Topband: Ferrites and verticals

[shristov]

Henk PA5KT <pa5kt at remijn.net>  wrote:

> Is a choke needed, and where do I place it on a regular 1/4 
> vertical and where on a HF[6|9]V?

The purpose of a choke is to prevent the coax shield
from becoming a part of your antenna.

Do you want to connect directly to your antenna
a long conductor with random length and position,
connected directly to your household appliances and
to other innumerable sources of noise?'

If not, place a "good enough choke" at antenna feed terminals.
Additional chokes may have to be placed elsewhere, but that's another story.

What constitues a "good enough choke" depends on circumstances.
In general, a choke with common mode impedance of at least 500 Ohm
is required for well behaved antennas such as symmetrical dipoles and beams.
Off-center fed antennas may require tens of kOhms.

A resonant vertical (not necessarily full size) with large full-size radials
has a very mild requirements on choke common mode impedance.
But an elevated "ground plane" antenna with 3 or 4 radials needs a very good choke, or several of them.
And a vertical having a few very short "counterpoises" needs an extremely good choke.
Such an antenna is essentially an off-center fed dipole, placed vertically.

So, one either studies in detail the electromagnetics of his own antenna,
and determines the necessary common mode impedance of the choke by calculation,
or just puts in a very good choke (with measured common mode impedance)
on the operating frequency, and hopes for the best.

A choke with unknown common mode impedance
can be likened to a screw of unknown diameter and pitch.
It may fit your purpose, but it also may completely fail to do so.


73,

Sinisa  YT1NT, VE3EA