The G1EXK Ultra Compact 60m Antenna
I live in a 'new build' property with a postage stamp sized tiny garden. Only 28ft long I can't even fit a 20m halfwave dipole without shortening the length (I use an Inverted-V setup).
I needed to find a way to work the low bands with an efficient, effective antenna that would fit in the tightest of spaces and work at low level (no big tall verticals).
I couldn't find a satisfactory existing antenna. So I modelled and built my own using a combination of different techniques.
Features and benefits of the G1EXK Antenna
Fits in a space as small as 25ft across, and can be mounted low level and still be effective for DX and NVIS.
Naturally tuned antenna that does not require an ATU on the resonant frequency.
Both Vertical AND Horizontal polarisation elements.
Vertical radiator gives low angles of radiation for DX even at low mounted height, horizontal radiator gives NVIS pattern.
Fed with Coax with low feedline losses due to the low SWR.
Balanced antenna that does not require a lossy ground radial system.
Balanced design also makes it tolerant to nearby objects such as buildings.
Cheap to build.
Low profile stealthy design.
Can be used on 80m with an ATU (SWR drop over the 80m band).
Build Guide
The antenna is fundamentally a balanced resonant dipole that has been shortened significantly and given both a vertical and horizontal radiating component.
The antenna is fed with a regular dipole centre with coax. The centre conductor feeds the vertical element and the ground feeds the horizontal (counterpoise) wire.
The vertical part of the antenna has been shortened via 'linear loading'. This technique involves folding the antenna back on itself with parallel spacing. Using an insulating support (I used a 6ft plastic pole), I spaced and dropped the wire down over a 6ft distance and a hanging height of approximately 15ft. I used enamelled wire that holds it's shape. The wire is tie wrapped and electrical taped to the tube at the top, and pinned, and held in place with string and tent pegs at the bottom. An alternative option is to use thin plastic tubing/spacers to hold flexible wire in place.
The horizonal wire functions like a normal dipole. This can be configured however fits your space. I have a Zig-Zag shape that takes it from the dipole centre to a back fence, and then a side fence. You can use a horizontal length and then a vertical drop at the end, or an L shape.
The Insulating support that 'hangs' the vertical element can be supported from either end. A mast is ideal (can be metal), with the dipole centre at the top, and the coax running down the mast. I'm even more restricted for space so mounted the support against the wall of my garage and the dipole centre on the other end of the support. The coax then hangs down from the feed point. See my diagram below for this configuration.
Further details and photos of each component are shown below.
Dipole Centre
Standard cheap commercial dipole centre. Cable tied to the end of the horizontal insulating support. Enamelled wire connected to the centre. Green 'antenna wire' connected to the ground side with strain relief. Wires are crimped and soldered before being screwed to the dipole centre. Coax connected with a M&P sealed compression plug, a layer of self amalgamating tape, then a layer of 'coax seal' putty. I really don't like water getting in my coax..
Enamelled wire crimped and soldered. Be sure to scrape off the enamel coating to ensure a good connection.
Vertical Radiator
I used enamelled copper wire. The wire can be bent into shape as required. I attached the top to the support with tie wraps and electrical tape. The bottom I pinned out and held in place with three tent pegs and some string. If you're using normal flexible wire you can create a lower horizontal insulating support and attach the wire the same as the top.
The vertical height of the support is not crucial (but higher is better), and the number of times you fold back the linear loading is not important (less is better). The diagram shows my setup. Squashed into a space 6ft long and 15ft high. All that is important is you keep at least 40cm (16 inches) between parallel wires, the ground, and surrounding structures.
How you terminate the wire depends on where it finishes. Terminate the end with an insulator and then some string/rope as you would with a normal dipole. This can then be tied to the ground (end of wire finishes downwards), to the support (upper or lower), or the wire can be pulled out sideways and tie to a nearby fence/tree etc.
