Topband: 160 meter elevated vertical

[Tom W8JI]

> Just DO the common mode choke on any vertical.  Don't tempt Murphy.
>

1.) He has a vertical with the base 60 feet above ground, and it sounds like 
the feeder drops vertically

2.) If the coax is 60 feet to ground, and has a surge impedance of around 
500 ohms in common mode (it's a thick conductor), the common mode impedance 
at the radial connection will be about 500 ohms and it will be inductive

3.) To add any significant common mode impedance to the existing CM 
impedance existing in the 1/8th wave vertical wire, he would have to add 
many hundreds of ohms. Even so, the impact on field strength would be 
essentially  zero because the system STARTS with a pretty high common mode Z

4.) By far the most effective second step in a system like that would be a 
good ground where the coax reaches earth. The least effective change is 
adding impedance more to the existing 500 ohms CM impedance. As a matter of 
fact if he does not establish a good ground at 60 feet from the antenna on 
the feeder shield, adding additional CM reactance could have a minimal 
effect or even hurt system CM suppression.

***If we neglect looking at our specific systems in their entirety, we often 
come up with some pretty wasteful rules. There seems to be an 
over-application of "you always need this" when the case actually might call 
for something else.  :-)***

If his feeder was 30 feet long to ground, or worse ten feet long, a 
different rule would apply.

Let me give another example using a dipole. This is actually what misled 
Jerry Sevik when he was thinking about baluns and dipoles.

If we mount a dipole 1/4 wave above earth and use NO balun at all, and bring 
the feeder vertical down to earth where the shield is grounded, the dipole 
becomes perfectly balanced without ANY balun.  If we add a balun to that 
system, the effect of adding a balun ranges from no change to causing worse 
balance! This is because the common mode is already at a peak impedance.

My 75 meter dipole 35 feet above ground with a vertically suspended feeder 
grounded about 45 feet from the feedpoint is almost perfectly balanced with 
no balun at all. Adding a coaxial choke balun makes it WORSE.

To the contrary, a 75 meter dipole 35 feet above ground with a suspended 
feeder grounded 100 feet from the dipole would have terrible balance issues.

Think about it, and you will see it is true. This is why for short leads 
inside the shack between grounded devices, a few beads with just dozens of 
ohms can be dramatic. The common mode impedance is LOW, so even a small bead 
impedance has a large effect. If we added the same bead to a 100-foot cable 
on 160, it might make it worse!

One carte blanch rule does not fit all when dealing with common mode. What 
happens with a very small fractional wave cable is entirely different than a 
1/4 wave cable, and a 1/2 wave cable is different than a 1/4 wave. Even 
where we are at along a cable makes a difference in what must be done.

73 Tom