An Improvised Directional Wire Antenna

IMG_1417 - Copy

A question from MT-01 over at Lower Valley Assembly:

I have a neighbor who was a special forces engineer sergeant in Vietnam. I helped him get his amateur license. Sometimes we talk about radio comms in Vietnam. He tells one story where the comm sergeant put the end of his antenna into a stream in order to make a contact. Is that feasible? I keep wondering if he misunderstood what he saw, and they guy was grounding something instead of using it as an antenna. If it didn’t ground the signal, I guess that I could see it being a really long wire antenna, but if it worked at all it’d just work as a NVIS I would think. I’ve tried googling the idea, but haven’t found anything. Thought I’d run it by you.

longwireThe antenna was most likely a variation of the Longwire, using the water as a reflector. It can work on any band, from HF to UHF, depending on what application you need and cut to the appropriate length. What’s nice is that it’s a somewhat directional antenna as is, but when you take the cold end and make it vertical with the hot end sloping towards the ground, you’ve got an even stronger directional antenna. Add a terminating resistor to the mix and now you have a completely directional antenna pointing in the direction of your sloping wire.

IMG_1418 - CopyYou may not be able to tell from the photo, but the same antenna that’s pictured in the diagram is also right in front of you. And that’s using clear coated, somewhat shiny speaker wire. One leg is running into a small stream at the bottom and the other strung across to the opposite bank. This is a VHF antenna, with the hot wire cut one wavelength for the VHF frequency I’m using (936/frequency= length in feet) coupled to a terminating resistor on the opposite end sloping into the ground on the other side.

IMG_1416 - CopyThe antenna itself is not for NVIS propagation at all- it does the opposite. The radiation pattern is for a strong groundwave pulled in one direction; think of a flashlight pointed in a direction and that’s very close to the radiating pattern.

On VHF this type of antenna is very easy to construct on the fly and including a terminating resistor pulls the current on the wire towards the end along with your signal. The cold end is running to the water and grounded out with an aluminum tent stake. The water itself serves as the reflector along with the cold leg of the antenna being vertical.

I built this antenna entirely on the fly out of my ruck on site just for the photos to demonstrate what can be done- the same way we do it in class. Is it perfect? IMG_1412 - CopyNope. Does it work? Well, the signal report on the repeater I checked into that’s quite a long ways away says so- and this transmitter spot is located at the base of two hills. That’s not always the perfect gauge of effectiveness, but it is one nonetheless. Improvements to make include soldering and sealing the crimps with shrink tubing for robustness and soldering the resistor in place. Immediate improvements would include electrical tape on all the potential points of failure.

There’s nothing quite like having the skills to build your own gear on the go and as needed. As any professional soldier knows, Small Unit skills go far, far beyond simply being a trigger puller who can flank a target. Communications is the third leg of tactical proficiency and is by far the least understood. We cover improvised antennas and much much more in the RTO Course, and I’ve got a few coming up- 14-15 July and 11-12 August. Email me at  brushbeater@tutanota.com for more details.

We’ll see you out there.

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28 thoughts on “An Improvised Directional Wire Antenna

  1. Hey brother. I have some follow up questions:
    1) Are you referring to the end attached to the transceiver as the “hot” or “cold end”?
    2) What are you using for a terminating resistor? My guess would be a dead AA battery, but I’m occasionally wrong, as my wife likes to remind me. 😀
    3) In the HF world, would this type of antenna exhibit preference for ground wave, NVIS (assuming the band and conditions permit), or low take off angle? As a VHF antenna, this example would seem to hint at something more than ground wave, if you made it over 2 hills to a repeater, and I know NVIS isn’t possible with a band that will hit the moon and bounce back. Maybe I just reasoned my way to an answer. Anyway, the only place I’ve seen an antenna such as this hinted at is in the SF Antenna Handbook, and details were sketchy there.
    4) When the end of the wire is put in water, does that also act to ground the antenna or just act as a reflector?

