Eric seeks advice: Building a directional mediumwave antenna for the RSP2 SDR

Many thanks to SWLing Post contributor, Eric McFadden (WD8RIF), who writes with the following question:

A request for assistance.

After comparing the reception of the RSP2 and 1/2 doublet to the reception of the County Comm GP-5 SSB and its little external ferrite-bar, I’ve decided I probably want to make some sort of directional antenna to use on AMBC with the RSP2. A wire loop, perhaps, or some sort of ferrite-bar thing, that connects to the RSP2’s Hi-Z input.

[Perhaps SWLing Post readers can suggest] options and then I can get the needed bits at the 2017 Hamvention.

Post readers: if you can offer Eric suggestions, or point him to antenna plans, please comment! I do believe he would rather build an antenna than simply buy one and he’s looking to permanently mount this antenna outdoors.

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15 thoughts on “Eric seeks advice: Building a directional mediumwave antenna for the RSP2 SDR

  1. 13dka

    If you get overload with a doublet, I think you really might be better off with a selective, dedicated loop for MW indeed! Crossing my fingers that it works out well for you, also remember there are some simple, ready-made attenuators built into adapters with all sorts of connectors available, the cheapest may be BNC female->BNC male adapters, usually around 10-20 bucks on eBay and some shops, 10-20dB of attenuation should suffice. The advantage is that they’re already shielded properly, and it may be less of a hassle and money than making one yourself with connectors, a metal box and whatnot.

    Reply
  2. Eric

    I’m the Eric that Tom posted this question for–thanks for the truly wonderful replies.

    Right now, I’m playing with a borrowed Tecsun AN-200 loop which I’ve discovered is quite useful with my portable MW receivers (e.g., ICF-SW7600GR) but isn’t large enough to serve effectively as a medium wave antenna for the RSP2, not when used inside my downstairs radio shack, anyway.

    While at Hamvention in just over a week, I’ll buy wire to construct a 3′ or 4′ 10-turn rotatable loop with 1-turn pick-up loop and see how that works with the RSP2. I think I already have a suitable variable capacitor–I’ll verify before Hamvention. (After Tom posted my query, I found a nice description of such a loop in Skip Arey’s book, “Radio Monitoring: The How-To Guide”, which is available here: http://www.naswa.net/journal/areybook; Skip’s suggestions dovetail nicely with the recommendations I’ve received here.)

    I have to say, Chuck’s “Superloop” is very intriguing and I might try one after I play with the 10-turn loop.

    I’ve already noticed MW overload of the RPS2 when using half of my ladderline-fed doublet through either of the RSP2’s lo-Z inputs but, as 13dka mentioned, the RSP2’s hi-Z input seems less prone to overload. Still, I will be sensitive–no pun intended–to the policy (oops) potential of overload with any loops I try.

    Eric
    http://wd8rif.com/radio.htm

    Reply
  3. 13dka

    Everything “loop” is pretty good on MW compared to the same amount of conductor put up straight as a doublet etc. Small active loops are the rotatable weapon of choice but nothing beats a simple passive wire loop in price and simplicity. I use a 60ft delta loop with the RSP2, works quite nicely on the AMBC band, actually the performance down there is quite impressive and it works generally pretty well from VLF all the way up to the 250MHz sat band for reception. Since there are no AM transmitters in Germany anymore, the risk of overloading the RSP2 is quite low on MW here(on the Hi-Z port tho, the SMA ports are more susceptible to overloading). I even used an amplifier with it for a while to increase SNR and that worked well too.

    60ft is still a “small” loop for medium wave, so it does have some directivity similar to small loops (“SML”) for shortwave (along the plane of the loop, not perpendicular to it) but it’s also borderline big for MW already. If you need more pronounced nulls, just make the loop smaller, at the cost of some gain of course, and the benefit of less tendency to overload the frontend.

    Alternatively to a preselector, you can also use a simple attenuator with high output antennas , after all you just need to drop the highest peak below the clipping point to stop the overloading mayhem, provided there are no really strong (e.g. local) signal sources inside the input filter passband (i.e. 0-12MHz on the RSPs). The gain reserve of the big (or amplified) loop, the sensitivity of the RSP2 and the high noise levels on medium wave make sure that using a 10-20dB attenuator doesn’t give you a penalty in form of losing stations. That all depends on your location of course, the closest MW stations are several 100 kilometers away here, if you live next to some AM station you’re probably better off with a different receiver to begin with, and a loop design with sharp nulls (ferrite loop) or even narrow bandwidth on top (passive, tuned SML).

    Reply
  4. TomL

    I still like Jeff’s (VE1ZAC) design which is broadband, outdoors and rotatable (look for his link to “Magnetic Receive Loop”). http://www.ve1zac.com

    He uses an broadband outdoor amplifier from active-antennas-eu which also has antenna PDF files.

    I built my own crossed-parallel loop using quilters hoops from a craft store and copper 0.1mm foil from eBay. Plus, I did buy a Wellbrook amplifier without their loop and it has dependable performance. But my loop is too flimsy for long term outdoor use. See: swling.com/blog/2016/10/guest-post-more-anti-noise-ideas/

    And also: swling.com/blog/2016/12/guest-post-backpack-shack-radio-listening/

    You can mitigate much of the overload problem with a good preselector in front of the RSP. I use a Palstar but it is not quite robust enough to tune the lowest part of the MW band around 650 kHz as effectively as higher using this antenna. Perhaps a physically bigger antenna would work better at the lowest frequencies and also in your case, a passive preselector with no amplifier circuit if there is such a thing.

