Sunday, March 17, 2013

Ch 7: Bottom Antennas

About Antennae
Electronic navigation and communication require antennae.  The great thing about composite, or plastic, airplanes as they are called, is that you can embed many of the antennae under the skin of the aircraft.  Since ATC radar can see right through foam and fiberglass, it doesn't matter that the antennae aren't mounted on the exterior like they need to be for metal aircraft.  The benefits are a nice sleek looking airplane with no unsightly antenna protrusions and less drag since there's nothing hanging out in the air stream to slow this bad boy down.  It's certainly not much drag but you would be amazed at how all the small sources of drag can really add up.

RST Antenna Kit
Per the plans, I'm installing RST designed copper tape antennae except instead of using RG-58 coax, I'm using RG-400 throughout the entire aircraft.  You can find several recommendations in the archives for it.   RG-400 is similar 50 ohm coax that has replaced RG-58 as the standard for aircraft antenna feed lines   It has a stranded center conductor and is double shielded which is needed for the transponder and GPS transmit lines...oh yea...and quite a bit more expensive.  The best price I found was at B&C Specialty at $2.55/ft or $2.35/ft if you buy more than 50ft.

I'm installing the following systems that will have antennae "buried" into the structure.
- Dual radios - 1 antenna in each of the vertical winglets at the end of the wings
- Dual Navs - 1 antenna in the canard, 1 antenna in a wing
- Marker Beacon - 1 antenna on the fuselage bottom
- Glide Slope - 1 antenna on the fuselage bottom
- APRS - Automatic Position Reporting System used for flight tracking via the internet - 1 Ant in the wing

Other non-copper tape antenna that will be installed...
- Transponder - Installed in the nose
- GPS - probably in the nose

Fuselage Antennae
MB Solder Joint
On the bottom of the fuselage, I'm installing the marker beacon (MB) and glide slope (GS) antenna.  After determining where to install them, I oogered out the channels in the foam for the coax keeping in mind RSTs guidance on proximity and size of metal objects.  You need to pay attention to where the nose gear will retract and the landing light if you're using the plans design...which I'm not.  Then soldered the coax to the copper tape of appropriate length based on RSTs instructions.  After everything is installed, I potted the solder joint and filled the channel with micro.

MB & GS Antennae trial installed just before
permanent install and fill.
GS Solder Joint
To the left is my antenna layout before final install.  The marker beacon is on the right and glide slope in the center.  I punched the coax through at a point just fwd and to the pilots side center of the instrument panel.  Be sure avoid the wheel well area.

Note the oogered channel running diagonal back to the front of the IP...I changed the routing of the MB coax to keep the appropriate distance from the tip of the GS antenna and any metal.  I'm not sure if I had to do this with feedlines but I figured I would play it safe.  You can see the re-routed coax now comes toward the front more and then over to the punchdown.

TIPs:  You'll need some tools.  Based on a recommendations from an RV builder site, RST, and other internet locations, here are the tools I purchased.  Coax stripper: I bought a Paladin 11255-LC CST and it worked great.  BNC crimper: I purchased an Eclipse Lunar 300-054 crimper frame and the 300-038 die.  For soldering supplies I got Kester 44 63/37 solder.

Now for the fun...time to glass the bottom.  Onward.  cab

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