Based on the best experiences found on the web, I made my own copy of the CHA-250B antenna. This antenna has many controversial opinions. I decided to do it because having a Hex-beam directional antenna and SDR radio – I needed a multi-band antenna with omnidirectional characteristics. In addition to the characteristics, the most important features are multiband and a construction that does not require counterpoise wires.
The unun transformer was made as described by G8JNJ. I used 2×3 EMI suppression rings made of type 33RI ferrite maunfactured by MULTICOMP, purchased from Farnell (part number 9640436). Two pieces of copper tube with a diameter of 18mm and a length of 65mm were screwed with smal copper tubes, which avoided troublesome soldering. The winding was made of the inner of a typical coaxial TV cable, so the cable insulation is strong and high-voltage resistant. The transformer has been closed in a metal box of 114x89x55mm (BS25MF), the plastic housing is not recommended due to the heat generated in the transformer.
Telescopic fibreglass mast was covered with aluminum foil on the length of 720mm (aluminum 50mm sticky tape for fireplaces, you can use copper foil as well). The output of the transformer was soldered to a piece of copper plate clamped on the mast by a metal band. Another one was used to fix the mast directly to a 38mm support tube. The mast was so light that the assembly did not cause any difficulty.
The SWR measurements with the meter in the MFJ-933B antenna tuner are shown in the table below (left column). Matching to the 50 ohm impedance is surprisingly good. As for the effectiveness of RF energy radiation is certainly far from ideal, part of the power heats the transformer, and lack of counterpoise wires must also have a significant meaning.
The listening sessions showed that the signal from my CHA-250B copy is 10-16 dB weaker than that from the W8010 or HexBeam antennas. Although I made some communication on it in all basic bands, including the 60m and 30m FT8 emissions, these results should be considered disappointing. 10 dB of loss on the transmitting side means a tenfold loss of effective radiated power!
G8JNJ has developed an improved version of the antenna with counterpoise wires and a modified transformer, it looks very interesting.
G8JNJ opracował ulepszoną wersję anteny z przeciwwagami i zmodyfikowanym transformatorem, wyglądała bardzo interesująco. According to the author, the new design gives a lot less losses at the expense of a larger SWR. I decided to check this concept. In addition, I added a balun in the form of several coaxial cables turns on four 33RI ferrite cores, as shown in the adjacent figure. I connected the transformer’s enclosure electrically to the mast and added 2 counterweights with the same length as the mast, ie 7.20 m. The SWR measurement confirms the expectations – it is larger (the right column in the table), while the overall effects after the change seem to be beneficial.