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I need a coil antenna for a small car module which takes its input from my cellphone and transmits it at 100MHz. I have thus far learned that I need 8 windings of 0.25 inch inner diameter. I presume I just feed the antenna output into this coil, and the other end of the coil to GND. Is this correct?

Cluebat bludgeoning is welcome, it is very annoying to spend time on making antennas that do not work.

It would not hurt to have a range of ~50 meters on this device, so it can be used in the garden as well.

Posted in EE because amateurradio is pretty silent these days, and most hams are here as well.

Edit: Coil build info came from this page. The FM device I am using is a small plugin module to my phone (microUSB), it states 50mW effect. It has a tiny coil, but its range is 50cm. Haven't measured current draw, but voltage is about 3.7-4.1, depending on charge level. Car antenna is on the roof, so there's no audiolove.

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  • \$\begingroup\$ It doesn't sound correct - where did your design details come from? Provide a link please. You didn't mention bandwidth or available power so predicting a range of 50 metres is impossible. Neither have you mentioned what the receiver is? Poor question really. \$\endgroup\$
    – Andy aka
    Commented May 12, 2017 at 9:20
  • \$\begingroup\$ The coil info is from this page: google.com/amp/www.instructables.com/id/… \$\endgroup\$
    – user2497
    Commented May 12, 2017 at 9:27
  • \$\begingroup\$ I've looked at several pages which describe coil diameter and number of windings. I do not know enough about the subject to determine which is right. Can you recommend a page? \$\endgroup\$
    – user2497
    Commented May 12, 2017 at 9:37
  • \$\begingroup\$ The coil is the inductance of the (LC) tuned circuit which sets the transmitting frequency - it has nothing to do with the range. \$\endgroup\$
    – JIm Dearden
    Commented May 12, 2017 at 9:46
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    \$\begingroup\$ Keep in mind that such transmitters for the FM broadcast are low power and have bad antennas for a reason. If your transmitter disturbs tge reception of licensed broadcasts, then the FCC (or your country's equivalent) can fine you. Those little transmitters operate in a (very dark) gray zone. You are not licensed to transmit in that band, so it is illegal to do. But, if you don't disturb anyone it is unlikely anyone will care. Owning such a transmitter is not illegal, but it is illegal to disturb a licensed broadcast with it. \$\endgroup\$
    – JRE
    Commented May 12, 2017 at 10:36

1 Answer 1

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Take a look at this picture that I have amended: -

enter image description here

Well, it's the antenna.

I need a coil antenna for a small car module which takes its input from my cellphone and transmits it at 100MHz. I have thus far learned that I need 8 windings of 0.25 inch inner diameter. I presume I just feed the antenna output into this coil, and the other end of the coil to GND. Is this correct?

No, that coil is not the antenna - the blue wire is the antenna and I reckon it should be about 75 cm long optimally for the 100 MHz band. This is what the coil does (see L1): -

enter image description here

L1 is part of a colpitts oscillator that is frequency modulated by amplifier Q1.

However, this design is totally illegal (as presented in the guide you linked to) so I can't encourage you to build it at all.

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  • \$\begingroup\$ I am not building that pile of s??t. I will add short wire antenna to my phone's fm-module. I have several 868/433M modules which I use with arduino etc, and these all have coils. So does my 5/8 UHF antenna. Have I led myself astray with these observations? \$\endgroup\$
    – user2497
    Commented May 12, 2017 at 10:19
  • \$\begingroup\$ I think you have led yourself astray. A coil of that size at 100 MHz will never produce a proper EM wave - it will generate a H field and this can be received but the attenuation of just a H field is proportional to distance cubed (as opposed to attenuation being related to distance for a proper EM wave antenna like a monopole or dipole). \$\endgroup\$
    – Andy aka
    Commented May 12, 2017 at 10:24
  • \$\begingroup\$ So I should stick with a quarter wave wire antenna? Is that your best advice? \$\endgroup\$
    – user2497
    Commented May 12, 2017 at 10:25
  • \$\begingroup\$ You can go shorter than 0.25 but the reactance of the antenna means you might need to series tune with an inductor and also the significant lowering of radiation resistance seen at the antenna connection might be troublesome to some circuits: aa5tb.com/efhw_13.gif \$\endgroup\$
    – Andy aka
    Commented May 12, 2017 at 10:28

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