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golfguruPosted - 4 December 2012 16:6  Show Profile  Email Poster  Edit Message  
No diode - ac voltmeter across inductor:
(r) = 19 ohms (80%)

Diode with 180k resistive load:
(r) = 19 ohms (80%)

R = ~76 ohms

Mmmm ... you beauty ... antenna Z of local (963kHz) might be around 75 Ohms - matches my test gear impedance and coax test leads.

.................

Edited by - golfguru on 12/4/2012 4:14:03 PM

golfguruPosted - 6 December 2012 15:44  Show Profile  Email Poster  Edit Message  
Mmmm.... the info in this thread migrated to my other "antenna resonance" post.

Back here now.

Don't know about this app now:
http://ngsir.netfirms.com/englishhtm/RLC.htm
It indicates that the voltage across the inductor halves with a doubling of the series resistance.
Might have something to do with still having the antenna C hanging across the A/G terminals in my situation?

It would appear, if Zim's calculation of my antenna/ground /Z/ (57.5 Ohms) is correct, that my original premise of reducing the voltage across the inductor by 70.7% with an additional series resistance was correct.

Anyway, moving on:
In my particular situation I was able to measure the same "70% drop in voltage" across a 120kOhm resistor in series with a 1N34A diode (as the load across the coil) when inserting a 56 ohm resistor in series with ground. 120K seemed to be a "sweet-point load". More, or less, resistance dropped the variation by a few %.
The high load R is an advantage in that the voltage is increased by (say) 6 times, making measurements more accurate.

I included a capacitor in the series tank this time to make it easier to peak signals.

Measuring the RF ac across the coil without a load would be the best method but it requires a high impedance RF voltmeter. My "Ballantine 303" has 10Meg input Z and is good to a few MHz.

Measuring the 70% drop across the 120k, noninductive resistor, as the diode load, at DC, using a digital voltmeter is within the average hackers reach.

Points to watch?

* The method only shows the apparent /Z/ at the tuned frequency.

* Different diodes may require different loads to be accurate - might even vary with frequency or signal power.

Others may like to comment and/or run some of their own tests at different frequencies which I am not able to do with my lack of stations (1 only).

....................

PS: Zim's resistor substitution method may be the most accurate "hacker" method if a simple formula could be produced.

...............

Edited by - golfguru on 12/7/2012 1:06:28 AM

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