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Antennas, the third characteristic of successful DXers*

Quick links

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


My antennas

I have two small towers with beams, acres of space and some tall fir trees supporting wire antennas. 

6m quad above tribander 2 cropped 743

Trimming the treesTower 1 is fixed, 12m high, and made from welded angle iron.  It’s heavy at about 150kg and probably strong enough to free stand but for safety and peace of mind, particularly when I’m climbing the thing, I’ve guyed it top and middle. 

Tower 2 is also only about 12m high and the base is a few metres lower but it more than compensates for that by being  a wind-up tilt-over design.   It’s ideal for testing and tweaking antennas - such as the Hy-Gain TH3 tribander and homebrew 6m quad seen above.

The beams are only about 12-14m off the deck, quite low in wavelength terms but it helps enormously to be perched on top of a 260m hilltop from which the ground slopes steeply away all round.

Despite having checked all the traps and replaced a loose screw, the TH3 didn’t work very well.  The bandwidth was narrow and the SWR was high unless it rained ... which was a clue to the cause: one of the element tips was so corroded at the junction with the 10m trap that it was open circuit.  It was stuck fast and wouldn’t shift even using a big pipe wrench.  In the end, I cut it off, drilled out the broken end, cleaned, greased and replaced it, then added a small piece of TV antenna tubing in the end to restore the proper length.  Bingo!  SWR, bandwidth and pattern are all normal, and I’m enjoying myself chasing the DXpeditions getting ready for CQ WW CW 2013.

The homebrew 3 element 6m quad is a recycled antenna, built from the remains of a Band 1 TV antenna, six 2m long stainless steel rods picked up cheap in a sale at  local hardware store, copper wire stripped from an offcut of mains wiring, a bunch of ring terminals riveted in place, Photo of Gary up the masta nice die -cast boom-to-mast clamp recovered from a quake-damaged commercial radio company in Christchurch ... oh and some superglue to stop the elements twisting in the wind, for a while at least.  It is fed with an electrical quarter wave of 75 ohm TV coax, passed twice through an FT240 mix 43 core as a choke, and a long run of low-loss coax to the shack.  Total cost excluding the feeder and tower was about $50 plus an enjoyable couple of hours in the workshop.  Unfortunately 6m condx are so poor it is not earning its keep and has now been replaced by a much more productive 12m loop.

Here’s my antenna line-up:

160m - nothing up at present.  I have started gingerly clearing low side branches from the big old fir trees along the track using a cherry picker, with the intention to install LF antennas at a decent height (the trees must be close to 30m high).  To be continued.

80m - an inverted vee dipole in the trees, with the peak about 20m up.  The antenna is oriented N/S and works well off the ends, judging by some extended greyline openings to Western Europe (up to 90 minutes long!).  It’s not so hot off the sides to the Americas though.  I could do with yet another antenna.2013 40m loop labelled 300

40m - a quarter wave wire vertical crudely supported on a 5m roach pole (a fibreglass fishing pole without rings) lashed to a 5m post, with a few wire radials laid on the ground, plus about 20 square metres of deer fencing laid on top of the wires, and a fence wire connected in for good measure.  This ‘temporary’ antenna has lasted about seven years so far.  It just works.  I’m planning either a 4-square or a 3-triangle to replace it at some point.  Meanwhile, I have put up a horizontally-polarized but vertically-oriented full-wave wire loop supported by two tall fir trees, with its highest point about 25m up leaving the bottom corner about 5m off the deck.  It is fed with an electrical quarter -wave of 75 ohm coax, wrapped 5 times around an FT240-43 toroid.  The resonant frequency moved about 100kHz HF as I hauled it from just above ground level up to its full height, and the SWR dropped to 1:1 as I adjusted the shape to maximize the enclosed area.  I may yet run a line over another fir to lift the other top corner higher, but for now it is supported on the end of my 80m inverted-vee

30m - a homebrew 4-square on the tin roof of my shed.

20+15+10m -  the HyGain TH3 tribander, now repaired and working nicely.

Tiltover tower 740

One element of the 30m 4-sq17m - I am trying out a 3 element Yagi on loan from ZL2AAA Morrie (above).  Morrie built it for the ZL7T DXpedition, and it evidently still has plenty of DX left in it.  It’s perched on my tiltover tower.  Long path 17m openings to Europe are fun!

12m - a full-wave wire delta loop supported on a homebrew welded bracket holding two weedy fibreglass spreaders and a central PVC pipe.  It works surprisingly well, even though it is ugly and not very high.

6m - a homebrew 3 element delta quad, currently in storage.

All my balanced antennas use homebrew QRO ferrite baluns.Photo - Ant switch box in a weather box

 

 

 

 

 

 

The antennas are selected using  homebrew remote antenna switches like this one.  They use “B1B” SPST vacuum relays from Russia.  The relays aren’t exactly cheap but it sure beats paying $$$ to coax-feed all the antennas separately.

Note the small dummy load on the rightmost antenna port, useful to check from the shack end that the feeder and switch are working properly.

The long feeders run partly underground.  It was a tedious job hand-digging the trenches but the thought of reducing the local QRM from our computers etc. kept me going. 

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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 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.  Here’s the chart I use to cut wire antennas to length (prior to trimming anyway):
Click to download the Excel version

    [Here is the Excel file, now including a sheet with imperial measurements
    as well as the original metric version, and some notes on matching stubs]

  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 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 wire verticals, it is possible to 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, vee-beam 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, although it does help protect the self-amalg from the sun’s UV).
  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.  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.
  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, preferably using a decent butane-powered soldering iron (yet another worthwhile investment).
  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 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 here.

