A rather simple aid to tuning an AM base coil loaded antenna is the RF current transformer. This report details a bare bones transformer which senses the RF current in a conductor and when connected in line with the feed to an antenna the RF current can be measured on an oscilloscope.
CONSTRUCTION
Several turns of wire are wound on a toroid with the number of turns required determined by the core material. A surplus powdered iron toroid was used for this project but a commonly available T50-2 core can be used but will require more secondary turns (perhaps 30 to 50 but this has not been tried). The coil used here has 9 turns on the secondary and the secondary is connected to a 50 ohm resistor and the scope input as shown in the sketch HERE. The primary winding is 1 turn and is the wire running through the toroid.
The first prototype was made using BNC connectors but most any wiring connections can be used. PHOTO.
The final transformer was mounted in a container from Pomona Electronics Co. as shown in this PHOTO and in this OTHER VIEW . The connection to the transmitter is on the left and the voltage pickup to the scope is made via the BNC Tee connector. The current signal to the scope is made to the BNC connector in the center and the antenna is connected to the connector at the right.
CALIBRATION:
The voltage from the transformer secondary is produced across the 50 ohm resistor for display on the scope. For calibration a load resistor (29 ohms was used but any nearby value will work) is connected to the antenna connector and the voltage across the 50 ohm resistor and the load resistor are measured and the current is calculated as I = (voltage across load R)/(load R). The calibration factor is calculated as K = (I)/(voltage across 50 ohm R). For the core used here the K = .326 A/V. There is some residual inductance in the toroid secondary winding which will show an inductive phase shift between the transmitter output voltage and the current waveforms with a dummy load resistor connected. Ideally this phase shift should be 0 degrees but in this unit it is -6 degrees. This is close enough to get usable readings. If during calibration the waveforms are 180 degrees out of phase then the secondary ground and scope connections need to be reversed.
USE
Edit to add: The transformer needs to be placed between the transmitter final amplifier and the loading coil. For transmitters with an internal loading coil this would mean tapping into the circuitry between the final amplifier and the loading coil. In theory the current could be measured at the connection between the loading coil and the radiating element but this is a high impedance point and severe detuning will most likely result. End Edit.
The transformer is connected between the transmitter output and the antenna feed. Since being near the antenna pulls the tuning off resonance, coax cable can be used to connect the transformer V and I signals to the scope. If this is done the 50 ohm resistor needs to be attached at the scope input. This is a PHOTO of the transformer assembly connected to my indoor test antenna and the coax leads in the photo connect to the remote scope.
On a dual trace scope the phase shift between the transmitter output voltage and the antenna current can be viewed together and the antenna can be adjusted to give a near 0 degree shift which indicates resonance. For practical use a phase shift less than 10 degrees is acceptable. If the current waveform leads the voltage the load is capacitive and more inductance is needed. Likewise, if the current lags the voltage less inductance is needed. This SCREEN CAPTURE shows a +29 degree phase shift between the current and the voltage which indicates more inductance is needed. The digital scope used here displays the phase shift but it can also be seen on an analog scope. The bottom trace is the current and it is shifted to the left relative to the voltage waveform by 29 degrees. The actual number for phase shift is not important since this is being used to indicate resonance.
This SCREEN CAPTURE shows the waveforms with the antenna system adjusted to resonance. Note the waveforms are aligned indicating a nearly 0 phase shift.
OTHER GOODIES
From the data on the resonance screen shot the power delivered to the antenna system can be calculated. The current is K x 0.1872 volts = 0.326 A/v x 0.1872 V = 0.061 amps = 61 mA. The power is P = V x I x cos(theta) but since theta ~= 0 the power is P = 1.352V x 61 mA = 82.5 mW.
At resonance the antenna system R = V/I so in this example the R = 1.352V/61mA =22 ohms.
This simple transformer and a dual channel scope provides a good way to check and adjust resonant antennas and I hope many are encouraged to duplicate this project.
Neil
This SCREEN CAPTURE shows the waveforms with the antenna system adjusted to resonance. Note the waveforms are aligned indicating a nearly 0 phase shift.
There is a parallel in this clever approach with the power factor (PF) in any a-c circuit.
When current and voltage are exactly in phase, then the PF is unity, and maximum power is delivered to the load.
PF is defined as the cosine of the phase angle between the current and the voltage. The cosine of a phase angle difference of zero degrees is 1 (unity).
If for example the phase difference was 60 degrees, then PF = 0.5, and only 1/2 of the available power would be dissipated by the load.
All day today I've been mulling a response to this interesting invention, and in lieu of that I'm just logging in to say that I like it a lot.
My brain is on so many things the full impact of the invention hasn't sifted into the inner chamber where I can actually comprehend what it is and what it does.
I'll probably build one, but I don't yet know why.
Great developments from radio8Z
Last night I spent awhile going over the photos and collecting my questions into a coherent list, with such things as "Why is there a T-connector" and "what happens after tuning is accomplished".
