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Help with antenna phantom power

Discussion in 'General Electronics Discussion' started by kruskal, Mar 8, 2017.

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  1. kruskal


    Jan 27, 2017
    I want to create a phantom power supply for an active car DAB-antenna (174 - 240 Mhz, 25mA), and I've been trying to read me up on how this is done. As you may know from my previous posts, I'm lacking a lot of the basics when it comes to electronics. So after some Google-fu, I found the following phantom power circuit for an active AM antenna at


    My understanding is that the 47uH inductor let DC pass, but stops AC. On I found the formula
    For the lowest AM frequency, 535 kHz, and 75 ohm impedance this gives a needed inductance of ~22uH. So it seems the 47uH gives some headroom. When using the formula for 174 Mhz and 50 ohm, the required inductance seems to be ~46nH. My proposed circuit then looks something like this:


    1. Is it actually this simple? No more filtering?

    2. Should the inductor be of molded type (as in the AM example), or can it be any type?

    3. Is there any problem selecting a higher inductance, lets say 1mH?

    4. Is it OK to use 0.1uF ceramic for C1?
    Last edited: Mar 8, 2017
  2. duke37


    Jan 9, 2011
    1. Not quite as simple. A choke has an internal capacitance between the turns so it will have a resonant frequency, It will act as a choke below this and a capacitor above this. If you have a grid dip oscillator, the resonant frequency of the 47μH choke can be measured. It is likely to be in the range of a few MHz.

    For higher frequency, use a smaller choke or put two chokes in series.

    2. Moulding holds the wire together so it is more robust. A VHF choke will have fewer turns and thicker wire so moulding will not be necessary.

    3. A higher inductance can drop the self resonant frequency. It must not drop below the frequency which is being received.

    4 0.1μF should be OK also 0.01μF. Keep the leads short to minimise inductance.
    Harald Kapp likes this.
  3. kruskal


    Jan 27, 2017
    Thank you for your reply!
    I found a RF choke calculator at, and with a frequency range of 174 - 240 MHz and 50 ohm impedance it suggest something like this:

    The first suggestion with a SRF over 240 MHz is actually way down the list (0805LS-182). So it seems the impedance is higher for other suggestions even if the SRF is below 174 MHz for those alternatives.

    I think I'll play safe and stick to your advice, going for the 0805LS-182 with a SRF of 246 MHz.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    246MHz is awfully close to your highest operating frequency.
  5. kruskal


    Jan 27, 2017
    Then I'm a bit confused why the RF choke finder displays so many inductors with a SRF below the input frequency (174 - 240 MHz) ?

    With it be better to pick the 0805LS-102 which has a SRF of 340 MHz? That one gives a inductance of ~1150 ohm at 174 MHz and ~1750 ohm at 240 Mhz.
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