Wednesday, January 22, 2014

Building a simple Ham radio antenna

Building a simple ham radio antenna....
Or, Someone got an antenna book for Christmas...Can you guess who?1

So, you want to try your hand at building your first antenna for ham radio use, but are unsure just where to start. All the tutorials you find online and all the math involved can be confusing and often frustrating, causing you to lose interest and move on to something else. It doesn’t have to be this way of course, but in this plug and play society that we live in it is sometimes easier just to spend a few bucks and purchase a pre-built antenna. The trade-off is that you gain a sense of accomplishment after you build your first antenna and find out just how easy it can be.

HF, or High Frequency is from 3 Megahertz to 30 Megahertz. Ham radio operators have privileges to transmit on designated frequencies from 3 to 29.7 MHZ in the HF spectrum.2

A dipole is the simplest ham radio antenna you can build and get's you on the air fast!

When looking at all the different designs that are available it is best to start off with the simple designs and move onto some of the more exotic antenna's later. A good starter wire antenna is a simple dipole3 that can give you the confidence you need to try other antenna's. A simple dipole antenna is just a ½ wavelength piece of wire, cut in half and fed with either coax or ladderline. It doesn’t have to be any more complicated than that really. Their are a LOT of other factors to building an efficient antenna system but a good start is with a simple ½ w dipole made of wire. By using the formula:

468/Freq=Length (It really is THAT simple)

You can come up with the basic length to get you in the ballpark of a working antenna system. Building an antenna for 80M in the CW portion of the band you would just divide 468 by 3.55 to get roughly 131.8ft (Total length). I personally add at least 1ft on each end for tie offs to the insulator also. From this point depending on ground conditions and configuration of the antenna, and type of feed-line that you use (Coax or Ladder-line) you should be very close to a working antenna.

Getting fancy.

If you want to get more advanced and have a more efficient antenna then you should try using Ladderline to feed the antenna. Not only does ladder-line have less loss equaling more power to the antenna, but it's characteristic impedance more closely matches a wire antenna, coming in at 400 to 600 Ohm. Ladderline is a bit harder to work with and depending on how close you have it to other objects will affect it.

If you want to improve on the dipole design even more you can add a 4:1 balun at the feed-point of the antenna. This will stop the vertical section (The part hanging down from the wire) from radiating any energy, making more of your signal come out the antenna.

And that is one of the simplest antennas that will get you on the HF bands and making contacts. It works best if at least 1/4w above ground, but will work fine if you can't achieve this.(For 80M this is about 66ft off the ground) Just be sure to set it up at least high enough to keep it out of reach so no one grabs a hold of it. (While your transmitting).

1 Reference for most of this material comes from “Practical antenna handbook 2nd Edition” by Joseph J. Carr.
2 HF band privileges range from LF (Low frequency) up into the Microwave frequencies.
3 A Dipole antenna has a omni-directional pattern, radiating equally in all directions.