The AM Forum

Hola,

I'm working on the bandswitching layout of the new rigs. 

One way is to bansdwitch/short the tank coil from the loading capacitor up the coil. This keeps the "Tesla effect" of high voltage auto transformer action to a minimum, but produces a lower Q coil and more losses from the shorted turn.

The other method is to simply tap the coil where it needs to be without shorting the end progressively.

I now use a big linear where I can get away with this "free tapping" method up to 20M without the bandswitch arcing on the 75 and 160M taps. But I think any higher band taps and there wud be problems.

Any opinions on the lower Q /losses with a "shorted  turn" coil?  I see the handbook designs do it, but wondered what others thought about this.  Plug in coils solve the problem, but I desire to bandswitch this time and keep a decent hi Q coil without arcing problems.

T

Tom,
I have always shorted turns with good luck. 4X3 has 4 20 amp contacts in parallel for low resistance. I use the radio switch wafer about 4 inches in diameter round with 3 ceramic pillers as section spacers. I have never had flash over problems. Q has never been a problem either with careful attention between tube plate and plate Tuning cap lead length.  I use 1/2 inch wide strap for each tap and 2 in parallel for the 10 meter tap. This type of radio switch switch is one of the parts I scope out at the flea markets. I usually take a pile of parts and build my own. The shaft is a grooved ceramic rod about 1/2 inch in diameter. it takes a visit to my brother when I need to cut one to the length I need. There was some mil rig that used one of these switches a foot long but was gear driven. I usually find the mechanical stop and 1/4 inch shaft the hardest part to find. Radio switch had 2 bigger units that I kow of.
This was the one about 6 inches ID with 2 large ceramic pillers and the one with 3 pillers like the tank switch in the GPT10K They have 3/8 inch shafts.
I knew this SK who did high voltage and power, He said you never solder high power
connectiond they are bolted or brazed. The switch I use has contacts that accept 6-32 hardware. I was lucky to find silver plated screws. I think  the taps are the high resistance point that kills Q. I keep them short and wide. A shorting switch helps but they are very rare. 
Break over back to the ant. Just came off the tower and got a nice little metal sliver on my small finger. hard to type.

Franz,

Vu said: "So solly of finger. You now need pick nose with left hand."


So, you feel there is no difference  between progressively shorting a tank coil compared to free tapping it without shorts?

The free tapping is what causes the unwanted autotransformer action, but I always thought it was a cleaner and less loss way of doing it????   Isn't there SOME circulating currents still in the shorted turns and jumper?

T

I found it worked ok as long as the taps were low inductance and low resistance.
two inductors in parallel. If one has a bad q say a small diameter wire then you have heating. GPT10k tank did 1/8 by 1/2 inch straps to the switch.
Need to look at finger under microscope worked till dark on antenna then watched football till the giants lost in ot.
I lucked out with the bow and got a branch in the top of the tree in the front yard
and the third support was just as good. feed point is about 50 feet and the ends a lot higher. 125 feet on a leg vee facing west.