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[?]Looking for an active antenna suitable for the home constructor.

Discussion in 'Electronic Design' started by David Chapman, Aug 8, 2012.

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  1. I'd be most grateful if someone could point me towards a suitable
    design for a 'roll-your-own' active antenna, with high dynamic signal
    range, suitable for use over the frequency range 10kHz to 30MHz (or

    I've been looking at the Avago range of FETs which have excellent
    performance in the GHz region with good dynamic range, but I have heard
    that they do not perform as well down in the KHz/MHz frequency ranges.

    Most of the active antenna designs that I've seen for the lower
    frequency bands, admittedly published in old ham-radio magazines, seem
    to use a J309 or equivalent J-FET as a front-end device.

    Before starting to build up one of these old active-antenna designs, I
    was wondering if anyone in this NG has any recommendations for a modern
    '2012'version with (possibly) better performance that would be suitable
    for an enthusiastic amateur to build. Maybe one of the higher-current
    wide-band MMIC amplifiers would now be better suited to driving the 50
    ohm output than the discrete component circuitry used in all the earlier
    designs that I've seen.

    I'm quite happy designing/building PCBs and I regularly work with SM
    components down to 0603. I don't have any professional-standard design
    software (or the ability to use it) but I do have a decent signal
    generator and spectrum analyser ready to hand.

    Any constructive (!) suggestions will be gratefully received.

    TIA - Dave.
  2. Guest

    Why would you expect that such circuits would actually exist ?

    Handling 1:10 HF frequency band with a single active antenna, might
    even be doable.

    For MF/HF, you might try some whip antenna with a low capacitance FET
    source follower.

    Below that, I would suggest some untunued (or preferably tuned)
    magnetic loop.

    Doing a 1:3000 frequency range with a single antenna does not make any
    real sense.
  3. Many thanks to those who have already replied to my posting and for
    the various web-site references mentioned. All very interesting and
    potentially useful.

    Let me try and explain my requirement in more detail:-

    I have an AR5000 receiver which I use for monitoring LF through to UHF
    but close to the house there is a very high level of electrical noise in
    the bands up to around 30MHz.

    This noise considerably as I move further away from the house so, for
    receiving these lower frequencies I'm planning to mount a suitable
    wide-band active antenna, with 1 metre long rod antenna, around 150 feet
    away from the house and power it over the same co-ax feeder that brings
    its signals back to the receiver.

    I'm hoping to find an amplifier design that has very good dynamic
    range to cope with a few strong broadcast signals, and assume that this
    means that it will need to draw a current of some tens of milliamps
    rather than the few consumed by some of the most basic designs that I've

    I've no serious interference problems with the antennas that I use for
    frequencies above 100MHz - it's only reception of the lower frequencies
    that I wish to improve.

    I hope this explains more clearly what I'm trying to achieve.

    - Dave.
  4. Guest

    A very short antenna is very reactive, in practice the capacitive
    reactance of a small (10 pF) capacitance. Thus, it is essential that
    the amplifier input impedance is as high as possible. With FETs, the
    input resistance can be made very high, but the problem is the
    capacitance from input to ground. Together with the antenna
    capacitance, this will form a capacitive voltage divider.

    If the amplifier input capacitance is the same as the antenna
    capacitance, there is going to be a -6 dB signal voltage loss.
    A small magnetic loop will have a very low radiation resistance
    (milliohms or even micro-ohms), thus the amplifier impedance should
    have as low input impedance as possible. In that link, a grounded base
    configuration is used. To reduce the impedance even further, multiple
    small signal transistors could be used in parallel or even some power
    transistors (e.g. in TO-220 case) could be used.
  5. Guest

    For proper operation, bandpass filters usually require quite well
    defined source and load impedances. A short whip will have a highly
    reactive impedance varying with frequency, so this must be considered
    in filter design. Not any text book filter will do.
  6. Martin Brown

    Martin Brown Guest

    Might it not be worth doing a careful survey of your household
    appliances first with a handheld receiver and then putting better RF
    suppression on the worst offenders? Likely to be electric motors,
    dimmers and various other kit that arcs and sparks.

    That has always been the approach taken at radio astronomy sites.
    You will be better off notching out the strongest local signals.

    Martin Brown
  7. Guest

    Why not simply build separate antennas for each band with dedicated
    amplifiers for each antenna and use relays to switch between the
    antenna/amplifier combinations.