Horizontal Radiator
The horizontal radiator can be treated like a normal dipole leg. I used a stealthy green flexible antenna wire (details in the parts section). You can run this however your space dictates. I used a Z shape from the back fence to the side fence on my small garden space. Higher is better, but it will work at lower heights. Mine comes down to about 5ft from the 15ft dipole centre.
Coax and Choke
RG58 is fine for relatively short runs (less than 25m or so). Loss is low at 5MHz and the lower bands, especially with a matched low SWR antenna. With the high noise floor of the lower bands a drop in overall signal will not effect the SNR ratio. I used a premium narrow coax called 'Messi and Paoloni Hyperflex 5'. It's lightweight, extremely flexible and double shielded. Perfect for my needs. Route the coax away from the antenna so that it doesn't run close and parallel to either the vertical or horizontal radiators.
It's a good idea to choke common mode currents along the feedline. This will stop the feedline becoming part of the antenna, reduce 'RF in the shack' and keep the antenna performing as designed. Using the G3TXQ Choke Charts. I calculated 15 turns on a Fair-Rite 240-31 toroid to be perfect for 5MHz. I simply wrapped the coax around the toroid before fitting the PL259 plug and attaching to the dipole centre. I use these on all my antennas now to great effect.
Insulating Support
I originally bought a 32mm diameter, 2m long 'waste' pipe. As seen in the photos, it flexed under the weight of the coax and toroid. This wouldn't be as big issue if the support was centre mast mounted. I've since bought a 40mm pipe, and found the 32mm pipe slides inside perfectly making it essentially double walled thickness. This arrangement is pretty solid. Any non conducting support will work. I connected mine with a right angle bracket to an existing antenna pole as shown below.
Dimensions
You may have noticed I have not included any element lengths above. This is because it can, and will vary quite a bit depending on your particular configuration. The vertical linear loading element will require a longer wire due to the way the loading works. The more loading you add the longer this element will become compared to the counterpoise length. Also other nearby objects will affect the length, as will the wire type used. I'll include ballpark figures for the 60m antenna I built, and also some scaled up numbers for the other low bands.
Ideally an antenna analyser is the best way to tune the antenna. Taking measurements with an SWR meter will work at a push. Because each side of the dipole requires a different length it can be difficult to work out which end needs resized. Shortening or lengthening either side will reduce or increase the antennas resonance (low SWR point). However as the dipole becomes further 'off centre' the impedance will raise. So the trick is to resize the side which changes the impedance towards 50 ohms and a good match.
The bandwidth and lowest SWR point will depend on your particular configuration and amount of linear loading. Linear loading is one of the most efficient ways to reduce the length of an antenna, but the trade off is reduced bandwidth. For 60m you should be able to get an SWR of 1.5:1 at just over 50ohms and a bandwidth of below 3:1 of over 200KHz.
40m Band: Vertical Element 11.9m, Horizontal Element 9.7m
60m Band: Vertical Element 16m, Horizontal Element 13m
80m Band: Vertical Element 22.9m, Horizontal Element 18.6m
160m Band: Vertical Element 44.6m, Horizontal Element 36.2m
Antenna Analyser Sweep for my G1EXK 60m antenna
Performance
So how does the antenna perform in real world testing? Pretty well actually.
During the day on 60m the antenna is slightly down (maybe 1s point) on a similar height mounted dipole. This is amazing as we've reduced a 27m long antenna to fit in a 9m space!
During the night European and North American signals are up as much as 3s points on a similar height mounted dipole. The good DX performance is due to the vertical polarised radiator and the low angles of radiation vs a very low level horizontal dipole.
Bonus 80m Band
For reasons not immediately apparent I noticed while sweeping the HF band with my antenna analyser that there is an SWR drop to around 4:1 across the 80m band. A good antenna tuner will tune this no problems. Efficiency will be down, but it will be possible to operate and make contacts on the band.
Kit List
Above links show some suggested UK based sellers