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golfguruPosted - 3 December 2012 21:47  Show Profile  Email Poster  Edit Message  
Might have asked this before:

Assuming a series L/C circuit is in resonance with the antenna on a strong local station, could a variable resistance, inserted in series with ground and varied to obtain a 70% RF-ac reading across the coil, (compared to the original reading and after repeaking the signal), indicate the approximate ground resistance when removed and measured?

Probably best at the low end of the band?

Thanks.

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

Edited by - golfguru on 12/3/2012 9:50:53 PM

gzimmerPosted - 3 December 2012 21:53  Show Profile  Email Poster  Edit Message
Interesting idea. But wouldn't it show the total resistance (Radiation + Loss + Ground)?

And why 70 % ?

Or you could use a noise bridge...

........ Zim

golfguruPosted - 3 December 2012 22:58  Show Profile  Email Poster  Edit Message  
I am thinking half power point?

Just tried it with a CRO to start with - modulated waveform was too eratic.

70% DC across the phones was achieved with 49 ohms in series.

>>>>>>> But wouldn't it show the total resistance (Radiation + Loss + Ground)?

Resistor halves the power to the system - it's value should roughly equal the "absolute value" of ground resistance + radiation resistance which I think people here have said is fairly negligable at the low end on a short antenna?

Would also include some "hopefully negligable" diode variation?

Not looking for single digit accuracy.

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

gzimmerPosted - 3 December 2012 23:23  Show Profile  Email Poster  Edit Message
Err, wouldn't an equal voltage across each resistor give half power?

Do a Google search on "Antenna noise bridge".
Lots of interesting reading.

......... Zim

golfguruPosted - 3 December 2012 23:40  Show Profile  Email Poster  Edit Message  
Probably a bit thick but if adding a series resistor to the resonating A/G circuit halves the system power (~proportional to the DC power drop across the phones?) then the value of the resistor is also close to the absolute impedance value of the system?

Dunno.

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

Edited by - golfguru on 12/3/2012 11:41:18 PM

Edited by - golfguru on 12/3/2012 11:41:39 PM

golfguruPosted - 4 December 2012 0:18  Show Profile  Email Poster  Edit Message  
Just got rid of the diode and phones and used an damped RF ac volmeter across the coil for pretty much the same value (~50 ohms).

Tried checking the voltage across the resistor but got low level - meter capacitance shorting out the resistor?

Still dunno.

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

_J_Posted - 4 December 2012 0:43  Show Profile  Email Poster  Edit Message
Thats a good approach to get at the A-G system resistance, but I use a smaller resistance so as not to disturb the loading as much and calc the indicated resistance. But as Zim says, it measures R of the whole A-G sys, much of which is Rg. But it doesn't matter, you have to treat the whole A-G resistance as the source R anyway.

John Davidson

gzimmerPosted - 4 December 2012 0:44  Show Profile  Email Poster  Edit Message
Forget Power.

Isn't it just a matter of equal voltage appearing across equal resistors?

.......... Zim

golfguruPosted - 4 December 2012 0:52  Show Profile  Email Poster  Edit Message  
Where are these "equal resistors"? (without using a bridge)?

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

gzimmerPosted - 4 December 2012 1:17  Show Profile  Email Poster  Edit Message
Assuming that the antenna consists of an R and a C in series to earth:

You measure the open circuit voltage (at a specific Freq), insert a series L and R to earth (R at the bottom), adjust the L for maximum voltage across the R, then adjust the R for half the open circuit voltage.

The value of the test Resistor equals the Antenna resistance (including Earth Res and losses).

Or you do it the easy way and use a noise bridge.

...... Zim

Edited by - gzimmer on 12/4/2012 1:19:28 AM

golfguruPosted - 4 December 2012 1:42  Show Profile  Email Poster  Edit Message  
>>> You measure the open circuit voltage (at a specific Freq)

How?

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

gzimmerPosted - 4 December 2012 1:46  Show Profile  Email Poster  Edit Message
Use a Selective Voltmeter, or a Spectrum Analyser. But you need a high impedance probe.

A noise bridge uses a coms receiver to look for a null. Much easier.

There are a few variations on the Antenna Bridge. Some use a wide-band noise source and a narrow band detector (coms RX), others do the reverse, use a narrow band source (sig-gen) and a wide-band detector (CRO).

........ Zim

Edited by - gzimmer on 12/4/2012 2:21:48 AM

golfguruPosted - 4 December 2012 3:3  Show Profile  Email Poster  Edit Message  
Thanks guys - guess I was trying to find an easy way that anyone could use.

So John, are you sure the "70.7% DC voltage after the diode" method would be reasonably accurate for calculating antenna /Z/, at a particular tuned frequency, using a series (non-inductive) ground resistor or pot?

(PS: I guess I was initally after "antenna /Z/" anyway, not "ground R").

Thanks.

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

Edited by - golfguru on 12/4/2012 3:26:06 AM

_J_Posted - 4 December 2012 9:42  Show Profile  Email Poster  Edit Message
That APPROACH will get an estimate, but it can be better.

Loading it to drop 30% can change the Q and change the signals (and measurements) getting through. I would load it less and calculate the indicated resistance.

John Davidson

golfguruPosted - 4 December 2012 14:45  Show Profile  Email Poster  Edit Message  
Mmmm........
This app seems to indicate that the voltage across the inductor halves (50%) when doubling the series resistance at resonance.

http://ngsir.netfirms.com/englishhtm/RLC.htm

A 20% drop in inductor voltage would therefore indicate that R would equal 4x "additional series (r)".

ie. R = 4.(r) (at 80%)
/Z/ = ~R plus other losses.

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

Edited by - golfguru on 12/4/2012 2:46:51 PM

golfguruPosted - 4 December 2012 15:34  Show Profile  Email Poster  Edit Message  
Tests using "DC after diode" and 80% voltage (across the load) varied with the DC load resistance value (probably to be expected).

100k (resistive) load - (r)= 22 ohms
3.9k (resistive) load - (r)= 33 ohms

ie. theoretical R varied from 88 to 132 ohms.

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

Edited by - golfguru on 12/4/2012 5:09:52 PM

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