From: reinier.I_HATE_SPAM.gerritsen@NO_SPAMchampionchip.nl (Reinier Gerritsen)
Subject: Re: Coil antenna
Date: Fri, 20 Sep 2002 07:24:31 GMT
Organization: Solcon Holding BV, Dronten, The Netherlands
References: <email@example.com> <firstname.lastname@example.org>
X-Newsreader: Forte Free Agent 1.11/32.235
On Thu, 19 Sep 2002 21:48:58 +0200, Stef Mientki
>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.
There is not much of modulation I guess. I have an old hart rate
monitor that gives a short burst of some 10KHz (may be 5KHz too) for
every hart beat. So modulation frequency is about 200Hz at most. Take
250 Hz as maximum. Then Q=5K/250=20. Probably possible with an air
>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.
Flas is ok. It gives you the highest inductance (best coupling between
the windings) and therefore the highes Q, assuming losses are equal
for a cylinder and a flat coil.
The voltage you receive is proportional to the number of turns, the
area of the coil and the Q factor. Keep in mind that the voltage goes
down with the third power of the distance, so moving from 1 to 2 meter
distance gives you 8 times lower voltage.
I'm not sure if you can have a range of 3 meters. Signal level will be
very low! You'll probably need a large antenna.
>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.
>> email@example.com (Mmsya) wrote in message news:<firstname.lastname@example.org>...
>>>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,