oSaddam (Yuri Blanarovich) wrote in message ...
I once had an occasion where the bottom of a coil did get warm, while
the top did not. Yet my RF ammeters showed the same current on both
ends. What was going on?


What was he measuring, where was the coil?
W9UCW used 100 mA at the bottom, I doubt that this would "cook" the coil. You
see the pictures, they are good quality coils and he use two meters and
flippped the coil to eliminate possible error as described here.

OK, I found the other website and saw the pictures. But this leads me
back to my previous post, which no one has commented on yet. To me,
the upper meter is actually measuring a point above the coil. But it
hard to tell from the pix exactly where the coupler is mounted. The
main question I would like answered is, would the presence of the
capacitive whip above the coil effect the current reading you got,
being you seem to be measuring slightly above the coil? To me, it
seems it would being you are not measuring inside the coil windings
themselves, but slightly above the coil. If this capacitance is the
cause of the decreased reading, then yes, it would be totally normal
to see the same results if you flipped the coil and meters. MK

But this leads me
back to my previous post, which no one has commented on yet. To me,
the upper meter is actually measuring a point above the coil. But it
hard to tell from the pix exactly where the coupler is mounted. The
main question I would like answered is, would the presence of the
capacitive whip above the coil effect the current reading you got,
being you seem to be measuring slightly above the coil? To me, it
seems it would being you are not measuring inside the coil windings
themselves, but slightly above the coil. If this capacitance is the
cause of the decreased reading, then yes, it would be totally normal
to see the same results if you flipped the coil and meters. MK


There is no coupler involved. Meter is inserted between the end of the coil and
remaining mast or whip. Thermocouple meters have negligible insertion effect,
they act as perhaps an inch of wire inserted in the circuit, which is easily
compensated for by retuning either the antenna or moving the frequency. Their
meter mechanism is virtually immune to any RF field distortion. They are
specially designed to measure the RF current with minimum impact on the
measured circuit and to be interfered with. If you can grab one at the flea
market get it!
You could measure current on each turn if you managed to cut it and insert the
ammeter. It would show cosine curve decrease across the coil. Measuring it at
the first turn, end of the coil or inch or two above or below the coil produces
virtually the same results, difference in the current there is really
minuscule.

Another close way of measuring the current is to fashion the current
probe/coupler made of (split) ferrite ring, have few turns of pickup wire,
rectifier and small meter. (There is a description on one of G something web
pages.) You could slide this contraption up and down the radiator and measure
the current. Of course you have to back off and not to touch anything in
vicinity, otherwise you will detune the antenna setup and get erroneous
results. The most accurate and practical way is the way W9UCW did it, he read
the meters with binoculars from the distance.

Yet another way is to use thermal effect, use thermal strips, paste it along
the coil, put some power to it and watch the colors change. Not terribly
accurate, but proof that meters do not disturb the circuit or distort the
measurements.
You can't use probes, scopes or anything with wires attached to it, it detunes
the antenna and gives useless results

Yuri, K3BU.us