07 What kind of antenna should I use? (Low Power Broadcasting)
Description
This article is from the Low Power Broadcasting FAQ, by Rick Harrison raredata@geocities.com with numerous contributions by others.
07 What kind of antenna should I use? (Low Power Broadcasting)
The antenna is the most important part of a broadcast station; it has
more effect on signal strength and station safety than any other
component.
Remember that YOUR ANTENNA CAN KILL YOU. Don't put up an antenna in a
place where it could fall onto a power line, or a place where a power
line could fall onto the antenna. Make sure your antenna mast is
properly grounded so that if lightning strikes, the lightning will be
conducted into the ground instead of into you. Falling off of roofs
and ladders can result in injury.
Factory-made antennas are available for the FM band; the 5/8-wave
vertical antenna made by Comet has gotten good reviews on the 'net. If
you need to build an antenna, the J-pole and the half-wave dipole are
good choices, and lots of FM antenna plans are available on the
world-wide web.
If you are broadcasting on AM or shortwave, you will probably end up
building your own antenna system. If so, you would be well advised to
get a copy of The ARRL Antenna Book, which is published by the
Amateur Radio Relay League and available from ham radio supply dealers
and Amazon (www.amazon.com).
* quick and dirty antennas
The following types of antennas can be built quickly and cheaply, and
will serve to get you on the air when you first start out. As you
learn more, you will want to upgrade to better antennas.
shortwave
For shortwave broadcasting, a horizontal dipole works well enough. Cut
two pieces of un-insulated copper wire; the length of each piece will
be 234 feet divided by your frequency in MHz. Example: for 6950 kHz,
each element will be (234/6.950=) 33.7 feet long, and you will need
two trees or other support structures about 67 feet apart. Solder one
element to the center conductor of your coaxial feedline, and solder
the other element to the outer conductor (shield) of the co-ax. (Note:
the solder joints cannot bear the weight of the cable; loop the cable
once over an insulator and provide some "strain relief".) Make a
little loop at the free end of one element, and tie a long piece of
string to that loop. Tie a small, heavy object (such as a lead fishing
weight) to the other end of the string. Throw the weight or use a
slingshot to launch it up into the branches of a tree so that it goes
over a branch and comes back down to earth; then hoist up that half of
your antenna. (Suggestion: don't hit yourself in the head with the
weight.) Repeat the process for the other element.
AM
In AM broadcasting, a vertical section of TV antenna mast, 10 or 20
feet tall, provides a quick antenna.
The center conductor of the coaxial cable from your transmitter is
connected to the bottom of this vertical mast; the base of the mast
sits on an insulator which sits on the ground. If the vertical
radiator is made of several sections of antenna mast, make sure the
sections are electrically connected -- try screwing some self-tapping
sheet metal screws into the joints. Obviously the mast will not stand
up by itself; use nylon rope or other non-metallic materials to
support it. The outer conductor (shield) of the coaxial cable is
connected to a set of "ground radials," which are pieces of copper
wire radiating out from the base of the antenna like spokes from the
hub of a wheel. (The radials are not connected to the vertical
radiator.) The radials can be buried a few inches below the surface
for a permanent installation. "Beware the lawnmower."
This antenna is only a tiny fraction a wavelength high, therefore it
will not be a very efficient radiator. There are ways to improve the
efficiency slightly (loading coils, capacitance hats, etc.) -- study
the ARRL Antenna Book and visit the Medium Wave Alliance web site
( http://www.geocities.com/ResearchTriangle/Lab/1635/equip.html ) for
more information.
[For those who use 1 watt or less, the web version of this FAQ
contains a section called "getting every milliwatt to radiate."]
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