[TowerTalk] Carolina Winsome and vertical radiator question

[Joe Subich, W4TV]

 > I didn't say anything about "forcing" CM to the braid.

"Forcing" was my term.  W4THU's write-up on the web site claimed the
"DMU" was designed to *enhance* current in the vertical radiator.

 > What you appear to be suggesting is W4THU's DMU is just a 4:1 voltage
 > balun. Is that right?

I suggested the DMU was an autotransformer based on what I recalled
from autopsying the DMU from a CW160 in which the PVC housing had
failed (all three "eyes" pulled through the PVC) after several years
of exposure to UV here in Florida.  I did not take time to unwind
the transformer to count turns in each leg.

73,

    ... Joe, W4TV


On 2015-01-26 11:21 AM, ve4xt at mymts.net wrote:
> 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 at 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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