Antennas

Constructing simple wire antennas

Simple wire antennas like dipoles and verticals are dead easy and very cheap to make: here’s how.

Parts required

1. A sufficient length of suitable wire. Stranded wire is more flexible and resilient than single-core wire, in my experience, but some prefer hard-drawn copper. Plastic-coated wire reduces corrosion but increases the size and weight of the wire. Use my spreadsheets (below) to figure out how much you will need for, say a halfwave dipole or quarter wave vertical. There are tabs along the bottom for:

• • Antenna lengths in metres;

  • Antenna lengths in feet and inches

  • Stub lengths in meters, and in feet and inches

  • Antenna lengths in fathoms and other curious units of measure. Did you know the 10m band has a wavelength just short of the length of a London bus?

2. A feedpoint insulator/connector, with a balun if you are feeding a balanced antenna with coax (e.g. a conventional dipole). There are several home-brew options here ranging from suitable pieces of wood, Perspex sheet, plastic or ceramic Tees or open-wire insulators, up to commercial centre pieces with built-in coax connectors. I make my own from IP56-sealed plastic boxes housing a ferrite toroid, using stainless steel bolts with stainless wing nuts for the antenna wires and either SO239 connectors or long coax pigtails terminated with in-line SO239s (again, to avoid unnecessary connectors and save the odd tenth of a dB - they all add up!). You should probably use ring terminal connectors of approximately 6mm to terminate and connect each leg of a dipole at the centre point, but soldered/tinned loops in the end of the wires will do. For unbalanced antennas such as wire verticals, simply solder the antenna wire and earth wires directly to the feeder centre and shield, respectively, then wrap the feed point with self-amalgam tape. “Chocolate block” screw-down electrical connectors can be useful when experimenting with new antenna designs or for field day lashups but don’t rely on them for permanent fixtures as they are hard to seal and corrode quickly.

3. Coax or open wire feeder, more than enough to reach from the antenna feedpoint once erected to the shack or remote antenna switch. Don’t go overboard but a few extra metres will allow for antenna movement in the wind and positioning the feeder to avoid garotting passing animals. Open wire feeder has negligible losses but the balun and ATU put the losses back in and add complexity, so open wire is only really worthwhile for a multiband doublet antenna, vee-beam or rhombic (dream on).

4. Antenna end insulators. Ceramic ones will last approximately forever (barring accidentally dropping them on the ground or over-stressing them) but are heavier and are quite scarce. Plastic ones will last up to a decade. Electric fence wire insulators are good if you have a farm supplies shop nearby, as they are designed to insulate tens of kilovolts for longterm outdoor use. Bits of Perspex or other strong plastic sheet can be cut to size. At a push, you may be lucky just using the plastic cord (see below) but it tends to start conducting when wet so the resonant point and match will change in the rain.

5. Self-amalgamating tape or coax sealant (NOT ordinary insulating tape - it won’t last more than a few days or weeks, although it does help protect the self-amalg from the sun’s UV).

6. A wooden winder board on which to wind the antenna. Make these from offcuts of plywood or thin MDF board approximately 30 x 15cm, with U-shaped notches in both short ends to hold the wire in place as you wind. Trust me, it’s quicker in the long run to make a bunch of these up and use them routinely than to untangle the wires and ropes/cords every time you go to erect a stored antenna.

7. Plastic cord to hoist the antenna. Fairly thin nylon or polypropylene cord is fine to hold out the ends of a small to medium-sized dipole or vertical, and lasts for ages if not over-stressed. Even nylon monofilament fishing line will do for small antennas, or maybe nylon strimmer line for larger ones. You will need thicker cord or rope (up to about 4-6mm) for large antennas and to hoist the balun and feeder of a dipole.

Antenna experiments

There are several parameters to adjust when it comes to antennas but essentially the choices come down to gain (in both horizontal/azimuth and vertical/elevation planes, don’t forget) and pattern. I’ve had good DX results with vertical quarter-waves and inverted-vee dipoles, with reasonably low take-off angles and more-or-less omnidirectional in azimuth. I’ve never had tall enough towers at home to make horizontal antennas perform as per the textbooks (except in local contests where the high-angle radiation is useful) but even ground-mounted verticals seem to work well.

I suspend miscellaneous wire antennas in the trees from time to time. Loops seem to work better than verticals and dipoles, presumably due to their higher radiation resistance - around 125 ohms according to the books - and hence higher efficiency. Some experts claim they have lower angle radiation than dipoles but I’m not sure about that, and both factors presumably depend on their height above ground. They do seem to be low noise antennas, although that’s purely subjective. Whatever, loops work well for me.

I was tempted to try 5/8^th wave verticals until I read a paper by W4RNL modelling them against quarter waves and vertical dipoles. The improvements in gain and low take-off angle just don’t seem worth it. I’m sure multi-element vertical beams would be better.

One day I’ll have to measure my ground conductivity and dielectric constant: meanwhile I use an estimate based on the ARRL Antenna Book’s values for my type of soil (around 6-9 inches of topsoil on a clay base on a forested hilltop), namely 5 to 6 milli-Siemens per metre conductivity and dielectric constant of 13.

My first antenna experiment in ZL was simply to add additional wires in parallel to the existing 40m quarter wave vertical, using the same coax feed and ground plane. I added 30m and 80m quarter waves - the 80m one makes an inverted-L. They seemed to work, after a fashion, but were noisy on receive. That’s a FAIL.

I’m considering an array of quarter wave LF verticals sharing a common feeder, earth mat and tree top support:

Yet another possibility is a quarter-wave vertical/L for 160m, end-fed on 80m with a relay-switched matching unit such as those described by AA5TB.

