Electrical RF conductivity of copper oxide

[Jackson Richter]

In my search, I have been looking for any data that might explain the RF resistance from 3 to 30 MHz range of the two basic types of copper oxides that form on the outside of bare copper wire. I believe these oxides become semiconductors and have large range of impedances. Due to the nature of skin effect at RF along with the combination of the oxides, will antenna efficiencies be affected and if so how much?

 

[zoki85]

I don't understand. Semiconducting property of a material is one thing while skin effect is another thing. Also be more specific about antenna structure

 

[Tom.G]

A brief search showed a huge range of resistivity of Copper Oxides (10,000:1 range).

If they are low resistance then they will have little effect, at high resistance they will act as an insulating layer just as any insulated wire would. Mentioned thickness starts around 3um after 4 hours at 300°C.

Since chemical reactivity changes by a factor of two for every 10°C temperature change, that would indicate at 30°C ambient it would take 1000± hours to build an oxide later 3um thick. (Somehow that doesn't sound right!)

Hmm... let's see if we can get a Ham radio operator to chime in... paging @berkeman

 

[zoki85]

Wide range of specific electrical conductivities of CuO_x thin films is the consequence of a way how the films are prepared. For instance, see some experimental results here
Natural *patina* is quite a different beast of mixtures (and genearally poor conductor). I have no idea why OP is interested in antenna and SW band as regards these matters

 

[tech99]

Silver plating is sometimes used to reduce the skin effect of copper wire used for inductors at VHF. The resistance of such inductors may be only a fraction of an Ohm, so the small improvement due to silver plating may be worthwhile.
For antennas, however, the feedpoint restistance of a dipole is relatively high, about 70 Ohms, and in practical cases we do not see a noticeable contribution from surface oxidation of the copper.
If the conductor has a high resistance coating, the current will tend not to flow through it.

 

[zoki85]

in practical cases we do not see a noticeable contribution from surface oxidation of the copper.

If natural oxidation occurs at joints of electrical parts that's a bad thing (increased contact resistance)

 

[berkeman]

Hmm... let's see if we can get a Ham radio operator to chime in... paging @berkeman

I'm not aware of any antennas that use bare copper. Bare metal is usually aluminum or other metal that is fairly stable when exposed to the weather.

 

[Jackson Richter]

My understanding of skin effect, RF will travel on or near the surface of a particular metal depending on the frequency in question. The skin depth at 4 MHz in copper is ~ .033mm. Copper oxides also develop on the surface of bare copper surfaces when un protected. This can happen when insulation covering the wire breaks down. After awhile polymers covering the wire can deteriorate along with other poor insulators and expose the copper to moisture and oxygen, creating oxides. I do believe some RF impedance may occur. When considering this effect on a wire antenna, its difficult to quantify or put a value on the amount of loss in the far field radiation. Sorry, this question may not be answerable because of lack of the many variables.

 

[Fred Wright]

I'm not aware of any antennas that use bare copper. Bare metal is usually aluminum or other metal that is fairly stable when exposed to the weather.

I constructed a three element vertical out of 3/4" copper tubing and used it for years on 80m-10m. It worked great at 100w with no degradation in swr...plenty of dx with excellent signal reports. Oxidation was a non issue.

 

[zoki85]

My understanding of skin effect, RF will travel on or near the surface of a particular metal depending on the frequency in question. The skin depth at 4 MHz in copper is ~ .033mm. Copper oxides also develop on the surface of bare copper surfaces when un protected. This can happen when insulation covering the wire breaks down. After awhile polymers covering the wire can deteriorate along with other poor insulators and expose the copper to moisture and oxygen, creating oxides. I do believe some RF impedance may occur. When considering this effect on a wire antenna, its difficult to quantify or put a value on the amount of loss in the far field radiation. Sorry, this question may not be answerable because of lack of the many variables.

Seems to me you don't understand the effect good enough and the question is answerable . If the surface of Cu wire gets covered by thin coating of a poorly conducting material the RF current still mostly passes thru the thin section of an unaltered copper beneath the coating. IOW, negligible influence. Only in the case of a VERY thin wire when coating thickness starts to dominate Cu thickness the effect isn't small

 

[berkeman]

I constructed a three element vertical out of 3/4" copper tubing and used it for years on 80m-10m. It worked great at 100w with no degradation in swr...plenty of dx with excellent signal reports. Oxidation was a non issue.

Interesting! Did you use Cu pipe just because you had it left over from a home plumbing project? I've seen a fair number of 80m-10m antennas at our local Field Day events, and I don't remember any made out of Cu tubing...

 

[zoki85]

Interesting! Did you use Cu pipe just because you had it left over from a home plumbing project? I've seen a fair number of 80m-10m antennas at our local Field Day events, and I don't remember any made out of Cu tubing...

Yeah, copper heavy, alu light

 

[Fred Wright]

Interesting! Did you use Cu pipe just because you had it left over from a home plumbing project? I've seen a fair number of 80m-10m antennas at our local Field Day events, and I don't remember any made out of Cu tubing...

I found the design somewhere... in an old 60s edition of CQ magazine..I think. I decided to try it out and was amazed at the results.

 

[berkeman]

I found the design somewhere... in an old 60s edition of CQ magazine..I think. I decided to try it out and was amazed at the results.

73!

 

[tech99]

Copper has often been used for antennas. The first rotatable beam arrays made by amateurs were in the USA around 1935 and were made of a wooden frame with copper elements; I think aluminium tubing was probably unobtainable. The traditional material for wire antennas is hard drawn copper wire, and when broadcasting started around 1930, every home had a copper long wire antenna.

