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h&jeuropa
Joined: 07 Nov 2006
Posts: 645
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 Posted: Fri May 02, 2008 3:47 pm Post subject: Comm Antenna |
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I'm having a problem with my comm antenna. It is a common copper tape antenna with toroids over the coax. Materials and plans came from RST Engineering. Installed in the rudder per Europa directions and tuned using a SWR bridge.
When I transmit, certain frequencies cause my ACK ELT to transmit. It is located on the bottom of the fuselage touching the rear bulkhead (under the elevator torque tube). If I disconnect the cable going to the remote actuator, there is no problem. (I had this problem previously and added toroids to the four wires but no change).
Also, when I transmit, certain frequencies cause the magnetometer in my GRT EFIS to swing heading 20 degrees. The magnetometer is mounted on the port side of the fuselage essentially on the fuselage parting line and just at the rear of the D panel bulkhead.
Have others had difficulity with the dipole in the rudder causing problems? Found a solution? How about using other antennas, like Bob Archer?
Thanks
Jim Butcher
N241BW almost in the air again
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paul.the.aviator(at)gmail
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| Posted: Fri May 02, 2008 6:30 pm Post subject: Comm Antenna |
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Hi Jim,
I tossed the GRT supplied harness and used a pair of shielded cables.
I seem to recall that I used a 4 core and a two core. Any how I
grounded one end back to my central ground point, and for each cable I
used a small toroid. I was able to get an addition loop through each
one although it was tight.
My suggestion is that you try the same for your ELT wiring. I know
that you mentioned that you were already using toroids, but try an
extra loop through for each wire, and add a large one over the whole
cable. If that doesn't work, replace the 4 core flat telephone cable
with a 4 core shielded cable.
I have a bunch of toroids left, let me know if you would like me to
toss a few in the mail. BTW, I am using a Bob Archer antenna. It
isn't much better, lots of stuff goes nuts when I transmit.
Cheers, Paul
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craigb(at)onthenet.com.au
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| Posted: Fri May 02, 2008 9:48 pm Post subject: Comm Antenna |
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You might also try twisted pair cable, that too helps reduce crosstalk
between parallel conductors, you can get shielded twisted pair as well, I
had
simmilar effect in one of my cars once, High power AM cb radio with the
aerial
directly in line the the ignition coil, About 50% of the time the CB would
cause the engine to miss fire when you keyed the mike to transmit. I ended
up
turning the coil on its side to stop it happening. Thats why most countries
have minimum separations and crossing at right angle requirements where
phone/data
cables are close to high voltage AC lines.
regards
craig
--
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h&jeuropa
Joined: 07 Nov 2006
Posts: 645
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| Posted: Tue Jun 10, 2008 4:39 am Post subject: Re: Comm Antenna |
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I seem to have these problems pretty well solved. Initially tried shielded cables (shield grounded to A/C ground point near inst panel - unconnected at other end) with lots of toroids to the ELT and Magnetometer. Also made the interconnect between the Primary Flight Display and the MFD a shielded cable. Had little or no effect. Then tried just shielded cable (no toroids) to ELT and wrapped the entire ELT in aluminum foil. Foil is connected to the cable shield and ground is as before. That solved the ELT problem. From other forums I learned that there are D Sub filters which have capacitors built in bypassing each connection to the shell of the connector. Acquired Spectrum Control Series 100 filters, 5600 pf (from Newark Electronics) for the Magnetometer and the PFD (main display). At the Magnetometer I connected the cable shield to the D Sub shell since the shell is floating inside the Mag and I needed a return for the filter. Found the PFD filter didn't have any effect but the Mag helped some. Finally wrapped the Mag in aluminum foil which is connected to the shield and that seemed to solve the problems. I think there is so much RF from the com antenna and no shielding because of the composite, that it overwhelms the ELT and Magnetometer.
I also found a local ham with an antenna analyzer (MFJ Model 269) and had a look at my com antenna. Found it had very low SWR but the resistance of the antenna changed if I walked near the antenna and the resistance was not constant or nearly so across the 118 to 136 Mhz range. We both suspected the toroid balun just didn't perform properly. So we decided to make a balun from coax as the ham radio books have shown for years. It is very much like the one on the Aero Electric website except the length of the balun section is an electrical half wavelength, so you must take into account the velocity factor of the cable. Using RG400 (vf=.695), the balun section is 16 1/4" long. Took off the toroids and installed the new balun. Had to trim the antenna to get the SWR low near the center of the band, but now the SWR is less than 2:1 across the entire band, the resistance stays constant and walking near the antenna makes no difference.
Hope this helps someone else.
Jim Butcher
Europa N241BW
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