Topband: Ferrites and verticals

[Tom W8JI]

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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