Q Meters
© Brooke Clarke 2013
I have an interest in inductors and part of that is high-Q inductors both air core and with magnetic materials. The latter are used for the Joule Thief circuit as well as for flux gate magnetometers or loopstick antennas. Air core inductors are used for things like Crystal radios.
I spent about two years full time winding coils and measuring their Q using the Boonton 160 Q-meter. Recently I found a Heathkit QM-1 and have just received it.
The
The key specification for a coil is it's inductance. This can be measured by an LCR meter such as the Stanford Research SR715, the HP4260A Universal Bridge, HP 4261A LCR Meter, HP4274 or 4275_LCR, HP 4332, the ZM-11/U bridge, General Radio GR 650-A or GR 1650B Impedance Bridges, Marconi TF-2700 Universal Bridge etc. But none if these can measure the Q of a coil when that value is much above 20. For higher Q values a specialized Q-meter is needed.
It may be that the HP 4395A can measure higher values of Q, but that remains to be seen. It has a big advantage over the above instruments because it can make a sweep frequency measurement instead of a single frequency measurement like the above meters.
A Q-meter typically has an test signal source that's adjustable in frequency over a wide range using a band switch plus a tuning dial and to a small extent power output in order to set a reference level. The inductor to be tested is brought to resonance using a variable capacitor that's part of the Q-meter. It's possible to add additional capacitance in parallel with the Q-meters internal capacitance to test a lower frequencies.
If the coil under test is fed with a constant RF current the the voltage measured across the coil at resonance can be displayed as Q.
The coil tuning capacitor can be calibrated both in it's own capacitance inductance if the test frequency is one of a few special frequencies.
The following list of what's measured by a Q-meter is pretty much in order of how often they are measured. Note that (3.) distributed capacity is very important since it determines the self resonant frequency of the coil. In order for a coil to operate at high Q it needs to have a load capacitance that's large compared to it's distributed capacity. A coil operating at self resonance has a Q of 1.
1. The inductance of an inductor at the test frequency.
2. The Q of an inductor at the test frequency.
3. The Distributed Capacity of an inductor at the test frequency
4. The Capacitance of a capacitor below about 25 nF (mmF) directly
5. The Capacitance of a capacitor above about 425 nF (mmF) when a known inductor is used.
Q vs. Frequency Plot
This is a very important plot if you want to understand what's causing the losses that degrade Q.
The slope of this plot is the secret to the loss mechanism. Some common causes of loss are: resistance in things like the coil former or wire insulation, Resistance in the wire caused by bulk resistance of the wire, skin effect (Wiki) or proximity effect (Wiki). These loss mechanisms show up as different slopes on the Q vs. Frequency plot.
If you want to measure a component with connectors designed to have a nominal impedance of 50 Ohms then a Network Analyzer, like the HP 4395A, allows fairly accurate measurements. But if you want to measure a component that's going to be soldered into a circuit, i.e. has no connectors, then you need to be concerned with not only the Device Under Test but also the electrical parameters of the fixture used to connect to the DUT.
I have a separate web page Z about impedance as well as another web page Zo showing why the impedance of a transmission line is NOT constant.
Self Capacity
When making a high Q coil how the wire is would has a dramatic effect on the self capacitance and since the load capacitance needs to be much larger than the self capacitance then minimizing the coil's capacitance becomes very important.
About the worst way to wind a solenoid type coil is a back and forth wind where layers are built up. For example think of a 2 layer coil with a total of 100 turns.
It has 50 turns on the first layer and 50 turns on the second layer. The start and finish wires are on the same end of the coil. The voltage difference between turn 1 and turn 100 is the full coil voltage and turn 1 and turn 100 are right next to each other. This makes for a lot more capacitance than other winding methods. If all 100 turns were on just the first layer (the start and finish wires on opposite ends) then the voltage between any two adjacent turns would be 1/100 of the total coil voltage resulting in a much lower self capacity.
One way to greatly lower the self capacity of a multilayer solenoid coil is bank winding.
Another way to lower capacitance is to arrange the wire crossings so that they cross near 90 degrees to each other rather than running parallel to each other. A coil winding machine is needed to this and a number of other types of winding. This is called Basket winding (Wiki). A similar winding is the honeycomb or diamond weave where pins are used on a cylinder and when the coil is finished and somehow stuck together the pins removed. Contra - wound coils use forming pins that are placed parallel to the central axis so that if one turn has a larger diameter then the adjacent turns will have a smaller diameter and so less turn to turn capacity.
A disk can have radial slots cut for the wire to be woven so that adjacent wires are on opposite sides of the disk. If radial pins are used to replace the disk and slots the coil is called a spiderweb coil.
Another way to lower the self capacity (and to allow for higher voltages without breaking down the insulation) is to seperate the coil into a number of series connected coils. One way of doing that is to place rings of insulating material along the axis of the coil and fill those with basket woven coils. This is the way RF chokes are made.
