Ferrite antenna com system

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

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

Hi Paul -

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?

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

Thanks - Henry

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

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