On 11/23/22 8:53 AM, W0LEV wrote:
The OCF set of wires is NOT a balauced (differential mode) source/load!
Why use a balun? A balun is a circuit element intended to transform
differential mode (dipole) to common mode (coax). An RF choke consisting
of multiple turns of coax through an appropriate toroid or a number of
clamp-on ferrites on the coax at the feed point are appropriate, but not a
"balun" as such. The proposed "fixes" will choke common mode currents on
the external surface of the coax shield.
Dave - W?LEV
One might want a transformer (which could serve as a balun or RF choke) on an OCF dipole, because the resistive impedance at resonance is typically quite high, compared to 50 ohms. OTOH, if the feedline is the *right* length and type, maybe it's ok.
You need an RF choke (or the functional equivalent) if you don't want your feedline interacting with the antenna. On the other hand, for casual use, you may not care - if the feedline (exterior) radiates, and you make a QSO as a result, it's all good.
There's more than one antenna out there that the feedline is an essential part of the radiating structure.
Traditionally, folks like to prevent the feedline from radiating because:
1) it can be unpredictable - it's hard to know if the system is working with simple instrumentation.
2) it can cause RFI and safety problems due to proximity (feedlines are close to people and victim equipment)
3) it can result in increased losses - imagine a radiating field line immersed in vegetation, the vegetation gets warm, and the power doing that isn't radiating.
4) the interaction of the field from the feedline and the field from the dipole (off center or not) can cause nulls and lobes. That can be good (lobe in good direction, null in noise source direction) or bad (null in the direction you want to communicate)
5) a variant of 4, if your feedline radiates, it also receives, and if you have a noise maker next to the feedline, that can be a problem
