An explanation of the Comet CHA250

The CHA250 is a 7m vertical conductor with a feed unit that accepts 50Ω coaxial cable and is claimed to have low VSWR feed from 80m to 6m. The manufacturer recommends that it be mounted at a height of about 11m.

Above is the CHA250BXII mount with matching unit (the lower black finned module).

Fig 0:

Fig 0 is from VK5ZD's dissassembly of a CHA250, see ANATOMY OF THE COMET CHA-250BXII for more images and info.It is not very clear, but the third vertical conductor from the left is outside the brass tube.

At lower HF frequencies, the magic component inside the CHA250 can be analysed as very short coaxial transmission line sections with toroidal cores on the outside to increase common mode impedance.

Fig 1:

Fig 1 is a simplified diagram of the conductor details in the matching device. Not shown in the diagram are three ferrite sleeves fitted over each of the two tubes.

Two key properties that allow a simplified analysis:

A property of a coaxial transmission lines at frequencies where skin effect in the outer conductor is fully effective is that there are no external fields due to the current flowing on the inner conductors, all electric and magnetic fields are contained within the space bounded by the inner surface of the outer conductor. There will be a current flowing on the inner surface of the outer conductor that is equal in magnitude and opposite in direction to the sum of the currents flowing on the inner conductors.
The transmission line sections are very short wrt wavelenth, so it can approximated by assuming that the magnitude and phase of current at one end of an inner conductor is identical to the other end.

If the current flowing to the 7m vertical tube above the feed unit is taken to be I₁, it can be seen that the sum of the currents on the inner conductors of the right hand line section is 2I₁ downward, and therefore the current on the inner surface of the right hand line is 2I₁ upwards.

If we take the input voltage to be V_i, a current flows upwards on the outer surface of the right hand line of V_i/Z_cm where Z_cm is the common mode impedance of the line section, essentially the choke impedance formed the the outer surface of the line and the toroidal cores surrounding it.

Now, applying Kirchoff’s Current Law (KCL) at the node formed at the bottom of the right hand tube where the inner surface of the outer conductor, the outer surface of the outer conductor and the centre of the coax connector join, I_i=V_i/Z_cm+2I₁.

Looking at the left hand line section, the sum of the currents on the inner conductors is 2I₁ upwards, so there is a current of 2I₁ downwards on the inner surface of the outer conductor. Considering KCL at the node formed at the bottom of the left hand outer conductor, the flowing out of the node from the winding wire is I₁, and the current flowing into the node on the inner surface of the outer conductor is 2I₁, so the current on the outer surface of the outer conductor must be I₁ out of the node (ie upwards).

Now we can treat the nodes at the top of the tubes. For simplicity, lets merge both nodes into one, so the paths and currents are:

inner surface of left hand outer conductor inner surface, 2I₁ down;
outer surface of left hand outer conductor outer surface, I₁ up;
inner surface of right hand outer conductor inner surface, 2I₁ up;
outer surface of right hand outer conductor outer surface, V_i/Z_cm up
outer conductor of input socket, must be I₁+V_i/Z_cm down.

Treating the feedline, there is V_i/Z_cm+2I₁ flowing upwards on the outer surface of the inner conductor, therefore V_i/Z_cm+2I₁ flowing downwards on the inner surface of the feedline. Given that there is I₁+V_i/Z_cm downwards from the feed unit to the shield, there must be I₁ flowing upwards on the outer surface of the feedline / support mast.

The common mode voltage drop due to I₁ in the outer surface of the left hand outer conductor is in series with the vertical radiator, so it effectively loads the radiator with a series impedance of V_i/Z_cm since its construction is the same as the right hand line section.

The doubled back turn in the right hand assembly is interesting, it has no effect under this analysis. In fact, in a more detailed, more accurate analysis at higher frequencies, it is a lossy short circuit stub that introduces a very small inductive reactance and some resistance.

Because the current into the vertical is I₁ and the current on the inner wire of the coax socket is 2I₁, the device is a 4:1 impedance transformer, albeit non-ideal.

Fig 2:

Fig 2 is an equivalent circuit of the 'matching device' where the resistance and inductance shown are the components of Z_cm, and an ideal transformer with 2:1 turns ratio, or 4:1 impedance ratio. The impedance of Z_cm is frequency dependent.

In summary then, the CHA250 would appear to be an 18m vertical, fed at 7m from the top by a non-ideal, lossy 1:4 transformer.

Links / references

ANATOMY OF THE COMET CHA-250BXII