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43′ Vertical Matching Network Debugging – con’t

 

43′ Vertical Matching Network Debugging – con’t

Filed in Antennas on Nov.02, 2010

I got the matching network back on the bench tonight.  I started by confirming that the Unun leads are as I expected.
I hooked a 220 ohm resistor across what I thought was the high impedance side, and connected the Rx Noise Bridge across the low impedance side.  I read very close to 50 ohms with zero reactance on the low impedance side.  So the Unun is working correctly.

Next, I wired the Unun back into the matching network.  Again, hooking the 220 ohm resistor across the output, I measured the input: Nearly ZERO ohms!  (This was with the relays de-energized, so the coils were out of the circuit.

So I suspected either the caps between the input and the Unun, or the choke for the relay power.  I disconnected both, and temporarily hooked up only the caps on the input to the Unun.  Aha! back to very close to 50 ohms through the caps to the low side of the Unun.  That means the caps are not the problem.

 
Testing the Isolation Capacitors on the input to the Unun

 
50 ohms resistance

 

 
Zero ohms reactance

So now I hooked the AADE LC meter across the choke.  What’s this?? 0.04 uh?? That seems a bit low for a 1 mh choke.

 
Low inductance choke reading

The formula for inductive reactance is Xl = 2*PI*F*L, where F = Frequency in hz, and L = inductance in Henries.  So at 160 meters (1.9 mhz) a 1 mh choke would have about 12K of inductive reactance.  But a 0.04uh coil would only have about a half ohm of reactance.  Any RF going to that would pass right through, like a piece of wire.  At 10 meters, it would be even worse. Normally 180 k of inductive reactance is instead 7 ohms.

So I suspect that when the wiring was incorrect, and we transmitted into it, the choke took the brunt of the 10 watts we were using to tune up.  It probably heated up and melted the enamal coating, shorting the wire turns together.

 
Bad Choke

So the next step will be to replace the choke, and make some more readings, eventually energizing the relays and using the noise bridge to determine a starting point for the taps on the coils on 160 and 80 meters.


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