Q - steel wire at RF

Some discussions we had on the QRP-L list March 4 through 13th of 2015 on the subject of RF resistance of steel conductors merged in well with my recent attempts to improve my ability to measure coil Q.

In order to compare copper with steel and also Copperweld, I'd need to wind some coils that were physically identical and test them.  I have a ceramic form I picked up at Dayton last year, which has grooves for the conductor, assuring constant spacing and diameter.  I also need to have the same diameter and type (solid conductor) samples for a good comparison.

Below is my coil with ten turns on it, bright steel #18 bare wire:

And below is the same coil with #18 solid copper, insulated:

OK, I'll skip photos of the other two since they all look the same.

You see my shunt measurement "fixture" (BNC tee with receptacle on 3rd port) and a silver mica capacitor chosen to resonate in the vicinity of 7 MHz.

I had two kinds of iron or steel wire, one bright and shiny and the other dark in color.  The shiny one may have been galvanized but I doubt it.  I used the dark one as well because I'm sure it's not coated in any way.  However, it was a standard increment smaller in diameter.

I also wound one coil with #18 Copperweld.  I was glad to have that form because that stuff is super springy and fights you every step of the way.  I probably got this piece from Burstein-Applebee back in the 60s:

Wow, 250 feet for $1.89!  That's a lot of dipoles.

Results:

In the table below I list the results for each sample.  The coils took about 8 feet of wire, so I extrapolated the effective resistance out to 100 feet.  I also show the ratio of RF resistance for each sample to that of copper.

What should we make of this?  Well, I'd say iron or steel conductors aren't good for antennas (especially long ones) or inductors.  (Unless "broadband" is your thing.)  We probably already knew that.  

One thing that kicked off this study was data from a web site showing huge RF resistances for steel.  The calculations were based on skin effect and the effect of permeability in making the skin depth very thin.  I wondered if the permeability of these materials remained high even at RF.

Interpolating from the data on the web site, the ratio of steel to copper RF resistance at ~7 MHz is 33.3.  I got ratios of 13 and 15.  So, somewhat less but still pretty damning for steel conductors at RF.

What about Copperweld?  One alarming potential outcome of RF resistance being primarily determined by permeability was that the skin layer of Copperweld might be very thin as well, resulting in high RF resistance, despite the copper cladding.  This didn't turn out to be the case, with Qs almost equal to copper and RF resistance only 24% higher for Copperweld.

Another observation ... with the steel samples the inductances I measured with my AADE meter were 10% or so more than values calculated from the resonant frequency with a known capacitor.  No doubt this is due to the low measurement frequency of the AADE.

I was a bit surprised that the darker steel (iron) wire had about the same Q as the bright shiny wire did, despite having a smaller diameter.  Maybe the bright stuff was alloyed with something that raised its resistivity as happens with stainless.  I don't think this wire would be classified as stainless though.

Error Factors:

I don't know the composition of either of my steel or iron samples.  (I called the dark colored one "iron" just to differentiate the two.) Both strongly attracted a magnet.

I think my Q measurement methods are reasonably accurate these days.  But this isn't NBS work by any means - just the efforts of an amateur experimenter.

It would have been useful if I'd measured the DC resistance of the samples, but I wasn't geared up at the moment for accurate small resistance measurements.

Nick, WA5BDU

4/21/2015

Links:

http://ve3efc.ca/wire_ohms.html

http://www.g3ynh.info/zdocs/comps/part_1.html