[TowerTalk] Test Fixture for Common Mode Chokes

[Brian Beezley]

K9YC wrote:

"A first approximation of the effectiveness of a choke in a specific 
installation can be computed in NEC by adding a resistive load equal to 
the resistive choking Z to the model, which includes the feedline as an 
element of the antenna, including the physical geometry of the feedline 
and it's connection at the station (for example, to an entry panel 
bonded to premises ground)."

Jim, one of the reasons I wrote the program is because I suspected using 
the resistive part of the choke impedance often was overly conservative. 
The small number of models I had done suggested that reduction of 
choking effectiveness due to the reactance of the choke combining 
unfavorably with that of the coax shield was not an issue. But the 
models were necessarily simplified, which leads me to the second reason. 
I think it is impossible to model the common-mode circuit for many 
antenna installations to any useful level of accuracy. The impedance of 
the coax shield depends on what it connects to and what it couples to. 
The connection part is hard enough when you consider all of the possible 
current paths. I think the coupling part is impossible to model. The 
coax may drop vertically next to a tower leg and other cables. At the 
ground it may approach other conductors. It may pass vegetation or 
structures that act like lossy a dielectric. Because the lengths 
involved are large, coupling can be significant. The separation to other 
conductors may be impossible to determine, either because it can't be 
measured for practical reasons or because it varies. The dielectric 
properties of objects are anyone's guess.

You can eliminate these uncertainties by measuring the impedance of the 
common-mode circuit. Doing so eliminates the necessity of guessing how 
much choking impedance is enough and whether you need to consider the 
resistive part alone. The equation I use allows the reactances to add or 
subtract.

You can easily add a Bluetooth module to a NanoVNA. This lets you 
control the VNA from a cell phone. You attach the VNA and split-core 
probe to the coax, hoist it into position, and make a measurement from 
the ground. You can do this at more than one spot to find the most 
effective location. No need to sever the coax to try a choke.

Finally, I doubt most people need to measure a choke let alone the coax. 
Installing a well-tested choke design at the antenna feedpoint will 
probably cure most common-mode issues. If problems remain, measurement 
and calculation can help to determine where to install a second choke.

Brian