80m Inverted-L Antenna (Base-loaded for 160m)
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My coaxial-dipole would just about tune up on 160m but its replacement, a standard 40m half-wave dipole wouldn’t. Given the small size of my garden even a long wire contorted with lots of bends wouldn’t easily fit. A bent quarter-wave vertical (in inverted-L configuration) seemed the best approach and the answer was inspired by G4EIE’s advice “you need a whacking big loading coil”. Garden aesthetics also demanded antennas have as low a profile as possible; consequently it would have to be base loaded as a large 160m loading coil swinging in the air and was out of the question.
I wouldn’t describe this antenna as being a DX antenna however within my small garden where true DX antennas would be impossible it has performed very well. I was mostly interested in 160m and worked into North Africa, near Asia and across Europe; I have also heard North American stations. I won the WPX Contest for 160m low power SSB England for 3 years using this antenna. On 80m, I have worked across Europe and a station in the USA. During a time when I had problems with my 40m inverted-V, I tried this antenna on 15 and 20m and found it can work pretty well on HF too. Receiving 57 reports from the US and Mexico on 15m and 20m, even in relatively poor propagation conditions.
At the furthest point in the garden, I hammered in to nice clay soil a 2m (6 foot) length of copper pipe – I had about 600mm (2 feet) of copper pipe above the ground. On top of this pipe I fitted some black plastic conduit which acts as a prop to take the weight of the loading coil. Above the loading coil, the vertical section is suspended by Kevlar cord attached to a 2 x 2 inch post. Overall the wire section above the loading coil is about 18m long (as long as I could get away with in free space). The feed-line is about 15m of coax, most of which is either laying on or just under the soil. Since then I have sunk a further 3 earth spikes.
The loading coil itself need not be accurate. The dimensions I used are above in the diagram but the important factors are:-
- use white uPVC pipe – avoid black uPVC pipe as that contains carbon
- use large diameter pipe at least 100mm (4 inch) diameter
- the length of the pipe should be around 200mm (8 inch)
- use 1.5mm flexiweave insulated copper wire
- wind about 40 turns of the copper wire on the pipe and form taps along it – I would suggest at 0, 2, 5, 10, 15, 20, 25, 30, 35, 40 turns. This will allow you to experiment by using varying numbers of turns on the coil
Initially I relied on my ATU and found I was receiving most stations 2 s-points higher and the background noise was 2 s-points lower (as compared to my dipole); a huge improvement! Feed-line radiation is low; this was always a problem on 80/160m with the dipoles (which are too short of course). However – Relying on the ATU for 80m brought me reports 4 s-points lower than the dipole on transmit. I carried out tests and found tap positions which gave me resonance on 80m and 160m and so decided to fit a switch between these two tap positions (160m = 28 turns; 80m = 6 turns). The transmit side now improved greatly on 80m, with stations reporting me 4 s-points higher than the dipole. As a M3, I’d never had reports of 59+20 before – what a treat! 160m, later that evening, delivered similar success with GB6OD in Weymouth (about 270 miles) giving my 10w a report of 55.
Noise levels on 160m can be horrific, often as high as s8. Getting a NES 10-2 DSP speaker at Christmas 2004 has made a big difference to reception and I was able to work Ukraine on 160m albeit at rs44 with 90w. The limiting factor now was again my antenna which wasn’t quite up to DX. I just about heard N4CQC but to work him on 160m would needs some serious thought!
Bigger and better – the initial design antenna came down on January 7th 2005 after a storm with 75mph winds. Not to be beaten, the antenna was up again by the end of the month, now with an increased the vertical section taking the vertical section nearer 20′ off the ground. I replaced the support lines with Kevlar; originally I used 60Lb nylon fishing line which frayed on the roof section. Increasing the height of the 2×2″ post gave me some anxieties watching it in the next storm – I found this useful guide of wind pressure to worry me further (HI!).
Some variations in the tuning arrangements were required to better deal with the comparatively low impedance that this antenna presents but the higher antenna appears to be a success (for it’s relatively small size).
In August 2008 a home-brew Aerial Relay Selector Unit was installed.
May 2011 – Aerial Selector Unit has now been redesigned and Mk 2 is operational.