CHA-250B antenna by SP5GNI

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.

500,30 1,50      2,60
3550,00 1,50      2,60
3600,00 1,40      2,50
3650,00 1,40      2,40
3700,00 1,20      2,40
3750,00 1,10      2,40
3799,30 1,00      2,40
7000,40 1,30      2,10
7050,00 1,30      2,10
7100,00 1,20      2,30
7150,00 1,20      2,40
7199,50 1,20      2,40
10100,50 1,6      2,10
10149,50 1,7      2,00
14001,00 1,70      1,90
14050,00 1,60      1,90
14100,00 1,60      1,90
14150,00 1,60      2,00
14200,00 1,60      2,00
14250,00 1,50      2,00
14300,00 1,40      2,00
14349,00 1,40      2,00
18068,50 1,5      1,90
18167,50 1,5      1,90
21000,40 1,30      1,80
21150,00 1,30      1,80
21300,00 1,20      1,80
21449,60 1,20      1,70
24890,50 1,3      1,80
24989,30 1,3      1,80
28000,30 1,20      1,80
28400,00 1,40      1,70
28800,00 1,30      1,40
29200,00 1,30      1,00
29699,32 1,50      1,30

Miro SP5GNI

10 Comments

  • Well, I’m using Hex-beam to call DXs.
    I understand that you have heard me, you also called, and you did them too!
    Mirek SP5GNI

    Reply
    • I only heard VK9, he was just answering you. I also called, but to no avail. As for the antenna you made. I wondered what its effectiveness compared to vertical with the antenna box used by the Mark ISZ.
      Zbyszek SP5JSZ

      Reply
      • Certainly worse. Matching the vertical right at the base, plus counterpoise wires is the maximum that can be done. In the case of my antenna is far from ideal.
        I will try again, both with counterpoise wires and with transformer modification. I am looking for a way to reliably compare the trialias. When SunSDR comes back to me, I will have more opportunities.
        Mirek SP5GNI

        Reply
  • For me to contact VK7AE the best is 4 x 2 × 2 Yagi Stack monobander – for each band a separate structure, and 6 Band 5 Element Quad (10-11-12-15-17-20m), then goes series V-Collinears monobanders very effective towards Australia-New-Zelaqndia-South-East Asia, Mauritius, Madagascar, South Africa, South America (but only the southern and central part), Caribbean faint, Stay and Canada – not at all.
    Hex-beams used rarely, clearly weaker ..
    Greetings from my Hamsphere antenna farm :-)]
    Chris SP5GNG

    Reply
  • In my Matrix 6 Band Hexbeam is also clearly weaker than 6 Band 5 Element Quad, but also clearly smaller…
    SP5GNI

    Reply
  • Then connect for example FT991 to your Hexbeam, and check what’s going on. And then connect 991 to your Quad. Can you hear something?
    If you want to be stubborn, you can probably connect a real radio to a virtual antenna and vice versa.
    SP5JSZ

    Reply
  • Hi! I started the adventure with CHA 250 in January 2018. I knew that this wonderful device has something special about it. Research and trials after 18 months have led me to such a simple and banal solution as a regular RG 100 feeder or other. Yes, you simply replace the wire that CHA is coiled with the feeder. Power to the hot wire, grounding to the braid. At the output, the hot wire is connected to any radiator, the braid remains unconnected.
    The effect exceeded my wildest expectations. The CHA 250 F has opened the the world in all bands, the listening session was comparable to the box tuned antennas.
    During transmitting all the energy is radiated.
    I currently work on two sets.
    One is CHA 250 F on ten cores with LW 20.5 m 2.5 mm Cu and connected in parallel the frame antenna 20.5 m made od 5 mm Al tubes, shortened by coils.
    The second more modest set is CHA 250 F on 6 cores and a straight helical antenna from the remains of the old RG58 feeders (old – connected) 1025 cm.
    I invite you to the experiments because it is worth it, even for the trips this construction will find many devotees, if the losses of RX-TX are close to 0-10%.
    More on QRZ.com/HF7ST
    Thanks to Mirek SP5GNI for help.
    Adam HF7ST vel SQ7HGY Poland 1997-2019

    Reply

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