In this article I dive into the common mode choke of my attic antenna presented in the previous post.
Why do you need a common mode choke (or "balun")? A coaxial cable has two spaces: the inside (everything between the shield and the center conductor) and the outside (everything outide the shield). The shield is the wall between these spaces. Electromagnetic waves propagate along the coaxial cable both on the
What does a common mode choke / balun do in this context? It is a lossy inductor that only affects common mode currents flowing on the outside of the coax.
The design is pretty standard: 13 turns (6 turns, one crossover, and 6 more turns) of RG316 coax around a FT240-43 core. To evaluate it, I made a small jig to perform a "S21 series measurement".
Figure X. Choke measurement setup
Here is the measurements of the choke impedance (in ohms) and S21 (decibels) vs frequency:
Band R [Ω] X [Ω] |Z|[Ω] |S21| [dB]
---- ----- ----- ------ ----------
160m 0.5k +2.6k 2.7k -28.6
80m 2.7k +4.0k 4.8k -33.8
40m 5.6k +3.8k 6.8k -36.7
20m 10.5k +0.5k 10.5k -40.5
15m 8.5k -5.5k 10.2k -40.2
10m 7.3k -7.5k 10.4k -40.4
The resistance component is the most important one. The reactance of the antenna can add to the reactance of the choke and cancel out, because reactances can be both positive and negative. Resistances are always positive and cannot be canceled out, so they are a better guarantee.
The |S21| dB value (which might seem to be the obvious thing to measure) also has the same pitfall as |Z| and might thus overrate the choking ability of the choke.
The choke impendance is connected in parallel to the antenna impedance (a current divider), so we want the choke impedance to be large in order to maximize the power into the antenna. A larger impedance makes the choke current small, which also limits heating in the choke from the resistive component.
How much impedance is enough? A presentation from Jim Brown K9YC says that before 2006, 500 Ω was considered good enough and in 2006, Chuck W1HIS showed that 5 kΩ was needed to minimize receive noise. When discussing high power transmission chokes, Jim Brown later says that 10 kΩ is plenty for balanced antenna chokes and that 30 kΩ might not be enough for unbalanced antennas.
Luckily, my current rigs only put out less than 10 W. So the high power issues don't affect me. For receiving and low power transmitting, it seems like the choke should do perfectly fine for 40m to 10m.
Jim Brown also argues that the #31 material is far superior to #43 for HF. I wasn't paying attention, apparently. Something for the next choke project, maybe.
=> http://audiosystemsgroup.com/CoaxChokesPPT.pdf Coaxial Transmitting Chokes (Jim Brown K9YC) => http://www.karinya.net/g3txq/chokes/ Amateur Radio (G3TXQ) - Common-mode chokes => http://www.karinya.net/g3txq/chokes/s21.pdf Derivation of R and X from S21 Magnitude and Phase for a device placed in series between the two ports of a VNA