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Equipment

Antennas

WB6BYU On Multiband Antennas: There are basically five approaches to a multi-band antenna:
  1. use a single antenna with a tuner
  2. use an antenna with multiple harmonic resonances that fall close enough to bands of interest
  3. use an antenna that has separate tuning adjustments for each band
  4. use a truly broad-band antenna that covers a wide spectrum
  5. use resistive loading to reduce the SWR by absorbing power
The first one easily covers all bands, though the radiation pattern will vary, and the feedline losses and impedances to be matched need to be carefully considered.

The second would include loops, G5RV, OCFDs, etc. where the wire and feedline lengths are chosen for reasonable performance across multiple bands. This often works well on some bands, but none of the ones I know of will give a low SWR on all of them.

The third would include multiple dipoles on a common feedpoint, trap dipoles or beams, multiple quad loops on the same set of spreaders, etc. Generally this allows individual adjustment for each band rather than relying on the harmonic resonances in the wire.

The fourth isn't as common in ham use, but has increased potential with the addition of the WARC bands. This would include log-periodic arrays, true fan dipoles, discones, conical monipoles, and even some HF quadrifiliar helix arrays that are much more common in military or commercial service. These might operate, for example, from 5 to 30 MHz with an SWR under 2.5 : 1. They tend to be more complex to build, but certainly are capable of what you are looking for.

The fifth includes antennas such as the T2FD or other broadband folded dipole designs with a large resistor in the middle, as well as some of the newer "dummy load on a stick" versions. It IS possible to make a plausibly efficient antenna like this that covers multiple bands, but most common designs achieve low SWR at the expense of signal strength.

Note that you can also have combinations of these methods in a single antenna.