I was contacted privately on this issue by someone whose engineering
ability I well respect. He confirmed what I have been saying, but
suggested that the link between choke dissipation and SWR is "best not
mentioned" for fear some folk would interpret it to mean that SWR
*causes* CM current.
I prefer to think that TowerTalk readers are sufficiently technically
aware that they can distinguish between "SWR affects choke dissipation"
and "SWR causes choke dissipation"; but for clarity, let me stress that
SWR does *not* _cause_ CM current: even when an antenna is perfectly
matched, CM current may still be present.
However, as we have seen, the magnitude of the antenna impedance (and
therefore the SWR) directly determines the differential-mode voltage at
the feedpoint, the CM voltage at that point, the CM current, and
therefore the choke dissipation.
Steve G3TXQ
On 19/04/2012 09:07, Steve Hunt wrote:
> Jim,
>
> Let me try to explain it more simply.
>
> Picture Roy Lewallen's "classic" diagram showing how the
> differential-mode current flowing on the inside of the coax braid splits
> two ways at the feedpoint - some flowing into the dipole leg and some
> flowing the CM path back along the outside of the coax braid. Suppose:
>
> Iant is the current flowing into the dipole leg
> Icm is the current flowing along the CM path
> Zant is the impedance looking into the dipole leg
> Zcm is the impedance looking into the CM path.
>
> We know that the current will split in inverse proportion to the two
> impedances, so:
>
> Icm / Iant = Zant / Zcm
> or, rearranging:
> Icm = Iant * Zant / Zcm
>
> If Zant changes, thereby changing the SWR, then Icm will also change and
> so will the choke dissipation.
>
> If the SWR is caused by a drop in Zant, the choke dissipation will
> decrease; if it's caused by an increase in Zant, the choke dissipation
> will increase. Either way it's quite wrong to say: "SWR has NOTHING to
> do with dissipation in a common mode choke."
>
> In a practical example like an HF wire dipole, you'll see that the
> increasing SWR as you move away from the resonant frequency results from
> an _increase_ of antenna impedance because the reactance changes at a
> faster rate than the resistance; a choke designer would be wise to allow
> for the extra dissipation that causes.
>
> Finally, if you don't believe my analysis, just try modelling the
> situation in EZNEC. I took an 80m dipole (#14 wire at 35ft) and added a
> 4k resistive choke to the CM path at the feedpoint. For 1000W applied
> power these were the choke dissipations at various frequencies:
>
> SWR=1.4:1 at resonance: 2.2W
> SWR=2:1 below resonance: 3.3W
> SWR=2:1 above resonance: 3.9W
> SWR=3:1 below resonance: 5.2W
> SWR=3:1 above resonance: 6.4W
>
> As you can see, the SWR directly affects the choke dissipation!
>
> 73,
> Steve G3TXQ
>
>
>
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