DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btopenworld.com;
  h=DKIM-Signature:X-Yahoo-Newman-Property:X-YMail-OSG:X-Yahoo-SMTP:Received:Message-ID:From:To:References:In-Reply-To:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MIMEOLE;
  b=d6Vy5YAft5wfmgPliLzfvS3F3oEv8SY8SDAbgv8/2LO/AB9bFnLoqWL65SAm45wNIQTtpOmPwjenRpGgL1HgpjzYpWvSJtTifQNgqmUtqZJinSm4P3eXGm50/OcBlpbGOUwQq/2z8IrH0YWA0BhnH03XQeWrbFEpeC8EjhrkcWM=  ;
Message-ID: <EE4DAAED21874BA5957F1FCB7C61722D@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4F286089.000027.05504@BOB-PC>
In-Reply-To: <4F286089.000027.05504@BOB-PC>
Date: Tue, 31 Jan 2012 23:35:33 -0000
MIME-Version: 1.0
DomainKey-Status: good (testing) 
Subject: LF: Re: Converting antenna
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-1";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
X-AOL-SCOLL-AUTHENTICATION: mail_rly_antispam_dkim-d277.1 ; domain : btopenworld.com DKIM : fail  
x-aol-sid: 3039ac1d41184f287b1327f3
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


Dear Bob, LF Group,

I guess as far as LF frequencies are concerned, the Butternut antenna is 
just a 10m vertical whip (provided you disconnect the coil across the feed 
point - otherwise it will be nearly a short circuited whip...). This will 
give you plenty of signal, provided the antenna is reasonably matched to the 
RX input. Matching requires tuning it to resonance. The simplest form of 
tuner is just an inductor connected between the feed point of the antenna 
and the low impedance RX input. The inductance needs to resonate with the 
antenna capacitance at 136kHz. A 10m vertical piece of tubing probably has 
about 100pF or so of capacitance, so quite a large inductance is needed, 
roughly 13mH. For receiving, small ferrite-cored chokes are OK as loading 
inductors - probably the easiest way to get this much inductance to to would 
be to get a small selection of 1mH and 2.2mH chokes and connect them in 
series - this makes it easy to adjust the inductance. This sort of 
arrangement gives a very sharp resonance, with a bandwidth of only 1kHz or 
so. For fine tuning, you can connect a variable capacitor say 100pF or so 
between the antenna and ground - this will reduce the available signal level 
a bit, and the required inductance, but is desirable due to the critical 
tuning.

The signal level from an antenna this size should be enough for reasonably 
sensitive receivers - as you tune the antenna to resonance, you will hear a 
dramatic increase in signal levels and band noise. However, some receivers 
have very poor sensitivity at LF, so a preamp is sometimes needed.

Hope this is helpful - good luck with LF!

Cheers, Jim Moritz
73 de M0BMU