Rapid Deployment Field Expedient Random Wire Antenna Ideas

The image shows amateur radio station VA3KOT/P inside its stealth enclosure – an old airline pilot briefcase made from stout, strong leather.

This is an almost complete station:

• Yaesu FT-891 QRP/QRO-optional in its field hardened steel 50-cal ammo box enclosure
• Bioenno 12Ah LiFePO4 battery
• 9.5ft tactical collapsible whip
• Adjustable loading coil for the whip
• PAC-12 capacitance hat for the whip
• LDG Z-11 Pro auto tuner (I like to think of it as a super fast L-match with a good memory)
• CWMorse aluminum paddles
• Heil Traveler headset (the microphone is feeling neglected due to not having been used for a very long time)
• Ham-made (by me) Guanella 4:1 balun
• Selection of coax cables
• Set of ground radials
• Random wire antennas (we’ll talk about those shortly)
• Operating table (yes, operating table too; see image below)

What’s missing? Just a chair. I have a selection of lightweight folding and collapsible camping chairs to support my delicate derriere.

Oh yes, the table; these pilot cases (NB: I am not a pilot) are so strong that, if stood on end, they make an excellent support for the radio.

When everything is packed inside the pilot’s case then it is a little on the heavy side (not to be confused with the Heaviside which is a layer of the ionosphere). The weight is manageable if it is only being carried a short distance, but I have a small folding cart for transporting it further away from my vehicle.

So what about a rapid deployment, field expedient antenna?

Rule 1: It has to fit inside the briefcase. Rule 2: If an antenna doesn’t fit inside the briefcase refer to rule 1.

I have 2 choices that meet Rule 1; a “tactical” (meaning it is painted green and looks like it might not be out of place on a Humvee) collapsible whip. It can be mounted directly on a 3/8x24tpi bracket fixed to the rear of the ammo box. It is usually deployed with a loading coil and capacitance “top” hat.

Rear connections. Note the bracket for mounting the tactical whip on the left. The antenna wire and radials are plugged into the magnetically attached 4:1 balun. The CW paddles are secured to a steel bracket which is also magnetically attached to the ammo box – a steel box is very handy for attaching accessories with small rare earth magnets.

Second choice is a simple wire antenna. I am currently in favor of random wires due to their inherent multi-band flexibility. Although I have never been a fan of auto tuners, for several reasons, their speed in finding and memorizing a match for multiple bands is very valuable for the kind of hit-and-run style activations that I enjoy. The LDG Z-11 Pro is an L-match which means it is perhaps more efficient than a typical C-L-C type of manual tuner.

Don’t risk disappointment

I have read a lot of blogs and watched a lot of videos in which an operator buys a commercial wire antenna and deploys it in the field without investigating whether the antenna is actually going to work in the manner they expect. We don’t all have the time or inclination to model an antenna to find out how best to use it, but it can lead to disappointment if we don’t do the homework first.

‘Tis a gift to be simple, ’tis a gift to be free

It is very simple to make a “Simple Wire Antenna” (SWA) and it can even be free if you can scrounge some leftover wire. Climb an old telephone pole and pull down disused POTS (Plain Old Telephone Service) wire if your neighborhood has switched to fiber lines. Okay, I’m kidding, don’t do that; some poles also carry high voltage electric cables.

I have had great success recently with a sloper wire 27 feet (8.23m) long supported from a tree, or my Spiderbeam 23ft (7m) pole, and worked against four 13ft (4m) ground radials. The LDG tuner easily finds a good match of 1.5:1 or less on my main bands of interest: 20m, 30m and 40m. Out in the field there is no lossy coax transmission line involved – apart from very short sections to plumb in the Guanella 4:1 balun and tuner. This wire is a little short for 40m but presents no problem for the tuner. I have received some very good signal reports using this wire.

Then I wondered, if the wire was longer, could the antenna work even better? EZNEC helped me decide. I modeled three random wires: 27ft (8.23m), 41ft (12.5m) and 84 ft (25.6m). To make it a fair comparison, the same band (20m) was compared and the three wires were modeled in exactly the same sloper configuration. The results were very revealing.

Model 1: 27ft random wire sloper

The elevation plot shows a good directional signal with a small amount of gain at an elevation of 35 degrees.

