LF Coil antenna impedance
LF Coil antenna impedance
(OP)
I'm looking for ideas or references that will help me design multi loop coil antennas for 1380Hz to 5000Hz range. The system is a transmit and receive system that puts out a bipolar pulse, and measures the Eddy current decay off of a piece of metal that passes between the antennas. I'm looking at what I can do to increase the sensitivity and improve the transmitted energy pulse to increase the eddy current induced in the metal for detection. Any ideas would be greatly appreciated.





RE: LF Coil antenna impedance
Sounds unusual from an antenna point of view. Why multi loop antennas. Helical, spiral and quadrafilar are most common at that frequency band.
If you want to concentrate a field at a certain point that you pass the metal between, you can make an elliptical reflector. I've had a 1" spot made at 2450 Mhz before to heat chicken. It used a one foot diameter elliptical and the spot was about one foot from the reflector. If you had back to back elliptical reflectors, that could be what you're asking.
kch
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance
Have you considered making a metal box with open ends (Microwave waveguide) and locating it around your conveyor. Then have antennas radiate inside this box through a slot in the box. When RF energy flow is concentrated in a metal box like this (similar to a microwave oven), field densities increase. Hence I'm hinting changing your environment to concentrate the energy.
Alternatively, if you metallize your conveyor line and have box shaped antennas just above the conveyor, it'd concentrate the energy better too.
These solutions are suggested in part because I'm not familiar with your present antenna design and test setup.
kch
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance
Putting a DSP to work to add some post processing can probably give you a major return. sub-bit averaging can give you more range, chopping may give you a better noise floor (moves you away from the 1/f noise).
You might be able to find a quieter/more stable/simpler preamplifier than one designed 40 years ago too.
Boy, I bet there was a lot more metal on the Ford than there is on that Dodge!
Z
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance
One technology emerging that will give your receive antenna sensitivity a boost of about ?30 dB is non-Foster matching network. Your short antennas are essentially a capacitor or inductor and OpAmp technology at the antenna output can create an active C or L to resonate with your electrically small antenna. It can make it a tunable C or L to sweep thru frequencies too I believe.
It's more difficult to do for transmit antennas, but works well on receive antennas.
I actually haven't made any, we've discussed it at work, and considered proposing them. I have a few writeups that I researched.
If you go that way I'd be interested in what you find. Maybe I can help out a bit.
Kevin
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance
here's the best article I could find, with improvements of 9, 18, 27 dB depending on antenna and frequency. Alot of good theory, but frequencies don't go down as far as yours usually.
htt
good luck,
kevin
RE: LF Coil antenna impedance
For transmit mode, to increase the magnetic field output you need to increase the amp-turns. The simplest way to do this is probably to add more turns, but at some point the inductance may impede your "high frequency"
I would design & treat the transmitter as a current source.
For receive mode, the optimum coil might well be different. In general, I think that you can use many more turns.
You don't necessarily need to match impedance for receive mode because the signal-to-noise ratio (SNR) at these frequencies is almost certainly set by the external environment (not the amplifier input).
Careful filtering might be essential.
Another thing that might be possible (depending upon how much time you have) would be to repeat the measurement and synchronously add-up the return signal. If the target is in 'the zone' for ten pulses, then a 10:1 (10dB) SNR improvement should be possible.
Note - all above is based on what I know about LF communications systems. I believe it is applicable, but stand-by for any corrections from others.
RE: LF Coil antenna impedance
Normal filters, like you look up in a table, have an input and output impedance, like 50 and 50 ohms. It is hard if the load impedance is 2 + J300 ohms!
RE: LF Coil antenna impedance
The reason for the original question is the documentation for the original antenna designs were incomplete. I.e. It's more of a trial and error to get the right number of windings to the correct current level. I've looked at the basic formula for an inductor, but with the size and seperation of the sides of the windings, it didn't quite come out to what is realistically comming from the circuit(even with worst case performance values). If I have a good starting point for the antennas I can make sure any changes I implement later won't detract from or I can design an active tuning circuit with a "Standard" turn antenna.
VE1BILL, you were close for the Amp turns, except. Reducing the number of turns increases the current. i.e. inductor length reduced along with bundle thickness. I also like the concept of noise canceling filtering. The timing isn't too terrible, however. Not sure I'm willing to do a major board re-design just yet
Hopefully I can find a version of the inductor formula that might be closer to what we have measureable from the antenna/coil/windings.
RE: LF Coil antenna impedance
This layer of analog filtering is likely to provide some advantages as compared to going straight into the digital domain (even the best software defined radios would benefit from roofing filters - excuse the 'comms' view).
And with a buffer, the signal source impedance need not affect the filtering.
The only tricky bit would be making sure that the huge transmit pulse and other noise sources don't overload the front end of the receiver. There are receiver techniques to address this, basically using preamps with lots of headroom if they're before the filtering and outside the automatic gain control loop.
With respect to the relationship between the coil and the current, it makes sense that the output current should be right up against the reliable limit of the transmitter. Thus the amps should be defined by the source, and then you design a coil with as many turns as you can without affecting the current.
In general, whenever I see people experimenting with extreme magnetic fields (Google: magnetic 'can crushing' and 'coin shrinking' for some insane examples), they always use just a moderate number of turns on their coil because otherwise the inductance impedes the current.
Sounds like a 'fun' project.
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance
Single layer
L = (r * N)^2 / (9*r + 10*l)
N = sqrt( L* ((9*r + 10*l)) / r
Multi layer
L = 0.8 * (r * N)^2 / (6*r + 9*l +10*b)
Single layer spiral
L = (r * N)^2 / (8*r + 11*b)
watch out for those lower case "L"; if it doesn't make sense as a one then it must be an "l".
Of course, someone else may have posted these on the web by now...
RE: LF Coil antenna impedance
CCCCCCCCCCCCCCCCCCCCCCCCCC
C C
C C ____
CCCCCCCCCCCCCCCCCCCCCCCCCC /
----------------------------------------C=====<
CCCCCCCCCCCCCCCCCCCCCCCCCC \____
C C
C C
CCCCCCCCCCCCCCCCCCCCCCCCCC
RE: LF Coil antenna impedance
RE: LF Coil antenna impedance