Henry Kiefer wrote:
I built a simple ferrite antenna communication system. Unfortunately it
won't work if I set the sender more distanced than about a meter. That is
even true with different transmitter configurations.

Here the details:
Transmitter:
ferrite antenna: diameter 8mm , 50mm long
frequency is 77.5KHz, digital modulation is AM 25%
bit-rate is 1 bit/sec (0 is 100ms carrier 25%, 1 is 200ms carrier 25%)
insulated copper wire coil 10 turns

The transmitter is self-constructed and delivers a very good signal.

Receiver:
same antenna copied, but a built-in resonating capacitor.
ready-to-use WWVB 77.5KHz receiver. Demodulated signal goes to scope.

The transmission works over about one meter without any shortage.


Now the problem is that I can change the transmitter parameters but I cannot
reach a substancial greater distance. I changed:
- the coil wound times
- output current to the antenna (measured across a series resistor)
- added an antenna current sensor coil to sense the antenna current and to
see if the ferrite antenna saturizes (NO! Very clean sinusoid)


Googling around to find theoretical aspects of ferrite antenne got no good
results. I spent several hours and read all I can read.


Have someone suggestions to try or good links to read? Especially for:
- when a ferrite or iron powder rod/bar goes in saturation?
- optimal rod dimensions
- optimal coil design (I suggest single layer, resonating with good Q
capacitor, about 3 to 10 turns)
- LNA design for such a low frequency?
- antenna field theory in near-field.

I can't really help you with ferrite antennas for transmitting, but can
tell you that if you google around for "lowfer" and the Longwave Club
of America http://www.lwca.org/ you will find a lot about antenna
designs that are suitable for this band. They will also might have
recommendations for frequencies of operation that are legal for
transmission in your home country (I don't even know what that is!)

LNA isn't really applicable here because there is so so so much
man-made and natural noise in this band.

I'm a little surprised that your achieved range was so small from a
ferrite rod antenna, actually. Did you really tune both antennas, in
place and in circuit, for resonance? The resonance is so so super
narrow that strays between design and circuit make a big difference. I
mean, CRT screens with flybacks, and faulty flourescent lamp ballasts,
and incadescent dimmers radiate all sorts of crap around the LF
spectrum for blocks, and they aren't even trying to be intentional
transmitters! And don't get me started about induction heaters and
welding machines, those can be heard across several states!

Tim.