    You may want to use real wood and laminated or glued 0.2mm copper foil.

    Just an idea for you!

    Reply
  5. Chuck

    I use a 16′ tall X 36′ long “Superloop.” This antenna is directional off the end where the feed point is located and works very, very will on MW.

    Here is a YouTube of me tuning the antenna. Know that WCBS, 880 is off the “back” of the antenna.

    https://youtu.be/ugLKjTuncsw

    Reply
    1. Guy Atkins

      Eric needs a direct coupled antenna to work with his RSP2. The excellent loops offered by Sarmento are for radios with a ferrite loopstick.

      Reply
      1. nickcarr

        Hehe, good point Guy. Yeah I guess I wasn’t paying attention enough to the receiver. ;o)
        Ah well, break down and buy a MW Welly loop! :p

        Reply
  6. Ashok Shankar Das(VU3VFU)

    HI,
    I would suggest Joseph J Carr’s Antenna books. Specially small loop antennas are very good.
    If i remembered correctly then he also mentioned a RDF antenna system. That is really good.

    Hope it helps.
    de VU3VFU.

    Reply
  7. Mike Agner

    While both are, as I understand it, excellent performers, I believe Eric wanted something he could put outside (lucky stiff – we cave dwellers are envious, for sure)….the K9AY design needs LOTS of space, but surely there are others that would qualify

    Mike

    Reply
  8. Guy Atkins

    My concern about a broadband antenna like many suggested above would be overloading of the RSP2 SDR. The amplified Wellbrook, W6LVP and Pixel models would be the culprits in this scenario.

    A tuned loop–rather than broadband–for MW would provide strong signal handling–an essential addition to the RSP2–as well as good gain.

    If Eric would like to build a loop, then the IRCA and NRC have plans for inexpensive air core loops he could build, and some of them have provision for a direct connection like the RSP2 requires (no inductive coupling through a ferrite bar antenna).

    Two ready-made tuned-loop options come to mind:

    Budget category: The C. Crane Twin-Coil Ferrite MW Antenna. These are often seen on Ebay for $50-100. These are a great *value* in a commercial tuned loop and can be used with a direct connection or inductive (loopstick) coupling.

    Serious Gear Category: The Radio Plus+ Quantum Loop 2.0 or 2.0 Plus+ models ($199 & $239). I’ve owned both of these before, and currently have a special Quantum Loop model with a 15-inch ferrite loop head. The stock 2.0 and 2.0 Plus+ models work excellently though and are highly recommended. I have not tried my own RSP2 with the Quantum Loop, but it should be an excellent addition due to its low noise and selective circuitry which provides natural preselection for the front end of receivers.

    Reply
  9. Mike Agner

    Assuming you are not interested in a Wellbrook, Pixel or W6LVP model, there’s a lot of information to be had on this subject. If you have lots of land, this is evidently worth a look

    http://www.hard-core-dx.com/nordicdx/antenna/loop/k9ay/index.html

    and I would recommend the late Joe Carr’s excellent book on the subject…I have some of his other publications and they’re well worth reading

    http://www.universal-radio.com/catalog/books/0016.html

    Of course, the 2 big MW DXing clubs in the US are the International Radio Club of America and the National Radio Club, both of which would no doubt have a lot of information on the subject

    http://www.ircaonline.org/

    http://www.nrcdxas.org/

    and speaking of the National Radio Club, some of their pubs can be found at the very bottom of…

    http://www.universal-radio.com/catalog/bk_ant.html

    Reply
  10. rtc

    I would suggest one of these with a one turn pickup loop:

    https://www.bing.com/videos/search?q=ng9d+loop+youtube&view=detail&mid=CFB9F7CB903025B23335CFB9F7CB903025B23335&FORM=VIRE

    The longwave version worked great here for European LWBC with a pickup loop
    thru a converter:

    https://www.bing.com/videos/search?q=ng9d+youtube+longwave+loop&view=detail&mid=19754959942898A294A919754959942898A294A9&FORM=VIRE

    Here are some details on the pickup loop:(this is a closed site but be patient;
    the info is archived)

    http://www.mds975.co.uk/archive/Content/aerials1.html

    The tuning capacitor is available from:

    https://www.tubesandmore.com/products/capacitors?filters=Type%3DVariable

    Get the C-V365.

    Don’t know how this will work with your SDR-Play but the impedance is
    around 75 ohms so all should be fine.

    If you prefer to buy,try here:

    http://amradioantennas.com/longwave_antennas.htm#

    Paul makes them for Medium wave and has amplified versions.

    Hope this helps…

    Reply
    1. rtc

      Oops…after a second cup of coffee here is an inexpensive,instant
      solution for you medium wave loop:

      https://www.amazon.com/Tecsun-Rotatable-Tuneable-Antenna-Battery/dp/B01M1EQA3Z/ref=sr_1_1?ie=UTF8&qid=1492949615&sr=8-1&keywords=tecsun+an+100+loop+antenna

      Note that it comes with a built-in pick up loop and cable,all you do is hook it up.

      (For future use…if you switch a 220 pf cap across this loop’s variable it will do
      400-530 khz very nicely,too for the new 630m ham band.)

      Reply

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