The formula for capacitance is ( k * Area)/Spacing
Although not in the capacitance formula the but it's easy to see that the voltage across the capacitor is very important.
- maximize the physical spacing between turns
- minimize the dielectric constant everywhere
- (Note: because of other considerations you usually don't have control of the wire diameter to lower capacity)
A single layer cylindrical coil where the turns are spaced one wire diameter apart (B&W Airdux) is pretty good. You can think of each wire to wire capacitance with some capacitance and so the total capacitance is the series combination of (N-1) capacitors. But if it's a two layer cylindrical coil then the first and last turns no longer have the same voltage relationship, the single layer coil has 1/(N-1) of the total coil voltage across each of the (N-1) capacitors, but in the two layer case the first and last capacitors have the full coil voltage across them, capacitors 2 and (N-2) have slightly less voltage, etc. in the end the voltages between adjacent capacitors is much higher. So:
- Wind the coil is such a way that minimizes the voltage differences between nearby turns. Hence another advantage of bank winding.
D/L Radio
The bulk wire resistance in an air core cylindrical solenoid type coil is a linear function of the length of wire used to make the coil. To get a given inductance you can wind a long skinny coil or a short fat coil, but the length of wire used will be very different. Wheeler's formula for the inductance (Wiki) is:
L = (r*r*N*N) / (9*r + 10*L) where:
L is inductance in uH
r is outer radius of coil in inches
l is length of coil in inches
N is number of turns
Length of wire (approximate) = 2 * (lead length inches) + N*2*PI*r
I say approximate because the turns are not rings of wire, but rather they have some slope between turns. This effect becomes more pronounced if there's a large gap between turns where the wire needed will be longer than the formula above would suggest.
As Far As I Can Remember (AFAICR) the shortest wire for a given inductance is when the diameter is about 2.4 times the length, i.e. a short fat coil, NOT a coil that looks like a toilet paper tube.
Note that most "Tesla Coils" you see have a tall skinny coil, but if you look at Tesla's Colorado Springs Notebook you'll see his coils were short fat ones AND the key thing most "Tesla Coils" get wrong is they operate the coil without capacitive loading. His coil experiments were a flop until he built the transmit mast AND loaded the mast with additional capacitance at the top i.e. it ended up being a top loaded vertical antenna which added a capacitance considerably greater than the self capacity of the coil. Another way of looking at it is to say the operating frequency was considerably lower than the self resonate frequency. A coil operating at self resonance has no Q multiplication.
Trivia: The units Tesla used to measure capacitors, inductors and frequency were all "meters".
Trivia: He made use of many Champagne bottles which in those days all came from France. I expect many cases were needed since they were used as capacitors by putting them in a tub of salt water and filled with salt water and they were used an insulators for the mast both at the base support and along the wood tower as lateral supports.
Spacing Between Turns
If a coil is would using insulated wire and the turns are all touching each other then there will be more turn to turn capacity than possible with other winding methods and the losses will be much higher because of the proximity effect (Wiki). So for simple solenoid type coils it's good to space the turns about one wire diameter. The fancier coil winding methods discussed above have even lower turn to turn capacity and hence lower proximity effect loss.
Wire Type
For most coils either solid magnet wire or Litz wire (Wiki) is used. Because of skin effect (Wiki) the AC current flowing on a solid conductor tends to travel near the surface of the wire. As the frequency increases you get to a point where you could have a hole through the center without adding to the resistance. This happens when the wire is about six skin depths (Wiki) in diameter. So for some applications theoretically you can get a higher Q coil by using Litz wire. I say that because you can get some very good results using plain old magnet wire. The trick is choosing the correct Litz wire for the specific frequency range and coil.
Notice that in high power RF transmitters or induction heaters where the coil is carrying very large currents the "wires" are really copper tubing with water cooling. If the wall thickness is more than three skin depths the AC resistance is about the same as if the inside of the tubing was all copper.
Trivia: The largest commonly available wire size comes about because of the skin effect at 50 or 60 Hz. I.e. the wire that's 6 skin depths in diameter at the line frequency is the largest one that's practical in an AC power system. If you need more power the next step is to lower the frequency, for example using 20Hz for a subway system. Or use DC where there is no skin effect.
Covers 50 kHz to 75 MHz. The 50 kHz is important because it allows testing WWVB 60 kHz loop antennas.
When I got my first 160 Q-meter the thermocouple was bad so I sent it to a guy in San Jose who installed a replacement thermocouple.
I've read recently that to allow the meter to have a longer useful life you might set the oscillator level to 2 instead of 1. This idea being to use a lower power which requires multiplying the displayed Q by 2 (or some other factor). But I just set it at full scale (1) all the time.
First 160 Q-meter
This one was rebuilt prior to using it for coil measurements.
Second 160 Q-meter
Got this one for spare parts to maintain my first working unit.
Need to check if the Banana sockets are the same pattern
on the Heathkit as for the 160. The 160 is the standard and
there are a number of reference inductors to fit it.
