Antennas: the third key characteristic of successful DXers*

Quick links

1. My antennas - up the tower and in the trees, the antennas I use most of the time
2. Constructing wire antennas - homebrew instructions
3. Antenna experiments - aerials I’ve tried or am planning to try
4. 2x2 quad for 12+17m - a 2 element 2 band nested quad design
5. Kites - using kites to life wire antennas
6. Optimising radials - pragmatic advice from a true amateur

* The first being skills and the second, location.

My antennas

At present, most of my antennas are attached to the tower, a 2-section 12m high square lattice tower, originally used for a farm windmill I suspect.  Being welded angle-iron it’s heavy at about 150kg and probably strong enough to free-stand but for safety I’ve guyed it top and middle:

The Cushcraft A3S and new rotatable 30+17m dipoles are on a ~4m steel scaffold pole sticking out of the 12m tower, putting them about 12-14m above ground.  They seem higher since, being a hilltop QTH, the ground slopes away quite steeply in most directions. 

The tribander is my main antenna on 20+15+10m .  I use wire antennas on the other bands:

160m - a dipole, currently in store.

80+30+12m - a temporary parallel-fed wire dipole just 12m AGL, hanging from the tower NW-SE but sloping down to the SE.  I’m currently trying a separate 12m wire loop in the trees near the shack, about 15m high.  It works great so will soon be moved to the tower area to replace the dipole.

40m - a quarter wave wire vertical supported on a roach pole (fibreglass fishing pole without rings) with just 6 wire radials laid on the ground.

30+17m - a new rotatable antenna  made of two wire dipoles paralleled on a common balun and supported about 1m underneath the beam on a pair of roach poles.  I couldn’t get a third paralleled 12m dipole to load up, presumably due to interaction with the 30 & 17m wires or perhaps the aluminium pole that holds the two roach poles in place.

I also suspend miscellaneous fullwave wire loops and dipoles 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.   Anyway, they work well.

The dipoles and loops use my homebrew QRO baluns.

The antennas are selected remotely using the homebrew antenna switch box shown here, or another recently constructed.  They both use “B1B” vacuum relays from Russia.  For SO2R or multiop contesting, one switch is near the tribander tower to the North of the house plot and the other will be in the trees to the South, giving some separation and flexibility.  The relays weren’t exactly cheap but it sure beats paying $$$ to coax-feed all the antennas separately.

The coax feeder to the remote antenna switches runs partly underground so I can mow the lawn and drive over the top.  It was a tedious job hand-digging the trenches but the thought of reducing the local QRM from our computers etc. kept me going.  Thanks to the Boxing Day sales, I’ve replaced a perfectly good TV due to the hash it puts out on 30m - by comparison, the LCD replacement is quiet as a gagged mouse.

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Constructing simple wire antennas

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

Parts required:

1. A sufficient length of suitable antenna/earth wire.  Read up on wire antennas or use my spreadsheet (below) to figure out how much you will need for, say a halfwave dipole or quarter wave vertical.  Aim for heavier wire for longer dipoles (40m to topband) that you don’t anticipate repairing every few months.  There is no such thing as “too many” earth radials for a vertical.  Here’s the chart I use to cut wire antennas to length (prior to trimming anyway):

[Click the spreadsheet to download the original Excel file]

1. 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.  Personally, I like to make up my own IP56-sealed plastic balun boxes using a toroid plus stainless steel bolts with stainless wing nuts for the antenna wires and either SO239 connectors or long coax pigtails (again, to save 0.1dB - 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.  If making up a wire vertical, don’t use an insulator or balun but solder the antenna wire and earth wires directly to the feeder, then wrap the feed point with self-amalgam tape.  “Chocolate block” screw-down electrical connectors can be useful when experimenting with new antenna designs but don’t rely on them for permanent fixtures as they corrode quickly.
2. 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 or rhombic (dream on).
3. 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.
4. Self-amalgamating tape or coax sealant (NOT ordinary insulating tape - it won’t last more than a few days or weeks).
5. 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.  It’s quicker in the long run to make a bunch of these up and use them than to untangle the wires every time you go to erect a stored antenna.
6. Plastic cord to hoist the antenna.  Fairly cheap, 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.  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.

Assembly instructions:

1. Measure out the exact wire lengths you will need and cut the wire.  I usually do this by clamping the end of the wire plus the end of a long tape measure (another toolbox investment) in the workshop vice (there’s another!) and walking them out into the yard.  If you have, say, a convenient fence or wall at least 10m long and some patience, you could measure and mark it permanently with paint or similar markers to use it as a giant ruler.  Err on the long side as you will trim the antenna down to resonance later but adding wire would involve soldering bits on.
2. Make up the feed point. 
3. Seal the coax feed point with self-amalgam tape or coax sealant.  Take care over this as water in the coax ruins it forever.
4. Label the feedpoint with the band/type of antenna.  I sometimes add short lengths of shrink -wrap on the centre ends of the antenna wires in order to label them.  Marker pens work for up to a year before fading in the sun.
5. Erect the antenna and test it for resonance.  It’s best and safest to erect it all the way to its finished position as resonance and match will vary nearer the ground.
6. Bring the antenna down, trim a few cm off the ends, put it back up, re-test for resonance and repeat until done.  If you get smart, calculating exactly how much you need to trim off, you will inevitably cut off too much at some point.  As I said, err on the long side.
7. Work lots of DX.

