Trapped Inverted L Antenna for 40/80/160m
This page describes my home made 'Trapped Inverted L' antenna, which is resonant on the 3 lower frequency HF bands 160m, 80m and 40m. I won't go on about how good verticals are or aren't, but having a back garden space which is really for the kids, and a desire to not festoon the whole house with antennas (a loose paraphrase of my wifes words !) everything had to be reasonably tidy and out of the way. Also, when I designed this antenna, I did a lot of modeling using NEC, and decided that whatever I was going to put up was going to be resonant. When you look at the radiation patterns of non resonant antennas, the energy goes anywhere but where you would want it to go. I think the small losses associated with traps on these bands are a small price to pay for a good pattern.
I installed a 5m long steel scaffold tube in the ground (as far as it would go using a sledge hammer) at the bottom of the garden, and then attached more scaffold tubes to it to get some height. The top of the scaffold mast stands at about 8m above the surface of the lawn. A small pulley block attached to the top of this enables a rope to lift up whatever wire antennas I like, and also makes tuning (by cutting the wires) easier! As the mast is only 8m high, it is quite a lot shorter than a quarter wave at 40m and doesn't make much difference to the radiation pattern. It makes even less difference on the lower frequency bands.
The support at the other end of the L is the chimney stack conveniently positioned on top of the house, which also sports a small pulley block and rope.
The inverted L described here is fed against ground. I managed to get a few radials into the lawn but not as many as I would have liked. Radials are really important for an antenna fed against ground; using an earth spike just won't work. The ground is fairly damp even in the summer, and the lack of radials doesn't seem to matter too much though I think I'll add some more when time permits. There is a coaxial cable which runs from the radio room in the house under the lawn and to a point about 2m away from the mast. The outer of this coax connects to the radials, and the inner is connected to the antenna. The wire goes up vertically as far as it can to an insulator and then horizontally away from the mast towards the house.
There are two traps, the first is about 10m from the feed point and is resonant at 7.1 MHz. The other is a further 8m away and is resonant at 3.7 MHz. The second trap also incorporates a small amount of extra inductive loading to make the last bit of wire (to make the antenna resonate at 1.9MHz) a bit smaller than it would otherwise be, saving me a bit of space so I could get a 15/20m fan dipole in the remaining space between the end of the inverted L and the chimney.
You are welcome to e-mail me with any comments if you like at mike at spikey-mike.com.