A discussion about balun in the last days made me create this text. Here are my simulation results, results of testing in the backyard and experiences from SOTA activations.
On summits with low hights and/ or bad radiation situation but enough room I’am using a dipole something around 20 m long. The antenne is optimised by simulation and by using a VNWA. Feeded asymmetrically the system provides a reasonable SWR on 40 m, 20 m and 15 m. In my opinion the experiences are also useable for symmetrically feeded dipoles, if they are build up asymmetrically versus ground. This may happen often during short portable operations.
The first balun is a transformer 4:1 (aka voltage balun) which provides an earth symmetrically voltage to a symmetrically load. Yes, I know, it doesn’t force an earth symmetrically voltage in case of a unsym load like my dipole is. Therefor comes the next balun in direction to the coax cable, a common mode choke (CMC, aka current balun). This CMC separates the outer part of the outer conductor of the coax from the antenna. The third balun is also a CMC and located between the coax and the station.
Why so much balun?
The transformer is necessary for matching, for the SWR.
The first CMC separates the dipole from the outer part of the outer conductor of the coax. Otherwise this part of the coax would act like a thick and isolated wire connected direct to the dipole. This extension would affect the radiation pattern and the feeding impedance of the antenna system and so the SWR, more or less. It depends from the frequency, the coax length and the enviroment of the cable. This was simulated and verified in the backyard using the VNWA.
The last CMC keeps unwanted rf away from some parts of the station. My FT-817 has no speech processor and so I’am using an external compressor between the mic and the TRX. The outer part of the outer conductor of the coax acts like a vertical antenna and receives a part from the radiated rf power. These rf would go via the TRX to one side of the compressor. The mic with cable and me, capacitive coupeld, is the counterpoise on the other side. The speech processor is located in the “feeding point” of the unwanted antenna system. The rf there makes a hugh audio distortion. I’ve heard about similar problems with other rigs and using a CW key. The CMC solve this problem.
Whats about the weight of these balun?
The transformer is good for 40 or 50 W in CW and SSB. Its a relative small ferrite toroid. The weight with a plastic box, two connectors for the antenna wire and a BNC connector is about 134 g. (Unfortunately there was no extra room for the first CMC in the box).
Each CMC consists of a piece RG174, a small EMI ferrite toroid and two BNC connectors. The CMC are good from 40 m up. The weight is 38 g each.
Of course, there are other possibilitys like EF antennas with lower weight. But my antenna works fine (two QSO from DL to W from a not so dominant summit on 20 m in February 2018 and much more contacts from more summits, also from difficult ones during poor conds, all with 5 W in SSB).