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Re: Topband: Choke Construction Info Needed

To: <philclements@centurylink.net>, <topband@contesting.com>
Subject: Re: Topband: Choke Construction Info Needed
From: "Tom W8JI" <w8ji@w8ji.com>
Reply-to: Tom W8JI <w8ji@w8ji.com>
Date: Thu, 26 Jul 2012 16:30:12 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
>I need to construct a heavy-duty choke to be installed from the feed point
> of my 160 meter vertical to ground, for static drain and for lightning
> protection. How large does the wire, form, and inductance need to be?

Phil,

This is like any shunt or series choke problem. The choke needs to be 
several times the impedance at the insertion point. In a case like this you 
don't want too much choke, which many people tend to do, because it:

1.) Can cause series resonance issues

2.) Reduces effectiveness of the system

A general rule is a choke provides **about** 1000 ohms per 100 microhenries 
on 160 meters.

With a 1/4 wave Marconi, a 50-100 microhenry choke would be provide more 
than enough impedance. With a half-wave end fed, the choke would have to be 
several hundred uH.

The problem in this application is having the choke stand water, surge 
currents, and transmitter voltage. Most commercial BC tower chokes are 
solenoid construction with moderately heavy wire, almost like a plate choke 
for linear amplifiers but encapsulated. As a matter of fact, a standard high 
power amplifier plate choke would work pretty well. Like one of these:

http://www.w8ji.com/rf_plate_choke.htm

What this choke can do is prevent the antenna from trickle charging during 
wind or foul weather, if the antenna has no dc path to ground. What it will 
NOT do is prevent lightning damage from strikes within a few miles.

If you want better protection, you could use a 1/4 wave stub, or heavy 
components in a parallel resonant circuit, with a spark gap to ground.

I use pipes as gaps here on a 220-ft tower:

http://www.w8ji.com/lifting_rohn_25g_45g_tower.htm

That tower also has a 130 foot long horizontal tower that works as a 
"grounding stub". It allows me to access the tower with unlimited wires and 
cables without "shorting out" the tower to ground on 160, and it serves as a 
trickle drain. You could use 1/4 wave of shorted coax as a grounding stub on 
a 1/4 wave vertical.


73 Tom






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