Ground SIGINT: Low Noise Receiving Loop Antenna

20160131_111626Part of the joy I get as an instructor and consultant is the diverse and quality interactions I have with students. Everything from their thoughts on what makes a good homestead or their successes as preppers to their favorite shortwave stations, its a blessed perspective that I get from everyone and something I like to be able to pass on to everyone else. In one of the recent classes a very brief conversation ensued just on the utility of shortwave and how these days its becoming more challenging to listen into good stations, both because of the number of stations going off the air and the sun cycle approaching its minimum, limiting propagation and increasing the signal to noise ratio. Reception can be tough, and just like how we want to ring the most efficiency possible out of a transmitting antenna, we need to do the same for reception. We do this through building a solid receiving antenna, and the best designs are just a tad different than the standard dipole, end-fed or vertical that most are used to. The two most popular ones, because they work, are the Beverage and the Ground or Grasswire Loop.

dx-394There’s a ton of reasons for building a good receiving antenna- from improved reception of distant shortwave and AM broadcast stations to digging out weak signal transmitters, the ability to listen and even pull out good quality signal is very important for the prepper and homesteader looking for quality information from a wide variety of sources. On the clandestine monitoring end, a certain amount of information can be inferred from the various utility stations on the air; anything from the numbers stations which populate the HF waves to the USAF Global HF net. Although many might wonder just what the value would be in listening to these is if we don’t know what’s actually being said, the number of times these stations go on the air and the amount of traffic can give us an idea that we might want to pay attention to the news, as something is going on or about to go on. But operationally, such as communicating over a region, having a good receiving antenna is important. From a transmitting standpoint a common clandestine weak signal transmitting method is to send a burst message on one frequency with a tiny amount of power and receive a response on another with the same type of weak signal, both times in an attempt to mitigate the direction finding (DF) threat. Having a good receiving antenna running to a dedicated communications receiver, such as the Radio Shack DX-394 pictured above, some of the higher-end SDRs or any receiver made by Icom or AOR, becomes an important tool to the Underground when establishing regional communications even when the solar cycle is not cooperating.

beverage.jpgThere’s two relatively simple antenna types that can be built at home and on the cheap- the Beverage (do yourself a favor and gloss over articles part I & II) and the Ground or Grasswire Loop. The Beverage is an excellent option for low noise and relative simplicity; the problem is that it requires a large space to be erected which most folks living under the thumb of HOAs cannot accommodate, and its peak reception is directional to end-fire of the antenna. In order to get 360 degree coverage, you’d need two. And maybe you don’t need that- which is something that requires preplanning- but then again maybe you do. A way to overcome this is through building the other option- the ground loop, also known as a grasswire loop, which has a much broader reception lobe.

loopMany of you who’ve read mine and various other QRP and NVIS articles may recognize the term grasswire– it gets its name, simply enough, from being a run of wire stretched out either just above the grass or laid on the ground itself. The advantage of this design is that it is very low noise due to its relatively high loss- meaning that it’s not a transmitting antenna by any means but can listen very well when paired with a good receiver. When used as a transmitting antenna, it would have great reflectivity from the ground which is an asset for working local NVIS propagation but loses quite a bit of signal (also known as negative gain) for going longer distances- possibly another asset for the same clandestine communications methods.

ground loop.jpgGrasswire Loops are relatively simple to build- all you really need is a run of wire, four insulators and a balun, which you can build yourself as well as purchase. And while the diagram shows 70 feet of wire, you can make the sides shorter as needed to suit your space requirement. Loop antennas as a rule should not be longer than one electrical wavelength (936/frequency= One Full Wavelength Length in Feet). And since receiving antennas are low noise by design, 75 Ohm coax that’s used for cable TV and found pretty much everywhere is perfect. Keep in mind that these normally have an F-Type connector. All of the components can be found in the local hardware store and built in a couple of hours- and the ability to build is a great skill to have when buying readily made equipment might not be an option.

Experiment with one over the weekend and find out what you can do. Better yet, inquire about an upcoming signals intelligence class I’m putting together where we might just build one, along with training on a lot of other neat techniques.

