Hi All, this is an observation from an innocent bystander:
With constant power level, if the antenna Z increases, The voltage
increases as well, right? Conservation of energy then requires the current to
decrease, right?
The rf voltage on the coax shield where it connects to the antenna would
be the voltage referenced to ground at that half of the antenna. The choke
current would be that voltage divided by the choke impedance (also
referenced to ground), Is that right?
So, one might think that a higher Z antenna = higher E = higher choke
current? If so, power loss is then I^2 X whatever resistive loss is present in
the choke.
2X Z = 1.414 X voltage = 1.414 X choke current = 2 X loss. if the above is
true?
73,
Gerald K5GW
In a message dated 4/19/2012 3:08:04 A.M. Central Daylight Time,
steve@karinya.net writes:
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
On 19/04/2012 05:47, Jim Brown wrote:
> On 4/18/2012 12:16 PM, Steve Hunt wrote:
>> Sorry to disagree again, but the CM choke dissipation DOES also depend
>> on the SWR!
> I don't buy your analysis, but I don't have time to pick it apart. :)
>
> 73, Jim K9YC
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