The azimuth plot shows a wide angle of radiation with ~5dB of Front-to-Back signal strength.

The 27ft sloper has been a solid, reliable performer with strong signal reports being typical.

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Model 2: 41ft random wire sloper

It would be tempting to throw a 41ft wire up into a tree and run it as a sloper down to the ground, but the results might be very disappointing.

Although the gain is increased, the elevation angle of maximum radiation has also increased to 75 degrees and the directionality has all but disappeared.

This would make the antenna a cloud warmer and much of the signal would disappear into space. Your CQs might perhaps be answered by extra-galactic radio operators a few thousand years in the future. How patient are you?

Model 3: 84 ft random wire sloper

If you have a much better throwing technique than mine, you might be able to get your throw weight up 62 feet in a tree to pull up an 84ft random wire sloper. But, if you look at these charts you may not want to do it.

Your signal now goes straight up into outer space. In most parts of the world 20m is not an NVIS band so almost your entire signal is wasted. As an aside, I recently experimented with a “V” wire antenna with 29ft arms supported at the ends by trees. I QSOd on it but I received a very disappointing 449 RST report for my 30 watts of signal. When I modeled the antenna afterwards I realized why – the radiation pattern was very similar to this 84ft sloper!

Why does this happen?

What causes the increase in radiation elevation? A big clue emerges when we examine the current distribution on the antenna wire. Remember, high current points along the wire are responsible for the majority of the RF energy radiated.

Current distribution in a 27ft wire Current distribution in a 41ft wire Current distribution in an 84ft wire

As we can see from the three charts, a 27ft wire has just a single high current point.

Along a 41ft wire a second high current point begins to emerge and the interactions raise the radiation angle.

When we look at the 84ft wire we can see that there are two full current maxima and the emergence of a third. Once again, these interact to raise the radiation angle.

Was this a fair comparison?

I think it was fair. None of the three wires is overly difficult to erect as a sloper. Some hams get a lot satisfaction from shooting wires high into trees on the assumption that higher and longer is better. In fact, on the lower bands an 84ft sloper might perform quite well. With the popularity of POTA, and the solar cycle at its peak, a lot of activity is to be found on 20m which is why I chose this band for the comparison.

What if …

Maybe, if we orient an 84ft wire differently, we can get it to be the outstanding performer we would expect on the higher bands. I have QSOd on several occasions with an end-fed 84ft wire hung about 5ft above ground supported by a long hedge, or low branches of small trees. Even at QRP power levels I got good results despite the antenna having no gain.

So let’s follow the old adage of getting as much wire as possible, up as high as possible, but with the proviso that it must abide by the limitations of being rapidly deployable and field expedient. How about an Inverted-L?

In fact, to make it RDFE (Rapidly Deployable, Field Expedient) I am going to call it a Lazy Inverted-L. The feed end of the wire connects to the rig (sitting on the pilot’s briefcase) at a height of 2 feet. The wire then goes to the top of my Spiderbeam 23ft (7m) pole about 3 feet (1m) away with the remainder sloping down to the far end supported 3ft (1m) high on a trekking pole. It might not be the best setup but remember it must be “RDFE”.

Model 4: 84ft Inverted-L antenna

The results are quite encouraging. First, the antenna has a decent amount of gain for a SWA. Second, the elevation angle has two useful components: a forward lobe at 35 degrees elevation with a -3dB point down to 15 degrees for a chance of getting some DX action.

There are two more lobes behind with a higher radiation angle that could yield short range contacts.

Where’s the gotcha?

The azimuth for maximum radiation changes from band to band. That would mean re-orienting the wire for band changes. Fortunately the RDFE, Lazy Inverted-L orientation makes that a fairly easy task.

Imperial or Metric?

I have tried to use both Imperial and Metric dimensions in this post. I was brought up using Imperial measurements, earned my Physics degree which was taught entirely in metric units and now tend to mix them up. Canada is officially metric but supermarkets still mark the prices of meat and produce in dollars per pound. Why? Because it sounds cheaper I guess! Okay fellow hams, what’s your favorite HF band? Mine is 65.6168ft. Yes, that’s 20m.

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