This morning I returned and read from the top, all at once grasping what this neat circuit achieves.
I'm going to guess that after the antenna/coil is peaked, this transformer must stay in place, or the tuning would be lost if it were removed.
Guess # 2... Since the 50-ohm resistor provides the load to the secondary, the oscilloscope can be safely removed without upsetting the antenna.
Curious Question: Is there a further monitoring function this transformer can provide after serving the initial job of tuning the antenna?
Here is more information in response to:
Why is there a T-connector The tee connector is there so the voltage into the antenna system can be seen on the scope along with the current waveform. This allows for a phase measurement. In other words it connects channel 1 of the scope to the voltage at the transformer input. The same could be done using scope probes but I find that coax is more convenient.
I'm going to guess that after the antenna/coil is peaked, this transformer must stay in place, or the tuning would be lost if it were removed. Good point but consider this transformer to be a measuring device such as a DVM. Both will affect the circuit when connected but the question is "by how much?". There was no discernible difference in either the transmitter input power or the field strength with the transformer in or out of the circuit. It could be left in place but this is not necessary and I remove it after the measurement.
Since the 50-ohm resistor provides the load to the secondary, the oscilloscope can be safely removed without upsetting the antenna. Yes, if you want to leave the transformer in the circuit. The value of 50 ohms was chosen to be low enough that the transformer acts as a current transformer and also to match the impedance of the RG58 coax I used to connect to the scope. This matching is why I recommend that the resistor be placed at the scope input.
Is there a further monitoring function this transformer can provide after serving the initial job of tuning the antenna? Yes. Most monitoring techniques are useful because they enable the operator to watch for changes which might indicate a problem. Also, the modulation appears on the current signal so it could be used to check this as well as the antenna.
In Summary: There is no need to use all the BNC hardware, I just did so because I had it in stock and it was convenient allowing the transformer to be placed in and taken out of the circuit without soldering. My intent was to only use the transformer for setup and checking and it is not in place while operating. Again, consider it as you would a voltmeter which can be used for a measurement then disconnected.
The operation of a current transformer can be a bit hard to grasp at first but it is just a transformer with a 1 turn primary and a multi-turn secondary. It will affect the circuit since it places an equivalent resistance in series with the antenna feed. This resistance is equal to (primary turns/secondary turns)^2 x (load resistance) = (1/9)^2 x 50 ohms = 0.62 ohms in my unit, so there will be a little loss with it in circuit but this is very low compared to other losses.
Neil
I will be looking at this one for
a while. A good educational piece,
I'm very sure.
Bruce, GNAT 90.9, SLUG 88.3
SLUG 88.3?
What is
How are
Could it
When will
Since I have somewhat
adjusted to our domestic
situations (for the moment) I am
back on the board, but
without a real Part 15 station.
(My kind of "real" Part 15 station
is some kind of cool vintage studio, and
an outside 3 meter AM stick.) That's
just my take on it - or - the way
I wanted to have it. I was intent
on rebuilding all of that, and making
it better - but that's probably not
going to happen under these circumstances.
(Things are likely to get trickier here
on and off domestically, so I may not
be on as much, as I mentioned.)
HOWEVER, in the Dog Radio tradition
of trying to add jokes and stories
to the board, well - the only
humor I could muster (?) was creating the
"callsign" to my "link" transmitter.
So in a couple of days, we'll all see
if good old "SLUGGY" is still alive.
Cross your fingers.
Bruce, GNAT 90.9, SLUG 88.3
Neil,
Good work.
I'm particularly intrigued by the capability to measure RF ground loss resistance. Ground loss resistance is the single most important measure of a good part 15.219 installation, and at the same time it is the most elusive thing to measure. It is a measure correlating to the effectiveness of the ground radial system.
It would be groundbreaking to have a simple device available to the average semi-technical part 15 broadcaster for measuring the effectiveness of the radial system.
On the down side, the current transformer can't be used with transmitters that have internal loading coils. Placing the current transformer over the antenna on such transmitters would severely mistune the system. It's OK on the input to the loading coil, but that point isn't easily accessible in transmitters with internal loading coils.
As a tuning aid, it won't result in any more precise tuning than you would get from a simple field strength meter or the meter terminals in commercial transmitters.
Get those brain cells grinding and come up with a simple way to measure ground loss resistance so users can optimize their radials! I've thought about that problem at times, but the "simple" part eludes me.
Neil wrote: At resonance the antenna system R = V/I so in this example the R = 1.352V/61mA = 22 ohms.
then PhilB wrote: I'm particularly intrigued by the capability to measure RF ground loss resistance. Ground loss resistance is the single most important measure of a good part 15.219 installation, and at the same time it is the most elusive thing to measure.