 

[Baluncore]

In my search, I have been looking for any data that might explain the RF resistance from 3 to 30 MHz range of the two basic types of copper oxides that form on the outside of bare copper wire.

You should consider humidity of the atmosphere as a variable. The thickness of the surface dielectric oxide layer determines the velocity factor on the wire, but that effect is small because the layer thickness is small. With time the oxide may be converted to a copper acetate which has more stable but different dielectric and hygroscopic properties as it matures over time.

The Velocity Factor of exposed bare copper wire is close enough to 1.00 for most practical purposes unless you are modelling the precise radiation pattern of long traveling wave antennas. Thirty years ago I measured the VF of a single strand of weathered beryllium copper wire as 0.997 ; (Note: dust from beryllium copper is carcinogenic). The catenary sag of a long wire between supports is a more confounding variable than is VF at HF.

I believe these oxides become semiconductors and have large range of impedances.

Yes, copper oxide is a semiconductor, but that is not important. It is only when a connection is made that pinches, but does not cut through the surface layer, that any harmonic generation can occur. Zinc oxide is also a semiconductor, so the same is true of galvanised and zinc flashed conductors.
Remember that a lose terminal can generate harmonics. Use stainless steel clamp bolts with dielectric grease to keep the water out of the metallic contact.

There may be microphonic wind noise voltages generated between multi-strand conductors that are not present on a single conductor. Single conductor wires vibrate more in the wind than multi-strand, which tends to ameliorate that problem.
Copper work hardens. If you use a single solid conductor you need a short rubber tube near the end as a damper to prevent wind vibration fatigue of the wire. Wire insulation can achieve the same reduction in vibration, but it significantly reduces the velocity factor.

Due to the nature of skin effect at RF along with the combination of the oxides, will antenna efficiencies be affected and if so how much?

Antenna efficiency will not be affected by the oxide. Much greater losses arise due to variation in height, or distance from support structures. Failure to taper the effective wire diameter to maintain constant transmission line impedance along the wire causes much greater losses due to progressive reflection. Tapered cage dipoles are often needed to make efficient HF antennas.

If you want low losses you must learn to analyse antennas as transmission line inductors in their capacitive environment. Analysing the impedance of a traveling wave antenna is a good start. Resonant antennas can become more complex.

 

[Jackson Richter]

Reading the comments from this engineering forum, losses are assumed to be minimal at the HF frequencies. I would assume at VHF and UHF any oxides might become more of an issue. I totally agree antenna efficiencies are very dependent on height and location. Proper conjugate matching and feed line losses are also big factors as well. Others in the forum had judged their antennas by their DX contacts and of course this is very non-scientific due to the variation in transmitting conditions. I appreciate all of the comments and yes my understanding of oxides on bare copper wire is minimal. If I understand your comments correctly, VF can be affected more by the oxides than typical IR losses.
Any loss in an antenna system is a "loss" and without setting up a bench test experiment I was hoping to find a metric or something tangible to measure the loss, if any. Thanks to all.

 

[Baluncore]

Any loss in an antenna system is a "loss" and without setting up a bench test experiment I was hoping to find a metric or something tangible to measure the loss, if any. Thanks to all.

Any loss is a loss, is a truism, unless it is a directional gain reduction which is not a real loss.

If I understand your comments correctly, VF can be affected more by the oxides than typical IR losses.

Resistive I²R losses convert RF energy to heat. Reactive variations reflect energy and redistribute the phase of the signal current on the antenna. As a result of the redistribution, the gain of the radiation pattern is changed, and the expected peaks are often reduced, which some incorrectly interpret as a resistive energy loss.

Signal currents do not flow through the oxide, they flow in the conductive surface below the oxide. The surface oxide is “short circuited” by the conductive surface below. The voltage gradients involved are insufficient to turn on any semiconductor junction in the oxide.

The oxide on the wire appears only as a very thin dielectric layer on the wire, not as a resistor. There is no real energy loss or heat generated in the oxide. The same is true at VHF and UHF. Your fascination with I²R losses in the oxide is not based on any reality.

 

[AldoPaolo]

The penetration depth is espressed as:

Pen depth = SQR RADIX (2 ro / u w)
ro is resistivity of a given material
u is permeability of the material
W is the pulsation

at a given frequency, the "pen depth" state were is concentrated the 70% of the current (esponential curve from outside conductor to the center). That means the remaining 30% of the current is distributed in the remaining part, practically must be null to the center.

On the other side, we can see that higher "ro" that means a more resistive conductor, cause an higher value in "pen depth" beeing to the numerator, this is what happens when the superficial resistance increases. On the other side, the current flows where the resistance is lower at any frequency, this do not means is better to have oxide but the problem may be not so severe...

For example with silver on a copper wire, @ 50°C, 400Mhz, pen depth= 3.38/10⁸, the 99.999% of current will be concentrated on periferic 23um of the Silver coating...

 

[Averagesupernova]

A common antenna for 80 and 160 meters, even 40, is a simple 1/2 wave center fed dipole. Often made from copper wire.

 

[Svein]

I seem to remember...
Wasn't copper oxide used in some early rectifiers (around 1960)? I seem to remember it being rather large and looked like a stack of red heatsinks...
I think they were repaced by selenium rectifiers which were slichtly better, but stank all over the place when they failed.

 

[Baluncore]

Wasn't copper oxide used in some early rectifiers (around 1960)?

Metal oxide rectifiers were used from the 1920s.
https://en.wikipedia.org/wiki/Metal_rectifier