| EA3AILHam radio, antennas and propagation |
| We can mount different types of antennas depending on the space we have. A vertical is always easier to fit on a small plot of land or a rooftop. If you install it on the ground in a yard, it's always easier, mainly because of the radials. Verticals are usually multi-band or, in some cases, single-band. Here we will describe the construction of single-band or dual-band verticals. Any antenna designed for one band will resonate at its nearest harmonic, and in the case of 40 meters it will resonate on 15 meters even if it isn't a true harmonic. The simplest is the quarter-wave vertical antenna. It should be well insulated from the ground; placing it at a height of between 15 cm and 1 meter will greatly improve its performance. We should always try to use as many radials as possible (a minimum of 4). Sixteen radials are sufficient. If their length is a quarter wavelength, all the better. If they're shorter, they'll put extra work on the tuner. It's much better to choose a radiating element of an appropriate thickness so it doesn't have so many losses and isn't too narrow in bandwidth. For example, a 30 mm mast, isolated at the base, of course. |
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| This type of antenna can be shortened (relatively) without losing much gain. To shorten it, we'll use inductances (coils), and their position relative to the radiating element will determine the antenna's parameters. In this case, it can be fed with a 1:1 balun. |
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| With the inductance at the center of the radiant element, we will have the characteristics of the diagram below.... |
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| We can also choose to place the coil at the base or at 90% of its height. Never at 100%, since the voltage peaks are extremely high and we would burn out the coil. |
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| We lose the harmonic bands, but we're left with a MONOBAND antenna that has very acceptable performance and a good launch angle for DX. A bit of resonance at 12 meters to play with the tuner. The 20-meter band is completely out of reach. Another option would be to build a hybrid antenna (I haven't tried this yet), like half a dipole with traps set up vertically—that is, a single branch. I'm sure it works; it's just a matter of trying it... You always have the option to run tests with the calculator on the website and play around with setting up antennas at your QTH...... Lluís Mas EA3AIL https://ea3ail.com Radio Club Cerdanya (EA3RCP) |
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