Using amplified loop antennas with portable radios?

SWLing Post contributor, Klaus Boecker, sports a homebrew magnetic loop antenna on his balcony in Germany.

Many thanks to SWLing Post contributor, Marty, who writes:

I have a question about loop antennas; specifically which type is “better,” passive magnetic loops or active electric loops?

I know, “It depends.”–?

I live in a ground-floor apartment, with a small porch, lots of RFI and restrictions against visible antennas. Also there are no trees within 75 ft of my porch, which faces on a parking lot. My radio is a Tecsun PL-660, which works okay inside with my 10-ft bare wire antenna hidden on the porch.

With a loop antenna, I’d like to mount the antenna on the porch at night and have a remote tuner/control inside because it’s very hot n humid here in Louisiana even after dark.

No doubt there are a number of magnetic loop antennas that could serve you well in your situation, Marty.

To answer your first question, though, you should search for a wideband amplified loop antenna since you’re only concerned with receiving.

Passive loops are great antennas on the shortwave bands–and easy to build–but they best serve ham radio operators who wish to transmit. Passive loops typically have a very narrow bandwidth that requires the operator to constantly tune the antenna when they tune the radio a few kHz. Most amplified wideband loops need no separate tuning mechanism.

Last year, I posted an article about choosing the right loop antenna for situations like yours where one has limited installation options.

Click here to read : Shortwave antenna options for apartments, flats and condos

Portables and amplified loops?

I do hesitate to encourage you to invest in an amplified loop antenna when your only receiver is a Tecsun PL-660. Some portables don’t handle amplified antennas well–they can easily overload and I imagine you can even damage the front end of the receiver.

I’m well aware, however, that there are a number of readers here who do couple their portable radio to an amplified loop. I have connected a number of portables to large wire antennas and found they could easily handle the extra gain, so I imagine an amplified loop would perform as well; the Sony ICF-SW7600GR, Tecsun S-8800, and Sangean ATS-909X come to mind.

But the PL-660 is a hot little receiver even with the built-in telescopic antenna–I’m not sure if amplification would help or hinder.

Please share your experience

This is where I hope the amazing SWLing Post community can pitch in and help us out here!

Does anyone here regularly connect their PL-660 to an amplified loop antenna? If so, what model of antenna are you using? Are there other portables out there that you regularly connect to amplified loops? Please share your notes, thoughts and experience in the comments section.

Thank you in advance!


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20 thoughts on “Using amplified loop antennas with portable radios?

  1. RonF

    “I live in a ground-floor apartment, with a small porch, lots of RFI and restrictions against visible antennas.”

    Given that, I’ll gently disagree with Thomas’ recommendation of an amplified wideband loop. I think an unamplified remotely-tuned loop would be a better choice. My reasoning:

    (Note: all of this applies to ‘electrically-small’ loops; the typical 30cm-1m diameter loop everybody thinks of as ‘shortwave receive loops’. Fractional wavelength or larger loops, as used by hams who like to boast of their large backyards and antenna farms, are somewhat different beasts…)

    – Any well-balanced loop antenna – amplified or unamplified, tuned or untuned – will provide both (a) directionality (the ability to null individual noise sources or maximise signal) and (b) a degree of immunity to immediately-local noise within the near-field distance of a given frequency.
    – Unfortunately, when mounted near (within 1/2 wavelength or so) of obstacles/reflectors – the ground, walls/brickwork, etc – directionality pretty much goes out the window. Best case (30MHz) at HF, “1/2 wavelength” is 5m.

    So in this situation we’re left with immunity to near-field (i.e. predominantly E-field) noise as a loop’s big advantage. That’s actually quite a biggie – it means that, unlike E-field antennas (e.g. whips, longwires, etc), they’re not _as_ susceptible to noise generated in your house (&, depending on distance, your neighbours’ houses). So +1 to loops in general so far.

