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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Mon, 29 Aug 2011 21:53:03 +0100
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Subject: Re: LF: Re: Ferrite wideband antennas?
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Dear Daniele, LF Group,

You wrote:
>So my rough summary is:
>- small-size (e.g. ferrite cored) loops need tuning = high Q to provide a 
>reasonable signal level.
...Or a more sophisticated preamplifier design that reduces loaded loop Q 
without also reducing SNR. But practically this becomes more difficult to 
achieve as the bandwidth increases.

>- intermediate-size (e.g. 1m diameter or so) loops can be wide-band but 
>require low-noise, very low-impedance pre->amps.
There are quite a few amateur and commercial designs around of this type, 
and they can be quite good. The reason for having a preamp with a very low 
input impedance is that the ratio between output signal voltage and input 
field strength is then nearly constant over a wide frequency range. This is 
useful, e.g. for making wide-band field strength measurements, but not so 
helpful for reception purposes, since it just effectively means the noise 
level at the output rises as the frequency is reduced.

>- bigger loops (e.g. > 2m diameter) can be wide-band even when connected 
>directly to a low-impedance receiver input
>(without a pre-amplifier), may be through a broadband transformer to 
>furtherly reduce the load impedance as seen by the >loop.
...But unless the loop is very large, good receiver sensitivity is still 
required - be aware that many receivers have degraded sensitivity at low 
frequency.

>In general, loop size is more important for sensitivity than permeability 
>of core material and number of turns.
The number of turns essentially only varies the source impedance of the 
loop, but does not change the available signal output power. High 
permeability cores have slightly greater effective area, but usually have 
higher losses.


>For practical reasons, for rx activity at home I'm building a ~1m wide-band 
>loop. The reference design for the amplifier is >M0AYF's (despite it is not 
>a very low-impedance pre-amp, I think).

>Nevertheless, the discussion about ferrite-cored antennas here on the 
>reflector is more and more pushing me to build my >own "loopstick" and 
>experiment with it.
>Could it be possible to tune a ferrite loop over a frequency range as wide 
>as 100-500KHz? Or at least over the EU NDB >band (say 300-500KHz)?
>Any practical suggestion (and/or references on the web) about how to build 
>it?

A narrow-band loop that can be tuned over a wide range of resonant 
frequencies is quite easy - A winding inductance of a few hundred uH could 
be tuned with a capacitance varying from a few hundred pF to several nF to 
cover that range. This could be achieved with a combination of switched 
fixed capacitors and a variable capacitor. Or using the same tuning 
capacitor with windings of different inductance, like a long wave / medium 
wave ferrite rod antenna. A high input impedance preamp like the one used 
with my prototype loop, connected directly to the tuned winding, would give 
adequate gain for reasonably sensitive RXs. If you want a wide-band loop 
covering 100kHz - 500kHz with no tuning, that is more difficult...


>About the ferrites, there are several sellers on eBay who offer a russian 
>MU400HH type. Would it be suitable for LF? >Are there some recommended 
>alternatives?

Probably the best choice is ferrite intended for medium wave antenna rods, 
which combines low resistive losses with fairly high permeability. The 
losses generally decrease at lower frequency, so if it works at MF, it 
should also be good at LF.

Cheers, Jim Moritz
73 de M0BMU