    I suspect the answers are going to give me one of those “why didn’t I think of that!?!?” moments. Also, for the folks reading this blog, don’t forget that this weekend’s ARRL Field Day presents a good excuse to get to the field and actually try this antenna out. To be honest, that’s why I’m asking so many questions about it myself. It also means that we’re about 99% likely to have some sort of rainfall, if everyone else’s luck runs like mine. An instructor I know put it this way, “If it ain’t raining, you ain’t training!”

    Liked by 2 people

    1. The hot end is the wire running from the Red connector for the split post BNC adapter. It carries the load. The cold/ground end goes on the black.

      The terminating resistor actually is a 450 ohm 10w carbon resistor. You can make a temporary one from a AA battery’s carbon core. It will corrode in time- so they work but it’s a one time use deal.

      On HF, this antenna actually would exhibit both groundwave and NVIS characteristics, especially being as close to the ground as it is and both in the direction of the resistor. Groundwave has a shallow takeoff angle, NVIS (vertical) has as close to 90 deg as possible. For VHF it’s a groundwave antenna (mostly, unless you run into a random E layer skip or tropo ducting) for communicating directionally in thick bush/jungle conditions.

      The water is a ground, and acts as a reflector. Water is a great reflector, especially salt water.

      And yes- Field Day. I encourage everyone to run it the way its SUPPOSED TO BE RUN…off-grid and more as a training event than a contest. But I digress. 🙂

      Liked by 3 people

  2. Cavguy

    Do you use pl259 on your cable or strictly BNC or mix and match?

    Rain here in southern Delaware for field day, all the fellas in the club I sometimes event with like their creature comforts. They’ll be inside, but I have to admit the food is good.

    Cavguy

    Liked by 2 people

  3. HUTCOMINT

    What would the radiation pattern of this antenna look like?
    Like the forward to back ratio, and how much is coming off the sides.
    Basically how would this compare to a handheld yagi type antenna for directionality?

    Liked by 2 people

    1. Smith charts for it are displayed in the USMC antenna handbook on the resources page of this blog.

      It serves the same function as the Yagi, just done with wire and a lower visible signature.

      Liked by 1 person

  4. Cavguy

    NCScout, I’m working an idea to change our ham radio demographics, targeting Home Schooled kids. I’m putting together some blocks of instruction for various age groups. Need some practical, simple ways to help kids visualize radio theories.

    Example 1; using a vessel to hold water drop a B.B. in so circular ripples represent an Omni-directional antenna pattern.

    Example 2; bounce a tennis ball straight up and down for NVIS, low and long bounce for long distance shots.

    I’ll use PowerPoint and video shorts as part of the classroom. Intent is not to teach the tech level class for testing. But to expose/teach the various concepts of Ham.

    Your help and you readership is gladly accepted.

    Please contact me directly.

    Cavguy

    Liked by 1 person

  5. Here is a spec sheet of the type of resistor you should be looking for:
    http://www.caddock.com/Online_catalog/Mrktg_Lit/TypeMS.pdf

    You want resistors that are non-inductive at RF, and can handle the wattage. In this instance, you are looking at the model MS322. They are a bit spendy new, YMMV in the used market.

    A pack of 4 D-Size Carbon Zinc batteries (BA-30 in military parlance) is $1 from the local dollar stores. Make sure you buy Carbon-Zinc (usually also called “heavy duty”), and not Alkaline batteries.

    Six Meters lends itself well to antenna experimentation, especially with all the military tactical VHF antenna stuff.

    Liked by 1 person

  6. Keypounder

    The long wire antenna grounded at the far end you guys already know as a Beverage antenna. It is commonly used for listening, as the configuration results in gains of -6 db at best for 3 + wavelengths, and less for most installations, but it does work for transmitting, too. Also known as a “Grasswire” antenna, this configuration provides good directionality, vertically polarized signals, and is easy and quick to set up, especially on VHF.

    Liked by 1 person

  7. Chris

    So what’s a long wire? A 100′ wire seems long to me. But a10′ wire would be long at 440 MHz. The key to remember is that lengths are measured in wavelengths for antennas. A longwire antenna is usually a couple of wavelengths or more. So that 20m longwire might not be a longwire for 160m.

