There's not much to say about a 40 meter loop apart from the fact that it performs brilliantly up to about 1500 miles and I am one of the strongest 100 watt staions inter-UK. However, this is one of the supports, a 16 foot wooden stair rail, stained a teak colour!
The feedpoint; 4:1 balun fed with coax back to the shack.
Half-way around the loop, there's a couple of shortened out insulators. My loop is also a triangular di-pole for 80 meters. Mind you I have to lower the loop and take off the shorting wires to convert it.
Here's a view of the other support from an upstairs window. I can lower the loop from here so that I can winch the tower over.
More views from upstairs.
Easy to blend this in. The neighbours love it!
The "lowering" mechanism!
"Height is Might", Nigel (G4NRR) exclaimed during his club talk on antennas during 2003. Of course he is right, but there is one style of antenna that does benefit from actually being closer to the ground; the NVIS antenna where we make use of the ground itself as a radiator. NVIS stands for "Near Vertical Incidence Skywave" and relates to a style - or type of propagation.
We all know the typical radiation pattern of a dipole mounted high in the air, it's like a doughnut. What happens to the radiation pattern of a dipole mounted closer than a quarter-wave to the ground, say an 8th? It starts sending out lobes into the sky. You can even put a radiator on the ground and improve the NVIS properties. I'd like to concentrate on one antenna and discuss my results with you as I really do believe that money can't buy better, that's assuming you have roughly a 10x 10 Meter back garden; the 40 meter horizontal Delta Loop.
What does it look like? Basically a delta loop; "Delta" - from the Greek alphabet is a triangle. It consists of a wire held in three (or more) corners at about 4 to 6 meters (around 12.5% of the wavelength) horizontally above the ground. I say "more" corners because you can have as many points on the loop as you like. The perfect antenna builder is actually trying to build a complete circle of wire, but a pentagon, square or delta loop will work just as effectively as far as the amateur is concerned. A vertical delta loop will also work, but you will need a very high support to hold up the highest point.
The only restriction is that you mustn't build your loop with internal angles greater than 180 degrees, in other words don't fold the loop in towards a figure of 8 for instance, although this might be in interesting experiment another day. Just remember to make some sort of loop but the exact shape is pretty irrelevant.
How big is it? It's just about exactly one wavelength of the lowest frequency you want to work with, so for 40 meters, it's 40 meters! Some theorists suggest that a delta loop should be 106% of the wavelength. My personal building experience demonstrates that you need a loop just equal to the wavelength in question, perhaps a whisker bigger. My best 40 meter loop is actually 41 meter in length, slightly shorter than the theorists and I get better than 1.5:1 SWR across the whole 40 meter band. It also tunes on 20, 15 and 10, but more about that later.
According to the Foundation License material, you can calculate the wavelength from the frequency by making a calculation: 300 divided by the Target Frequency. So let's say we target 7.1 MHz as being the middle ground for our loop, the calculation will be:
300 / 7.1 = 42.25 (meters)
In essence if you were to build a loop with 4 sides, all 10.5 meters in length, you won't be far off. I chose to have a three sided loop with one side 16 meters, one 12 and the other 13. Just remember that experiments show that the optimum loop is actually 106% of the above calculation, so maybe it's best to build it larger and chop off half a meter at a time to "tune it in".
How do we feed it? You have a choice but my favourite is based on the assumption that a full wavelength closed loop antenna has an impedance of around 150 ohms. A 4:1 balun helps match this design to the 50 ohm impedance required by modern day rigs. A balun does something else that is very important, it stops all that RF whizzing around the loop and coming back down the outside of the coax into the shack. Not only would it completely muck up your dimensions, but the capacity for causing interference is now exaggerated a great deal.
So basically, feed coax out of the back of the rig and up to the point where you want to feed your loop - probably at one of the mechanical supports, connect the coax to the 4:1 balun via a PL259 and connect each side of the loop of wire to the two points on the balun. Job done.
You might also get away by building a basic balun out of coax, just make about 8 to 10 turns of RG58 around an 8 inch imaginary former. This can stop RF current coming back down the outside of the coax, but will not particularly help match the impedance. For testing though, this shouldn't matter if you have a tuner. Connect the inside of the coax to one side of the loop - and the braid to the other side. A home-brew dipole centre will be sufficient and an inbuilt ATU will cope with tuning this up, so you might be able to build this for next to nothing.
Regarding performance, the main difference will be around two S points over a G5RV on transmit and a huge difference on receive, even on par with my 3 element mini-beam on 20 meters. On 40 meters, you will be one of loudest stations in the UK and will comfortably work right across Europe, from Finland in the North down through the Ukraine into Greece and Turkey in the far South East. Your signal will be 9+10db on 100 watts for at least 25% of days and you will easily compete with lofty 40 meter dipole stations working 400 watts and more.
Bearing in mind that most of the RF will be going skywards, there's a surprising amount of DX you can work at a pinch, although the same lofty 40 meter dipole stations will now overtake you as you stretch your legs into the 3,000 mile diameter DX circle. My loop has worked into CQ Zone 4 and 5 in the US down through Zone 8 and Zone 9 in Venezuela, Zone 10, Bolivia and also into New Zealand at 5:30am in the morning, breaking a pile up. Quite amazing!
Last edited by Callum, M0MCX, 22nd November 2004.