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

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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