The antenna system R determined by using the current transformer has four contributors:
1. Current transformer loss
2. Radiation resistance of the radiating wires of the antenna system
3. Loading coil loss
4. Ground system loss
Undoubtedly Neil can calculate or measure items 1-3, the sum of which would need to be subtracted from this 22-ohm total to find the loss in the r-f ground connection.
For example, Neil reports an ESR of 0.62 ohms for item 1. If radiation resistance is 0.12 ohms, and the coil loss is 15 ohms, then the ground loss is 22 - (0.62 + 0.12 + 15) = 6.26 ohms, approx.
When Neil first posted the physical definition of his new outdoor antenna setup, I modeled it using NEC-4. For an assumed earth conductivity of 8 mS/m, d.c. 13 (as shown on the FCC M3 chart for that general location), the calculated r-f ground connection loss was 5.47 ohms -- which is in quite good agreement with the 6.26 ohms determined using the estimates in the paragraph above.
So this is one example where measured, real-world hardware performance was predicted by pure mathematical analysis.
Rich's post above sums up the situation regarding the use of the transformer in measuring ground loss. To do so, the loading coil resistance needs to be known and this was measured indirectly by using Ben Tounge's method of measuring the coil Q and calculating the resistance.
The data I cited was from measurements made for my indoor test antenna which is below ground in my basement and which therefore has a large capacitance to the "universe" and thus requires a comparatively low value of inductance from the loading coil.
The data from the outdoor antenna system measurements is probably more representative of a typical installation and indicates a higher ground resistance compared to the indoor antenna system which uses a cold water pipe ground connection. The outdoor coil resistance is 19 ohms and the total R calculated from V/I is 57 ohms at resonance. Thus, neglecting the insertion resistance of the transformer and the radiation resistance the ground resistance is 38 ohms. This is higher than expected but we have not had rain here for over a month (until last Saturday) and the soil is very dry. In the last two days we have had 0.42 inches of rain with more expected today and if I have time I will make a measurement to determine if the resistance has changed.
Phil mentioned the placement of the transformer and it is correct that if used with a transmitter with an internal loading coil it would need be placed between the final and the loading coil. The connection from the loading coil to the antenna is a high impedance point and the stray capacitance added by the transformer will have a large effect on the tuning however one reference I checked on the topic claims that with the use of a Faraday shield this can be done (though I have my doubts). In the photo I posted the transformer is shown placed between the transmitter output and the loading coil but this could have been made clearer in the writeup..
There exists on the net some references for measuring the ground conductivity by means of a four wire test rig but all that I read lack an explanation of the method of calibration so I deemed this not worth pursuing. With the transformer method I described a reasonably accurate means of measuring the coil Q and R is needed and this does require some sophistication in the test equipment and use. Lacking this, it is reasonable to assume that a 3" diameter air core coil of about 250 uH will have a resistance of 15 to 20 ohms which can be used in the calculation. If it is accepted that this is reasonable then the ground resistance measurement can be made directly as described without the need to simulate the system using the ground conductivity. This gets right to the effect of interest, namely the ground resistance of the antenna system.
Neil
What could be done if we placed such a transformer on the ground wire between the transmitter and the earth?
If the current is measured in the ground lead going from the transmitter ground to earth ground the current should ideally be equal to the current in the lead to the loading coil but because of other possible paths it probably will not. I would expect this current to be less due to the current returned by the audio/power leads for example.
Neil
There are a bunch of RF current meter circuits, build it yourself plans and one cheap commercial unit from MFJ, http://www.mfjenterprises.com/Product.php?productid=MFJ-854
I did the search on "measure RF current".
Some of them (including the MFJ for $119.95) have clamp-on style sensors. The MFJ says it can accommodate cables up to 1/2 in. diameter.
What I am thinking is that if you bunch the ground wire and power/audio feed wires together inside the sensor, you should be able to get a reasonably accurate total current measurement. If there is an additional connection at the transmitter to a metal mast pipe or tower, any RF current flowing there will be excluded, so that's a disadvantage. But, it should work if the transmitter is only connected to a ground wire and the audio/power feed wires.
The MFJ meter Phil linked should work at part 15 AM levels. The current in a resonant antenna system usually ranges from 20 to 50 mA which this unit is spec'd. to cover. The price is not bad but a good working meter can be built for much less though it will have to be calibrated if actual current measurements are wanted. If only used for peaking then calibration is not needed.
Tuning for resonance could be done with such a meter but it is much more accurate and sensitive to tune to the phase angle rather than a current peak. This requires a two channel scope. Measurement of the antenna system equivalent resistance requires actual voltage and current readings, so the instrumentation depends on what one wants to do.
Here's a LINK to another home brew current transformer. Analog meters are a bit expensive but a simple circuit as shown in the link can be used with a DVM. Since most DVMs don't work above 10 kHz or so the diode circuit is needed with the meter set for DC ranges. Last week I purchased a DVM at Harbor Freight for $4 which is much less $$ than a new analog meter.
Neil