I was tempted to try 5/8th 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:

Drawing: fan verticals to a 1953 design

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 or low-pass filter feeding the 160m vertical maybe, a high pass filter for the 40m vertical, a remote relay at the base or a completely separate 40m antenna).

Yet another possibility is 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!

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Four squares

4sq supermarketI’ve been planning 4-square antennas for ages.  It took me so long to get around to it that I thought about just buying one from the local Four Square supermarket -->

The design is easy enough: four identical quarter wave vertical antennas set at the corners of a square with quarter wave sides.  Using the simplest feed arrangement, the antenna fires across the diagonals giving one of four directions: it needs a negative phase shift on the forward antenna relative to the sides, while the rear antenna needs a positive phase shift.  The pattern is cardioid.

Patrick TK5EP published a simple circuit for a 4-square hybrid coupler using three DPDT relays and a 3-core control line to switch directions.  I created an Excel spreadsheet using textbook formulae and the manufacturer’s AL values for various Micrometals powdered-iron toroids to confirm TK5EP’s component values for 40m.  By my calculations, T300-2 cores give exactly the right inductance and can handle more power than the T200-2’s favoured by TK5EP ... but for practical reasons to do with the length of the fibreglass roach poles I had on hand, I started with a 30m version to prove the concept.

4 verticals on the shed

Element base Base fitting The tin roof of my workshop is the perfect size, now that I have built a woodshed extension to square it off. 

The antenna elements are wires attached to fibreglass roach poles using rings of heatshrink tubing.  The poles are a push-fit into surprisingly strong adjustable angled base fittings made to hold Sky satellite dishes, a very convenient arrangement.  The purple wire is the earth connection to the coax outer, relying on metal to metal contact of the fitting to the galvanized steel shed to couple it in to the main roof area.

The calculated inductance values in the hybrid coupler on 30m require 8 turn coils on T300-2 cores, but for some as-yet unknown reason, my shiny new LCR meter from Hong Kong measures them at more than twice the target inductance (the calculations and inductance values are correct, but the meter is wrong, although it measures small potted inductors of a similar value correctly).  The coupler needs two 156pF capacitors: I could only find 100pF HV capacitors in my junk box, so added some coax tail capacitors in parallel in the first version. 

Here’s the coupler with the original 100pF capacitors in place, just before I added the coax tails.  The toroids are cable-tied to a chunk of PVC pipe to keep them in place.  The device on the right is a 50 ohm ceramic power resistor bolted to an over-sized heatsink, since I didn’t know how much power would be lost due to imbalances in the system.

Hybrid coupler guts

Phasing box under roof The hybrid coupler phasing/switching box is inside the shed, screwed to a rafter under the tin roof.  It’s more accessible and waterproof there than it would be sitting outside.

It works!  I get about 2 or 3 S-points difference front-to-back and signals are about 1-2 S-points lower off the sides.  At last I can tell whether signals from Europe are coming via the long or short path.  Direction switching is instant.  I get good reports with 500W out, so I’m happy with that.  I’m not quite so happy to be mobbed by rude EU stations calling over the top of my QSOs but in time they’ll learn as I pointedly ignore them to work weaker but far more polite DXers through their obnoxious QRM.

 

My pal Morrie ZL2AAA has since given me some 150pF mica capacitors and confirmed the calculated inductance values with his LCR meter (thanks Morrie!), so I will shortly fit those and test it again.  Perhaps I should dust off the old scope to check the phasing too ...

Next on the drawing board is a 40m version - well two actually, one for home and one for the club.  Longer roach poles are on order already from Neil, a very nice man at Kilwell.

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2x2 nested 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:

2 by 2 quad design

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

2 by 2 quad gain figs

Azimuth plots show reasonably clean patterns predicted on both bands:

2 by 2 quad plots

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:

 

Inches

Metres

17m reflector

687.2

17.45

17m driver

660.8

16.78

17m element spacing

101.4

2.58

12m reflector

505.92

12.85

12m driver

475.2

12.07

12m element spacing

75.54

1.92

Total wire length

194’ 1”

59.15

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:

Part

Imperial

Metric

Wire: copper house wire?

194’ 1” plus a few inches for joins

59.15 m plus a few cm for joins

Boom: 2”x2” steel box section

~1’

~30 cm

Spreader supports: angle iron

8 x ~2’
= ~16’ total

8 x ~60 cm
= ~3.8m total

Stub mast: 1 3/4”? steel pipe
(needs to be a sliding fit inside the
2” steel scaffold pole mast)

1’

30 cm

Spreaders - bamboo

8 x 10’

8 x 3 m

Cable ties to secure wire to spreaders

x 16 plus a couple for the feeder

Sleeving to support wire corners

16 x 4” long

16 x 10 cm

Locking pin - offcut of Number 8 wire

x 1

2:1 toroidal balun
with SO239 coax connector

x 1

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

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 are the takeaways :

  • Use sixteen to thirty-two λ radials on the ground , or at least four elevated λ radials.
  • If you don’t have the space for λ 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.
  • 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 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 (such as the radiator, the feeder, the rig, the operator and the QTH - plenty of improvement opportunities!) rather than the radials.  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, say, 32 radials .  Those guys who lay out 100+ radials 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.

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 λ 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 - in other words, half a circle of radials means half the ERP.  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.

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