On the top of the circular vent plate the labels talk about
using patents of AT&T, WE and RCA.
The 8-pin 5W4 power supply tube is missing.
The 5Y3GT can also be used.
The 6-pin Boonton special number 105-A tube is missing.
This is a special tube that's similar to the Boonton special
number101-A or 101-B.
BRC535B is a selected 1659 (Radio Museum)
The 1659 is a selected 2A6 or 75.
You can see it's plate cap just behind the front panel.
This is the vacuum tube voltmeter that measures the Q.
Instructions for Replacing Type 165-A Thermocouple unit in
Type 160-A Q-Meter.
Licensed by Electrical Research Products Division, Western
Electric Company, Inc., under U.S. patents of American
Telephone and Telegraph Company and Western Electric
Company, Incorporated, for use only for testing or measuring
apparatus or phenomena or for other scientific or technical
investigation, development, or instruction, except for testing
or measuring physiological phenomena and except as part
of or in association with apparatus for the transmission or
reception by wire or radio, of intelligence of any kind, other
than in association with, but not as part of, radio broadcast
transmitting or receiving systems for testing and measuring
the transmission or reception thereby or programs.
The main oscillator 4-pin tube is missing.
Type 45 or later Boonton special number 102-A
BRC536A is a selected #45 tube
This is the meter I used for a number of years. It covers down to 50 kHz and so can be used for WWVB loop antennas as well as for other H.F. coils. Boonton also made a VHF range Q-meter but I haven't used it. HP bought Boonton in 1959 and continued selling Q meters for some time.Introduced by Boonton in 1934.
TM11-2635 TS-617B/U 3 January 1945
TM 11-6625-471-24P-4 TS-617C/U March 1977
The TS-617 is the military version of the 160 Q-meter. It includes a case of 14 inductors covering the range of 1 uH (8 to 20 MHz) to 25 mH (50 to 140 kHz).
The oscillator tube is a 5763. The VTVM and Bucking Diode use a 5726/6AL5W tubes. The balanced amplifier uses a 5814A tube. The AC rectifier is a 5Y3W GTA tube.
Manuals
TM 11-2635A Q-Meter, TS-617B/U, Feb 1956 with changes 1 to 5 (Aug 1976).
TM-11-6625-471-24P-4 Org, DS, GS, Maint Repair Parts for Q-Meter TS-716C/U, March 1977
about Feb 2016
Covers 150 kHz to 18 MHz, does not cover WWVB loop antennas at 60 kHz.
Maybe introduced in 1951. This appears to be very similar to the Boonton 160.
The Q standards come in two types. One has a removable metal can and in the other the can is hermetically sealed.
Both types have Banana plugs spaced to match the Banana sockets on the top of the Q-meters.
A quick look in a very full shipping container did not uncover an example.
You can see them on Google Images.
I phoned NBS and asked the inductor guy how they calibrated the Q standard and the answer was by comparing it to their in house "gold" standard inductor.
HP bought Boonton in 1959. HP developed the 4342A Q-meter (military TS-617) that covers 22 kHz to 70 MHz.
The military TS-617C was a full mil spec version of the 4342A and may be the ultimate Q-meter.
The HP 42851A Precision Q Adapter has a motor to turn it's internal resonating capacitor, I haven't use this instrument and must by used with an HP 4284A LCR meter.
This is a very expensive setup, even used, since it may be the most accurate (lower total errors) way to measure Q.
2137787
Method and Apparatus for Electrical Measurements, Harold
A Snow, Boonton
Radio Corp, Nov 22, 1938, 24/654, 324/653,
338/61
This appears to be the starting patent for the model 160
Q-meter.
The early part of the patent is about how to make a 0.04 Ohm
resistor that has a small amount of inductive reactance up to 30
MHz.
The Heathkit QM-1 above uses a capacitor instead of the special
resistor used by Boonton.
27 is the oscillator tube and 35 is the VTVM tube.
Inductors
flux gate magnetometers
General Radio GR 650-A or GR 1650B Impedance Bridges
HP4260A LCR meter
HP 4261A LCR Meter
HP4274 or 4275_LCR
HP 4332
HP395A HP 4395A Combination Network, Spectrum, Impedance Analyzer
Loopstick Antennas
Marconi TF-2700 Universal Bridge.
Joule Thief
Stanford Research SR715 LCR meter
Xam Crystal Activity Meter
Xec Crystal Unit Equivalent Circuit
Xtal Electronic Crystals
Xtal1800 Crystal Radio 1800? & Brooke Clarke #1
Xtc Crystal Temperature compensation Patents
ZM4 ZM-4 Bridge battery powered
ZM11 ZM-11/U Capactance-Inductance-Resistance Bridge, line powered
Z - Impedance requires test fixture and and way to de-embed the fixture's parasitics
Zo Transmission Line Zo vs. Frequency
Fun with Tubes - Test Equipment: The Neglected Q Meter. -
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Page created 12 Oct 2013.
