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|golfguru||Posted - 1 December 2012 18:34 |
Is the "resonating of antennas" a "croc" WRT crystal radios?
I suppose the proof is out there (that they do) but I have to wonder (as many have wondered here) why the "Q" of the primary A/G circuit is so poor.
Just measured my variable ferrite coil "resonated" against antenna C and calculated "Q" to be 5.2 @ 963kHz.
I have to wonder why it's so poor and whether the "peaking" experienced is simply due to better antenna impedance matching at a particular frequency?.
ie. Is the basic, barebones, single tuned, A/G crystal set circuit, merely an antenna "Z" match unit? (and is that all it has ever been?).
NOTE: Not meant to blow out to a huge discussion - just wondering
|gzimmer||Posted - 1 December 2012 20:51 |
If you add inductance to bring the antenna to resonance (assuming it's less than 1/4 wave), then the Q is decided by the antenna resistance (earth + losses + radiation resistance).
In general the Q of a full-size dipole is fairly low due to the ~50 Ohm feed impedance.
However a very short antenna has a very low radiation resistance, so its Q will be very high (if there are no losses).
The impedance of free space is 377 Ohm, which is the mean of a dipole's center impedance (current node = low Z) and the end impedance (voltage node = high Z).
The Q of a full size dipole is about 100.
You can't NOT resonate an antenna. Any piece of wire in the air will be resonant as some frequency. Better to make the antenna resonant at the frequency you are interested in.
|gzimmer||Posted - 1 December 2012 22:40 |
But yes, as you suggest, resonating the antenna doesn't make the signals stronger. It just makes the power transfer easier, simply because we are using a resonant transformer to do the matching.
This was discussed before, but here's an example: My long wire antenna is a half-wave at the top of the band, and a quarter wave at the bottom. If I use a broadband toroid transformer to couple the antenna to the diode, I can receive stations at the bottom of the band with a low impedance tap (against earth), and different stations at the top of the band with a high impedance tap. And they are just as loud as they would be on a more conventional set.
But this is a special case: It is at the two places where the antenna is already resonant (eg resistive).
If the antenna is not already resonant, the matching device must produce a complex impedance (a conjunctive match). Which is what an antenna tuner does. So if you like, we are tuning the antenna.
|golfguru||Posted - 2 December 2012 0:15 |
Yeah. Guess I just had a brain explosion.
Comes back to loading and coupling in the end.
Power should be at a maximum though (a previous thread).
|homebrew||Posted - 2 December 2012 9:23 |
Q looks way too low.
My 30 to 50 ft series tuned antenna circuits with a ferrite core coil and variable typically calculates 40 to 80 Q.
Usually in the lower ranges mid BCB.
|golfguru||Posted - 2 December 2012 15:13 |
|golfguru||Posted - 5 December 2012 4:19 |
Used a cap in series with the coil - it lifted the calculated Q to around 9.5 at about the same sensitivity.
|gzimmer||Posted - 5 December 2012 6:36 |
Something weird going on...
Could you describe your antenna and Earth and lead-in please?
And could you measure its resistance to earth with your multimeter?
|golfguru||Posted - 5 December 2012 14:4 |
Initial Q=~5 measurement was made by marking the ferrite rod at the 3dB points and checking the freqs on the HP q meter.
Q=~9.5 was made by measureing the capacitancve settings at the 3dB points and calculating the freqs.
* 60ft long
* Outer conductor of cheap, insulated, TV coax
* "Z" Shaped, from dining-room table, up to my 9ft high picture rail (~12 feet above ground level), 6m along one VJ hoop pine wall, 5m at right angles along the adjacent wall, 6 meters at right angles up the passage-way in free air.
* Insulated 7/.063 power cable, soldered to 5 foot length of galv. 1" water pipe at edge of leaking 20000 gall concrete tank.
* lead-in is buried 4" deep, for 5m, to house and comes up through wooden floor.
Radio ground to house ground R = 3.3kOhm (very dry here).
Just rechecked the coil (no cap) by measuring the L on the AADE at the 3dB points.
Hmmm ... my bad, better do a couple more checks.
|golfguru||Posted - 5 December 2012 14:46 |
Some more figures:
Measured A to G capacitance at 184.5pF
Which should need 148uH to resonate at 963kHz.
Actual L to resonate at 963k was 129.9uH indicating a theoretical antenna C of 219pF.
So ... does my antenna have 18uH of inductance (20uH is shown in "dummy" antenna test setup circuits) or is there extra (non A/G) capacitance as well?
Edit: I suppose diode and phones across the coil contribute C or L (or both)?
These could affect bandwith calculations.
Anyhow ... rechecked:
Using the dubious 219pF "equivalent" antenna C value here:
the 3db freqs calculated at:
Which was the ~original calculation
Edited by - golfguru on 12/5/2012 2:53:04 PM
|golfguru||Posted - 5 December 2012 14:59 |
Measured series diode-phones combo at 12.5pF (may mean nothing).
|gzimmer||Posted - 5 December 2012 15:46 |
Firstly I don't understand how you are using the HP Q-meter to measure the antenna Q. Can you explain again please?
Secondly, an indoor antenna as you describe will inevitably have very high losses. I wouldn't be at all surprised if those low Q figures are actually correct.
What resistance do you see between antenna and earth (pref with an analog meter, a digital meter may read crazy due to mains hum, RF, etc).
Cheap LC meters will also read crazy on antennas due to noise and because they actually do the reading at audio frequencies (not at RF).
|golfguru||Posted - 5 December 2012 16:42 |
Analogue meter not working at moment.
Antenna-ground resistance was >20Megohms as in post above, using digital meter (10Meg impedance).
>>> Firstly I don't understand how you are using the HP Q-meter to measure the antenna Q. Can you explain again please?
Don't even know why I did it now (must have been some hair brained reason at the time).
EDIT: I have had trouble with component adjustments (L & C) moving when trying to disconnect and measure using AADE. Thought the HP method might be more accurate but not so. The last reading is the more accurate one.
The calculated Q was of the "tuned circuit" so I guess it also represents the "Q" of the antenna (at least when connected to that particular coil and detector load). Never thought of it that way.
I marked the 3dB points WRT the coil former on the ferrite rod using tape.
Hooked the coil only (nothing else) to the HP.
Set the core to an arbitary mid point between the marks and tuned it to 963k with the HP cap.
Using that C setting I adjusted the core to the 3dB markings and found the frequencies that peaked on those inductance values.
B/W was the difference between the two frequencies.
Q was 963kHz divided by B/W(kHz)
Edited by - golfguru on 12/5/2012 4:44:40 PM
Edited by - golfguru on 12/5/2012 5:03:09 PM
|gzimmer||Posted - 5 December 2012 20:13 |
Your method sounds reasonable.
Only one thing I can think of: Do you have a Bennie? (sorry if you've already covered this)
Perhaps the strong local is resulting in sufficient diode current (eg diode goes low-Z) to broaden things out considerately...???
|golfguru||Posted - 5 December 2012 20:37 |
I suppose a diode tap or antenna tap would tighten it up. Not using a benny.
Just playing around with stuff.
Been trying to find a post of yours where I though you had similar Q levels, with a series circuit, and were lamenting the fact?
|homebrew||Posted - 5 December 2012 23:16 |
I would say that your antenna is too lossy.
The newer coax shields have high (AC)resistance.
Edited by - homebrew on 12/6/2012 8:07:08 AM
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