Ferrite antenna com system

[Henry Kiefer]

Sorry "Roy" -

The effective permeability of a rod is dictated largely by the air gap
in the magnetic path, which is a function of the length/diameter ratio
of the rod. Powdered iron in general has very low permeability compared
to ferrite. If you really wanted to apply a huge amount of power to a
rod antenna, powdered iron might be a better choice because of its high
saturation flux density. But I doubt you could get the Q of a ferrite
rod antenna at the frequency in question, so it would be considerably
less efficient. You'd probably end up with less power radiated than if
you ran less power to a ferrite rod antenna, and a less efficient
antenna would impact your received signal. You'd have to crunch some
numbers or make measurements to find out for sure.
Surely.


The presence of even a small air gap has the effect of reducing the
effective permeability of the core and therefore the inductance of the
winding. It also dramatically reduces the core flux density for a given
number of winding amp-turns. This makes it very hard to saturate.
Inductors used for power applications commonly have a small core gap for
this reason. A rod has a very large air gap in the path -- from one end
of the rod, curving around outside the rod, to the other. And for many
ferrites used at radio frequency, the material loss is high enough that
the core would be hot enough to explode well before you reach a flux
level anywhere close to saturation. This isn't true of all materials at
all frequencies, of course.

The radiation takes place from the field outside the core, i.e., in the
air gap. If you didn't have a gap, you wouldn't have any significant
radiation.

I'm not so theory funded to explain why a gap in the core transmits
electromagnetic wave. That is an open question for me.
I even don't understand why a ferrite rod will accumulate magnetic field
lines. There is the explanation of "shorten" the field lines way. Some
mysterical!

If you talk about explosion: What is the power level you mean?
Currently I have a ferrite rod of 9mm diameter and 50mm length, driven by
40mA against 2Vpp.
Do you think it is impossible to pump maybe 5watts into the rod?

- Henry

 

[Roy Lewallen]

. . .
If you talk about explosion: What is the power level you mean?
Currently I have a ferrite rod of 9mm diameter and 50mm length, driven by
40mA against 2Vpp.
Do you think it is impossible to pump maybe 5watts into the rod?

Dunno. If it had to dissipate most of the 5 watts (which is likely), it
would get as hot as a resistor that size dissipating the same power, and
that would be pretty hot. Just how hot depends on how well insulated it
is, how good the air flow is around it, and how much heat is conducted
away through the wires or any other physical connection. The first thing
I'd check would be the Curie temperature of the ferrite. If you reach
that temperature, the material will lose its magnetic properties, so the
antenna impedance will abruptly and dramatically change. At some higher
temperature, the ferrite will fracture, maybe violently.

If that gets to be a problem, a lower loss ferrite might be necessary.
That usually means lower initial permeability, and probably a lower Q
inductor. You'll have to determine what the optimum trade would be.

Shouldn't be any trick to feed some power to one and watch its
temperature with a thermocouple.

Roy Lewallen, W7EL

 

[Paul Keinanen]

How much band noise should I expect?

When listening at the signal e.g. through an SSB receiver, it is quite
easy to know the difference. The equipment noise is more or less
constant "hiss", while the band noise is mainly through numerous
distant lightnings.

Some field strength measurements made in England at 73 kHz during the
summer, using a calibrated meter indicated 25 uV/m in 200 Hz
bandwidth, which would produce about 120 dB more power from a full
sized (2 km) dipole than a single matched resistor at the receiver
input.

Thus, even if the actual antenna efficiency was -100 dB and the LNA
noise figure as bad as 10 dB, the band noise would still be stronger
than the amplifier noise.

While an antenna with -100 dB gain would be usable for receiving, such
antenna would be useless for transmitting.

Paul OH3LWR

 

[Henry Kiefer]

Hi Paul -

Some field strength measurements made in England at 73 kHz during the
summer, using a calibrated meter indicated 25 uV/m in 200 Hz
bandwidth, which would produce about 120 dB more power from a full
sized (2 km) dipole than a single matched resistor at the receiver
input.

My receiver bandwidth is about 10Hz because of the 77.5KHz quartz crystal
filter. So the sensitivity is better. Is 10*lg(200/10) here correct?

Thus, even if the actual antenna efficiency was -100 dB and the LNA
noise figure as bad as 10 dB, the band noise would still be stronger
than the amplifier noise.

While an antenna with -100 dB gain would be usable for receiving, such
antenna would be useless for transmitting.

It is interesting to learn, that antennas can be noise limited. So
reciprocal theorem is not all to know.

Thanks -
Henry

 

[Henry Kiefer]

Hello Roy -

I will try your suggestions when I build a more powerful power amp.

As the modulation is AM with known 25% there is a interesting measurement
methode possible:
Ramp the amplifier from low to high power and make a curve of the AM
modulation depth at the receiver or just with a measurement coil at the
transmitter ferrite rod. It should show a saturation if the ferrite goes in
saturation. I will try this.
There is an open question what a time constant the ferrite will have if it
goes into saturation or out of it. And if this is very different if iron
powder is used.

- Henry