Hi Steve,
Thanks for the explanation, but I just want to clarify you're answering my
question, since you appear to be countering points I didn't make.
I didn't say anything about "forcing" CM to the braid. CM is like Forrest
Gump's dog feces: it happens. My point was certain transformers will also choke
off CM, which, if it happens at the feedpoint removes the CM from play on the
vertical radiator of the Carolina Windom.
What is different inside the DMU (W4THU's term) from, say, a 4:1 voltage balun
is never clearly explained, though it's clear he's not trying to choke off CM
at the feedpoint, but at the choke (or line isolator, as he calls it) 24 feet
down the line.
What you appear to be suggesting is W4THU's DMU is just a 4:1 voltage balun. Is
that right?
73, Kelly
ve4xt
Sent from my iPad
> On Jan 26, 2015, at 9:20 AM, "Steve Hunt" <steve@karinya.net> wrote:
>
> 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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