Tuesday, September 2, 2014

Simple Ham Radio Antennas: Antenna Basics, part II. Post #292

Source:  Field, Van (W2OQI). "HF Antennas 101." "QST", September 2004.

Comment: Russ Roberts (KH6JRM).

Summary:

This article is a follow-up to my last post, where I quoted James R. Duffey (KK6MC/5) from his article entitled "Antennas--Some Rules of Thumb for Beginners."  These two articles will give new licensees as well as us "old timers" a gentle reminder of some basic truths concerning antenna design and theory. Thanks to Dean Manley (KH6B) for running off a copy of this helpful article.  Like the previous essay from James R. Duffy, I will place this in my antenna reference library.  

Article excerpts:

Here are 10 tips and truisms that every ham should know:

1.  An antenna does not have to be resonant to work. The only reason to make an antenna resonant is to eliminate the need for a impedance-matching device such as an "antenna tuner."  Actually, a non-resonant wire dipole antenna fed with open-wire line and an antenna tuner is a great multiband antenna.

2.  Two wires are needed to power a lamp.  The same is true of your antenna. The best antenna configuration calls for feeding energy from the transmitter to a balanced antenna, such as a dipole. If you can do this with a balanced, parallel-wire feed line, so much the better.

3.  Antenna "gain" is derived by shaping and aiming RF where you want it to go. For example, the so-called "beam antenna" gets its name from the fact that it concentrates RF energy in a particular direction, like a flashlight.

4.  The function of an antenna tuner is to effect a match between the output of a transmitter and the input of an antenna system.  Antenna tuners are variable-impedance transformers that allow you to transform the antenna system impedance (which can be almost anything) to the 50 ohm input of the transmitter.

5. A wire antenna doesn't always have to be center fed.  For example, you can feed a long wire at the end with a two-wire feed line.  Connect one conductor of the feed line, but not the other.  This type of antenna used to be called an end-fed Zepp.  To work well, however, the ground side of the antenna tuner needs to be connected to a network or radial wires, or a counterpoise system.  Another "oldie but goodie" is an off-center fed dipole, called a Windom.  Cut a wire a half wavelength long, find the center, and connect a single wire 14% off center.  This also requires a counterpoise for good results.  The impedance is about 600 ohms, so you'll definitely need an antenna tuner.

6.  A dipole antenna does not have to be perfectly horizontal.  The antenna can be on an incline, or even vertical.  The shape of the antenna and its height above ground will affect its impedance at the feed point, so you may need to experiment to obtain a low SWR, if you are feeding the antenna with coaxial cable.

7.  Vertical antennas shorter than half wavelength need a ground system.  This usually takes the form of radial wires, either elevated or buried.  

8.  With vertical antennas, there is no such thing as too many radials.  The more radials you install, the more efficient your antenna becomes.  Yes, you can reach a point where the benefits of adding more radials levels off, but that number is somewhere around 100!

9.  Having a 1:1 SWR does not mean you have a good antenna.  A 1:1 reading says nothing about how well your antenna is working.  For example, a vertical antenna with a poor ground system can be tuned to the point where you'll measure a 1:1 SWR at your station, but the antenna is so inefficient, most of the RF is being wasted as heat!

10.  Always use the best feed line you can afford.  Resist the urge to be penny wise and pound foolish.  This is particularly true of coax.  Better (less lossy) coax will cost more, but this is the cable that is carrying your precious RF signal to and from your antenna.  A good investment now will pay off in better antenna system performance.
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So, there you have ten additional tips to make your antenna more efficient.  Combined with James Duffy's "Rules of Thumb for Beginners", you will have the necessary facts to build and use an efficient, cost effective wire antenna that will give you hours of enjoyment.  Building wire antennas with materials found around your shack or from the nearest hardware  store or home improvement outlet is one of the most enjoyable parts of amateur radio.  Have fun.

Resources:

Ford, Steve (WB8IMY). "The Classic Multiband Dipole Antenna."  "QST", March 2004.

Duffy, James R. (KK6MC/5).  "Antennas:  Some Rules of Thumb for Beginners." http://www.norcalqrp.com/features/bgnsant.html. Dated 31 January 2004.

Field, Van (W2OQL).  "HF Antennas 101".  "QST", September 2004.

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Until next time,

Russ Roberts (KH6JRM).
Laupahoehoe, Hawaii (BK29jx15).