From: Stef Mientki
Subject: Re: Coil antenna
Date: Thu, 19 Sep 2002 21:48:58 +0200
Organization: The University of Nijmegen, The Netherlands
References: <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: Thu, 19 Sep 2002 19:48:59 +0000 (UTC)
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.2a) Gecko/20020910
X-Accept-Language: en-us, en
which reminds me of the theory I studied years ago, but how about the
(I'm facing an equal problem even at the lower frequency of 5 kHz, Polar
If I understand you well,
first thing to do is open the receiver or transmitter to see if it has a
coil (which is very likely in my case).
Then we've to make a resonant LC-circuit with a not too high Q, because
of the modulation.
So that's an air coil of with the largest possible diameter, in my case
I can accept about 5 centimetres.
How should the coil look like:
- should it be a flat coil (say about 5 mm high) or more a cilinder (a
few centimeters high) ? My feeling says that a flat coil is more
omni-direct, but I don't know if this gives a significant in practice.
And as I remember well the shape influences the induction, but I can't
remember the relation.
Tom Bruhns wrote:
> Some factors to consider:
> If you only want to receive 25kHz, you can make a coil, resonated
> with a capacitor, with very high Q (= very narrow bandwidth). But if
> you are trying to receive a modulated signal, you need to keep the Q
> low enough that the circuit passes the modulation sidebands well
> enough. In addition, if the circuit is very high Q, you must be aware
> of drifts. A 1% drift in a circuit with a Q of 50 is probably not an
> issue, but in a circuit with a Q of 500, it is very much an issue.
> You won't get high Q at 25kHz if you make a small air-core coil. If
> the core is ferrite, the Q _may_ be high enough that you need to worry
> about it.
> At only 3 meters distance from the source, you are distinctly in the
> "near field" and you should not try to analyze the coupling as
> "electromagnetic". Instead, you should look at either magnetic or
> electrostatic coupling. That is, if the source is a loop, you should
> receive with a loop; if the source is a probe, you should receive with
> a probe.
> You should be able to use very basic electric or magnetic field theory
> to know what signal the source voltage or current (and associated
> transducer) will induce in the receiving transducer, and from that,
> whether it's enough for your needs.
> firstname.lastname@example.org (Mmsya) wrote in message news:<email@example.com>...
>>Please help. I am trying to build an antenna made of a coil to receive the
>>signal from a 25KHz. transmitter. I would like to have the coil to be as small
>>as possible and to be able to receive the signal within 3 meter distance. I
>>have the following question:
>>a- What type of core material should I use for the inductor?
>>b- Is wire guage important?
>>Any other factor should I be concern?
>>Thanks for all your help in advance,