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|golfguru||Posted - 4 October 2012 4:57 |
Just did a little test on variable inductance tuning in a basic "series" single tuned setup versus a series LC setup.
A 1N34A in series with 4k phones was placed across the coil in both cases. Voltage was read across the phones.
This represented a power loss of 8.9dBm (87%). This would be even greater at larger values of L.
In situations where maximum power transfer was desirable, for antennas <~100ft (capacitive across the MW band - see earlier thread), the variable ferrite coil might be an option to consider.
The variable L circuit, rather than appearing to be a "series" circuit would appear to be behaving as a "parallel" circuit with the A/G capacitance, in which case, a low L:C ratio is favoured over the normally promoted high L:C ratio.
Theory: Using all the A/G capacitance to resonate the coil results in maximum antenna resonance whereas a series LC ciruit in parallel with the A/G C will resonate in it's own right, and resonate the antenna more weakly. Put another way, the A/G capacitance puts a capacitive load (short) across the resonant LC series circuit causing losses.
Edited by - golfguru on 10/4/2012 5:11:35 AM
|gzimmer||Posted - 4 October 2012 5:40 |
I think what is happening is that you are adjusting the tuning but not the matching.
Or rather you are changing the tuning but you can't adjust the matching to suit.
If you have one of our broad-band multi-tapped toroids, you could try the same experiment, but with the antenna coil connected to various taps, and the diode connected to the top, and earth to the bottom.
Sliding the antenna coil still tunes the antenna, but shifting the tap now adjusts the matching. I think you'll find one setting which clearly gives maximum output.
If you don't have one of our broad-band toriods, you can use a conventional tank with taps, but you will have the inconvenience of having to adjust the parallel C each time. Along with interaction and double humping.
Edited by - gzimmer on 10/4/2012 5:43:17 AM
|gzimmer||Posted - 4 October 2012 8:10 |
Another thought is that the Q of the tuned circuit will vary markedly as the ferrite is inserted.
The more ferrite, the lower the Q, as more energy is absorbed by the ferrite.
Edited by - gzimmer on 10/4/2012 8:11:16 AM
|golfguru||Posted - 4 October 2012 15:11 |
>>>> Another thought is that the Q of the tuned circuit will vary markedly as the ferrite is inserted.
The more ferrite, the lower the Q, as more energy is absorbed by the ferrite. >>>>>
In a series LC tank, the Q should theoretically rise as the L:C ratio increases.
Q experiments with ferrite cores and a HP Q meter (series resonated) indicate that, up to a point, the Q rises with increased inductance. The coil in question has a Qu of at least 400.
Edited by - golfguru on 10/4/2012 4:13:49 PM
|golfguru||Posted - 4 October 2012 15:22 |
The impedance mismatch of the diode/phones to the coil should also improve with higher inductance and voltage?
|golfguru||Posted - 4 October 2012 16:18 |
>>> If you have one of our broad-band multi-tapped toroids, you could try the same experiment, but with the antenna coil connected to various taps, and the diode connected to the top, and earth to the bottom. >>>
Any chance of a hookup circuit?
|gzimmer||Posted - 4 October 2012 18:34 |
> Q experiments with ferrite cores and a HP Q meter (series resonated) indicate that, up to a point, the Q rises with increased inductance.
Yes, but that would be with varying turns centered on the ferrite. I vaguely remember that if you slide the coil along the rod, the Q varies with how much rod is inserted.
|golfguru||Posted - 4 October 2012 18:44 |
>>>> I vaguely remember that if you slide the coil along the rod, the Q varies with how much rod is inserted. >>>
Yes, this happens but, as previously stated (and checked this morning) the Q rises due to the increasing inductance in a series circuit (a-la HP).
Edited by - golfguru on 10/4/2012 6:45:11 PM
|gzimmer||Posted - 4 October 2012 19:2 |
quick and dirty
|_J_||Posted - 4 October 2012 21:24 |
My thought is that any combination of reactances that include the reactance of the A-G in its circuit, series or parallel or some arbitrary network, all together resonate at the combined resonance. The amount of influence the antenna reactance has on the system resonant frequency affects the resistive load placed on the antenna (Z matching). So seems to me you are varying the R load and resonance X just by changing reactances around. If you go through the pain of calculating the Z of the antenna as transformed by the attached reactances you can see how it changes the resistive match.
|golfguru||Posted - 4 October 2012 23:20 |
Disconnected the diode and phones and measured RF with HiZ amp and Ballantyne AC voltmeter.
It appears that the RF does increase with the capacitor in series. Seems to be a sweet spot for the L & C for max. voltage.
|gzimmer||Posted - 4 October 2012 23:38 |
> It appears that the RF does increase with the capacitor in series
Well yes, with the cap in series more inductance is required. Greater impedance means more voltage across it.
I think you need to be matching the antenna into a constant load. E.g. measuring Power, not Volts or Amps.
|golfguru||Posted - 5 October 2012 2:4 |
Measured the voltage at the phones with Zim's
circuit as drawn and again directly across the series ferrite coil only.
Sounds like I am getting the maximum power output for this particular set of circumstances with the ferrite rod only.
My original statement:
Others are welcome to try the experiment at different freqencies - I only have the one channel to play with.
Edited by - golfguru on 10/5/2012 2:06:04 AM
|golfguru||Posted - 5 October 2012 2:33 |
Tried replacing the phones with 3900 and 100k resistors.
Capacitor increased the max. voltage (to 940mV) with the 100k load and power was increased by ~2dBm over the 3.9k load.
"Horses for courses".
Edited by - golfguru on 10/5/2012 2:45:34 AM
|gzimmer||Posted - 5 October 2012 3:23 |
Try the toroid again, but with the series capacitor in circuit.
The current should go down, but you should be able to find a different tap which will bring it back up again.
Edited by - gzimmer on 10/5/2012 3:25:26 AM
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