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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 fine.

I was tempted to try 5/8th wave verticals until I read W4RNL’s paper (one of many) 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 seem to work, after a fashion.  I’m getting quite a lot of noise pickup on the verticals though.

I’m planning a 4-square, probably on 40m at first to compare against the dipoles and single verticals I’ve used before.  The design of the 4-sq antennas themselves seems quite simple in theory: 4 x identical quarter wave vertical antennas each with identical earth mats consisting of at least 16 quarter wave radials, set at the corners of a square with quarter wave sides.  Powering the antennas is also not too hard in theory: the front diagonal needs a negative phase shift compared to both diagonals, while the rear diagonal needs a positive phase shift.  However, actually splitting the power and obtaining those phase shifts is not quite so simple in practice. 

There are basically two common designs: one as used in the Comtek 4-square box, and the other a W8JK design.  W8JK criticised and improved the Comtek design, obtaining a bit more gain and a better azimuthal pattern with a reduced rear lobe.  Given that direction switching will be more or less instantaneous with the right switching (so if I want to work someone off the back, I can just click the switch), I’ll go with W8JK’s design for mine.

Patrick TK5EP has published a simple circuit for the 4-square phasing and control boxes, using 3 x DPDT relays and a 3-core control line (or 2-core bellwire plus the coax braid, maybe?).  I created an Excel spreadsheet using textbook formulae and A_L values for various Micrometals powdered -iron toroids to confirm TK5EP’s component values for 40m, using textbook formulae.  My calculations show T300-2 cores give exactly the right inductance on 40m and can handle more power than the T200-2’s favoured by TK5EP.  Now to find a source for T300-2’s and 50-ohm coax with a velocity factor above 0.66 ...

... Or maybe I should just use 3/4 wave phasing lines, like OK1RD’s somewhat impressive RX array for 160m.  Not quite a rotatable 3 ele Topband yagi 100m up like those crazy Finns.  Boy oh boy.

Meanwhile, I’m planning a simpler array of quarter wave LF verticals sharing a common feeder, earth mat and tree:

The design is based on a receiving antenna shown in The Radiotron Designers Handbook of 1953.  The 160m vertical will be an inverted L as even our tallest trees are not quite 40m - more like 20 -30m at a guess.  The earth mat will be a combination of deer fencing and wire radials.  The antennas will be separated by a convenient distance along the cord - a few metres each I guess (I’ll have to lay the lines out on the ground to set it up).  I could use two cords from the top, alternating the verticals  to spread them laterally as well as along the cord, but the single cord design looks easier.  I’m hoping the magic of resonance will effectively feed one vertical per band, although on 40m the topband vertical will accept some RF so the match may need adjustment on 40m and the pattern may be high angle - in which case I’ll just have to try something different (an L-match/low-pass filter feeding the 160m vertical maybe, a high pass filter for the 40m vertical, a remote relay or a separate 40m array).

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The 2x2 quad for 12+17m

An article in Antennex by LB Cebik W4RNL “Adjacent band quad behavior” 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 slightly distorts the driver of the larger quad and affects all the design figures, but modelling shows minimal effects:

The calculated free-space gain (around 7dBi) and front-to-back ratio (20-30dB) are quite respectable for such a simple design:

Azimuth plots show reasonably clean patterns predicted on both bands:

Feedpoint impedances are calculated to be around 100 to 125 ohms being mostly resistive with just a little reactance.  I guess a 2:1 toroidal balun at the common feedpoint would present a good match for 50 ohm coax, reducing the common mode current if a balun were not used.

Here are the element dimensions in inches from W4RNL’s follow-up Antennex article, with the metric equivalents:

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.

So, the bill of materials looks like this:

... To be continued when I’ve obtained the parts ...

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Kites

Big parafoil kites are ideal for lifting wire aerials, given quite a bit of wind and lots of space.  Parafoils are 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+) - I struggle to pull it down.   For lots more ideas on kite and balloon-hoisted LF antennas, visit G4VGO’s site.

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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?

So, it’s a refreshing change to read a proper 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 test various theories against practice.

If you have the time and interest, I recommend studying all 7 articles in the series.  If not, here’s some of Rudy’s key conclusions for now:

• Try to use at least sixteen 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 need a good ground system.
• A surface-radial ground system will affect the resonant frequency and you may have to adjust the vertical height for that.
• 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 loss 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 and, as the last bullet suggests, we are probably better-off optimising other parts of the system than the radials.  Unless you are absolutely desperate to squeeze the last tenth of a dB out of your earth mat, there is certainly no practical value in going beyond, say, 32 radials.

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 improve 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 if you can only lay radials in a semicircle.  Substituting the missing radials with more, shorter ones helps somewhat.

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).

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Hawke’s Bay
North Island
New Zealand

39^o 39’ South x 176^o 37½’ East

Locator RF80HL

260m ASL

IOTA OC-036

CQ zone 32

ITU zone 60

Member of:
ARRL
CDXC
G0FBB
G-QRP-C
M6T
NZART
ZL6QH
ZM2M