied 90
deg. Garmin/Apollo fitting and runs with other wiring through center
floor tunnel to rear of plane, passing within about 12" from pitch
servo. Dipole is two aluminum bars, 1/2"x1/16"x20" long for each
element (I know this is a bit short- packaging issues. Optimum would
be about 44" overall length, I believe)
On com transmission, several systems are affected- Ray Allen trim LEDs
dim, indicated EGT/CHT temps rise (EIS), and most excitingly, the
autopilot (TruTrak) will occasionally re-direct the aircraft if
engaged, usually in a pitch-up of variable amounts depending on
frequency transmitting on. Usually worse at lower com freqs.
My own research (Wikipedia has a pretty good article on dipole
antennas(ae?) with several balun examples at end of article:
http://en.wikipedia.org/wiki/Dipole_antenna ) indicates the coax is
radiating lots of nasties off the shield, which I assume is making the
mischief.
Material from the Aeroelectric site (
http://www.aeroelectric.com/articles/BALUN/Balun_Fabrication.html )
describes a balun for nav- is this same approach suitable for com?
We've tried several ferrite ring arrangements, clamped around coax at
various locations, rings near the antenna end etc. Most effective has
been using a "ring", actually a small block that has two parallel holes
through it- the center conductor at the antenna end is passed through
one hole and back through the other in a tight "U" turn then connected
to the element. I suspect this is helping simply by reducing the
overall power output as it warms to the touch after a few transmissions
by absorbing some of the RF energy in the conductor?
A complicating factor may be that the rudder is attached by a contiuous
stainless steel hinge pin that parallels the antenna full-length a bit
over one inch away from it, but even with this pin removed, ground
testing does not seem to indicate much of a change in the symptoms.
Perhaps separating the upper and lower half of this pin and connecting
the coax to them as the antenna elements might be a neat experiment?
Also read
http://www.aeroelectric.com/articles/Morris_Com_Loop_Antenna.pdf and
curious if anyone can comment. This would fit into the tailcone
nicely, but place the antenna loop within about 12-14" of the
magnetometer and pitch servo.
Any advice/insight/suggestions appreciated!
Mark
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jetboy
Joined: 22 Jul 2006
Posts: 233
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| Posted: Sun Jan 18, 2009 5:01 pm Post subject: Re: Feedline Radiation in Composite Airplane |
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At least one of the elements should be close to 24" so if you can extend the length of the lower one - it does not matter whether its laid over by 90 degrees or zig zag along the length to make room.
as you have found ferrites on the actual conductor will not work well. You may need 2 or 3 ferrites over the coax cable at the dipole end to decouple the cable from being a radiator. Your best friend here will be a reflectometer or someone who has one and knows how to use it. Otherwise known as SWR meter or directional wattmeter like Bird model 43 or a simpler example for the amateur radio 2 meter band.
The rudder hinge may also be a problem if it is longer than 24' and too close to the antenna. i would not try to use it as a radiator unless you know that the composite resins will not burn up from the heat. I do assume the composite is f/glass, not carbon?
Its also possible for the instruments and servos to be getting interference directly from a properly connected antenna, if it is situated too close - on some certified aircraft I've worked with that can be as much as 10' away.
Once the antenna VSWR has been confirmed in limits across the frequency range (certified antennas often incorporate a 12.5 ohm resistor inline to achieve this) you can go chasing affected servos and instruments to add ferrites to these, as a last resort.
Ralph
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Ralph - CH701 / 2200a |
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h&jeuropa
Joined: 07 Nov 2006
Posts: 645
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| Posted: Mon Jan 19, 2009 5:11 am Post subject: Re: Feedline Radiation in Composite Airplane |
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Had a similar problem with our Europa. Made a balun similar to the AeroElectric you mentioned. Also tuned the antenna using a Antenna Analyzer.
Contact a local ham radio club to see if anyone has an Analyzer. The balun that Bob describes, I found most references say that the the balun stub needs to be an electrical quarter wave, so you have to shorten by the velocity factor of the coax.
Balun will probably help a lot. I never could get ferrite beads to do much. I also shielded some items with aluminum foil that is grounded to the aircraft ground point and used shielded cable for wiring.
