"Henry" wrote in message
...
Hello Mike -


Thank you for the very long explanation!

Indeed, I already experimented with (unknown) ferrite rods but I'm just
waiting here for new specified material (better) ferrites.
So I have time to search for more info...

Your mentioned link is one of the best I already found on the net.

In transmitting state the ui seems to fall to near 1. You can see it
because the resonance frequency is going much higher (and that
will also increase harmonics)!

I'm not sure if a ferrite rod is a dipole? There is another structure
possible: Ferrit-loaded helical antenna.

Unfortunately that is a kind of black art. Not much on the net to find.

The very low radiance resistance of the ferrite rod antenna is not the
main problem. You must match the impedance to the
transceiver. That is easier if the antenna is an integral part of the
system end amplifier, TX-RX switch and pre-amp.

If I understood you right, the "miracle whip" is not a antenna, more an
impedance transformer. Whereas the typical ferrite rod
antenna is an autotransformer by itself - simply by adding or tapping
another coil on the rod.


regards -
Henry

Hi Henry,

The receiving (and transmission) patterns of ferrite rods are similar to a
dipole. In actual fact, the field pattern of a ferrite rod antenna is at 90
degrees to that of a dipole if both antennae were mounted with their long
axis parallel to one another. In practice, the influence of nearby buildings
and structures probably distort the pattern so much that the ferrite antenna
becomes pretty much omnidirectional.

You are absolutely right about the low impedence of small transmitting
antennae. Matching 50 ohms to perhaps 1 ohm or less will always be a major
problem. The low impedences, lead to high currents and high resistive
losses, which result in the ferrite heating up as well as the
interconnecting wiring.

The miracle whip type of antenna, uses a toroidal autotransformer
transformer which happens to be wound on ferrite to reduce the size of the
unit for portable operation. There are other ways to transform impedence to
get a match to a 54 inch whip antenna. A decent transmatch ATU will load up
just as well, but makes for a much bigger unit to carry around.

The ferrite rod is used to concentrate lines of flux within the wire coil
winding around the rod. While this works well in a receive situation, as I
understand it, the situation is effectively reversed when transmitting. The
coil of wire concentrates the transmitted signal within the ferrite rod
core. Because of the way ferrite is made up, it becomes extremely lossy when
saturated with RF and most of the energy is dissipated as heat. The higher
the frequency, the more energy is absorbed by the ferrite.

As losses are lower at lower frequencies, I would anticipate that a workable
transmitting and receiving antenna could be produced for use on the 160 and
80 metre bands, with the possibility of being able to use this type of
antenna on 40 metres. Certainly ferrite can be used to make a receiving
antenna for use up to 7MHz or so.

I would suggest experimenting to get something that receives well on the
frequency of interest. Unfortunately, receiver front ends are nearly all
high impedence so this won't necessarily guarantee a match to 50 ohms. The
proof is if you can manage to get a low SWR reading when trying to transmit
(at low power!). The ferrite rod will be doing it's level best to act as a
dummy load, but you may get enough power radiated for the system to work.
The addition of a retractable whip coupled to one end of the coil may
enhance transmission by allowing the ferrite rod and coil to provide the
matching and the whip to do the radiating. But then we are almost back to
the miracle whip idea.

If you haven't already tried one, I think you should try building a miracle
whip type of antenna. It will certainly give you an idea of the performance
you might get using a ferrite rod antenna, certainly in terms of received
signal strengths and probably transmitted output too. Buying all the
components brand new comes to under a tenner. Two twelve way switches, a
35mm diameter ferrite ring, a 54" telescopic whip antenna, a roll of 0.9 mm
enamelled copper wire and a couple of metres of hook up wire, terminals as
required and a miniature toggle switch. Don't worry too much about neat
wiring, it will all end up looking like a rats nest when all the connections
are made. Stick it in any old plastic box, you don't care if the whole
circuit radiates and receives. You can eliminate the telescopic whip and use
a random length of stiff wire if you want to reduce costs further. The grade
of ferrite ring doesn't seem to matter too much, I used one I found in my
junk box and I have no idea of the properties.

Having a good earth connection makes a world of difference to the way these
miniature antennae behave. Water and central heating pipes work okay, but
you may pick up quite a lot of local electrical noise as well. Be prepared
to hunt around for a decent quiet earthing point.

Mike G0ULI