This is the copy of an article by Jim Duffey, KK6MC, (aka Dr. Megacycle).
You can find the original "Antenna Rules of Thumb" on KK6MC's webpage.
Every now and then somebody asks one for antenna suggestions. Quite often these people asking are beginners who are afraid of making the wrong choice. In order to help QRPers choose antennas wisely I have compiled a few "rules of thumb".
As with any rules of thumb, these are general and there are some exceptions to them. A few may be somewhat controversial and I am sure alternate views will be given by those with opposite views. However I intend these guidelines to point one in the right direction rather than providing a detailed map of what to do.
Any antenna is better than no antenna. Rather than agonizing over an antenna choice, just put one up and operate. After operating with it for a while you will become aware of your operating habits and the shortcomings of the antenna you have erected. That will give you some hints as to which direction you should go with another antenna. You can loose ½ of your power in poor antenna system efficeincy and ony be down an S unit or so. I hear lots of S9 QRP stations. They would still make fine QSOs at S8. I am not advocating antenna inefficiency, but you can live with it. It is better than no antenna at all.
Higher antennas generally out perform lower antennas. A vertical on the roof of a one story house is probably a better choice than one on the ground in the backyard. A dipole whose end is tied to a 5 or 10 ft mast on top of the house will out perform one whose end is merely fstened to the eave.
Most people will be happier with a low dipole than with a vertical. Verticals require a bit more attention to work effectively and beginners can become frustated in dealing with ground issues.
It pretty much doesn't matter what kind of copper wire you use in an antenna. Thick or thin, insulated or bare, stranded or solid, they will all perform fairly well. Any effects due to these characteristics will be "second order". The old formula for cutting a half wave dipole, 468/frequency(in MHz), may be a bit diferent for various combinations, but this formula is only an approximation anyway.
Whatever antenna you chose, if it is fed with coaxial cable you should use a choke balun. This will prevent the feedline from becoming part of the antenna which can cause all sorts of problems. There are many designs to chose from. My favorite is an air core balun wound from coax. These are described in the ARRL Handbook and in the ARRl Antenna Book. You don't have either?
Then purchase a Handbook or ARRL Antenna Book and study it. Antennas don't change much, so even an old copy of the Antenna Book will be very useful. These show up at Ham Fests occasionally. You can also special order ARRL publications from good bookstores.
Outdoor antennas perform better than indoor ones. If all you can erect is an indoor antenna, fine, but try to see if there is a way to get up an antenna outside. A thin wire supported an inch or more away from the building will be much better than one inside. If you can dangle a wire out a second story window, feed against a counterpoise, that will be a pretty good antenna.
Don't scrimp on feedline. Good, low-loss feedline does not cost much more than the antenna it is feeding.
Most single band antennas can be made into multiband antennas by feeding them with a balanced feeder like window line and using a tuner. This applies to loops as well as dipoles. For an inexpensive low loss tuner see Cecil's method of changing the feed line length to achieve a match W5DXP's No-Tuner, All-HF-Band, Horizontal, Center-Fed Antenna.
If you have antenna restrictions consider a temporary antenna. The SD-20 Blackwidow Crappie Pole can be erected with a wire of choice to make a vertical in a matter of seconds. With a few radials or a chain link fence as a ground, this can give a good account of itself. If somebody complains about it take it down and next time erect it where they can't see it.
Consider your operating practices in choosing an antenna. If you can only operate in the evening, then even a high 10 meters antenna will not provide you with much operating time. The band will usually be dead after sunset. On the other hand, a 40 meters dipole will provide you with a number of contacts late into most evenings. It can also be used on 15 meters for those occasions when you can operate during the day.
Avoid the temptation to "have it all". Multiband antennas are often attractive to new comers. So are electrically "small" antennas. They are by necessity compromises, and usually don't work as well as single band antennas. I suggest erecting a single band dipole and using it for a while. As you get used to operating or have desires to try out other bands you can erect another antenna, or feed the one you have (if it is a dipole) with ladder line for multiband use. You can build and feed a lot of single band dipoles for the price of an R-7000!
Homemade antennas are better than commercial ones. Ask anyone on who has built one!