[TowerTalk] Carolina Winsome and vertical radiator question

[Steve Hunt]

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
>
>
>