Antenna

Since my article was released I have done some homework on antenna design.  This came about primarily from an interest in using RF for directional beacons.  Good reference material for this comes from the HAM radio community.

As a result I constructed new antenna for my PC base station and my temperature transmitter board.  So far operation has been rock solid (based on 100' separation, outdoors, through some trees) so I'm assuming I did the right thing.  Here are photo's of the two installations.  The tripod mounted antenna is called a "ground plane" design (from a HAM radio book).  The 4 legs create a ground plane which helps divert more energy horizontally toward the receiver.  A plate attached to the bottom of the temperature transmitter antenna would help this as well.  The RF module folks also have good literature on the subject.  If you want directionality then you need to go to dipole or Yagi designs. Use 50 ohm coax if possible, most is 75 ohm which is a compromise.  The antenna length is measured from the point it protrudes above the metal sheath of your coax fitting.  The length of the legs in the tripod design is important as well, should be 90% of antenna length, set at 45 degrees, 4 legs.  Make sure the antenna is vertical - any offset reduces performance.

My original antenna were simply copper wires attached to the board.  This did not work as well as the new ones, based on lost packet count.  It's difficult to quantify the performance difference without an S meter, however.  I plan to build one using the RSSI (Received Signal Strength Indication) output from the latest module I purchased.

I still have a dilemma re sharing the antenna between the transmitter and receiver.  This is not supposed to be a good thing but it's much simpler than two antenna.  I'll do some testing when I get my RSSI working.  The transceivers all have a built in antenna switch whcih handles this problem.

Base station                                                                   Temperature Transmitter