    The big downside to electrically-small loops is they’re inefficient; their absolute output level is very low. That may lead you to think that an amplified loop – moar powa! – is better than a non-amplified loop. Rarely are things that simple… :

    – Unless your radio is deaf (i.e. faulty, or with poor sensitivity or high internal noise level due to poor design), the limiting factor for reception is the local RF noise floor.
    – Any amplified loop will amplify both the wanted signal, and all the noise & other signals within the antenna’s passband. For a wideband loop, the antenna’s passband is the loop bandwidth (realistically, let’s say 500kHz-20MHz)
    – Outside the edge cases of the local RF noise floor being below the radio’s internal noise floor (unusual, especially at LF/MW/HF), or the radio being particularly deaf, you’ve achieved nothing except make both the wanted signal – and all the noise + unwanted signal across 20MHz – louder. If you started with a wanted signal 10dB above the local noise floor, after amplification it’s still going to be only 10dB above the noise floor. Or, in practice, a little less – since any amplifier will add some noise…
    – The higher wideband noise level, though, makes the receiver’s job harder – the general amount of noise getting through the filters is higher, there’s more possibility of overload/intermod/other distortion, etc, etc.
    – A tuned loop, on the other hand, (a) adds no noise, and (b) both reduces the full-bandwith noise (i.e. increased selectivity) and increases the wanted signal presented to the receiver by a factor related to the Q of the tuned circuit. Effectively, they operate as a very narrow preselector: Tuned circuit = higher Q = stronger wanted signal, less unwanted noise. All pluses.

    The only downside to tuned loops – at least, the only one compared to loops in general – is that the loop needs to be tuned to track the receiver. Which is why people will often use a random-wire or whip for initial tuning, listening carefully to find a faint signal, then switch to the loop to drag it out of the noisy mud.

    So, to summarise the story so far: sitting on a small porch will bugger up one advantage of a loop (directionality). Having to have it so close to the house, however, brings a loop’s insensitivity to near-field noise to the fore. And, unless your radio is deaf – or you’re in the possibly unique situation of being in both an apartment *and* a place with a low RF noise floor – an amplified loop will hinder as much as it helps, while a tuned unamplified loop will help more than it hinders.

    And if I’m wrong, you can always add a small externally-powered amp afterwards… 😉

    Which leaves you with the problem of your body corporate/HOA not wanting externally-visible antennas:

    – Well, loops are easy to make portable – mine’s a 1m diameter loop of 3mm thick flat aluminium bar mounted on an old broomstick stuck into the cast-iron base of an old lamp, and could be easily moved inside (if I wasn’t on the body corp and therefore able to shut down any objections to it!).
    – Buy one of those wire loops Thomas posted recently, and get creative! Buy a cheap outdoor table, screw the loop down to it, paint it some blending-in colour, wrap it in some plastic flowers, connect it up with some discrete RG-178, and tell any nosey neighbours it’s a feng shui enhancer…

    Reply
    1. Thomas Post author

      Very good points, Ron! And, you know, a setup with a simple random wire and passive loop combo could be incredibly affordable if home-brewed.

      Reply
    2. TomL

      How would one remotely tune this to use with a portable radio like the reader question describes? Creating one’s own remote tuner takes some amount of electronics knowledge most people do not possess. Is there a commercial product that one can purchase?

      Reply
      1. RonF

        Same as you’d tune any varactor-tuned receive loop – either an adjustable bias tee feeding a control voltage up the coax, or a second pair of wires for same. Connection to the reader’s PL-660 is via the external antenna socket on the side – 3.5mm to BNC adaptor not supplied ;).

        Plenty of designs available on the web to DIY, and they’re not that hard (if you can wind a balun and solder connectors for a proper random-wire, you’re skilled enough). Certainly easier than building, say, a PA0RDT mini-whip kit.

        There used to be a couple of pre-built remote-tuned SW loops available, but they seem to have gone by the wayside – perhaps not surprising, because there was no ‘secret sauce’ to explain away the exorbitant pricing, and they lack the illusion of sensitivity that amplified loops give. Though I don’t really follow the commercial world, currently the closest things I know of offhand are the remote-tuned PK loops – but they’re MW-only.

        RH Electronics makes a kit that appears to be a copy of the Charles Wenzel tuned *active* SW loop mentioned by Andrew below, which could be modded as per the instructions on his site for remote tuning (using an extra pair of wires). That design was actually my introduction to small loop antennas, and I have fond memories of it – but, due to both its design and the choice of amplifier, isn’t really an optimal example of either a tuned or amplified loop. It’s also considerably more complicated to build than a simple remotely-tuned loop. It does work fairly well, though, especially if you’re not yet convinced that a non-amplified tuned loop has advantages over an amplified wideband loop 😉

        But, ultimately, the best thing a SWL’er can do in either – or any – case is to get their antenna out & above the near-field noise, and deal with the ambient local noise floor. It’s where you can’t – as in Marty & my case – where tuned unamplified loops come to the fore.

