80 m Radio “O” Receivers

Filed under: Latest News,Radio Orienteering by Joseph on November 30th, 2017

BOK received the first four of our 80 m ARDF (Radio “O”) receivers from Vadim.  BOK will be renting these receivers at our Radio “O” events. (Bring your own headset.) I put them through a test and they are excellent.

  • They are very lightweight, just under 9 ounces, which is important if you are holding it up and waving it around for an hour or two.
  • The sensitivity is excellent, I can hear my practice 50 mW transmitters from over 1k.  Typical competition transmitters transmit closer to 1 W.
  • The sense differentiation is exceptional – no volume change when you activate the sense switch when the receiver is pointing toward the transmitter and almost silent in the reverse direction.  On many receivers the difference is quite subtle.  It is even excellent close to the transmitter.  With some receivers when you are close to the transmitter the sense differentiation is lost because it is overpowered.
  • Very tight null discrimination.  When you are taking a bearing to a transmitter the signal is very quiet when you are looking through the loop, (as in the picture) and loud when you are holding the receiver sideways looking edge on to the loop.  The quiet signal, looking through the loop, is very narrow.  Just a couple of degrees off of perpendicular you get a noticeably louder signal so you can take an accurate bearing.  When you are looking edge on to the loop it is about the same volume plus or minus about 10 degrees, so an exact bearing is difficult to determine.  You would use this edge on orientation if you were far away and had a very weak signal.
  • Frequency drift is sometimes a problem with analog receivers.  This test was over an hour in cool weather and there was no drift.  This will be more critical when we test it in the summer.  Frequency drift requires that you re-tune the frequency knob from time to time.
  • Whoopie is an advanced feature on a receiver.  When the signal strength increases to a certain level you can hear a series of clicks.  As the signal increases further the clicks come faster and faster until when you are really close it is a screech.  (Waiving the receiver back and forth when you are getting close the sound goes from clicks to screeches, and thus the name Whoopee.) As you turn down the gain (volume) it will go back to being clicks again.  To set this up to measure distance you make marks on your gain knob to show where the clicks begin at different distances like 500, 250, and 100 meters.  This will give you an approximate distance to the transmitter.  Since different transmitters have different power outputs you need to calibrate your marks when you are using different transmitters.  This is why at major events there is always a practice period for people to experience the characteristics of the transmitters that are being used.  The distance is just approximate because the density of foliage, moisture on the leaves and in the air, differences in antenna setup, and topology between you and the transmitter all affect the measurement.  You also need to be certain that the frequency setting hasn’t been bumped a little or drifted.  With my 50 mW transmitters the whoopee starts at about 100 m.
  • The frequency knob is very sensitive so a tiny movement makes a big change.
  • The receiver uses a standard 9v rechargeable Li battery and has a socket for the cord that snaps to a standard Li 9v charger.  When it begins to lose power in a couple of years it will be easy to replace the battery ourselves.
  • The receivers use a standard cell phone headset.  That is a 3.5mm plug with 3 contact rings (2 black stripes).  The headsets with microphones have 4 contact rings (3 black stripes) and will not work.  When you rent one of the receivers you should bring your own headset.

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