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If you want to learn more about communications for preppers and survivalists for both the small unit and across a region off the grid, consider scheduling a slot for the RTO Course. It is a class unlike anything offered anywhere else. Open Enrollment dates for the Summer and Fall of 2018 will be scheduled soon. I also am willing to travel for private groups. Feel free to email me at brushbeater@tutanota.com for more details.

Reprinting and/or republishing of this article is highly encouraged however only under the conditions that credit is given to the author with a supplied link to this page.

 

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25 thoughts on “Ground SIGINT: Low Noise Receiving Loop Antenna

  1. Mitch

    What a timely article. I was just thinking that if the SNR gets any worse on 40m, I’m going to have to break out a beverage antenna to even hear net control on my favorite nets. I’ll take this article as confirmation of that idea. In the past I have had some success digging out SW stations with a simple stretch of telephone wire clipped to my gutters, with the other end of the wire stripped and wrapped around the antenna of my Tecsun SSB-capable SW radio. It’s not the best solution, but one even non-HAMs can use to increase their listening ability with stuff they likely already have at home. Low tech > No tech! Carry on brother.

    Liked by 1 person

    1. 🙂

      The only note I’ll add it that a large loop *may* overload the receiver on the Tecsun (or County Comm GP-5). You’ll know this is happening when you hear a lot of stations where they shouldn’t be. That’s the main drawback of small or handheld receivers.

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  2. Jon

    Brushbeater

    Front end overload protection is a problem in my QTH. Not enough room to separate transmitter and receiver antennas. I burned out one SDR because I didn’t think bout it in advance.

    So, what do you recommend as front end protection for a dedicated receiver in a location where a 100 watt transmitter will operate.

    Thanks

    Liked by 1 person

    1. I have a three way coax switch that I use. Separate transmitting and listening antennas that I switch between, but I’m primarily using my 7200 as a receiver also. If the receiver or SDR is located in the shack, have a separate switch for it also and when you’re transmitting switch it to the outlet that’s not connected to anything.

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    2. You need a disconnect that grounds the receiver’s signal input and ideally places a load in line with the input downstream. I’ve used a coax switch, and that works well, (if you use a switch like the MFJ 1704 or the similar Daiwa switches, both of which give >70db isolation) but if you are doing some serious operating and running both the transceiver and the receiver at the same time, that is a bother.

      MFJ makes such a box and I picked one up to use with my station’s receiver; it can be directly switched from the transceiver or RF sensed. Direct switching with the delay for the box programmed into the transmit memory is more reliable, IMHO…… Anyway, using that and a signal splitter, I can listen with both receiver and transceiver off one listening antenna input, or have the receiver on another antenna, and transmit without having to manually switch anything, protecting the receiver.

      You can also make up a switch box using relays to control multiple antenna inputs and receivers. Some of the serious low-band operators use such a system to detune the transmit antenna to control noise coupling between transmit and receive antennas.

      Hope that helps!
      Keypounder

      Liked by 1 person

  3. As always, a great article NCScout!

    I often hear from non-ham preppers that, “I can’t receive squat on my Grundig (or insert other manufacture here) radio!” Well like anything in the world of amateur radio, it comes down to ‘the antenna’.

    Even a simple ‘T’ type antenna attached to their SWL (Short Wave Listening) radio will start to bring in signals better than the typical telescoping antenna attached to these rigs. Just think what their reception would be if they constructed one of the two antennas you outline in your article.

    A really good book that is free on the intewize to take a look at is, “Practical Antenna Handbook” by Joseph J. Carr – Fourth Edition. Just ‘DuckDuckGo’ (I am boycotting Google) it and you will have 625 pages of pure magic when it comes to antennas. Chapter 13, “Antennas for Shortwave Reception”, outlines in more detail, what NCScout describe’s.

    Thanks again NCScout for putting these articles together and publishing them. As Mitch wrote. “What a timely article.”

    73 & Peace folks!

    Liked by 1 person

  4. For those interested, the new “Listening Antenna” book from ARRL is definitely worth the money. It mentions horizontal elevated Waller Flags as good options, which I had not considered, and there is a good section on the K9AY loop, which I have been interested for some time, and with which I’m going to be playing around this summer. If any of you are interested, K9AY has a good section on his website on arrays of K9AY loops.

