Recently, the club’s email reflector has been buzzing with discussion of the best wire for use in dipoles, both temporary and permanent installations. Many opinions were bandied back and forth, the details of which can be found in the reflector archives of YahooGroups under the subject line, “Dipole Wire”.
Tim VK2BT offered some of the more technically detailed commentary, which is worth repeating here:
I would NOT recommend using stainless wire for a “low impedance” antenna (such as a simple dipole). I learned this lesson the hard way myself when I recently constructed one for myself at home, and found that I could NOT match it properly. I asked (genius) Ross Beaumont what was happening here – he responded with a 6 page written report that he created with graphs, EZNEC antenna plots etc etc. The abbreviated bottom line is that (quote):
4. I would NOT recommend using stainless wire for a “low impedance” antenna (such as a simple dipole). I learned this lesson the hard way myself when I recently constructed one for myself at home, and found that I could NOT match it properly. I asked (genius) Ross Beaumont what was happening here – he responded with a 6 page written report that he created with graphs, EZNEC antenna plots etc. etc. The abbreviated bottom line is that (quote):
Initial results were computed for solid wire and tabulated conductivity of 18/8 stainless. No tabulated magnetic permeability was found for stainless steel but a sample showed distinctive ferromagnetism. Impedance at resonance for the dipole was a strong function of wire magnetic permeability.
5. A sample of s.s. wire showed an apparent effective permeability µ ≈ 5 f 0.26 at f MHz (approximate over the HF band) and a fill factor of 0.6. Apparent frequency dependency of ‘µ’ is an approximate correction for the excess increase in resistivity with increasing frequency. Better results are achieved if ‘µ’ is assumed constant and a separate resistivity vs. frequency term is introduced.
6. Final computed results using permeability and resistivity results derived for the sample gave values that were consistent with measured results on a 7 MHz dipole constructed using s.s. wire rope.
A sample of stainless steel wire rope, 3.3 mm overall diameter and 130 cm. long, was tested for DC and RF resistance. Alloy type was unknown but at least mildly ferromagnetic, hence likely to be the less expensive 304 grade. Construction appeared to be 6 strands, each of 7 wires, wound around a single strand core of 7 wires (6/1) in standard right hand lay at approximately 20º lay (twist) angle. To reduce contact resistance and the effects of the oxide layer that invariably forms on stainless steel, connections at both ends were made using silver solder. A soft soldered copper wire connection was then made to the silver solder surface.
Measurements at DC and at audio frequencies gave resistance ≈ 0.204 Ω and inductance ≈ 1.35 µH. Given the published resistivity ρ = 72 . 10-8 Ω-m. the fill factor ≈ 0.6, as estimated from assumed resistivity and the measured resistance including correction for lay angle. Fill factor is close to the theoretical value of 0.603. Combined effect of the fill factor and lay angle is to multiply the apparent DC resistance by a factor ≈ 1.9 when compared to solid wire.
Measurements at RF were made by forming the wire rope into a single turn loop using a General Radio type 916-AL bridge intended for measuring MF antennas, with types B601 and B602 Kerr Laboratories general purpose bridges, intended for carrier frequency components, with a Siemens R277 line impedance measurement bridge, and with Q-meter techniques for higher HF frequencies.
He provided data that showed:
•Stainless Steel Dipole @ 3λ/8 Height, the impedance at 7.1 MHz resonance = 135 + j 0.5 Ω
•Copper Wire Dipole @ 3λ/8 Height Impedance at 7.1 MHz resonance = 88 – j 1 Ω
•Copper Wire Dipole @ λ/4 Height Impedance at resonance = 80 – j 1 Ω
Basically the skin effect of a ferromagnetic material is greatly magnified, leading to a *much* greater RF resistance (as opposed to say copper wire).
[…] stainless is not recommended for a “low impedance” antenna (such as a simple dipole). However, it would be OK for an antenna with a higher RF impedance than a single dipole (e.g. 300 ohm or greater antenna, (e.g. folded dipole etc.)) asthe higher RF impedance would act to swamp out the affects of the RF impedance of the stainless.
The “best” wire to use for a wire antenna must have the (conflicting) properties of:
1.Lowest possible price.
2.Lowest possible RF resistance. NB because of skin affects at RF, the low resistance only *needs* to be the outer coating whilst the centre (if required) can be a different metal (e.g. steel for strength).
3.Lowest possible visible impacts (to not draw attention to yourself). This implies the thinnest possible wire, and further implies uninsulated wire as insulation adds to the diameter but does not add to any beneficial purpose (unless you are operating in a corrosive environment!)
4.Lowest possible weight per metre
5.Highest possible strength so it doesn’t snap when blown by the wind, when pulling it up into position, or when struck by falling branches, perching birds, etc.
6.Highest possible resistance to stretching when under tension. Eg hard drawn copper is very resistant to stretching.
7.Highest possible flexibility and resistance to metal fatigue – as wire antennas continually move and vibrate in the wind. Stranded or woven wire is usually better.
8.Easiest possible to terminate – e.g. whereas aluminium is light weight and is sometimes used in the electrical distribution industry for aerial mains reticulation, it is difficult to solder.
Like most things in life, this is all a case of trade-offs! Every metal that you could think of would satisfy some (but not ALL) of the above criteria! From my experience in the professional RF industry, we exclusively use hard drawn copper wire (either “100 pound”, or “200 pound” (i.e. either 100 pound, or 200 pound per 1000 feet) depending on the span) for big antennas. NB I ended up using hard drawn copper wire to replace my stainless dipole that never worked satisfactorily at home.
In the past at home I have used (ex military) bare, stranded copper wire – but it would always fatigue at the joints after a year or so in the air (to be fair, the military only used these antennas for a portable (temporary) use. For my portable random length antennas I use (ex military) stranded copper wire that is woven with some form of fibre. This makes a very flexible antenna – suitable for ease of erection, disassemble, rolling up, not kinking, etc. I also have a silver-plated wire antenna (ex Russian military) that I bought via Sean via one of his Ukrainian eBay contacts. NB for a given volume silver has the lowest resistance of all metals, followed by copper.
Anyway, at the end of the day – my final comment is – *Any* antenna is better than *no* antenna!
Ross Beaumont has given us permission to reproduce his report (PDF file), with the comment “the more we stimulate experiment and research the better”. Thank you, Ross!
Some internet links to products of possible interest also came out in the discussion:
- Solid copper wire from The Original Wire Man
- “DX-Wire” from Peter DK1RP
- Stranded copper wire from Radio.net.au
- “Figure-8″ flex wire from Altronics (reportedly cheaper than a similar product from JayCar)
(PLEASE NOTE: the mention of the above products does not constitute an endorsement by the the Manly-Warringah Radio Society or any of its members. Decide for yourself if the product suits your purposes.)