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Re: Topband: Ferrites and verticals

To: <topband@contesting.com>
Subject: Re: Topband: Ferrites and verticals
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Wed, 13 Aug 2014 09:40:56 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
The answer below by Sinisa is one of the best and most factual I have ever seen.

I would only add that when the coupling through air greatly exceeds the coupling through common mode, there is no reason to increase the choke impedance. Elevated radials are a different story, because common mode coupling to lossy ground increases losses.

I have many cases where just 20-50 ohms is more than enough, and in a very few others proper impedance is nearly impossible. In that case, changing cable lengths or modifying grounding corrects things.

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
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