I've managed to find time to read it (or rather, scan it). My first
reaction (I might be mistaken) is that he claims familiarity with the
works of Hertz and Tesla but seems not to be terribly aware of a guy
named Maxwell who who only managed to unify some laws of electromagnetic
radiation theory.
Should we, at this point retire to a quiet hostelry and discuss Black
Holes and singularities over a cold beer or three?
On Sun, 2011-08-21 at 23:47 +0100, Alan Melia wrote:
> 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
> >>
> >>
> >>
> >> -----
> >> eMail ist virenfrei.
> >> Von AVG überprüft - www.avg.de
> >> Version: 10.0.1392 / Virendatenbank: 1520/3835 - Ausgabedatum: 15.08.2011
> >>
> >
> >
> >
>
>
--
73 es gd dx de pat g4gvw
qth nr felixstowe uk
(east coast, county of suffolk)
|
