An antenna for your station is probably your best investment after you have a purchased a radio. But how do you choose an antenna , What are these letters after the gain number? What's the difference between dBd and dBi? What's the difference between a beam and an Omni directional antenna? Should you buy or build an antenna?
Your first consideration for an antenna is space. Take a good look at the area you plan to put that first antenna and have a good idea of large an antenna you can accommodate. Look for power lines or other obstructions prior to picking an antenna. For those of us in urban areas, you need to be aware of antenna restrictions or height restrictions. For those of us in the Garland area, the restriction is 35 foot. As I interpret this ordinance, that's from the ground to the top of the antenna.
It is not simply enough to buy the antenna with the highest gain number, but an understanding of how manufactures define gain in an antenna is important. The are two references used buy most manufactures when defining antenna gain: dBd and dBi. Gain referenced to a dipole is listed as dBd. Gain referenced to an isotropic source is listed as dBi. Of the two, gain listed as dBd is generally the better antenna What is an isotropic source you might ask? If you pick an imaginary point in space and from that point all of your power is transmitted, that is an isotropic source. Gain referenced to this point is dBi. When gain figures are listed as dBd, they are using a 1/4 wave dipole cut for the center frequency of the band of interest as the reference. Be careful when looking at a catalog. I was looking through a Cushcraft catalog as I was writing this and some antennas are listed dBd and some are listed dBi on the same page.
The Omni directional antenna puts you signal into the air in all directions with about the same signal strength. The beam antenna forces the signal travel in a single direction A good choice for that first antenna would be an Omni-directional. They are usually not to large, easy to construct, easy to mount and give good coverage. This type of antenna will get you on the air quickly and easily.
The beam type of antenna has some hidden pitfalls. Yes they have more gain but are larger, harder to mount, come in box with a hundred loose parts and you will need some kind of rotator to get the best use out of beam. Remember that a beam or Yagi antenna is very directional by design, you will need some kind of steering device to get the best use out of this kind of antenna. If you cant reach you local repeater with an Omni, then go to a beam.
For those of us who want to use packet radio, An Omni is a good choice here. With an Omni, other stations can Digipeat (Digital Repeat) through you. If you have a beam up and the station wanting to Digipeat through you is to the back or side of your antenna, it might make for poor re-transmittal of the data.
Antennas are mostly just wire. For a first construction project, this is what most of us attempt first. Dimensions and detailed construction drawings for antennas such as 1/4 Wave Ground Planes, Dipoles or a Verticals can be found in almost any Amateur Radio Publication.
Building an antenna is usually a rewarding experience. Making that first contact on an antenna you built make all the cutting, measuring, drilling and soldering worthwhile. There are several beam antennas that can be built such as a Quad-Yagi, that can be built for a few dollars out lumber and wire that have excellent performance. Larger antennas beam antennas as found in ARRL handbook require specific sizes of aluminum tubing and the dimensions are usually very specific, down to the 64th of an inch, and should be attempted only after some experience has been acquired building other antennas.
That first antenna is going to be the benchmark that you reference all other antennas you might try. Remember that if you hear the other station you can probably talk to them. Mount the antenna with care and
please be careful. Something else to remember is that although an antenna is a thing of beauty to us it might not be to you neighbors. Think about starting off small and working up to something else larger after they have gotten use to the idea of having an Amateur Radio Operator in the neighborhood.
73's and GL
Copyright 2005 Gerald Crenshaw WD4BIS. All rights are reserved.
Permission in advance is granted to those who use this for non-profit Amateur Radio club newsletters as long as it is used unmodified including this copyright notice and that notice is given to the author via email (firstname.lastname@example.org). In addition, please forward a copy of any newsletter this appears in to: Gerry Crenshaw WD4BIS, c/o GARC, 1027B W. Austin St, Garland, TX, 75040
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