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#21
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VLF from the amp
On Fri, 6 Oct 2006 17:24:40 +0200, "Henry Kiefer"
wrote: You can use ferrit antenna rods. That works for receivers very fine in VLF band and for transmitters too - as long as the ferrit won't go into saturation. That is a magnetic antenna and it is much shorter than the equivalent mentioned. Ferrite rod antennas with perhaps -50 to -60 dB gain are useful since the extremely high band noise at VLF. It does not matter very much if both the wanted signal and the band noise is attenuated by 60 dB, as long as the wanted signal at the antenna terminals is stronger than the preamplifier thermal noise. For transmitting, such antenna would be ridiculous. In order to get 1 W EiRP as allowed on the 135 kHz amateur band in many countries, you would have to feed 1 MW into an antenna with -60 dB gain. The ferrite rod would saturate at much lower power levels such as 1 W or a few mW. Most likely you might get a few nW radiated power without saturating the rod. Paul OH3LWR |
#22
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VLF from the amp
Sure Paul. It is as a transmitting antenna for short-range only. Do you know
about: http://www.ancom.no/ products? They claim their ferrite rod styled antenna works as good transmitter. See their patent application. regards - Henry "Paul Keinanen" schrieb im Newsbeitrag ... On Fri, 6 Oct 2006 17:24:40 +0200, "Henry Kiefer" wrote: You can use ferrit antenna rods. That works for receivers very fine in VLF band and for transmitters too - as long as the ferrit won't go into saturation. That is a magnetic antenna and it is much shorter than the equivalent mentioned. Ferrite rod antennas with perhaps -50 to -60 dB gain are useful since the extremely high band noise at VLF. It does not matter very much if both the wanted signal and the band noise is attenuated by 60 dB, as long as the wanted signal at the antenna terminals is stronger than the preamplifier thermal noise. For transmitting, such antenna would be ridiculous. In order to get 1 W EiRP as allowed on the 135 kHz amateur band in many countries, you would have to feed 1 MW into an antenna with -60 dB gain. The ferrite rod would saturate at much lower power levels such as 1 W or a few mW. Most likely you might get a few nW radiated power without saturating the rod. Paul OH3LWR |
#23
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VLF from the amp
On Fri, 6 Oct 2006 22:27:47 +0200, "Henry Kiefer"
wrote: Sure Paul. It is as a transmitting antenna for short-range only. So we are talking about a near field system. In the far field the ratio between the electric (E) and magnetic (H) field is constant and is 120 pi or 377 ohms. This field is inversely proportional to the square of distance since both the electric and magnetic field is inversely proportional to the distance. However, very close to the transmitting antenna (near field) the E/H ratio is no longer 377 ohms. The magnetic field is inversely proportional to the square of distance and the electric field inversely proportional to the cube of distance. There is much debate were the near field ends and were the far field begins. For simple antennas, the distance is less than one wavelength, the value lambda / (2*pi) often appears in literature. For large parabolic reflectors or lasers, the far field begins at hundreds or thousands of wavelengths. Near field communication systems have been used for decades e.g. in induction loop systems for the hearing impaired, in which a magnetic loop is surrounding the room and audio frequencies are fed into the loop. The headset will pick up the field, amplify the signal and feed it to the ear of the person with reduced hearing. Of course, such systems are now more or less useless due to the stray magnetic fields caused by various inverters. Do you know about: http://www.ancom.no/ products? They claim their ferrite rod styled antenna works as good transmitter. See their patent application. These seem to be near field devices even at MF, where they seem to operate. Paul OH3LWR |
#24
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VLF from the amp
Thanks Paul for the long explanation!
I don't found a good source how ferrite rods works for transmitters. All say it cannot work because of big power eating in the ferrite. How much power is possible? I think 1 watt should be possible. I google on... http://www.ancom.no/ products? They claim their ferrite rod styled antenna works as good transmitter. See their patent application. These seem to be near field devices even at MF, where they seem to operate. No! They are far-field devices. I even saw a explosion picture of a mobile phone antenna 900MHz helix construction as composite in a (ferrite?) filled outline. Does an antenna exists, which receives BOTH field-vectors? If the antenna covers only one /E or M), half of the transmitting power is lost - correct? - Henry |
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