antenna coil, RF measurement problem

andis_inq1

Joined Jun 16, 2006
1
Does anyone have an idea how to measure accurately the loss resistor (Rs) and inductance (Ls) of a coil at a high RF drive level?

I have to do that with a ferrite core coil (L=2.0uH, core dimensions 8x65mm) at a RF drive level of 50 Vrms or 265 mArms and f=15MHz.
The maximum measurement error should be for Rs = 10% and for Ls = 1 %.

Note: Both Ls and Rs normally depend on the material properties of the ferrite core, the AC flux density and the frequency f. I am also interested how to measure changes of the amplitude permeability and the loss (both vs. RF drive level) of the core material at 15MHz.

Measurements of Ls and Rs at very low RF drive level are no problem for me, because I can do this with an impedance analyzer (Agilent 4294A). But the 4294A current drive capability is limited to 20mArms and cannot be boosted.

Papabravo

Joined Feb 24, 2006
16,795
Originally posted by andis_inq1@Jun 22 2006, 02:52 AM
Does anyone have an idea how to measure accurately the loss resistor (Rs) and inductance (Ls) of a coil at a high RF drive level?

I have to do that with a ferrite core coil (L=2.0uH, core dimensions 8x65mm) at a RF drive level of 50 Vrms or 265 mArms and f=15MHz.
The maximum measurement error should be for Rs = 10% and for Ls = 1 %.

Note: Both Ls and Rs normally depend on the material properties of the ferrite core, the AC flux density and the frequency f. I am also interested how to measure changes of the amplitude permeability and the loss (both vs. RF drive level) of the core material at 15MHz.

Measurements of Ls and Rs at very low RF drive level are no problem for me, because I can do this with an impedance analyzer (Agilent 4294A). But the 4294A current drive capability is limited to 20mArms and cannot be boosted.
[post=17843]Quoted post[/post]​
A little tongue in cheek, I'm certain the Agilent salesman would be glad to sell you a new, improved and more expensive instrument.

On a more serious note I have some general observations.
1. If this particular inductor causes a shift in frequency as the inductance changes then measuring the change in frequency of the conducted or radiated signal might be fruitful. On the other hand those instruments might be eaually costly.
2. I'm assuming that the loss resistance can be detected by measuring input power and output power. The problem is that accurate power measurements are difficult because there are many sources of loss and you can't be sure that you've accounted for all of them. You might be able to get some idea of the scale of Rs by inserting an intentional series resistance and trying to measure its effect.

Not much help, I know, but it is what it is.

beenthere

Joined Apr 20, 2004
15,819
Hi,

Can you get an AM transmitter? 15 mHZ is down near the old HF bands - possibly you could look at the ARRL site for a schematic. All you would need is the oscillator and a keying circuit. Don't know how easy it will be to hit the power spec.

You could probably measure indirectly. Use a dummy load on the transmitter output to determine power out by temperature rise. Put the inductor in the circuit, and tweak out the inductance with a coupler and SWR bridge - that will give you the inductance. The Rs term should then drop the power into the dummy load, which will show up as a lesser heating of the 50 ohm resistance. Or as heating in the inductor - your choice.

Papabravo

Joined Feb 24, 2006
16,795
Originally posted by beenthere@Jun 22 2006, 10:49 AM
Hi,

Can you get an AM transmitter? 15 mHZ is down near the old HF bands - possibly you could look at the ARRL site for a schematic. All you would need is the oscillator and a keying circuit. Don't know how easy it will be to hit the power spec.

You could probably measure indirectly. Use a dummy load on the transmitter output to determine power out by temperature rise. Put the inductor in the circuit, and tweak out the inductance with a coupler and SWR bridge - that will give you the inductance. The Rs term should then drop the power into the dummy load, which will show up as a lesser heating of the 50 ohm resistance. Or as heating in the inductor - your choice.
[post=17858]Quoted post[/post]​
So where exactly are the new HF bands, I can't wait to find 'em. An amateur transceiver has the 20M band from 14.000 to 14.350 MHz. I don't think the commercial products will allow out of band transmission. My IC-706 will put out 100W AM on the 20M band, although almost nobody uses AM outside the 75M phone band.

beenthere

Joined Apr 20, 2004
15,819
Hi,

Sorry, not a ham. Same old HF freq's. I seem to recall times when the spectrum wasn't quite so constricted.

Notice I mentioned using a dummy load - no out-of-frequency-band problem.

Papabravo

Joined Feb 24, 2006
16,795
Originally posted by beenthere@Jun 22 2006, 05:33 PM
Hi,

Sorry, not a ham. Same old HF freq's. I seem to recall times when the spectrum wasn't quite so constricted.

Notice I mentioned using a dummy load - no out-of-frequency-band problem.
[post=17868]Quoted post[/post]​
No I saw the dummy load part, what I meant was the transmitter won't even turn on at exactly 15MHz. so such a transceiver could not be used as a cheap RF source.

beenthere

Joined Apr 20, 2004
15,819
Hi,

That's why I mentioned the ARRL site. Hopefully for older design before frequency synthesizers. That way there's usually plenty of room to tune up a megajiggle or so. Don't need a tranceiver, just a keyed amp outputting an unmodulated sine at 15 MHz.

The other way is to have a really, really, really good linear amp and just feed it with a 15 MHz sine wave. Sometimes a linear DC power supply regulator will do as well....