Kelly,
Maybe this will help understand what is happening:
Take a look at the SPICE schematic here - It models the feedpoint of a
low(ish) 80m OCFD fed one third the way from one end through a 4:1
voltage balun:
http://www.karinya.net/g3txq/temp/ocfd/80m_ocfd_spice.png
Points A and B are the feedpoint connections to the Long dipole leg and
the Short dipole leg, respectively.
If the dipole had been fed at the centre, the two leg impedances to
ground would have been resistive and equal to around 25 Ohms. But by
shifting the feedpoint to one side of centre, the Long side is now
significantly longer than a quarter-wave - its radiation resistance has
increased to 100 ohms and its individual impedance to ground has become
highly inductive [100+j319 Ohms]; whereas the Short side is now
significantly shorter than a quarter-wave, its radiation resistance has
dropped to 12 Ohms and its impedance to ground has become highly
capacitive [12-j319 Ohms].
As far as a differential signal applied across the A-B feedpoint is
concerned, just as we would expect the impedance appears to be a
resonant 112 Ohms because the reactances of the two legs cancel. But
those high reactances are key to understanding the properties of an OCFD!
The schematic shows a 100vpk source being applied differentially through
a 4:1 voltage balun to the dipole feedpoint. A differential voltage of
200vpk appears across A-B, and a current of 1.785Apk flows in the dipole
legs. Nothing new there!
But now look at the effect the 1.785A has flowing through the
individual leg impedances to ground: it causes the feedpoint to "float"
to a very high voltage with respect to ground; Point A goes to
597vpk/72.6degrees and Point B goes to 570vpk/92.2degrees. Point C - the
centre-tap of the balun where the braid is connected - floats to
575vpk/82.2degrees.
So - applying just 100v across the input of the balun forces the braid
balun connection to float up to 575v above ground !!! The explanation is
*not* that the feedpoint offset has caused the Short and Long leg
impedances to be very different from one another; rather, it's that the
individual leg impedances have become highly reactive.
Resistor R3 has been included to represent the impedance looking back
along the outside surface of the braid to Ground. It has been set to a
very high value so that the fundamental operation at the feedpoint can
be demonstrated without being affected by a large current. But you can
see that setting that braid path impedance to something realistic (a few
Ohms to a few hundred Ohms, and complex) will likely result in very
significant current flowing because of the high 575v at the balun.
You can swap the balun connections around to make it 4:1 UnUn, but not
much changes - you still get very similar voltages at the braid connection.
It's actually a bit misleading to say that a voltage balun or an unun
"forces" the CM braid current to flow; it doesn't - the driver for
braid current is the high voltage generated because of the dipole leg
reactances; the voltage balun or unun simply "allow" (fail to impede)
the CM current. Only a true current balun with high CM impedance can
substantially reduce the braid current.
Hope that helps.
Steve G3TXQ
On 25/01/2015 15:50, Kelly Taylor wrote:
So, the question is: while a 4:1 balun is the correct choice for an OCFD,
would it necessarily replicate the matching unit in a CW? If it's designed
to prevent CM current on the coax, maybe not.
I don't know the answer, which is why I'm asking.
73, kelly
ve4xt
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