|PA8W Amateur Radio|
the PA8W V2,3 Radio Direction Finder
the PA8W doppler RDF is absolutely no rocket science.
There are some points however that need a little attention to achieve the best possible results.
First of all we have to realize that the doppler principle only works in conjunction with a FM receiver.
It can succesfully be used on AM or other signals as well, but only with the receiver in FM mode.
So if you want to track an AM station, you tune in to that station precisely with the receiver in AM mode and the RDF in antenna test mode.
Having done that, switching the RDF to tracking mode and the receiver to FM will give you the bearing of that signal.
Tuning during tracking mode can be done as well but it will take a little more practice because the 500Hz doppler tone together with the stations modulation will be a little more difficult to judge.
The 500Hz doppler signal will sound "spiky" at both edges of the signal, and a little more round and dark when properly tuned.
Proper tuning of the radio is rather important. When you are off frequency too much,
the audio of your radio will be distorted because parts of the FM signal will run out of the passband, and therefore the RDF will give a faulty bearing.
Normally, there's a few kHz of tolerance before the bearing really starts to show serious errors, but it's a good habit to try to be right on spot tuning the receiver.
After determining the exact carrier frequency, you can investigate what tuning tolerances can be accepted,
simply by tuning carefully up and down until the bearing starts to drift.
Doing this will give you an idea about the tolerances that your receiver can cope with.
So let's walk through the hookup and calibration process step by step:
Plug the array control cable into the RDF and plug in the 12V supply.
Connect the array coax to your FM receiver and listen to a known station using normal audio volume. Be sure you are tuned to the station precisely as described above. Feed the audio from the receiver into the RDF,
If you use your receivers earphone socket the built in speaker will be switched off. Therefore, I equipped my RDF with an on-board speaker to make the audio audible again. The speaker switch on the RDF has to be in the ON position of course.
Set the Mode switch to the check position; the four antennas will be selected in a slow sequence so you will have normal reception only with small changes in field strength when the next antenna is selected.
This is normal, and generally due to signal reflections.
Each antenna will be selected for about 0,5 seconds before the next one is selected.
If that sounds ok then switch the test switch to the RDF position; normal operation wil start.
You will hear the 500Hz doppler tone.
Set the Threshold to maximum.
Set Q to approx. 1/2 of maximum.
These settings should now yield a solid bearing on the LED pelorus display.
But it will not be the right bearing already, we will have to calibrate first.
Just turn the Calibrate knob until the bearing is correct for your known station.
If this is impossible, switch the phase reverse switch to the other position. This shifts the calibration 180 degrees. Now you should be able to finish calibration.
You could check calibration on one or two other known stations and if ok then you're ready to track the direction of unknown stations as well.
Receiving a good signal you can turn the Threshold fully counterclockwise, and then slowly clockwise a little beyond the point where the corresponding red LED will go dark.
When te signal dissappears, the LED should light and the pelorus should freeze on the last received bearing.
Be careful if you use a transceiver for the RDF:
If you hit the transmit key, your array switch may be history...