        Reply
    3. Andrew

      “Buy a cheap outdoor table, screw the loop down to it, paint it some blending-in colour, wrap it in some plastic flowers, connect it up with some discrete RG-178, and tell any nosey neighbours it’s a feng shui enhancer…”

      Or either, he may build this LW/MW loop antenna

      https://randombio.com/loopantenna.html

      it’s a passive, remotely tuned loop; tuning is achieved through varactors and the loop covers approximately a range going from 150KHz to 2MHz, to hide such an antenna, one may enclose it into some suitable casing (wood or the like will do) and disguise it for a porch table, one may even place a vase with a plant over it 🙂 judging from the author, the loop is very sensitive and the tuning is quite sharp, for example, when tuned to 158 kHz, the antenna only responds to signals between about 155 and 161 kHz, this, along with the ability to null-out interferences by orienting the loop may allow to obtain a nice reception, plus the antenna will be totally stealth 🙂

      Reply
  2. Mark

    I frequently use a W6LVP “Experimenter” loop with an Eton E1 on trips to a local park to get electrical quiet with no problem. In fact, I did just that on the 4th. Maybe next time I will try it on the Tecsun 660.

    Reply
    1. Martin

      Hi Andrew. No, a flagpole antenna isn’t possible. I’m not allowed to put up anything that extends beyond the porch.
      Tenants who want dish antennas for cable TV have to get permission first before the antenna is installed.

      Reply
      1. Andrew

        Here I am, Martin, back at you.

        First of all there’s a BIG question, that is… BUY or BUILD ?

        I mean, while there are a number of commercial active antennas around, some of them are quite costly and the cheaper ones don’t offer the same level of performance (although will still be better than nothing); on the other hand, if you are able to use a soldering iron to put together some relatively simple circuit, then there are quite some good active antenna projects which may fit and will give you quite good performance

        A very simple, yet nice performing antenna is the “miniwhip” found here

        http://www.techlib.com/electronics/antennas.html#Improved%20Active%20Antenna

        it can be powered through the coax and is untuned, yet, thanks to its design it offers performances which may be compared with a full size vertical antenna, sure, it won’t be as “quite” as a loop, yet it’s easy to build and cheap and may be possibly mounted as a “stealth” antenna in your porch 🙂 !

        Another option may be a “roof butterfly” like the one depicted here

        https://i.pinimg.com/originals/b3/bb/d1/b3bbd1e7bcdea676221d11d43599353c.jpg

        or you may just use the above idea to lay some length of wire around the roof (or under it, for that) w/o configuring it as a “butterfly” but leaving the “loop” open (with enough distance between the start and end of the wire); notice also that, if using a “random wire” antenna, you’ll need a balun to match the impedance to the receiver’s input, for such a task you may use this cheap commercial balun

        https://www.nooelec.com/store/balun-one-nine.html

        in such a case, pick the balun with the circuitry up and the coax connector toward you, connect the antenna wire to the left hand clip and connect the other clip to a (as short as possible) run of wire going to a good ground, then use a coax to connect the balun to your receiver

        Reply
        1. TomL

          I have a mini-whip on a tripod I use at the beach, away from any e-field noise. Mini-whips pick up every stray electronic noise possible. A loop would be better for an apartment porch. Just my experience.

          Reply
          1. Andrew

            Well … yes, an activewhip will for sure pick up more noise than a loop, yet the one I suggested has a “tweak” just scroll up to see the simpler circuit and check out the description for the BUFFER circuit 🙂

            Then, at the top of that page there’s also a circuit for a remotely tuned activeloop, but that’s designed to work in the 3 … 30Mhz range, so while I know for sure that it works really well (built a number of those) I’m not sure it may be adapted to work on MW

            As for MW the same link also shows the design for a compact, directional, active ferrite rod antenna, that may be worth a try 🙂

  3. Martin Kraft

    Thanks everyone! You’ve given me some great advice and ideas to try out… at night… when the apartment manager ain’t watching!

    Reply
    1. rtc

      My experience with an amplified PK loop and a portable did not go well.
      The loop put out too much gain and swamped the portable despite full
      attenuation etc.
      Reducing the loop’s power supply voltage is a workaround but seemed
      rather silly after all the time,effort and expense.
      A passive loop won’t do this.