    I’ve also been looking at more advanced NVIS setups this book suggests, including circularly polarized NVIS antennas, a’ la HAARP. *Definitely* a subject of interest, and possibly an article once I get it up and running.

    But the big push this spring has been direction finding. I have been building and testing various DF equipment since February, and that will be an article sometime before the 4th of July.

    Keypounder

    Liked by 1 person

  5. Mike KN4FFK

    I have a couple of questions.
    1) In the diagram it shows 300 ohm wire going to a Balun. In the text you mention 75 ohm coax. If you use the coax, do you still need the balun?
    2) What is the max height you could hang one of these and still have it work well? (If I put it in the back yard, I need to be able to mow under it)
    As always, good stuff. Thank you
    Mike

    Liked by 1 person

    1. The 300 ohm wire in the first diagram is ladder line. It works well and is low loss, but connecting ladder line to your receiver requires some soldering. You can construct the whole thing to just use 75 ohm TV coax and keep it all simple.

      Something else I do with small loops is have an F-type to BNC or SMA adapter on hand, so I can quickly plug it into my Alinco DJ-X11. Although the loop in the diagram would be FAR too large and overload the receiver, a smaller one can be constructed just as easily and use an attenuator (something I specifically didn’t cover) to fine tune the loop.

      You can go high with it, but part of the reason it’s low noise is that the proximity to the ground ‘loses’ the ambient RF noise. So the higher you go, the more susceptible you’re going to be to power line noise and other RF issues. There’s no specific answer however, just try it and see what works for you.

      Hope this helps!

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  6. I’ve used something similar for approx. 20 years.

    The loop is 525 feet overall. I usually have it up 10-15 feet above ground. Learned early on to drop it down to about 5 feet during noisy band conditions. It works well both receive and transmit.

    Liked by 1 person

      1. Yes, 160. Actually started out as a “bigger is better” experiment trying to make a gain antenna for 80. 🙂 I’m down in a valley back in the mtns so I was trying to get out of the hole. All the ‘experts’ at the time said it wouldn’t talk five feet because it was too low to the ground. Yet it serves the intended purpose.

        The only problem is keeping it in the air. Winds here regularly hit 60 -80+ mph. .

        Liked by 1 person

  7. My experience is similar to what you spoke about. There have been times when a storm put the wire on the ground. Atmospheric noise and strong signals dropped, but weak signals I didn’t realize were there suddenly became readable.

    Liked by 1 person

  8. Doc Raydio

    300 ohm TV ‘twinlead’ works great as does those old VCR 4:1 baluns that coincidentally have an ‘F’ connector on the low impedance end and screws with piercing washers to attach to the twinlead. You wouldn’t want to put more than a couple of watts into one, but they’re great for a receiving setup.

    Liked by 1 person

  9. Jon

    Scout and company

    I’m struggling with the Grasswire receive design.

    Why is the LC-tank and transformer required at the feed point? I understand why it’s needed for XMIT to match impedance and reduce SWR.

    I don’t completely understand why it’s needed on receive only antenna. SWR isn’t an issue there.

    Perhaps there’s some line loss on incoming signal due to impedance mismatch and reflections that accompany it?

    Can you help me a little with the theory?

    I will build one of these after this summers PCS move.

    Liked by 1 person

    1. Jon, no problem! The transformer is essentially a choke- whatever RF noise is left gets choked off by the torroids. Its not so much about SWR as it is the antenna ‘hears’ better the closer to resonance it is for the frequency set you’re listening to. That Balun/RF choke makes it easier to listen to a broad range easily and also chokes off any common mode noise you might have (such as a neighbor’s bad ground, power line noise, etc.).

      Always feel free to ask, and Godspeed with that PCS- how I don’t miss it. 🙂

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      1. Jon

        Thanks. That makes perfect sense. RF choke of some sort is always a good idea. I use a 1:1 on my dipole just for that reason.

        And the LC network tank serves to tune to resonance on the desired frequency. I will have to play with that a little to find a way to tune remotely from the shack. I’ll experiment with it on both ends of the feed line.

        PCS is a pain. I think this is our last one. Fun meter is pegged. We are over weight allowance so a partial DITY is in our future.

        Thanks again for sharing your knowledge. This distance elmering thing is truly helpful

        Liked by 1 person

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