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Message-ID: <004801cc6055$87313f20$4001a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <16BC8B3CA8672445BC2A29B4C14A26D4379ED2AAB4@exlnmb01.eur.nsroot.net><4DF9EFD1.5010208@iup.uni-heidelberg.de><1313780109.51443.YahooMailNeo@web111907.mail.gq1.yahoo.com><9CD1E11E8BC9402CB4AECECAC4088443@JimPC><00f801cc6004$c2282bd0$1502a8c0@Clemens04> <CAJqZy6yoySSmxUwW9nS6u6b7p9PK6nm4XQbFbKpKydjB70-1vw@mail.gmail.com>
Date: Sun, 21 Aug 2011 23:47:54 +0100
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Subject: Re: LF: Re: Re: Ferrite wideband antennas?
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It might also be unsupported hype !! to judge from the tone of the text !!
Alan G3NYK

----- Original Message ----- 
From: "Tony Magon" <temagon@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
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 <cpaul@gmx.net> 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" <
> 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
>>
>>
>>
>> -----
>> eMail ist virenfrei.
>> Von AVG überprüft - www.avg.de
>> Version: 10.0.1392 / Virendatenbank: 1520/3835 - Ausgabedatum: 15.08.2011
>>
>
>
>