Pi network RF current

[W8ACR]

When designing a pi network tank, the loading capacitor is typically a low voltage (close spaced) variable capacitor. I presume this is feasible because this is a high RF current, low RF voltage point in the circuit. If this is true, then I would also presume that this is a good point in the circuit to put an RF ammeter to get an idea of the maximum RF antenna current.

Lastly, can I use this current value to determine my power output? For example, if I measure 2 amps of RF current at or near the loading capacitor, can I assume that my power output is 2x2x50=200W? Or am I missing something?

Ron

[KC4VWU]

Hi Ron,
            Remember the ART-13, GF series, etc, mil transmitters. Should work FB. Accuracy dependent upon resolution; probably better than most of the so-called "wattmeters" on the market now.

[W2JBL]

The Rf ammeter goes in series with the output of the pi network, between the loading cap and coax socket. if you get a decent meter it will read power out quite accuratly.

[KA2DZT]

The amp meter goes between the loading cap and the output connector.  I have one in my HB xmtr and with a resistive load it is very accurate.  It is less accurate with a reactive load.

I used the amp meter from an ART-13. It reads to 5 amps, BUT the meter is only a 250 ma movement and works from a current xfmr that is in the ART-13.  In my HB xmtr I made a current xfmr to make the meter work.  I calibrated the current xfmr to have the meter read at twice the current.  So, a reading of 2 amps is really only 1 amp.  This gives better resolution at the lower power levels.

Fred

[W8ACR]

Hi Fred,

Wondering if you could describe the components of your current transformer, or even better, post a schematic.

Thanks, Ron

[KA2DZT]

Ron,

The current xfmr is simple to make.  I use a 4 turn coil made from #12 wire on a 3/4" dia form.  The coil is about 1.5-2" long, the form is about an inch longer.  This coil is connected in line between the loading cap and the output connector.  I then placed a two turn link, made from insulated wire, on the form near to the other coil.  This link is connected to the meter.  The link can be close space two turns.  You can move the link along the form to make the meter read higher or lower.

Using a dummy load and an accurate Bird meter you can adjust the link to read the correct current to match the power read on the Bird meter.  Once calibrated I then varnished the link in place so it couldn't move.  You can use glue to do this.

The size of the coil probably can vary a bit.  In fact, I simply used a four turn silver plated coil that I already had on hand, but the coil can be made from any #12 gauge wire.  The link can be made from any size thinner wire. #18 or #20 should work.  The important thing is to affix the coils on the form so they can't move.  So, glue them in place.

You may have to vary things a bit, but what I described should work.  This set up is for the RF current meter from the ART-13 which should be a 250ma movement.  You can check your meter by looking at the face of the meter, it should say FS=250ma. along the bottom of the face.

Fred

[W8ACR]

Hi Fred,

OK, thanks for the info. A couple of questions. Is the meter a DC milliammeter or an RF milliammeter? If it's a DC meter, I would suppose that you need a diode in the circuit somewhere. Also, I would think a four turn coil might affect the tank tuning on the higher bands. Do you use this system on 21 or 28 mHz?

What you have described is what I would have called a "pick up coil". I guess when you said "current transformer" I was thinking of a toroid with the rf line going through the center of the toroid. The secondary is then attached to a circuit with a resistor, diode and meter.

This is similar to what I use to neutralize final amplifier tubes, see circuit below.

Ron

[KA2DZT]

Ron,

The meter is a RF ampmeter, probably uses a thermocouple.  No need for any diodes.  Just connect the link directly to the meter.  Now that I think about it, I'm sure it's a thermocouple meter because with heavy modulation the meter will slowly increase.  Takes a bit for the thermocouple to respond.

I only use the meter on 40 and 75.  On 10 and 15 there may be an issue with the added coil.  I would think on the higher bands the added coil makes the pi-network into a pi-l network.  On the lower bands that small coil doesn't change much.

Not exactly sure what your output tank coil looks like, but, you should be able to just add a link off the 10M coil and not use the 4 turn coil.

An added note, my 4 turn coil and link are not in the xmtr.  Mine are in another deck directly above the main xmtr.  It is connected with coax leads.  So, I didn't give any consideration to any detuning of the pi-network.

You should be able to get the meter to work correctly.  You don't have to do exactly what I did as I would think there are a number of different ways to feed power to the meter.  Whatever way you set up the meter, remember you have to be able to adjust the link pick-up coil.

Fred