Message-ID: <00f801cc6004$c2282bd0$1502a8c0@Clemens04>
From: "Clemens  Paul" <cpaul@gmx.net>
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>
Date: Sun, 21 Aug 2011 15:18:01 +0200
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Subject: LF: Re: Re: Ferrite wideband antennas?
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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
>
>
>
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