DIY Moxon Antennas and LPI

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Concerning local networks and infrastructure in the Survivalist communications realm, it’s vastly important to recognize the assets provided by Low Power, also known as QRP, and highly directional antennas to both focus our radiation pattern along the azimuth or direction we want it to travel and to make the very most out of our transmitting power. Because of these reasons, directional gain antennas such as Yagis and Moxons are very important to know how to build and rapidly field. Omni-directional (radiates in every direction) antennas such as the Jungle antenna described in this blog before are great for fixed position signals, such as a hasty TOC, but offer no directivity and thus very little security. Directional antennas on the other hand offer inherent security from a potential eavesdropper having to be in between the transmission path to be able to intercept-

thus the term, Low(er) Probability of Interception, or LPI.

Additionally, one can get away with running relatively little power on a point to point relay network, which conserves energy and reduces you electronic signal footprint. All of these factors culminate in a relatively good communications infrastructure, using many components you likely already have. It beats the tar out of simply using a rubber duck on that HT. For this project I chose the Moxon because it’s very easy to build, takes little time, but most importantly, can be rapidly broken down and stuffed into a ruck, taking up very, very little room on 70cm.

Why 70cm?

70cm is a UHF Band- it’s versatile in that it can be used in both Urban and Heavy forest environments with ease, antennas are small, nearly every dual band on the market has 70cm frequencies, directional transmissions behave rather predictably, and last but most importantly, there’s a huge spread of 30mHz to find a quiet spot. For point to point communications over reasonable distances, it’s a good choice.

Moxons can be built moxon.pngfor every band out there- from very tiny antennas at microwave frequencies to 160m beams- you’re only limited by how much wire you have and your own ideas on how to construct it.

The diagram shows the basic layout. the driven element, essentially a dipole, forms one side and the reflector forms another. C marks the insulators needed to separate the two wires (pay careful attention to the insulator distances…it’s important). The rest should be self explanatory. Moxon Antenna Project is an excellent resource, and contains a freeware modelling program that will calculate all of the dimensions for you- all you need to do is plug in the frequency you want to use and write down the values. As for the actual construction, I used a couple pieces of scrap 16AWG wire and an old cobra head from the connector box, wrapped along the outside of scrap PVC pipe bolted together. Form a small air choke with your coax(wrap the coax a few turns in a tight loop near the feed point), connect to the antenna and radio, and you’re set. After a nifty coat of Krylon, getting it in a tree is as simple as tossing a line and quickly hoisting it, keeping with the rough bearing I want to transmit. Take note that the driven element must be forward to the intended direction, and can be polarized either vertically or horizontally.

As you can see from the photo above, for 70cm it’s a small antenna, was rapidly put together, and it works well for the intended task. It can be used as a decent SIGINT antenna to get a rough bearing on signals as well, although I’ve found that a Yagi is generally more reliable for this. Building projects, at least for me, have always been the greatest teacher and instils the greatest sense of accomplishment; but all that aside, it builds knowledge, which weighs nothing and can never be taken away from you.

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11 thoughts on “DIY Moxon Antennas and LPI

    1. Range depends on a lot of variables. But considering nearly all of the power is reflected forward, it would be reasonable to suppose one would make it to the radio horizon on much less power than compared to an omni-directional antenna.

      Liked by 1 person

  1. shocktroop0351

    Thanks for some more good information. Looks like another project to play with. Does it matter which way your feed line runs relative to the antenna (like on a dipole)?

    Liked by 1 person

  2. PR

    To test out how LPI such a system is, you’d need something like this: https://www.rohde-schwarz.com/us/product/ums300-productstartpage_63493-56146.html
    It sweeps at 12 GHz/s, so the probability it intercepts a transmission is really high unless you get the transmitted power below the sensitivity of the signal monitoring equipment by using spread spectrum techniques. Another option is to use other ham bands at 10 GHz or 24 GHz which is out-of-band of most signal monitoring equipment:
    http://www.kwarc.org/10ghz/24g.html

    Another option is to re-purpose some of the millimeter-wave devices coming out for automotive radar for radio.

    They re-calculated the atmospheric attenuation versus frequency and found it was way different than originally measured in the ’70s. I’ll have to find the paper later.

    Governments are investing hundreds of millions in signal monitoring and DF equipment.

    Liked by 1 person

    1. Or a really cheap way is to take what is Common-off-the-shelf(COTS), rig up your antennas, get familiar with EZNEC, and do it.

      Red Cell your system; try to monitor it during exercises, both in between points and outside of them to see what it produces, write it down, and improve from there.

      LPI by definition refers to Microwave, which is doable but for most of the readership an abstract concept that most won’t embrace. But if one realizes how to make best use of the kit they have now, much room for improvement is allowed.

      Like

  3. Wilbur

    You probably know this, but some of your readers may not – the Moxon has a rather broad forward direction but a deep null towards the rear, much like a cardioid microphone. It is best used for direction finding by orienting the antenna for greatest signal null rather than maximum peak. The null is sharper and easier to find; then the direction of the signal is towards the back of the antenna.

    Liked by 1 person

    1. You’re absolutely right. The direction finding aspect was to be a subject coming later; the primary focus is getting the readership’s feet wet with building rudimentary designs, assuming little is known aside from simple plug and play procedures.

      I’m focusing on how to maximize what’s already in the hands of most- knowing that while far from ideal, does have a significant amount of flexibility and many, many possibilities outside the box.

      Like

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