Saturday, July 20, 2013

Ten Basic Antenna Truths. Post #214

Sometimes knowledge comes from the strangest places.  Take today, for instance.

While my van was having a routine maintenance check at my local Honda dealer (I have an Odyssey van), I ran into an Dean Manley (KH6B), one of my oldest friends and former station engineer at Hilo radio station, KHLO-AM.  I worked at this station from 1976 to 2011.  We discussed a variety of antenna ideas, including improving the homebrewed antennas we use at our stations.  I consider Dean an expert in this area.  He has spent many years building and upgrading antenna systems for  AM and FM stations, both commercial and nonprofit.

Anyway, we spent a good half-hour discussing some ideas, including his current project--a 40-meter vertical beam which Dean uses for the Hawaii Afternoon Net.  Before we parted, he gave me a copy of a handout he made for members of the Hilo Amateur Radio Club.  The one-sheet article, entitled "HF Antennas 101" by Van Field (W2OQI), appeared in the September 2004 issue of "QST".

What's remarkable about this informative piece is the easily employable principles of HF antennas--concepts that we often use without thinking, but are valuable in designing and building effective antennas.

Briefly summarized are the "10 tips and truisms that every ham should know."

1.  An antenna does not have to be resonant to work.  Van says the sole reason to use a resonant antenna is to remove the need for an impedance-matching device, such as an "antenna tuner."  The only time I've used an antenna tuner is when I go on 30 meters with my backup rig--a Ten Tec Argosy II.  My old Drake MN-4 doesn't cover 30 meters, so I try to use resonant antennas cut for 10.100 MHz.  Van notes that open wire feeders used with an antenna tuner and a basic dipole make great multiband antennas.  All of my inverted vees, loops, and slopers use 450-ohm ladder line in conjunction with a 4:1 balun and a 50-ohm feedline to the old Drake MN-4.

2.  Two wires are needed to power a lamp.  The same is true of antennas.  In this paragraph, Van says the use of tuned feeders to a balanced antenna such as a dipole eliminates some of the rf feedback problems of unbalanced feed lines using coaxial cable.  Of course, coaxial cable feed lines can be used successfully if operators use choke baluns or 1:1 baluns at the center of the dipole.

3.  Antenna "gain" is derived by shaping and aiming RF where you want it to go.  Beam antennas are the most common example of directing energy to a particular area.  Beams can be horizontal and vertical.

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.  Van suggests the use of an antenna transmatch to compensate for the narrow ranges of most internal tuners.

5.  A wire antenna doesn't always have to be center fed.  Van cites the "end-fed Zepp" and the Windom antenna as successful off-center fed dipoles.  However, these antennas require a radial or counterpoise system attached to the ground side of the antenna tuner.

6.  A dipole antenna does not have to be perfectly horizontal.  Dipoles can be configured to fit the available space.  I've used inverted vee dipoles, half-wavelength slopers, and even vertical dipoles with excellent results.

7.  Vertical antennas shorter than half a wavelength need a ground system.  This usually takes the form of buried radials, surface radials, or elevated radials.  Elevated radials are easier to install.

8.  With vertical antennas there is no such thing as too many radials.  My friend Dean (KH6B) says most commercial AM radio stations install up to 120 buried 1/4 wavelength radials.  Having installed a radial system at an AM radio station, I can verify that the project entails a lot of work on a large expanse of land.  For most hams, a modest system of 20 to 30 on- surface or buried radials should boost antenna efficiency.

9.  Having a 1:1 SWR does not mean you have a good antenna.  It only means that you have an impedance match between your rig and your antenna system.  A perfect match says little about how efficiently your antenna is working.  A vertical antenna with a poor or non-existent radial system can be adjusted to measure a 1:1 SWR, but the antenna is so inefficient that a large portion of the RF heats worms.  Even a dummy load can measure a SWR of 1:1.

10.  Always use the best feed line you can afford.  Better coax will cost you more money, but "this is the cable that is carrying your precious RF signal to and from your antenna."  A low-loss cable "will pay off in better antenna performance."

So there it is---some fundamental guidelines that should help you design an efficient, cost effective antenna that will give you many hours of enjoyment.

REFERENCES:

Personal conversation with Dean Manley (KH6B) on 20 July 2013.

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

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

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Aloha es 73 de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.