Monday, February 22, 2016

Simple Ham Radio Antennas--HF Antennas 101. Post #696.

HF Antennas 101
(http://www.sgcworld.com/Publications/Downloads/antennas.pdf.
Accessed on 23 February 2016, 03:07 hrs, UTC.
Author:  Van Field (W2OQI).
Special thanks to Dean (KH6B) for running off a copy of this outstanding article.
Please click title link or URL address to read the full article.

I'm always searching for interesting antenna articles to add to my reference library.  Sometimes, the greatest antenna articles can be found in the "QST" archives.  Such is the case here.

"HF Antennas 101" by Van Field (W2OQI) appeared in the September 2004 issue of "QST" published by the ARRL.

Van's concise, accurate article lists "10 tips and truisms that every ham should know."

Here's the list for easy reference:

An antenna doesn't have to be resonant to work.  According to Van, a non-resonant dipole fed with television twin lead, 450-ohm ladder line, or homemade open-wire and an antenna transmatch (i.e. "tuner") "is a great multiband antenna.

Antenna "gain" is derived by shaping and aiming RF where you want it to go. Beam antennas get their name because they concentrate RF energy in one direction.

The function of an antenna "tuner" is to effect a match between the output of a transceiver and the input of the antenna system.  Antenna tuners "are variable-impedance transformers that allow you to transform the antenna system impedance...to  50 ohms for the transceiver."

A wire antenna doesn't always have to be center fed.  Such antennas as the end-fed Zepp or the classic Windom remain popular today.  You'll need an antenna tuner because the typical impedance for these antennas is around 600 ohms.

A dipole antenna doesn't have to be perfectly horizontal.  Your dipole can be sloped, bent, zig-zagged, or used as an inverted V.

Vertical antennas shorter than a half wavelength need a ground radial or counterpoise system.  This "missing half" of a vertical antenna usually takes the form of  a buried or elevated configuration of evenly spaced wires below the vertical element.

With vertical antennas, there is no such thing as too many radials.  Van says, "The more radials you install, the more efficient your antenna system becomes. Of course, you'll reach a point of diminishing returns, "but that number is somewhere around 100."  Commercial AM broadcast stations often use 120 or more buried wires beneath their vertical antennas to establish a decent ground radial system.

Having a 1:1 SWR does not mean you have a good antenna.  All that 1:1 SWR means is that you have an impedance match between your transceiver and your antenna system. Val adds that a 1:1 SWR "says nothing about how well your antenna is working."  For example, a vertical antenna without a ground radial system may show a 1:1 SWR, but most of the antenna's signal is being used to heat the ground.  A vertical without a good ground radial system is a fancy dummy load.

Always use the best feed line you can afford.  Val says "Better (less lossy) coax will cost more, but this is the cable that is carrying your precious RF signal to and from your antenna."

Hopefully, this list of basic antenna "truths" will help you design, build, and use better antennas.

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Aloha es 73 de Russ (KH6JRM).