    For the lowest bands, you could even use motor driven variable
    capacitors in magnetic loops at the lowest bands. Switching out some
    of the loop turns with a relay, at least a 1:10 tuning range should be

    If you are using remote antenna locations, please remember to use some
    ferrites around the coaxial cable near the house, in order to avoid
    any interference currents flowing on the outside the shield (which
    could then reradiate into the antenna).
  8. JW

    JW Guest


    Can you localize where the noise is coming from? Seems to me it'd be
    easier to fix the noise source...
  9. Two problems there:

    Whilst I can locate and (probably) fix any interference generated by
    computers and other equipment actually in this house, some is coming
    from houses to either side of ours. This QRM is much harder to
    pin-point exactly and MUCH more difficult to fix without spending a lot
    of time and dealing with a lot of unwanted hassle with neighbours..

    .....and secondly, the reason I'm looking for a physically separated
    active antenna using a short (1 metre long?) rod is because I don't want
    to go the route that involves the erection of unsightly long wire
    antenna(s) adjacent to the house.

    I KNOW that an active antenna will do the job for me since I've tried
    one temporarily and it works very well when positioned more that 100
    feet from any nearby house and connected to the receiver by co-ax.

    Before deciding to build a design that was produced 10-20 years ago I
    thought I'd ask in this NG to try to find out if modern semiconductors
    have enabled any worthwhile performance improvements for this

    Maybe today's technology has nothing more to offer in this particular
    application of course, in which case I'll just build up one of the
    original J309 FET designs, but I thought it would be worth asking the
    question at the very least.

    Thanks again for all the interesting comments and suggestions.

    73 - Dave.
  10. amdx

    amdx Guest

    Hi Dave,
    Send me a private message to
    Be sure to removethis.
    I couldn't get your address to work.
    I have some info for you.
  11. No, not ferrite (I've never seen one that long either) but just a
    straight length of stainless steel rod around 2-3mm in diameter.
    Yes agreed, and I fully accept that your proposes course of action is
    certainly the correct scientific way to resolve the interference
    problem, but I really don't have the time or inclination to spend hours
    locating and sorting out problems that I already know can be largely
    overcome by using an active antenna 100 feet or so from the house and
    running a co-ax feeder from it to the receiver indoors. Life's too
    Understood and accepted, but I don't need gain, only Hi-Lo impedance
    transformation and a 50ohm co-ax driver. As I said earlier, I know that
    an active antenna will work sufficiently well to meet my needs - I just
    posted to this NG in order to ask the experts if there have been any
    significant improvements in semiconductors during the last decade or so
    for this application.

    Having read and digested the various interesting and informative
    responses to my posting I'll now probably go ahead and build an active
    antenna to one of the earlier designs since I've got lots of J309 and
    Siliconix E310 as well as E430 (dual) J-FETs in my spares box.

    Many thanks for the references to useful reading on other web-sites.

    ATB - Dave.
  12. Guest

    If this antenna would be used for transmitting, it would require a
    large (and lossy) base loading coil, in order to present a resistive
    50 ohm load to the transmitter. The efficiency might be below 1 % and
    the bandwidth perhaps only 1-2 kHz.

    However, the OP is talking about a receive only antenna and since the
    background noise level is quite high at this frequency (not to mention
    VLF/LF/MF bands), so the efficiency is not an issue. Please remember,
    a typical MF (AM) ferrite rod antenna might have a gain of -50 dB
    (0.001 % efficiency) and still you are able to receive broadcast
    stations. With a voltage follower after the highly capacitive antenna
    rod will make the system quite broadband.

    The problem with unmatched active antennas is the stray capacitance
    between mains wiring and the antenna element. Any interference voltage
    in the mains wiring is easily coupled to the antenna element.
    Loran-C is at 100 kHz i.e 3 km wavelength, thus, the element would be
    1/1000 wavelength.

    What is the stray capacitance from the house wiring and the antenna at
    30 m ? I guess it is quite low and hence the capacitively coupled near
    field interference is quite low.
    Please state clearly that it is a receive only application, since
    otherwise most readers would make suggestions about tuned narrow band

    If you are interested in frequencies below 300 kHz, it would make
    sense to check the various LowFER pages.
  13. Yes, my requirement is definitely Receive only - my AR5000 receiver
    doesn't transmit at all well ;-)

    - Dave.
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