I have heard of interesting ideas around this frequency for lightening detection. You might try a search with those key words. Otherwise, there is the ham radio books on generic low frequency design.
Please, post any interesting information you get. I have freinds interested in 110 kHz for liightening detection, but it is for a teaching aid, not for industrial use.
Active Antennas work pretty well at such LFs. Basically a whip feeding a Hi-Z FET input. Not directional.
If you want directionality, then you're looking at some sort of loop (perhaps ferrite core).
Note that I have assumed that your application is Receive Only. If you need to transmit then you've got a "whole 'nother ball of wax" where efficiency becomes The Main Issue. For receive application, efficiency isn't really an issue at these frequencies.
I'm interested in finding a source for a low frequency antenna design and fabrication. The application for the antenna is for testing tire pressure sensors that are installed on automobile tires. Communication to the sensor
is by means of a 125 kHz signal (+/- 10%). The sensor is tested under pressure in a ferrite vessel whose inner diameter is approximately 4" in
diameter. Therefore size restrictions need to be considered as well. Connection to an external signal source is to be accomplished by an SMA
connector.
It would be simplest and cheapest to procure one copy of the normal on-board receiver package (as found on the usual vehicle equipped with such a system) and simply 'hack' into the front end of it to extract the signals you desire.
Ferrite never struck me ;-) as a particularly good material for a pressure vessel, presumably charged to at least 30 PSI and perhaps as much as 200 PSI.
Hmmm, I'll be over there, crouching behind the filing cabinet...
I also hear ferrite needs a protective skin to take the pressure.
When I was looking into buying ferrite loaded antennas on the web, they warned about cracking and they only sold the antennas with encapsulated ferrite cores placed in tough PVC pipe for protection.
kch
PS: After review, I wouldn't go with the 0.75 mile railroad dipole
Ferrite (of the normal variety) would be more or less hard-up against the wrong end of the Resilient-to-Brittle scale.
This thread reminds me of the old joke about the bidders conference where, just as the industry people were getting up leave with their notebooks full of scribbles listing all the system requirements, the government procurement agent suddenly said, "Oh, by the way, one more thing... The entire system needs to fit through a tube with an inside diameter of 4.00-inches and with a 90-degree elbow half-way along".
At this frequency, especially given the size constraints, you do not need exactly an "antenna" but more of either a "capacitor" or one side of a "transformer". You are at a tiny fraction of the wavelength for this frequency. In other words, Capacitively or inductively couple to your device-under-test.
If you try to capacively couple, then think of your pressure transmitter as one plate of a capacitor. What you need is a probe inside the pressure chamber to pick up the signal. Follow the probe with a amplifer with a resistor to set the output impedance for 50 Ohms (I am assuming you are trying to connect to test equipment).
If you try to inductively couple to the device-under-test, put a coil inside the pressure chamber to pick up the signal. Again, you can follow it with a amplifer. With a coil pickup, you can use a differential amplifer, or a single-ended amplifer.
In either case, the amplifer can be a op-amp with sufficient gain/bandwith well above the 125 kHz you are interested in.
Coupling inside a metal chamber is typically much higher than in open air, at least at higher rf frequencies, all the energy radiated and received stays inside the metal walls and there is no space spreading loss.
You may not need much of an antenna.
I wish I knew more about inductive/capacitive coupling antennas, but once again, a beginner am I.
kch
Thing is that a capacitive coupler will almost certainly have very low capacitance due to plate dimensions and spacing, so a relatively high voltage drive is needed and the receiver can be a simple ultra high impedance bi-fet amp with low capacitance input Z.
If inductive coupling is used then it's a low Z drive and low Z receiver input. My bet would be with the inductive coupler.
I'd make the RX antenna from either a ferrite rod or 1/2 a potcore. eg, an inductor of 740uH wound on an Amidon R33-037-400 ferrite rod (.5" dia x 4" long) tuned by a 2.2nF capacitor will resonate at 125KHz. Couple into your RX amplifier with a tap at about 10% from the earthy end of the inductor and you have an antenna.
The TX antenna will be similar but you will play with the coupling tap point (which could alternatively be a seperate coil wound over the earthy end of the main inductor) for best results.
This is a non-critical design but do choose stable capacitors to resonate the inductors, silver mica are best such as Digikey 338-1055-ND.
