Topband: Antenna matching question

[Tom W8JI]

I have an amplifier with a pair of 3-400Zs I built in the 60s. When I built 
it it was for 20, 15 and 10 meters.
I used 3 of the 5 positions of a BC375 tuning unit switch. Everything work 
fine. I added 160, 80 and 40 meters.
160 was switched in and out with a RJ-1a vacuum relay. When I went to 20 and 
up meters the switch contacts arced.
I added another RJ-1a to short out the unused lower band coils. I switch the 
RJ-1a's in and out with 2 mini toggle
switches. One says 160---other bands, the other switch says low bands---high 
bands. It works fine on all bands. >>>>

Roller inductors and other systems where large inductors or strings of 
inductors are tapped but not progressively shorted almost always have that 
problem.

The mechanism causing arcing is the unused area of coil self-resonates at or 
near an operating frequency. The National switching system is one example, 
and large roller inductors commonly do that (they have one tap that moves). 
Roller inductor tuners sometimes have that problem on higher bands.

The mechanism is the same mechanism causing RF choke failures on some bands. 
The lower band tank with stray capacitances near the inductance center acts 
like back-to-back L networks at some higher frequency.

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

This is why almost all amplifiers use pick-up-and-hold switch contacts. It 
can be a problem with matching networks, too.

73 Tom