"Ceriel Nosforit" wrote in message
news
Hehe. Well yeah, I figured as much, but why doesn't it work? Too high
impendance? Why is that something the amp can't handle, if I maybe resist
the urge to crank it up to eleven?

No. The problem is that wavelength of a, say, 10kHz signal is 3km in free
space. You need to have an antenna that's a decent fraction of that length
for the antenna to look "distributed" enough (that is, the phase of your 10kHz
signal at the far end of the wire is significantly different than that at the
near end) in your to start getting any radiation. You can go through the math
for all this -- look up the input impedance of a short dipole on Google -- and
you'll find that the radiation resistance of a reasonable length speaker wire
is probably going to be in the milliohms, yet the finite (real) resistance of
the wire is going to be 10-1000 times as much, and the reactance will be as
well. Hence, all three parameters are against you: The high reactance will
keep the amps from being able to put much power into the cable in the first
place, and of what does get there, the vast majority will be eaten up as heat
rather than radiating.

To make matching tenable (so that you can actually get power into the cable),
you need a *very* long wire. To make things efficient, you need low loss
conductors -- nice, thick metal rods. Hence, doing a good job becomes rather
spendy, and thus you don't see that many people outside of the military
building VLF systems.

I think I'd almost suggest trying something like a 1.3GHz moonbounce system
prior to building your own VLF transmitter...

On Fri, 29 Sep 2006 17:13:28 -0700, "Joel Kolstad"
wrote:

No. The problem is that wavelength of a, say, 10kHz signal is 3km in free
space.

This must be a typo, since the free space wavelength at 10 kHz is 30
km.

Paul OH3LWR

Hi Paul,

"Paul Keinanen" wrote in message
news
On Fri, 29 Sep 2006 17:13:28 -0700, "Joel Kolstad"
wrote:

No. The problem is that wavelength of a, say, 10kHz signal is 3km in free
space.

This must be a typo, since the free space wavelength at 10 kHz is 30
km.

Oops, sorry... yeah, you're correct -- my mistake.

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.

- Henry


"Paul Keinanen" schrieb im Newsbeitrag
news
On Fri, 29 Sep 2006 17:13:28 -0700, "Joel Kolstad"
wrote:

No. The problem is that wavelength of a, say, 10kHz signal is 3km in
free
space.

This must be a typo, since the free space wavelength at 10 kHz is 30
km.

Paul OH3LWR

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

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

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

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