Message-ID: <000c01c2c0c8$c4b6ed30$9801a8c0@gigacom.com.br>
From: "PY2PLL" <py2pll@py.qsl.br>
To: rsgb_lf_group@blacksheep.org
References: <23.2aadfb3f.2b5c5522@aol.com> <5.1.0.14.0.20030120171137.00b158e0@gemini.herts.ac.uk>
Date: Mon, 20 Jan 2003 18:12:49 -0300
MIME-Version: 1.0
Subject: Re: LF: Loops x Q
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Sender: <owner-rsgb_lf_group@blacksheep.org>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

Hi James . . .

> It seems to me this is quite a large loop, and will generate a large
output:

Yes, it´s large ... at least the bigest one I ever did. And it´s a
prototype. I´m using PVC covered wire ... not soft enough to have evenly
"nice" turns spacing.

>
> For your loop, and a f/s of 1uV/m, this gives 0.21uV @ 400k, 0.85uV @
1600k.,
> With a Q of 200, the voltage at resonance will be increased to 42uV @
400k,
> 170uV @ 1600k

I think I´ll built a smaller one, just to make some "real" FSM and then
generate a table with the levels obtained with the bigger one. I have an SLM
from G&W which have a balanced 6K / unbalanced 75ohm input Z.

> The loop noise depends on the RX bandwidth, but in 10kHz BW it will be
> roughly 6uV @ 400k, 12uV @ 1600k. So for 1uV/m, SNR would be 17dB @ 400k,
> 23dB @ 1600k.

The thermal noise in my tables are simply the "coil" thermal noise @ 250Hz
BW. I used the article formula´s.

> The tuning is a problem - 4:1 tuning range needs 16:1 capacitance
> variation, including the stray capacitance of the loop. If L is about
446uH
> from the table, this means 350p - 22p. The stray capacitance of the loop
> may be greater than 22p, so you may not get the full tuning range. Using a
> varicap diode, it is unlikely the minimum- to maximum capacitance ratio
> will be high enough. it may be necessary to reduce the number of turns and
> use a larger tuning capacitor.

The loop turns are spaced by 5mm. I used G4FGQ´s software to estimate stray
C ...

> The other problem is that local stations will give very large signals -
eg.
> 10mV/m from an 1MHz MF broadcast station will result in about 1V across
the
> loop at resonance, which will probably lead to severe intermods from both
> the preamp and the varicap.

Ooopps ... this is true ... air-insulated C is winning ...

> You could connect the preamp across only 1 turn
> instead of all 9 - this would still give several uV out for 1uV/m f/s,
more
> than enough for a decent receiver.

I already did this way. I couldn´t resist to test it with an air-variable C
and no pre-amp so I added a 1 turn loop inside the main one, about 3 inches
between coils.

> In fact, since the loop output is so
> large, you could probably eliminate the preamp by using a single turn
> coupling loop to a 50ohm RX input. The air-variable tuning capacitor would
> also be a good idea for this reason, unless you live a very long way from
> any broadcast stations!

I guess I´ll keep this 1 turn solution ... the local BCs - nearest is about
5Km away - are pushing Kachina´s S Meter from 9+40 to 9+70 with internal
pre-amp off.

I´ll start the 136KHz prototype and compare it´s performance using 153KHz BC
and DCF39 as signal sources.

Thanks on the comments;

73
Marcus
PY3CRX/PY2PLL
http://py.qsl.br
Active from "DC" to Laser ...