Dear Peter, LF Group,
It is certainly possible to produce a ferrite rod
antenna with increased output by having multiple rods side-by side/end to end,
but it has to be quite big to compete with the usual sizes of air-cored
loops.
This is my viewpoint on ferrite rod RX antennas:- A
ferrite rod acts as an "area multiplier" when it is inserted into a coil. The
multiplying factor is called the "rod permeability". A ferrite rod antenna has
an effective area equivalent to an air-cored loop with area (rod
cross-sectional area x rod permeability). Rod permeability depends on the
length/diameter ratio of the rod, and the permeability (mu) of the ferrite
material (see the graph at the bottom of the page at http://www.amidoncorp.com/aai_ferriterods.htm
). For short, fat rods, the l/d ratio is the dominant factor, while for
very long rods, the rod permeability nearly reaches the ferrite material
permeability; the ferrites used in BC receiver rod antennas have
permeabilities in the hundreds. The upshot of this is that there is an upper
practical limit to rod permeability achieved by making the rod longer. There
will also be an optimum trade-off of length against area, since as the length
increases the volume of ferrite increases linearly, but the rod permeability
levels out. Therefore, a point is reached where less ferrite would be used to
make a rod with a larger area and shorter length, with the same factor (rod
area x rod permeability).
A rod permeability of the order of 200 seems to be the
optimum practical value for broadcast band materials; an air-cored loop with
about 1m^2 area can give a good SNR on 136 with an appropriate preamp, so a
ferrite rod of 0.005 m^2 cross-section would be expected to provide the same
signal output (assuming it had the same Q as the air loop, which is roughly
true). This would require a round rod of 8cm diameter. The length/diameter
ratio would be of the order of 20:1, so about 1.6m long - this would weigh
about 40kg.
This is only a very rough calculation, the rod
permeability is only an approximation and it may well be possible to come up
with better geometry requiring less ferrite. But it does show that a ferrite
rod antenna with good performance on 136kHz would have to be quite
bulky.
I have used an antenna with a single ferrite rod for
field-strength measurements; sensitivity is OK for signals above a few 10s of
uV/m, but certainly no good for DX reception. For SAQ and other VLF reception,
I have used a large ferrite rod about 3cm x 3cm x 35cm, made by sticking a lot
of surplus smps "U" cores together with silicone, enclosed by a casing made of
?formica? sheet to hold it all together - it works well at VLF, but has rather
low Q at 136k, due to the type of ferrite used
-----Original Message-----
From:
[email protected] [mailto:[email protected]]
On Behalf Of [email protected]
Sent: 05 January 2006 09:33
To:
[email protected]
Subject: LF: ferrite rods
I have a pile of ferrite rods that I have rescued from
radios over the
years.
I have been thinking about trying to make them into
one larger antenna rod
for receiving only.
Would I get any advantage over a normal short single
rod, or do the
advantages from one aspect get cancelled out by other
aspects. I'm sure
this must have been tried out before any advice would
be appreciated.
If I was to put them into an overlapping bundle
should the separate bars
be
a)in physical contact with each
other
b) completely isolated from each
other
c) if glued together is there a specific "glue" that
should be used
happy new year to all on list
regards
peter G8AFN
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