Coastal DXing with the AirSpy HF+ Discovery and a homebrew passive loop antenna

Last week, we packed the car and headed to coast of South Carolina.

The trip was a bit impromptu but through the creative use of hotel points, we scored a two bedroom ocean front unit with a fantastic little balcony.

The vacation gave me an excuse to test the new passive loop antenna my buddy Vlado (N3CZ) helped me build recently.

The loop design came from AirSpy’s engineer and president, Youssef Touil.

This passive mag loop takes advantage of the new AirSpy HF+ Discovery‘s exceptionally high dynamic range. Youssef had reported impressive results, so I had to build one.

Vlado had a length of Wireman Flexi 4XL that was ideal for this project. The only tricky part was penetrating the shielding and dielectric core at the bottom of the loop, then tapping into both sides of the center conductor for the balun connections.  Being Vlado, he used several lengths of heat shrink tubing to make a nice, clean and snag-free design.

The results were truly exceptional. I spent most of my time on mediumwave from the hotel balcony because I was determined to catch a transatlantic signal.

Check out the spectrum display from my Microsoft Surface Go tablet:

Our ocean front hotel was inundated with noise, but I still managed to null out most of it and maximize reception using the passive loop. I simply suspended the loop on the balcony rocking chair–not ideal, but effective and low-profile.

Want to take a test drive?

If you’d like to experience this portable SDR setup, why not tune through one of the spectrum recordings I made?

Click here to download the spectrum file [1.7GB .wav].

The recording was made on November 17, 2019 starting around 01:55 UTC–I chose it at random and have yet to listen to it myself. You’ll need to open this file in AirSpy’s application SDR# or a third party SDR app that can read AirSpy .wav files.

Stay tuned…

I’m writing an in-depth report of the HF+ Discovery, my experiments with this setup and AirSpy’s soon-to-be-released passive loop antenna for the January 2020 issue The Spectrum Monitor magazine. Spoiler alert: I am truly impressed with the wee little AirSpy HF+ Discovery. It’s a powerhouse!

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18 thoughts on “Coastal DXing with the AirSpy HF+ Discovery and a homebrew passive loop antenna

  1. John Diggins

    Hi I built one of these loop antennas and I am very impressed with the results.
    I made it from, as suggested, 1.5 mtrs of coax.
    There is a lot of interference in my neighbourhood here in Essex, UK.
    We have a BBC station, Radio 4 LW that goes out on 198 Khz.
    Interreference is normally so bad that I cannot receive the station at all, but with this loop antenna I can receive it with no interreference at all.

    I did wonder if using a longer length of coax would improve signal strength but I have read, I think on this website, that changing the length would affect the resonate frequency.
    I have read elsewhere that the length doesn’t matter too much as it is a wide band antenna anyway.
    I have already experimented with one made from 4 mtrs of coax. It worked exceptionally well on the UK FM band, 87.5 – 108.0 MHz.

    Anyone got any thoughts on the matter?


  2. N6LPO

    Please show a step by step on how to build this loop! I want to make one, too, but I am a little confused on your description in the article. 73!

  3. grantbob

    Thanks for the review Thomas. I was looking forward to mine and ordered one on Monday but yesterday I received a box from them with nothing but packing peanuts. 🙁 Looks like the box was opened somewhere along the line and re-sealed with tape.

    Naturally doesn’t want to do anything as they have confirmation something was delivered and told me to deal with USPS. They’ve at least been responsive and are helping me with info to give to the post office.

    Maybe they’ll “find” it… I haven’t given up hope yet.

      1. grantbob

        Well.. a few months later. No Airspy Discovery but only out $30ish. I did all the footwork with the USPS to attempt to trace down the missing/stolen item filling out all the forms for a “missing mail search” etc. and finally a claim. They approved the claim and gave me $100 which is the default insurance for priority mail. I asked if they have records from mailing show it was insured for the full value of the product. They said they’d check. It’s been a few weeks with no response.. so I assume they don’t. I’m just taking what I can get back and moving along. I picked up an RSPdx at HRO.

        1. Donald Chitester

          I received an HF+ Discovery a few days ago, after listening to sm5bsz (Lief) reports on YouTube. By that time I completely forgot the lack of software for android, and apparently there is some obnoxious feud between Youssef and Martin about propietary algos stolen one of them days, dating back to 2013. At any rate I could let you have it. I am in Boulder. [email protected]

  4. Andrew

    Just in case

    add some wood/pvc cross shaped support and some wire for the loop and there you go 🙂 just ensure to cut R1/jumper

    to obtain a 9:1 isolation transformer 😀

  5. Mike Frisco

    I wish someone would just start manufacturing and selling these Youloops. I’ve read such great things about them, but I don’t want to buy all the equipment to build one.

    1. Andrew

      There’s a way around it; just check out this design

      Now, one may be intimidated by the fact that it needs a balun (ok a transformer), but it shouldn’t be the case, just a matter of picking a nooelec balun and cutting the antenna side “0 ohm resistor” and the result will be an isolation transformer just like the original one; add a cross shaped structure (wood or pvc pipes) for the support, some wire (insulated one will do) for the loop and you’ll have your antenna; pick some coax and if needed “sma to whatever” adapters and there you go 🙂

  6. Andrew

    Maybe my poor brain is farting (all in all 36 hours w/o sleep may have some effect), but looking at the loop schematic, the “balun” seems to be a 1:1 one (4 turns / 4 turns), is that correct ?!?

    1. Ian BROOKS

      I’ve experimented with a few of these simple loops and have tried 1:1, 1:2 and 1:4 ratios, using BN73-202 cores which I think are twice as big as the 302 size. I’ve noticed very little difference in results. At present I have a 75cm vesion, mounted on a child’s hoola-hoop with my HF+ and regularly receive the more common US East Coast stations on MW during the night here in the UK. As the antenna’s impedance varies over the frequency range, a range of different transformers would probably be needed in theory, but 1:1 seems to work fine.

  7. Al Holt

    You’ve rekindled my interest in building a mag loop using LDF-4 with this design. I enjoy using my HF+ Discovery with a Raspberry Pi 3 running Spyserver Network.


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