[TowerTalk] Common Mode Choke Locations

[Grant Saviers]

What works for me - ie I hear better than talk even with 5+5 yagis and 
big towers at QRO.  But that is not definitive since I can only guess at 
the noise/QRM situation at the other end.  OTOH, #31 chokes on HF are so 
cheap (compared to towers and yagis) and well behaved that why not 
liberally use them?  I follow the K9YC cookbook and also measure what I 
buy or build. And use them on every coax and cable.

In spite of a good ground system, the tower and the coax/wires going up 
are still an antenna, so they pick up CM RF that can be introduced into 
the antenna terminals.  So chokes at the terminals help reduce Rx noise 
as well as the Tx benefit to balance the feedpoint current.

At the tower base, there may be noise from the shack on the coax, so a 
choke can help prior to the bonding.  As you note safety bonding isn't 
an RF ground, there is impedance to that system.   Coax/cables suspended 
above ground shack to tower are very good antennas so chokes there are 
very important. Dirt is a lossy medium so cables buried reduce pickup 
and it acts as a CM choke. Even laying on the ground helps a bit.

I think an about equal case can be made for chokes either side of the 
entry bonding panel to reduce shack noise from entering the coax/cable 
run to the tower.  The choke shack side before arrestor case is to keep 
shack noise from exciting the entry ground.  Versus assuming the shack 
ground is noisy and a choke on the tower side keeps the shack ground 
noise off the coax/wires. If the coax/wires aren't buried to the tower, 
the choke on the antenna side of the arrestors might be the better bet. 
Plus there is some advantage to inductance being there if there is a surge.

Some additional experiences with stations I've helped with chokes and 
grounds have shown significant reductions of noise getting into the 
antenna using these principles.

Grant KZ1W


On 1/26/2023 10:52, Art Greenberg wrote:
> I seek a better understanding of the proper application of common mode 
> chokes.
> 
> First, the setup:
> 
> I have several coax-fed antennas on a crank-up tower. The coax shields 
> are bonded to the tower at the top of the tower (at the end of the rotor 
> loop) and near the bottom of the tower. The bottom of the tower is in 
> turn bonded to a fairly extensive lightning ground system. The coax 
> leaves the lower bonding point and is bundled with rotor and control 
> cables, all of which go under ground in conduit to an entrance panel 
> where the coax encounters lightning arrestors mounted on a copper plate. 
> There are also lightning arrestors for the rotor and control cables 
> mounted on the copper plate. There is also a copper strap that runs 
> alongside the conduit to bond the tower to the copper plate. The copper 
> plate is bonded to additional ground below it which is also connected to 
> the mains ground.
> 
> The questions:
> 
> I realize that the bonding and lightning ground system isn't an RF 
> ground, but I wonder if it influences the location(s) of common mode 
> chokes?
> 
> I -think- a CM choke is appropriate at the antenna feedpoint, despite 
> the bonding setup. Is that true?
> 
> Is an additional CM choke appropriate near the bottom of the tower? On 
> which side of the bonding at the bottom of the tower?
> 
> Is an additional CM choke appropriate before the lightning arrestor? Or 
> between the lightning arrestor and the equipment?
> 
> TNX
>