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[TowerTalk] Top Hat Switch

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Subject: [TowerTalk] Top Hat Switch
From: (Tom Rauch)
Date: Mon, 16 Jul 2001 20:53:53 -0400
> (duh!) and am concerned about flashover.  A latching relay would work best
> but I may have to fabricate something in the shop to accomplish this. 
> Near as I can figure, there is no such thing as a HP latching relay (MSC,
> Newark, etc.).

You are begging for problems. The relay is at a voltage point, and 
worse yet you have a vertical! 

The voltage can be as much as twice the voltage on a dipole at the 
end, assuming you have a good ground system. That means you 
could have the equal of what a dipole would do with 6 kW applied 

You can model this on EZnec by putting a second source at the 
top just below the hat and setting that source to zero current. That 
causes the source to appear as an infinite impedance, like an open 
switch. A look at the source data will tell the voltage that would 
appear across an open connection at that point in the antenna. 

I did this quickly using a 2" diameter vertical, and there was about 
3 kV RMS with 2 kW of applied power. That's over 4kV peak 
voltage, that you'd have to handle in all kinds of weather. You're 
talking HV vacuum latching relays, which use a very high pull-in 
current. You have to be sure the path across any insulation, when 
wet, would remain open. 

The voltage would increase with a thinner vertical near the top, and 
decrease with a thicker vertical near the top. A fat antenna near 
ground tapering to a thin antenna at top would produce even more 

I'd switch a series inductance in at the base. There would be no 
measurable decrease in efficiency, and switching would be easier!!

73, Tom W8JI 

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