Topband: How to Measure Beverage Common Mode Noise

[Tom W8JI]

> I have run about 400 ft of F6-type flooded coax from my shack to one end 
> of
> my Beverage antenna.  My question is: do I need to install a feedline
> current choke about 20 feet from the end of the Beverage antenna (the
> current choke would have its own separate ground.

While a dummy load test for signals (as Mike says, noise is often too weak) 
might confirm a poor shield connection, especially if the cable is connected 
to normal grounds, it certainly does not test for the most common problem.

There isn't any test I can think of that will properly confirm common mode 
sensitivity except one:

Connect the antenna through a 400-600 ohm resistor to the ground terminal of 
the beverage matching transformer, while the antenna post of the beverage 
transformer is also tied to that ground, and see what you hear.

Most common issues, other than defective connectors or shield connections, 
are caused by using auto-transformers or "un-uns" to match the antenna, 
rather than a primary-secondary isolation transformer. The shield should 
****NEVER**** have a low impedance path to the beverage antenna ground. The 
test above tests for that defect, as well as other common mode issues.

If you look at transformers, especially before I brought this issue to 
public attention, most feed system designs tied the shield to the antenna's 
ground. That was a horrible method of matching that never should have 
started. Unless the ground is nearly zero ohms RF resistance, the shield 
becomes an inevitable extension of the antenna. Yet most early articles 
suggested un-un's.

73 Tom