Re: LF: Re: Ferrite wideband antennas?

[Daniele Tincani <danieletincani@yahoo.com>]

 

Hello Jim, LF,

 

thanks to all of view for feedbacks to my inquiry.

 

So my rough summary is:

- small-size (e.g. ferrite cored) loops need tuning = high Q to provide a reasonable signal level.

- intermediate-size (e.g. 1m diameter or so) loops can be wide-band but require low-noise, very low-impedance pre-amps.

- 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.

 

In general, loop size is more important for sensitivity than permeability of core material and number of turns.

 

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?

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?

 

Sorry for the (usual) long list of questions and thank you for reading.

 

Best regards

Daniele

From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Saturday, August 20, 2011 12:07 PM
Subject: LF: Re: Ferrite wideband antennas?

Dear Daniele, LF Group,

Regarding bandwidth, the first thing to note is that the same principles essentially apply to both air-cored loop and ferrite rod cored loop antennas - the main difference is that air-cored loops are wide and flat, but ferrite rods are long and thin ;-).

Assuming you can make a preamp with a low enough noise level, the minimum usable signal level "sensitivity" of a loop antenna depends on the ratio between the induced signal level, and the level of thermal noise produced by the resistance of the loop windings, core losses, etc. So this sensitivity depends on the construction and size of the loop/rod, and in principle it does not matter if it is tuned for narrow-band resonance or loaded to produce wide bandwidth, provided the tuning or loading arrangements do not introduce additional noise. But in practice, tuning/loading and preamplifiers will introduce some additional noise.

The big advantage of a tuned loop is that the resonant circuit can provide a high "passive gain". So Stefan's rod antenna probably produces an EMF in the nanovolt range for usable received signal levels, but the high Q circuit it forms with a parallel capacitor increases this voltage by more than 50dB The actual signal power level is not increased by the resonant circuit, but the much higher signal voltage is easily handled by a simple preamplifier with insignificant additional noise introduced. The resonant circuit also has a very narrow bandwidth - this might be an advantage for attenuating strong out-of-band signals, but is a drawback if wideband reception is required, or remote tuning of the loop is needed.

In many commercially available wideband loops, the loop is loaded by a preamp with a very low input impedance. This provides a flat frequency response, since the loop EMF rises in proportion to signal frequency, but the signal current at the preamplifier input is maintained constant by the reactance of the loop inductance, which  also rises proportional to frequency. This flat response is very popular for measuring applications and wideband reception. But the preamp design is much more difficult, because the input signal  amplitude is effectively attenuated by the combination of high loop reactance and low preamp input impedance. So careful preamp design is needed, to provide a low input impedance, very low noise voltage, and a low noise figure when fed from a highly mis-matched, relatively much higher source impedance. The "noiseless feedback" techniques such as "Zwichenbasis" amplifiers mentioned by DF6NM or "Norton" feedback amplifiers can be usefully used. But even with careful preamp design, relatively large loops (~1m) seem to be neccessary to achieve a reasonable sensitivity. Of course, if loop size is not an issue, one can simply increase the loop area  to produce a greater signal amplitude, and all that is needed is a large wire loop terminated by a low impedance receiver input.

In my view, for communications reception purposes, creating a flat output voltage vs. field strength relationship for a wideband loop is not particularly useful - the background noise field strength decreases with frequency, so if you keep the "natural" signal EMF-proportional-to-frequency response of a loop, the background noise at the receiver input remains fairly constant with frequency. I have used 2x2m and 4 x 5m loop antennas where the loop inductance forms the input inductor of a low-pass filter with cut-off frequency of about 550kHz, in order to attenuate powerful broadcast signals. These give reasonable results from VLF to 500kHz without any tuning adjustments.

Cheers, Jim Moritz
73 de M0BMU