A correspondent asked about whether the proprietary copper clad VHF whips are some kind of marketing ploy.

Lets consider a quarter wave whip for the 2m band. Made from 2.4mm stainless steel, it should deliver lowest VSWR(50) at about 483mm in length.

The properties of stainless steel whips will vary, but lets consider the case of one made from 17-7 PH Stainless Steel, a quite tough material that is popular with manufacturers of mobile whips.

17-7 PH Stainless Steel has higher resistivity than copper, but worse, it is weakly ferromagnetic which affects RF resistance.

Above is a calculation of the effect of skin depth, R_{rf}/R_{dc} is very high at 172, mostly a consequence of the permeability.

Above is a calculation of the end to end RF resistance of the whip, 15.3Ω.

The current distribution on the whip is not uniform, it is approximately a sinusoid, and the average of I^2 is 0.5I, so the equivalent conductor resistance at the feed point is 0.5*15.3=7.7Ω.

This resistance appears effectively in series with the radiation resistance which should be somewhere around 35Ω for such a whip mounted in the centre of a car roof, so the whip conductor loss is almost 1dB.

If you work these numbers for a whip with sufficient copper cladding to behave like solid copper, the loss resistance is tenths of an ohm.

Similarly, if you run the numbers for a non-magnetic Stainless Steel (eg 3,1), the losses are quite low.

So, the copper clad whip should perform better than a 17-7 PH Stainless Steel whip. The properties of the Stainless Steel have a large bearing on the outcome, if the material is weakly magnetic even, it is an unknown quantity..

The above results were verified with NEC-4. Note that NEC-2 does not natively model ferromagnetic conductors.

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Last update: 30th September, 2015, 12:32 PM

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