It might also be unsupported hype !! to judge from the tone of the text !!
Alan G3NYK
----- Original Message -----
From: "Tony Magon" <[email protected]>
To: <[email protected]>
Sent: Sunday, August 21, 2011 5:01 PM
Subject: Re: LF: Re: Re: Ferrite wideband antennas?
Hi All
This article may be of interest
http://gmweb2.net/The%20FS%20Loop.htm
73
Tony VK2IC
On Sun, Aug 21, 2011 at 11:18 PM, Clemens Paul <[email protected]> wrote:
> Antenna engineers use to say about design goal limits of antennas:
>
> Small
> Efficient
> Wideband
>
> Pick any two (meaning you can't have all three...).
>
> 73
> Clemens
> DL4RAJ
>
> ----- Original Message ----- From: "James Moritz" <
> [email protected]>
> To: <[email protected]>
> 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
>>
>>
>>
>> -----
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>>
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