Envelope-to: dave@picks.force9.co.uk
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From: "peter.cleall@virgin.net" <peter.cleall@virgin.net>
To: rsgb_lf_group@blacksheep.org
Date: Fri, 6 Jan 2006 04:16:25 -0500
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Subject: RE: LF: RE: ferrite rods
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Original Message:
-----------------
From: james moritz j.r.moritz@herts.ac.uk
Date: Thu, 5 Jan 2006 18:03:08 -0000
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: ferrite rods


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: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of
peter.cleall@virgin.net
Sent: 05 January 2006 09:33
To: rsgb_lf_group@blacksheep.org
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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