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Designing an antenna for the 5000m band
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney
wrote: First, the need for impedance matching between an antenna and a receiver. My understanding is that a resonant halfwave dipole will have an impedance around 73 Ohms; unfortunately, unless I can obtain research funding from the just-passed Congressional Economic Stimulus bill I'm going to have trouble paying for 2.5km of copper wire, some towers, a crateload or two of porcelain insulators,and the land to build it on. (Hey, I promise to dump it back into the economy ASAP. Really! grin!) So any non-loop antenna I can construct will necessarily be a "short wire" or "electrically small" antenna (two useful search terms). But how does one go about calculating the impedance of a coat hanger or an extension cord ("short piece of wire")? Hi Frank, One doesn't try. The simple solution is the conventional one - you use a tuner. The tuner provides the matching (providing it has sufficient inductance and capacitance - you will have to investigate designs) and adjacent signal rejection (which could seriously de-sense your received signals). Your antenna doesn't need to be very big, but it might help to have it very remote, if there are noise sources nearby (like motors, aquarium heaters, bottle style TVs, ...); and the line sufficiently choked. A good ground too, tying into the service ground through a separate wire to reduce coupling of noise from shared grounds. This last may introduce a ground loop if your Mohican is so vintage as to have had relaxed design standards. A little research online reveals it is battery operable. You may want to fully exercise that option. 73's Richard Clark, KB7QHC |
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