- March 5, 2018 at 11:18 am #31833
Solid vs. Stranded wire in coils by PA1ARE
In the past I used loading coils to improve the efﬁciency of a short dipole on 80m. The results, however, were disappointing. The main reason seemed to be the low Q factor of the coils.
These coils were made from 1 mm stranded wire. In the literature stranded wire is considered just as good as solid wire with the same crossection. Some even believe that stranded wire is somewhat better because it has a larger surface area.
Skin effect and Proximity effect
It is well known that at high frequencies current is concentrated in a thin area on the outside of a conductor. This is called the skin
effect. In a coil the combined magnetic ﬁeld of neighboring windings forces the current to ﬂow on the inside of the coil. This is
known as the proximity effect. A non-concentric distribution may be a problem in stranded wires. Since the individual wires in a
strand spiral around the center of the strand, the position of a wire will change along the length of the strand. In order to stay at the
inner side of the coil the current must transfer from one wire to the other. The contact resistance between individual wires may
therefore play an important role in the total resistance of the coil, and hence its Q-factor.
To investigate the difference between coils made with stranded and solid wire several samples were measured. Coil A is one of the
original loading coils. It is made from 0.75 mm stranded wire. Coil B is purpose build with 1.5 mm solid wire. Coil C is made from 0.75 mm solid enameled wire closely wound, and coil D is made from 0.75 mm stranded wire on a large diameter form. All Q measurements were made at about 3.4 MHz.
Although it is not straight forward to compare the results of the different coils, it is clear that the ones made with solid wire are
better than the ones made from stranded wire. For instance, the difference between coil A and B is remarkable. The wire diameter of B is only 1.4 times higher than that of A. If the ac resistance is predominantly determined by the skin effect only, the ac resistance of A would only be about 1.4 times higher, somewhere around 8 ohms. In reality it is almost 3 times higher. Even compared to the more compact coil C, having the same wire diameter, coil A performs worse. Except for coil C the spacing between the wires created by twice the thickness of the insulation helps reducing the proximity effect. Nevertheless the proximity effect is strong enough to cause the extra loss in stranded wires.
In any application where Q-factor and efﬁciency is important : avoid stranded wire !
You must be logged in to access attached files.March 5, 2018 at 11:21 am #31838
Layne AE1NMarch 5, 2018 at 5:45 pm #31840
Litz wire is a type of stranded wire specifically designed to have good Q for RF coils in the KHz and low MHz range. The idea is that the strands are insulated from each other and the strands are woven so that each spends some time on the surface.
If you look at old lightning arrester downlead wire, you see the same woven structure.
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