Rick,
It's not difficult - just a bit tedious - to work this stuff out. Let me
walk you through the example of a feedpoint 1:1 balun for a 40m beam:
1) Assume the power level is 1kW and the beam feedpoint impedance is a
nominal 50 Ohms
2) Then we know the differential voltage across the feedpoint must be 224v
3) If we assume the beam is inherently well balanced wrt ground, the
voltage to ground from the coax at the feedpoint must be about 112v.
How much balun CM impedance do we need for adequate current balance?
4) Assuming again the antenna is well balanced, the impedance from the
braid side of the beam to ground will be 25 Ohms
5) Assume a worst case, that the CM impedance back along the coax braid
is very low
6) Then if we aim to keep the current balance in the beam to better than
-20dB, we need the choke CM resistance to be a minimum of 250 Ohms
*** Unfortunately that's where most folk stop - they ignore the choke
dissipation considerations ****
7) Say we choose a choke with a measured CM impedance of 4800-j1000
(typical of 12 turns on an FT240-31) - clearly it meets the current
balance requirement easily
8) The current through the choke will be 23mA (112 / Zcm) and the power
dissipation will be 2.5W (0.023^2 * 4800)
9) I generally work on a cautious "rule of thumb" to limit dissipation
in one FT240 core to 10W - if you want to look at the detail, look at
the Fair-Rite thermal data. We are well within that limit at 2.5W!
However, re-visit some of the assumptions to see their impact:
10) If the beam SWR was not 1:1, but 2:1 at the band edges that could
increase the feedpoint voltage by SQRT(2) and increase the choke
dissipation from 2.5W to 5W
11) If the beam was not inherently well balanced wrt ground, that could
also increase the feedpoint voltage and increase the choke dissipation
further
We're now beginning to approach the 10W limit, however because it's a
thermal limit we can de-rate by the duty cycle of the transmission mode.
So I would be very happy to use this choke for 1kW SSB or CW in that
beam application.
As an aside, if we'd simply focussed on current balance and chosen a
choke with a CM resistance of 250 Ohms, re-working the maths it could
dissipate 100W !!!
You might like to work through the same analysis for a 1:1 balun used at
the output of an unbalanced tuner looking into a worst-case ladderline
impedance of 4000+j0 when feeding a multiband doublet - finding a
satisfactory balun for that application is a much greater challenge :)
Hope some of that helps,
Steve G3TXQ
On 24/04/2013 08:18, Rick - DJ0IP / NJ0IP wrote:
I would like to see some real life examples:
How much do we need in a beam?
How much do we need in a dipole?
How much do we need if the choke is between an openwire fed dipole and an
asymmetrical matchbox?
SPECIFICS please. "Enough" is not very helpful.
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