SUMMARY & CONCLUSION ON SHORT ANTENNAS
(for the Low Bands)
Definition: Situational Antennas are antennas that are chosen due to
the specifics of the local environment.
The situational antenna described here is nobody's dream antenna.
However for some hams, this short antenna has turned out to be
a dream come true antenna
because it has enabled the OM to finally have a decent low band signal, dispite his lack of what is considered required space.
SOME ENGINEERS will surely criticize the concept presented here. That is a pity. They have let their high level of technical knowledge distort their understanding of the real world around them.
Worse yet, their negative comments may convince some newer hams not to try this antenna. As a result, they do nothing at all and fail to experience the joy in low band communications.
FOR MOST PEOPLE, "Low Band Operation" means working NVIS (stations in your own state or country, etc.). In general a symmetrically fed horizontal antenna will work substantially better than any kind of short vertical antenna for this task, even if the horizontal antenna is very short - provided you match it correctly.
AVOID solitions such as mobile antennas mounted to a base in the yard, with several short radials. Thesse will definitely be much worse than the short horizontal antenna described here.
YOU MUST TUNE YOUR MATCHBOX
FOR MAXIMUM RF CURRENT
NOT MINIMUM SWR
IMPORTANT: I do not mean the you should completely forget about SWR, just don't overrate it. The level of SWR has NOTHING to do with the level of performance. Sometimes when tuning, you find that the point of maximum RF Current and minimum SWR are not the same. In such cases, as long as the level of SWR is still acceptable and within the operating range of your transmitter, tune to the point of maxium RF Current, not the one with lowest SWR.
HOW TO BUILD YOUR OWN "BEST POSSIBLE" ANTENNA:
- Identify the place in your yard where you can hang the longest dipole possible.
- Build the dipole as long as possible and feed it in the middle with a good quality balanced feedline.
- Homebrew ladderline will work best but if you don't feel confident to build that, then use WIREMAN (or similar) window line.
- Run the openwire from the feedpoint of the antenna to the antenna matchbox in the shack.
- If need be, convert to thinner feedline for running inside the house.
- Once in place, determine the impedance at the shack end of the feedline by either measuring with an analyzer, or through modeling. Measuring only the SWR is useless here.
- Choose the type of circuit needed (i.e. series or parallel) and build your own simple matchbox using the method and guidelines described here.
- Adjust parameters (i.e. inductance, capacitance and position of the sliding link coil) for maximum RF currento into the two legs of the feedline.