So many antennas in mind, so little time!

2x2 nested quad for 12+17m

In the Antennex article “Adjacent band quad behavior”, LB Cebik W4RNL used antenna modelling to calculate and optimise the characteristics of two-element two-band (hence “2x2”) nested spider quads for various combinations of the upper HF bands. In a follow-up Antennex article “Sneaking Up on 2-Element Common-Feed Quads - Part 3: Dual Band Quad Beams With Common Feedpoints” W4RNL adapted the design for a common feed located at the normal feed point half way along one side of the smaller quad.

This arrangement (below) slightly distorts the driver of the larger quad and affects all the design figures, but modelling shows minimal effects:

Using the spider quad design (i.e. spreaders radiating from the centre point of the array), the spreaders would need be about 10 feet (3m) long. I should be able to fabricate a spider centre using a short boom of square steel box section with angle-iron supports welded on at 62 degrees to attach the support arms. To attach the antenna to a round steel mast (a 2” diameter steel scaffold pole in my case), the end of the mast will need to be grooved to fit the corner of the box section steel, and then welded on: in practice, I’ll probably weld on a short stub of smaller diameter steel pipe that will sit snugly inside the main mast, with a locking pin through the main mast and stub to stop the antenna turning relative to the mast.

Kites as sky-hooks

Big parafoil kites are ideal for lifting wire aerials, given sufficient, steady wind and plenty of space. Parafoils are lightweight, spar-less, nylon kites that fold up into a small rucksack. Their aerofoil section (rather like a skydiver's parachute) gives loads of lift and they are fairly static in the sky (no good for aerobatics or fighting, ideal for hoisting aerials). My 5 ft-wide parafoil kite will lift a top-band quarter-wave wire in a decent steady wind (Beaufort force 3 or above), and I've used it to hoist vertical dipoles, inverted-Vees etc. using slotted plastic twin-feeder rather than coax (it's lighter and lower loss). In fact, it gives so much lift that it's a real handful in a strong breeze (force 5+) - a struggle to pull it down. For lots more ideas on kite and balloon-hoisted LF antennas, visit G4VGO’s site.

Optimising radials for verticals

There’s a lot of information (and some misinformation!) circulating in amateur circles about radials for verticals. Much of it is based on decades-old research conducted for commercial broadcasters, who have the space and resources to install and maintain over 100 radials per antenna. A fair amount of more recent advice draws on purely theoretical studies using antennal modelling software ... which presumably also draws on the decades-old research just mentioned. Quite a lot of what we read (... including my own musings ...) is essentially anecdotal (“It works really well: I work loads of DX!”). Unfortunately rather few hams have done the legwork to demonstrate, conclusively, what does or doesn’t work in a typical amateur setting, or to answer very common yet basic questions such as:

• How many radials should I put down?

• How long should they be?

• Should I lay them on the surface, bury them or elevate them?

• What if I can’t lay them out symmetrically?

So, it’s a refreshing change to read a scientific study of radials for vertical antennas by a dedicated amateur, Rudy Severns, N6LF. It’s obvious from Rudy’s excellent articles in QEX that he knows his stuff. He has carefully researched the professional and amateur literature and then designed and painstakingly conducted the controlled experiments to check out various theories in practice.

If you have the time and interest, I recommend studying all 7 of Rudy's articles in the series. If not, here are the takeaways :

• Use between sixteen and thirty-two ¼λ radials on the ground , or at least four elevated ¼λ radials.

• If you don’t have the space for ¼λ radials, lay down a larger number of shorter ones.

• The shorter your antenna, or the poorer your soil, the more you depend on the ground system.

• A surface-radial ground system will affect the resonant frequency so you may have to adjust the vertical height to compensate.

• Aside from the earth system, work hard at making the antenna itself more efficient. In other words, use high-Q loading coils, use top loading to minimize the size of loading coils, minimize conductor losses and so on.

Note that sixteen is not a magic number for on-ground radials. The experimental data indicate marginal improvements with more radials but rapidly diminishing returns probably make the effort pointless above about 32 radials and, as the last bullet suggests, we are probably better-off optimising other parts of the system (such as the radiator, the feeder, the rig, the operator and the QTH - plenty of improvement opportunities!) rather than the radials.

Let me say that again: unless you are absolutely desperate to squeeze the last tenth of a dB out of your earth mat, there is no practical value in going beyond 32 radials. Those guys who diligently lay out 100+ radials per radiator will do their utmost to convince you that it was well worth the effort, and in psychological terms they may be right but, scientifically-speaking, they are somewhat deluded. Just because 120 radials happens to be the numeric standard in broadcasting doesn’t make it appropriate, necessary or worthwhile for amateur stations.

If you use too few on-earth radials (e.g. just 4), their lengths become critical - in other words they are resonating, which leads to the counterintuitive finding that reducing their lengths to hit resonance may actually improve their efficiency!

Experiments simulating installations where there is no room for ¼λ radials right around the base of a vertical indicate that symmetry is fairly important. Efficiency is slightly reduced if one quarter of the circle is empty, and losses mount to as much as 3 dB (half your power!) if you can only lay radials in a semicircle. However, substituting the missing radials with more, shorter ones in the directions that you can lay radials offsets the effect a bit.

Another fascinating insight from Rudy’s experiments is that numerous factors affect the effectiveness of the radials, including for example the soil conductivity which varies with geology, moisture levels and band. Unless you can measure and control these factors, theoretical calculations from antenna modelling may not hold true in real installations, hence the value of careful experimental studies like Rudy’s.