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[TowerTalk] Tuners

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Subject: [TowerTalk] Tuners
From: (Tom Rauch)
Date: Wed, 08 Apr 1998 12:58:08 +0000
Hi Rag,

> In my experience the rotary switch is a weak point in tuners, supposed to
> handle 2-3 KW.

The only requirement is that the RF switches have to exceed the 
absolute breakdown voltage of the capacitors by a reliable safety 
margin. That's true for all the RF switches in the tuner.

Some tuners used inductor switches that improperly placed the 
highest voltage point of the switch (the lowest inductance 
positions) next to grounded screws. Others placed a HV contact right 
next to a LV contact.

HOW the contacts are laid out and wired are just as important as the 
physical size of the switch, unless you want to waste money on 
needless oversizing.

Properly designed and wired, there is no reason to waste money on a 
drastic-overkill switch. The capacitors will limit the voltage 
nicely, and automatically serve as "spark gaps" to protect the 
more fragile and easily damaged switch contacts. 

If the tuner and also if you (when you set the knobs) use enough 
capacitance, the maximum voltage is greatly reduced. The leading 
cause of arcs is not using enough capacitance in a T network, rather 
than component  voltage rating.. assuming there are no design 

The very same tuner that will handle 1500 watts when properly 
adjusted (using maximum C that allows matching the load) can fail 
with a hundred watts or less if improperly adjusted (using maximum L 
and miniumum C)!

73, Tom W8JI

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