    Liked by 1 person

    1. A longwire is a terminated full wave antenna. In the post I pointed this out using the formula for antenna length in feet. Used with a grounded resistor in lieu of a matching network at the feedpoint, it becomes a vertically polarized antenna with a radiating pattern in the direction of the resistor and representing a low SWR.

      Like

  8. Bob

    Would attaching the negative end of the above antenna to a woven wire/barbed wire fence serve the same purpose as the creek? Also, could the above fence serve as a reflector for a low horizontal antenna for nvis? Thanks for all your great writing.

    Liked by 1 person

  9. Daniel

    Does the coating on the speaker wire help prevent signal problems as it comes in contact with branches and damp things in the bush? Would bare wire perform similarly? Does wire coating/insulation even matter?

    Much appreciated – I have some interesting Field Day observations to type up and send to you.

    Liked by 1 person

    1. The coating on the wire helps. As anyone who’s troubleshot livestock fences knows, touching a branch can ground out the circuit. The same thing happens with antennas. Bare wire or anything that conducts can work for an antenna, but coated wire is better in improvised environments.

      And send those notes on!

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  10. Daniel

    Here is the report I promised. Use it where/however you like. As I said I was able to attend my first HAM Field Day this year. Several groups were operating near me and I visited two of them. The first one was the local club. All older fellas, mostly engineer types, working from a luxury motor home parked behind a VFW lodge. The 40′ mast holding up the 80′ double bazooka antenna was raised and lowered pneumatically with the push of a button. The only real “work” was carrying the donuts up the entry steps. They had one ICOM 7300 for voice and a CW station. The CW guys were positively ancient, the youngest being in his late 70’s. Barely able to walk, but still have their mad skills clipping along at 30+WPM. Respect. This bunch was all about chasing the contacts and I was able to log on the voice side for a while. To be honest, it became boring and pointless after about two hours. As an aside I mentioned my intention to build an 80 meter dipole with the Jetstream 4:1 balun and speaker wire for NVIS use. “Won’t work! Marginal at best. NVIS does sound interesting, though.” All were friendly and I was well treated.

    I eventually left there and sought out an ARES group on the far side of the county. They were all much younger and led by a female medical doctor. They set up in a remote forest preserve under heavy tree canopy without electricity or anything else. No donuts to be seen. The mosquitos were so thick the solar panels wouldn’t register. I’m pretty sure I caught diseases ranging from Zika to the Plague from being stung. They weren’t trying to log as many contacts as possible but were doing a lot of listening, having roughly ten different antennas set up in various configs running to 5 or 6 HF radios, mostly Yaesus of varying flavors, one running digital FT8, and a single CW key. They’d just finished a several hour long practical drill simulating a Godzilla incident, with operators scattered for miles calling in reports as Big G stomped and chomped his way across 40-odd miles before disappearing into a nearby lake, followed by heavy comms simulating the aftermath of dealing with casualties and damage assessment. Most excellent. Radios overheated and batteries ran low and died as they pushed the setup to wherever it would break. Their CW capability was much lower than the first team, using a huge poster showing the dots and dashes as nobody there really knew the code. Very slow but it worked. Again I mentioned my dipole plan and they said, “Oh yeah, that’ll work. We do NVIS all the time – in fact those two antennas are set up for it right now.” Again, very welcoming and helpful folks.

    As you can see the two groups were night and day apart but I learned from both of them.

    Liked by 1 person

    1. If you are interested in practical uses of NVIS take a good look at this well established operation. Good old fashioned Kiwi ingenuity at it’s best. Trampers take 1W sets in to the bush with them which gives adequate voice comms back to monitoring stations in each of the main islands. Basically a 100m footprint.

      http://www.mountainradio.co.nz

      Liked by 1 person

  11. Brian

    Yep, thought you would like that. Also note the beacon function which is basically a challenge & respond feature to see if you are getting your QRP signal into base – very sensible.

    Last time I saw one of these I wasn’t really interested in radio. I will be back out there later in the year so will try and hire a set to use on a bush trip and feedback idc.

    Liked by 1 person

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