The AM Forum

Hi
These though is always on my mind

It's common that we tap the loading coil to find the perfect match ,for the resonance frequency ,like in pi-section for example.

But is there some power to be lost. Like picture A it's common to short/bridge several winding/ turn of the coil to find the coil turns that matches/resonate at our Frequency

Like picture B ,actually this turns (red) if not shorted produce enough voltage that might lid/turn on a bulb,or a voltage across it.

By shorting it ,is there power wasted in the coil.

As for power transformer if we shorted several turns of the winding .it become hot /and maybe burned out.

Gito.N

On the other hand, the open end of the coil will act as an autotransformer and will have very high voltage at its floating end.  Perhaps enough to arc over at a switch contact.

I've always been uncomfortable about this also.  I just took a piece of miniductor stock, and threw it on my LCR bridge.  It read 3.9 uH with a Q of 120 at 1 MHz.  Shorting turns from either end didn't affect the Q much, but shorting two adjacent turns in the middle reduced the Q quote a bit - down to 60 or 80, depending which turns I shorted.  I note that this meter is pretty close in measuring L but its Q calibration is completely unknown, but it's useful for relative readings.

I think perhaps the air core and high initial Q of an RF coil has something to do with this.  I'm betting Don K4KYV would know!

The age old question - tapping or shorting.

I have tested both and could not see any difference in output power at the 1KW level doing either. The commercial amps all short the coil.

The problem with tapping it, as Johnny said, is the Tesla coil effect. If you plan on more than two bands with a bandswitch, it will probably arc over the bandswitch with the path from 40M to 160M. On 20M and higher, the problem is even worse.

I think the corona effect loss and the shorted turn effect are a wash and show the same loss no matter which is used. I use the shorting method on all my amplifiers here.  I have felt the shorted part of the coil and felt a small amount of heat with Q's of about 12 overall.   I do see the logic you stated for the transformer windings analogy.

If you are manually tapping without a bandswitch, the tapping method is best, but you will need to isolate the coil from everything around it well when tapping the higher bands. The sustained corona at high power can sometimes be seen with the lights out and is a power loss in itself.


** The solution is to use plug-in coils to eliminate both problems. As one well-known amplifier builder once told me, "World-class amplifiers use plug-in coils".   ;D

T

Look at the short as an inductor of low value in parallel with the turns you shorted.

Frank, that's an excellent explanation, and it also explains why, when I just shorted adjacent turns with a screwdriver (which has MUCH less inductance than the wires from the taps thru the shorting switch, etc) screws up the coil Q but shorting it with a 2" wire doesn't affect the Q much at all.

Hi

I can be wrong,
the way I see it.it's a like coupling coil( part of link coupling)but in one coil.
If point X is couplet to an antenna ,it will transmit some of the RF power.
It is low impedance source,that has a voltage across it,so if we short it (depends on many turns we shorted ) does it not load the transmitter

Why I asked this ,
I build a small MW transmitter( 833C final) and used a PVC tube for winding the pi-section coils.(home build}.

Taping the coils to find the perfect uH for resonance and loading.
Since MW transmitter transmit at least 10 hour a day not like ham user ,I find out the PVC tube is deformed at  a certain place ,because the heat close to shorted turns.(  the shorted turns heat)
First I think it's because too high Q that I choose for the output pi-Section output,but looking at the values of the L and C it's a "normal" Q system

It happens to the transmitters I build.

So I change the tapping of the coil like the picture B and the problem is gone.

Gito.N
 
a picture of the loading coil before deformed
 

Hi

Frank You are right if it is a stand alone coil paralleling it with low inductance (shorted it ),it become a very small inductance.

But it's not a stand alone inductance,but it's apart of the coil with an electromagnetic field (mutual coupling?},so there must be a voltage coupled into it depends on the turn ratio,since this  voltage is shorted ,than it produce heat.

Gito.N