Antenna Filter
(If you don't have a spectrum analyzer scroll down.)

Description: The antenna filter is a filter that notches out a frequency between 87.5 and 108 MHz.

Note:  You must use a general purpose VHF transistor in this curcuit on the NPN transistor labeled "VHF".

1.  Build the circuit on the right, and attach a signal generator to the antenna input.

Note: connecting the signal generator directly may overload the AF stage.
(If so just unplug the probe and attach two separate wires from the antenna side of the filter and the probe.)

Note: changing the 22 Ohm resistor adjusts the RF input level, remove it for max, however if it is too high it will feed back into itself and become an oscillator.

2.  Attach a spectrum analyzer to the output of this circuit and set a range close to 87.5 - 108 MHz.

3.  Adjust the gang and the signal generator, there should be an adjustable peak between 87.5 - 108MHz (if the local oscillator drowns the AF out, disable it, and re enable after this .)

4.  Turn the gang to it's lowest point and spread out the five turn coil off the collector until it peaks just below 87.5 MHz.

5.  Turn the gang to it's highest point and adjust the signal generator just above 108 MHz, adjust the trimmer capacitor on the back of the gang until the frequency is just above 108 MHz.

6.  Repeat step 4 and 5 until the range is a little wider than 87.5 - 108MHz.

7.  Connect the antenna filter to the mixers AF input.

8.  Connect a 3 foot wire to the antenna input as shown.

Tracking accuracy sync alignment
(and alignment without a spectrum analyzer)

1.  Connect a oscilloscope probe to the output of the third IF (0.1 us 0.001 Volts).

2.  Turn the gang all the way to the bottom and starting up try to tune some station that you can see on the oscilloscopes display (if the station is too powerful try taking a measurement off the second IF, or first).

3.  Expand and collapse the 5 turn coil off the collector in and out with a plastic tuning tool until maximum envelope is reached. (You may have to adjust the gang while doing this)

4.  Turn the gang all the way to the top and start down and try to tune a station.

5.  Adjust the trimmer capacitor on the back of the gang until maximum envelope is reached.

6.  Repeat 4 and 5

Realignment and Testing

1. See if you can get FM static from the speaker when there is no channel tunned by adjusting the volume and detector gain.

A. If you can, move on to detector fine tuning

B. if you can't:

A. Try realigning the radio or see construction tips, your radio might be feeding back into itself.

Ratio Detector Fine Tuning

1. Turn the detector gain all the way down and turn the volume all the way up.

2.  Slowly turn up the detector gain until you can hear FM static.

3.  Adjust the bottom transformer for minimum static (You may need to turn up the detector gain after this).

4.  Adjust the top transformer for maximum static

5.  Repeat from step three until it is perfect.

While turning up the detector gain now the static should slowly now come up.

The radio should now function normally but be a little difficult to tune because there is no AFC, the next steps are optional features of the radio.

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