Our BC-121.5 DF Antenna

BC-121.5
$165.00 USD
MK4 Sniffer
$295.00 USD

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The BC-121.5 Antenna is a 3-element Yagi beam antenna. It is specifically designed to be a hand held, fold up, portable radio direction finding antenna for use on 121.5 MHz.

The BC-121.5 is designed to be compatible with a wide range of equipment, including both the older and newer models of the L-Per, the MK-4 Sniffer (shown below), and any hand held receivers (receivers may require an antennuator).

 

My objectives were that the antenna:

  1. Fold up neatly for storage
  2. Have a good front-to-back ratio
  3. Have no side lobes at resonance
  4. Have good forward gain
  5. Specifically resonant at 121.5 MHz
  6. Have a standing wave ratio (SWR) of less than 1:1.2 @121.5 MHz
  7. An X-factor of less than 3
  8. Have an impedance of 50 ohms + or - 2 ohms @121.5 MHz.
  9. Have a balanced feed point
  10. Have good adjacent frequency rejection.
The idea for this antenna came when I wanted to upgrade my direction finding arsenal with a precision 3-element hand held beam antenna to use with my L-Per or my MK4 Sniffer on 121.5 MHz. After talking to various antenna manufacturers, I was surprised to learn that none of the major companies made a 3-element beam specifically designed for the search frequency. They made antennas for the aircraft VHF band, but not for the person searching in the field.
Folded:
Unfolded:
After deciding what I wanted the antenna to do, I spoke with several experts in RDF to get their input, including Joe Moell, K0OV, who wrote the book, “Radio Direction Finding Simplified,” and who was a regular writer for 73 and CQ magazines; as well as Bob Thornburg, an RF engineer and transmitter hunter.
Using the EZNEC 4.0 antenna program to model and optimize the antenna, I then tuned it using an antenna analyzer.
Then I had a friend test the antenna at the Jet Propulsion Laboratory in Pasadena, California, to verify the antenna characteristics and see that the specifications were met.