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Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Small Antennas - General Notes

All of the information presented in previous notes is interesting and important for a general understanding of how antennas work and what controls their performance. But, as radio amateurs, we are interested in actually making the antennas work. As usual, the theory is important, but the practical aspects of antenna design and construction are important, too. This section collects some guidelines for use in putting together a working antenna system in cramped quarters.

Site Survey. Perhaps when one thinks about a site survey for an antenna, the first thing that comes to mind are surveyors and lots of area to describe. While that may be true for professional installations, that isn't neccesary for most of us. What we really need is a description of the area we have available for installing antennas, along with some notes on the good and bad points. We need to keep in mind any constraints and also take advantage of any positive factors.

Remember that it's easy for most people (me included) to be negative and see the problems. You may have to force yourself to be the "eternal optimist" to find some advantages, but you can be sure that most every situation does have some advantages. Don't overlook the good points, no matter how insignificant they may seem.

Keep an Open Mind. As was pointed out in the previous notes, anything will work as an antenna to some degree subject to 3 major constraints:

  • You can load it
  • It doesn't have lots of internal resistance
  • You don't lose too much energy to the ground

Ask for advice, too. Several times my wife, who knows nothing about radio, has suggested something that I've been able to turn into a workable antenna. Anything from curtain rods to toilet brushes may be of use in figuring out how to get something to radiate.

If It Isn't Broke, Don't Fix It. That advice is a hallmark of many engineering activities, but in this case, it is really bad advice. My opinion is that every antenna installation can be improved and if we don't try to continually optimize, we'll never get a better working system. Of course, if you are already on the DXCC Honor Roll using limited space antennas or have already talked to everyone you want to have a QSO with, maybe you've reached the peak of optimization.

The problem with evaluating your own antenna is just this: What are you comparing it to? Normally we have nothing to compare with directly, so if we make lots of contacts, we assume the antenna is great. However, we almost never know how many contacts we didn't make. It isn't until you improve the antenna and make the additional contacts that one can look back and say that the improvement was worth it.

First Make It Work, Then Make It Work Better. That's an old adage that I learned doing computer programming and it holds true for antenna work, too. You'll find that it is often easier to modify an antenna system than it is to build one in the first place. At least with a working antenna, one can figure out where it is lacking and what it does well. Once we understand that, we're well on our way to modifications with a fair chance of actually improving things.

For example, rather than spending lots of time worrying about ground losses or feed line radiation, put up a dipole and see how it works. Once it's up and working, then see if it acts like there's too much ground loss or if it acts like the feed line is radiating. Once we know how bad those problems are, then we can figure out how to fix them.

Learn How to Model. With modern PC's and readily available software, one of the major tools available to hams is antenna modeling software. With the right software (and an understanding of how to use it!) it's possible to model most antennas that hams use. This provides a powerful tool for evaluating our antenna systems and understanding how to improve them. My personal approach is to build some sort of antenna and observe and measure its performance. Then model it and adjust the model until it agrees fairly well with my observations. Once that is done, then I can change the dimensions, feed point, wire size, or anything else in the model to see if it makes things any better. And when I find a way to make it better, I've got a better than even chance that my changes will actually work.

But at the same time, remember that an antenna model is just a model. An antenna model won't work DXCC - you have to build, install, adjust and use the actual antenna! So don't get too caught up in the modeling. Just don't forget that modeling is a tool, but the goal is to get the signals on the air.