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Forums | Crystal Radio Think Tank ** Advanced Forum | Is "resonating antennas" a &qu Post Reply Send Topic To a Friend
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gzimmerPosted - 5 December 2012 23:28  Show Profile  Email Poster  Edit Message
Well, I had a series coil and did have a low resistance, but that is good. Points to an efficient antenna.

Your problem must be that you have a relatively high resistance, which drags down the overall Q.

What resistance do you calculate given the known Q, inductance, etc?

I get ~160R, which sounds about right (say, 40R for antenna, 60R for losses, 60R for Earth).

...... Zim

Edited by - gzimmer on 12/5/2012 11:44:14 PM

golfguruPosted - 6 December 2012 0:45  Show Profile  Email Poster  Edit Message  
>>>>>>
What resistance do you calculate given the known Q, inductance, etc?

I get ~160R, which sounds about right (say, 40R for antenna, 60R for losses, 60R for Earth).
>>>>>>>

Sorry - don't know how to calculate those values. I got a value of 76 ohms at resonance for the lot? see other post, page1.

http://www.midnightscience.com/rapntap/topic.asp?topic_id=5505&forum_id=9&Topic_Title=Ground+resistance&forum_title=Rap+N+Tap+Clubhouse+-+Basic+Crystal+Radio+Fun!&M=False&S=True

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

Edited by - golfguru on 12/6/2012 12:47:25 AM

gzimmerPosted - 6 December 2012 1:8  Show Profile  Email Poster  Edit Message
Yeah, I did a quick calculation based on the loading coil value. But that ignores the rest of the inductance in the antenna itself.

I think you need to build a noise bridge.


............. Zim

Edited by - gzimmer on 12/6/2012 1:22:31 AM

golfguruPosted - 6 December 2012 1:20  Show Profile  Email Poster  Edit Message  
Might be getting out of my depth here - feel free to abort the discussion :-)

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

gzimmerPosted - 6 December 2012 1:30  Show Profile  Email Poster  Edit Message
Here's another method:

With a series loading coil, put a low value (~10 ohm) resistor to ground, peak the coil, and measure the volts across the resistor with your CRO.

Then substitute a higher value resistor (~100R), measure the volts again, and with a little mathematical jiggery-pokery calculate the third and missing resistor (eg the antenna resistance).

Should keep you quiet for a bit :-)


.......... Zim

golfguruPosted - 6 December 2012 4:49  Show Profile  Email Poster  Edit Message  
10 Ohm = 7mV
100 Ohm = 30mv

Simple case of proportionality?

Too good for me, can't form a connection between the two. Not enough info?

Current through ground is the same as that through each of the respective resistors but not equal in the 2 cases.

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

gzimmerPosted - 6 December 2012 10:22  Show Profile  Email Poster  Edit Message
Gawd, Simultaneous equations make my head hurt.

I get
Total Antenna resistance = 57.5 Ohms
and
Antenna voltage source = 47.25 mV

Now that you have series R, as well as Freq and Q, you could calculate the theoretical antenna L and C and compare it with what you measured earlier.


.......... Zim


Edited by - gzimmer on 12/6/2012 10:52:26 AM

golfguruPosted - 6 December 2012 12:59  Show Profile  Email Poster  Edit Message  
Wow, your a "freak-in" genius, Zim.

Q=1/R*sq.rt(L/C)

Substituting assumed values for R and L (previous posts),

Q=1/57.5*sq.rt(18uH/184.5pF)
Q=5.432

Measured value (3dB B/W method)
Q=5.42

QE "freak-in" D

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

Edited by - golfguru on 12/6/2012 4:59:36 PM

gzimmerPosted - 6 December 2012 18:14  Show Profile  Email Poster  Edit Message
And how do the calculated values for L and C compare with the measured values ?


.......... Zim

Edited by - gzimmer on 12/6/2012 6:16:15 PM

golfguruPosted - 7 December 2012 1:36  Show Profile  Email Poster  Edit Message  
>>>> And how do the calculated values for L and C compare with the measured values ?

Don't understand the question?

Can't measure antenna L, that was calculated from previous experiment, see thread. Antenna to ground C was measured and inserted in the Q formula, along with the calculated Ant. L and your calculated R.

C & L of tank was not measured in this final experiment.

Measured the "unloaded" coil Q, on the HP (at the current core setting), at 963kHz, for a Q = ~900.
(Yes, that is my best core, 17mm diam x 240mm long.)

Obviously the Q of the antenna swamps the coil Q.
ie. With a good coil, the measured Q of a single tuned series set would be the Q of the antenna?

"Real" Q of the antenna is probably fractionally above that measured ( probably by a factor of 5.4/900? (0.6%).

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

Edited by - golfguru on 12/7/2012 1:42:51 AM

gzimmerPosted - 7 December 2012 6:26  Show Profile  Email Poster  Edit Message
Just curious to know how the measured, guessed at values of L and C work out with these new numbers.

How accurate the measured C is?

And the inductor measured in the Q meter?

...... Zim

golfguruPosted - 7 December 2012 14:28  Show Profile  Email Poster  Edit Message  
Any measured L's and C's were done with the AADE which I believe is very accurate.

Measuring L on the Q meter is not as accurate and "real" values have to be recalculated from graphical approximations.

If you need me to do a special test and measure particular values of components under particular circumstances, you would have to list them in very basic terminology.
(There are two L's and two C's involved in the inductively tuned set.)

As requested in the other thread, anyone willing to corroborate my findings at other frequencies would be appreciated (it's a pretty simple setup for anyone with the right equipment).

(Can't understand the deathly silence - I hereby enter the words "free energy" and "oatbox" as a discussion stimulant.)

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

gzimmerPosted - 7 December 2012 20:2  Show Profile  Email Poster  Edit Message
> Any measured L's and C's were done with the AADE which I believe is very accurate.

That's what I was curious about.

The AADE measures at DC or audio (I think) so I was wondering if measurements at the operating frequency would differ.

The capacitance for instance is distributed along an inductive wire...

But I guess that would require a RF bridge or similar...

........... Zim


Edited by - gzimmer on 12/7/2012 8:03:39 PM

golfguruPosted - 8 December 2012 2:51  Show Profile  Email Poster  Edit Message  
I see your point, Zim.

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

It would be interesting to know if the method is actually viable across the range of less than 1/4 wave antennas though.

As long as there was reasonable accuracy I would be happy.

...........

Edited by - golfguru on 12/8/2012 5:41:32 AM

gsparksPosted - 13 December 2012 7:45  Show Profile  Email Poster  Edit Message
A resonant antenna on a crystal radio receiver will fight against you. It will tune the resonant frequency so you can just use a diode and headphones and get one frequency. any real measurements are almost impossible to make due to the RF in the vicinity. almost any test will have a diode somewhere, and now test will have voltage to deal with, and with radio signal varying amplitude. The crystal radio is a system, with many variables, including antenna / ground resistance. This is what makes them so much fun to experiment. Take your best set and try at a friends house, or different antenna / ground, notice any difference. My favorite set has no selectivity and different frequency range at home location with 40 foot wire thrown over tree, and the beach shack with end fed 120 foot end fed and ground rods in salt water soaked sand.
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