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Author Topic: Heathkit AM-1 impedance meter  (Read 5676 times)

Posts: 23

« on: December 29, 2012, 06:48:28 PM »

Hi,   Is this piece of equipment still useful in this day and age. Or would one better to spend some more money for the MFJ meter.

Thanks,   Jim  AF6EC

Posts: 545

« Reply #1 on: December 29, 2012, 08:12:37 PM »

Nice instrument;

It's a simple bridge that requires a source of RF, usually from a GDO in the "olden days".

Millen made a similar one to complement their GDOs.

You can build a simple bridge like this to measure scalar impedance; a "throttled-back" transmitter can be used as an RF source.

I think there were designs in the Handbook some years ago.

Posts: 5

« Reply #2 on: January 01, 2013, 12:04:57 AM »

1)Nice NewYear to all
2)Probably someone may give me an advice about using SWR meter in non-standard situation?

I have problem to match SWR between 27,12 MHz oscillator 3kW with inner impedance (calculated) something like Z=1-j58 and load Z=0,2+j4,7 (+/-80% accuracy, library data).

They are connected by ultra-short line (L~1/1000 lambda) thus by theory, mirroring into this <<wrong>> transmission line makes less as cos 0,001=0,0000000nothing, thus I dont need to build up the 0,2 Ohm spec-cable, or worther, go up from 1 Ohm to 50, and then down from 50 to 0,2.

But here is one stopper - my SWRmeter of course is designed for measurements in 50 Ohm cabling.What will happen if I shall use this meter in such short line? Am I true, that reflections from wrong cable end will disturb my results only for 0,000nothing??

Surplus, I cannot imagine to use any kind another methods as only SWR measurments for matching, because the load itself is just the cubicle of material in which happens plasma, there are no input wires where to put RF voltmeter.

Alternative, I thought over, is to transform load up to 50 Ohm, but then for my humble calculation, cabling voltage ought to be sth like 0,1 Megavolt in peaks, I am not sure about my SWR meter can withstand that, and even most dickest cable.

Thus-do You have any ideas how to indicate the right matching box state with this SWRmeter?

Posts: 5

« Reply #3 on: January 01, 2013, 01:19:26 AM »

P.S. My SWR reader for this occassion is Comet CMX-200

Posts: 17476

« Reply #4 on: January 01, 2013, 08:29:27 AM »

In that case, a standard SWR meter is NOT the right tool.

A common method is to calculate the matching circuit and build it, then
adjust it for maximum power into the load, consistent with proper operation
(current, etc.) of the source.

Some SWR meters designed for CB use have a connection for an external
whip antenna to use as a field strength indicator.  If not, you might be
able to add a whip to your existing meter for that purpose.  Then adjust
the matching for maximum field strength (there will probably be enough
RF floating around to indicate on a meter.)  Or you may be able to
observe the effects in the plasma chamber to maximize output power.

The clue is that, for a fixed load impedance, the voltage (and current)
in the load will be relative to the output power (as long as you are monitoring
on the load side of the matching network) and maximizing either one will
correspond to maximum power into the load.  This doesn't work on the
transmitter side of the matching network because the impedance it sees
changes as the matching network is adjusted.

In this case, if the impedance were 1 ohm, we'd have about 55V at 55A
for 3kW.  You wouldn't want to  put any resistance in series at such a
current, but the voltage would be well within the range of a meter with
a series resistor and diode.  A small pick-up loop to sample a bit of
current would also work.

In fact, some CB meters just use an RF voltmeter across the line to
measure power.  You might get enough of an indicator if you connect
a series resistor between the load and one of the terminals on the
SWR meter:  don't run the 3kW though the meter, but use it as an
RF voltmeter instead, with the series resistor to reduce the impact
on the circuit under test if a whip antenna doesn't pick up enough

Posts: 5

« Reply #5 on: January 01, 2013, 10:17:40 AM »

1)Thanks for ideas.
2)However , the load itself is inductivelu coupled piece of gelly in midst of serial resonance coil, so there are no leads to solder, principially. And load impedance changes as function of amperage, when in the coil is something about 80 Amps, on the coil stays around 100 000 Volts. When jelly is inside, current grows until 95-120 Amp, but voltage falls down about until 4000-5000 Volt. Actually, this Voltage ought not to be falling more than FOR 55 Volt, leaving 99945 Volts, thus I know sure, there are giant insertion losses.
3)But just hour ago I found, probably, only publication over the world, in russian languge, what told about using a standard SWR meter in very short transmission line, and played heavy math about that a)the wrong amplifier impedance never changes the result of meter, b)the wrong short line never is making BIG impact on measure results, thus in conclusion, standard mesurer may be used, with own risk and hope that maybe it shows correct.

Thus, do You are absolute sure, that it may be not used. Especially, do You are absolutely sure, that in-built Wattmeter will show wrong too?

Posts: 5

« Reply #6 on: January 02, 2013, 02:29:36 PM »

For better thinking just two analogies:
1) acoustical system of steel rail in which is measured falling and reflected waves and iron-made loudspeaker and iron-made microphone. But between them stays very very tiny layer of porous gum. Probably micrometers thin gum. Will this gum really will disturb the normal reflection picture in such steel rail??

2) optical system containing glass rod which lights, the glass rod in which we measure the reflections, and third glass rod which is kinda of solar energy panel. But in between all the rod gap is put the ultra thin layer of half-mirroring semi black plastic film. So thin, that it absorbs only 1/1000% of light. Do those wrong reflections will disturb the normal reflection picture until that reflectometer will give the clearly wrong readings?

As more I am meditating about my problem, as more I dont understand, do those russian antenna-men are right, or You.
Probably the best solution is just battle, as soon I shall receive that SWR meter, I shall switch on and if the readings will strongly contradict to plasma brightness, then I shall know, that You had been right.
 I shall give the message what I have out after some weeks.
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