Jim Butcher
Europa XS N241BW
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Mon Jan 19, 2009 10:59 am Post subject: Feedline Radiation in Composite Airplane |
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At 05:48 PM 1/18/2009, you wrote:
| Quote: |
I'm trying to help a builder kill the dragon in the subject line-
Specifics: Garmin SL-30/40 with RG-400 to dipole on vertical stab
spar immediately in front of rudder, terminated with ring
connectors/screws/nuts. Coax terminated at radio using the supplied
90 deg. Garmin/Apollo fitting and runs with other wiring through
center floor tunnel to rear of plane, passing within about 12" from
pitch servo. Dipole is two aluminum bars, 1/2"x1/16"x20" long for
each element (I know this is a bit short- packaging issues. Optimum
would be about 44" overall length, I believe)
On com transmission, several systems are affected- Ray Allen trim
LEDs dim, indicated EGT/CHT temps rise (EIS), and most excitingly,
the autopilot (TruTrak) will occasionally re-direct the aircraft if
engaged, usually in a pitch-up of variable amounts depending on
frequency transmitting on. Usually worse at lower com freqs.
My own research (Wikipedia has a pretty good article on dipole
antennas(ae?) with several balun examples at end of article:
http://en.wikipedia.org/wiki/Dipole_antenna ) indicates the coax is
radiating lots of nasties off the shield, which I assume is making
the mischief.
Material from the Aeroelectric site (
http://www.aeroelectric.com/articles/BALUN/Balun_Fabrication.html )
describes a balun for nav- is this same approach suitable for com?
We've tried several ferrite ring arrangements, clamped around coax
at various locations, rings near the antenna end etc. Most
effective has been using a "ring", actually a small block that has
two parallel holes through it- the center conductor at the antenna
end is passed through one hole and back through the other in a tight
"U" turn then connected to the element. I suspect this is helping
simply by reducing the overall power output as it warms to the touch
after a few transmissions by absorbing some of the RF energy in the conductor?
A complicating factor may be that the rudder is attached by a
contiuous stainless steel hinge pin that parallels the antenna
full-length a bit over one inch away from it, but even with this pin
removed, ground testing does not seem to indicate much of a change
in the symptoms.
Perhaps separating the upper and lower half of this pin and
connecting the coax to them as the antenna elements might be a neat experiment?
Also read
http://www.aeroelectric.com/articles/Morris_Com_Loop_Antenna.pdf and
curious if anyone can comment. This would fit into the tailcone
nicely, but place the antenna loop within about 12-14" of the
magnetometer and pitch servo.
Any advice/insight/suggestions appreciated!
|
It's not likely that you're suffering from the effects of
feedline radiation.
Most avionics devices for the TC aircraft side of the
house have deep roots in designs optimized for metal
aircraft. A conductive airframe offers considerable
isolation between potential victims inside the airplane
from energies emitted by antennas outside the airplane.
With the advent of composite aircraft, our brothers
have found it prudent to qualify their products at
much higher levels of radiated susceptibility.
Virtually every instance in my experience for mitigating
a radiated susceptibility symptom involved combinations
of reduced radiation (move victim/antenna further apart)
or improving on the victim's ability to stand off the
more aggressive antagonist.
Ferrite beads over a coax are almost useless for
de-coupling the shield radiation of a poorly terminated
coax. I witnessed a demonstration in the lab wherein
a technician first terminated a 50 ohm coax with a
200 ohm load (4:1 swr). You could watch the swept
frequency SWR display and witness a small change in
presentation when you grabbed the coax with your hand.
This happens only if there are components of the
feedline energy flowing on the outside of the coax.
He added a half dozen donuts to the feedline right
at the end. There was no discernable benefit for
having added the beads. He then put all beads he
had on the coax . . . something over 20 pieces. You
you see SOME benefit but it was still not zero.
But in any case, the ratio of energy radiated
from a poorly matched coax versus radiated from
the physical antenna is huge. It's most likely
that you're suffering from the effects of sitting
inside sphere of strong radio frequence energy.
Adding a balun would not hurt anything.
http://www.aeroelectric.com/Reference_Docs/Antenna/BALUN_Analysis.pdf
http://www.aeroelectric.com/articles/BALUN/Balun_Fabrication.html
Note that this balun does NOT use energy carried
inside the coax as part of the reactie network.
Thus you'll note that dimensions for the 1/4 wave
balun element and the antenna element are the same
length (open-air 1/4-wave).
Shorten the 26" dimensions to 23" for the comm
antenna. Then adjust ends of antenna elements for
lowest swr in center of range of interest (125 mhz).
Now, assuming this does not produce the hoped-for
result, you'll need to look at individual victims for
ways they can be made more tolerant of the environment
in which they're expected to perform.
Bob . . .
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