Unlike standard baluns, ANTENNA TUNER BALUNs are deployed in a heavily mismatched impedance environment. Consequently, special attention must be paid to their correct application.
ON7FU ANTENNA TUNER BALUNs are current baluns. These work according to the Common-Mode Choke (Current Transformer) principle.
Current BALUNs have the advantage of having excellent current balancing (Common-Mode Current Rejection) properties over a wide frequency range.
Using ferrite cores, baluns with a reasonable power rating can be built in a compact housing.
Current Baluns however have their limitations. Current transformers don’t like open circuits or very high load impedances at their antenna terminals.
Even with power levels of 100 Watts or less, with one, or both terminals left open (e.g. with broken transmission line wires) or when using antennas with a very high feedpoint impedance, the ferrite core can heat up, causing thermal runaway, and eventually resulting in an overheated or broken core.
Therefore, when designing an antenna (e.g. doublet fed with symmetrical transmission line) the user should be aware of the impedance values presented at the antenna terminals . These impedance values vary widely depending on the used frequencies (bands).
These widely varying impedances at the antenna terminals, are in turn transformed to yet other values at the feedpoint end of the symmetrical transmission line. This is dependant on the length and the impedance of the feedline.
Knowledge of the impedance values is important, in order to determine operating conditions which are safe for the balun (and TUNER).
Therefore, the user should, measure or simulate the load impedance presented at the balun terminals, at the different wanted frequencies or bands.
As a rule of thumb, when deploying doublet antennas, impedances in excess of some 500 -600 Ohms at the balun terminals must be avoided at all times. Some tuners will probably be able to tune it, but the balun won’t like it.
For those who prefer not to calculate or simulate, or who are in doubt, it’s advised to simply use the antenna lengths and transmission line dimensions of a classic G5RV or ZS6BKW type of antenna and place the antenna tuner with BALUN at the ladderline input. This will yield very manageable and easy tunable impedance values at the HAM bands the antenna is designed for.
Furthermore, during transmission, the VSWR value should be continuously monitored. An unstable VSWR, (e.g. VSWR rising during transmission), or an automatic antenna tuner, continuously re-adjusting during transmission, are indications of the balun’s core heating up.
In such case, transmission should be stopped immediately and the impedance at the end of the transmission line should be verified, and if needed antenna and/or transmission line length should be modified.