      Reply
  4. Jason

    I might suggest first investing in the necessary adapter, some coax, and a choke of some sort for the far end (e.g., a 1:1 Guanella balun on a small torroid, or just 5-10 clip-on ferrites good for shortwave frequencies). The reason being, that these three things will be necessary/useful for the future amplified loop or magloop, but you could start with just a small dipole hidden in the same place as your random wire, and work up from there. The random wire is likely picking up all sorts of noise from indoors, so by moving the feedpoint outdoors, you benefit from that inverse square law. If the radio is easily overloaded, a short dipole may work best, however, if you experiment with larger dipoles and it works well, then you may be fine with an amplified loop. Lots of experimentation for the cost of a roll of cheap speaker wire and a few minutes time.

    Reply
  5. TomL

    Too bad no one has figured out how to create an automatic/remote antenna tuner that does NOT need an illegal transmitter. Then, you could keep using the 10 foot wire outside and tune it to any frequency as needed.

    I use something similar to Chuck Hutton’s broadband idea of a ferrite sleeve loop but the cost and time to build it might be offputting and it does not work as well as a manually tuned loop. https://dxer.ca/articles/92-gary-debock (look at the bottom “Addendum”)

    I also echo Thomas’ observation that the outside wire antenna can work better than an indoor loop. Getting away from the noise is key!

    Reply
    1. RonF

      “Too bad no one has figured out how to create an automatic/remote antenna tuner that does NOT need an illegal transmitter.”

      How automatic do you want? 😉

      I’m currently in the middle of playing with a prototype autotuner design for my FRG-7700 & ‘passive’ (varicap-tuned) loop. It does require info from the radio though (tuned frequency), which on the 7700 can be derived from the band setting & VFO signal available through the memory unit interface.

      Config is simple – manually-tune the antenna at a couple of spots on each band, & the autotuner records the radio’s bandswitch setting, VFO frequency, & the tuning voltage I’ve dialled up. From that the autotuner’s micro calculates the tuned frequency & derives a curve-fitting function to match frequency to tuning voltage. In regular use the micro reads the Yaesu’s bandswitch & VFO settings, generates the tuning setting required, and drives a DAC to generate the voltage to tune the antenna.

      So far the basic idea seems to be working fairly well, though I need to tweak the hardware & software a bit before I’ll call it finished. The same basic idea should be easily-adaptable to any receiver that has tuning info externally available (e.g. the old Yaesu hardware interface, the serial-based Yaesu CAT / Icom’s CI-V / others) or easily-added (e.g. I reckon the early R-series Kenwood radios would be good candidates), and it wouldn’t be hard to interface it to any other remote-tuned loop or design a controllable antenna tuner to suit a long/random wire antenna.

      Of course, it’s not much use if you’re using a portable without any sort of interface available – but that wasn’t part of your spec… 😉

      Reply
  6. 13dka

    The PL-660 has a built-in attenuator that makes it much more apt for high-gain antennas than some other radios, for example the S-8800 (I mentioned that in the review I think). I have used it quite often with the voltage-spitting 8m flagpole at the beach (inductively coupled) and tried it with my dipole, the 60ft passive loop or the ML-200 amplifier with a small (80cm diameter) rigid loop at home. Without the 3rd position on the sensitivity switch it sure would have some trouble with most “large aperture” and active designs.

    That being said – if you can’t run coax to the distant trees (which could have the advantage of getting the antenna away from the nearfield RFI-“halo” surrounding buildings) your other option might be a somewhat bigger, non-resonant (no tuner) passive loop utilizing the geometry of your front porch – maybe you can string up some wire (maybe color matched for extra stealthiness) around the whole thing to form a not-so-small loop? With a sensitive radio like the PL-660, any lack of voltage coming out of such a loop isn’t a big deal and that would be considerably cheaper and more convenient than an active loop you’d have to put up and take it back in.

    Tuned passive loops with remote control are usually even more expensive than active wideband loops, with no benefit for receive-only usaage, so that’s likely not a good option for you.

    Reply
    1. 13dka

      Dang, forgot to add why you may want such a wire loop anyway – even with one side of a quad loop lying on the floor, that contraption will likely perform better than a straight stretch of wire at the same place. Even if you could bring the same length of wire into the air, the low elevation and asymmetric design would likely give you less signal and more RFI pickup.

      Reply

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