document the fintenna design

diff --git a/rockets/airframes/YikStik3/index.mdwn b/rockets/airframes/YikStik3/index.mdwn
index 4d696e487a26dba4cd1381e3833e2ddb3a7d6cc8..63c124a37472f8f1091499dfed619fa324555b29 100644 (file)
--- a/rockets/airframes/YikStik3/index.mdwn
+++ b/rockets/airframes/YikStik3/index.mdwn
@@ -67,6 +67,44 @@ with other capabilities.
 Installing the electronics in the fin can bay is a *huge* challenge due to the
 restricted space between the MMT and airframe.
 
+### Fintenna ###
+
+On 2YikStik, one of the big disappointments was a very poor signal from the
+UHF transmitter on the TeleMetrum board mounted in the fin can.  This isn't
+surprising, since the antenna was just a wire whip running in parallel with 
+and very close to the motor casing.  For this build, I decided to try 
+something completely different.
+
+The quick summary is that I'm loading the two fins that don't have thermistors
+in them as an antenna.  And it works brilliantly!
+
+Before bagging the fins with the CF and glass lamination, I put 1 inch wide
+copper foil tape on two of the fin substrates, running from near the tip
+down to the root.  Then the fins were bagged with 2 layers of 5.7 oz 2x2
+twill carbon fiber and a 6 oz glass sanding veil layer on both sides, all in
+one bagging operation to ensure all the layers formed a strong chemical epoxy
+bond.
+
+After unbagging, while the epoxy was still somewhat leathery, I carefully 
+cut the CF back at the root edge of each fin exposing the copper foil.  The
+fins were then allowed to cure and get sanded normally.  When installing them
+in the airframe, I oriented them so that the thermistor fin is on the anti-rail
+side of the airframe, and the two copper-foil-equipped fins bracket the rail
+side, with the copper foil sides facing each other across the rail-side valley.
+
+After applying fin to fin glass across the motor mount, additional copper foil
+tape was used to join the two fins, soldering the new foil to the foil embedded
+in the fins.  I then cut a "gap" in the center of the space between the two
+fins, and installed a 1.5-20pF piston trimmer capacitor with the screw aimed
+aft, and a piece of small diameter teflon coax with two ferrite beads on the
+forward side of the gap.  The coax was run through a hole near the root of
+the fin separating this valley from the ebay and terminated with an SMC 
+connector to mate with the TeleMetrum board.
+
+I'm pleasantly surprised at how well this seems to work.  With a Pro75 6xl
+case installed, the variable cap allows the antenna to be tuned to about 1.3:1
+SWR, and the resulting transmitted signal seems strong...
+
 ## Build
 
 Parts gathered during late 2011 and early 2012.  Built underway in early