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Idiot's guide to antennas
Preferred type of antenna is affected by several factors, but mostly by transmitting
site. In the middle of the area you want to cover you'll need an
omnidirectional antenna which transmits equally all ways, while outside
your coverage area you can beam the signal in with a directional
antenna. Before going on air get a low VSWR by adjusting the position
of the antenna and any of it's adjustable pieces. Aim for 2:1 or
less. Use low power into the antenna when tuning it up and making
adjustments. If you were using 100's of watts and a bit of the antenna
came off in your hand the VSWR could be so bad as to blow the final
transistor. For the same reason check the DC continuity of the antenna
with an ohmmeter before plugging it in, to be sure it's what it's
meant to be, either a short circuit or an open one, depending on
the antenna type. A dipole shown below should be an open circuit.
A PIECE OF WIRE OR TV ANTENNA IS NOT SUITABLE FOR FM BROADCAST BAND TRANSMITTER!
You have to realize
that antenna was, is and will always be crucial part of the system.
Special care has to be taken! It is usually good idea to place antenna
away from your transmitter, power supply and audio system. If you
cannot meet these requirements, you could experience feedback and
other RF problems. Interestingly, RF energy can make CD players
and other digital devices go bezerk. Try placing 30W-driven antenna
next to yours.
Dummy Load
Dummy load is not actually an antenna, it dissipates all transmitted power
in a form of heat. So what's the use of it? Well, it is presents
an ideal match for an output of your transmitter (usually 50ohms).
Since all power (virtually 100%) is transverted into heat there won't be
any interference to your neighbors while you do tuning and testing.
This is what dummy load is usually used for; testing and tuning
transmitters. If you don’t have dummy load, you can build one easily
from a BNC or other RF connector and the proper wattage/value of CARBON resistor(s).
DO NOT USE WIREWOUND OR METAL FILM RESISTORS! A useful one can be
constructed with 4 -220 Ohm 1/4 watt resistors in parallel (220/4
= 55 Ohms) with center conductor to outershell (ground) of an RF
connector. That is pretty close to 50 Ohms and if you use 1/4 watt
resistors you get a nifty 2 Watt Dummy Load for testing your equipment
without an antenna. Commercial Dummy loads are available at our website, check under Transmitter accesories.
The simplest possible antenna for VHF is known as the Half-wave Dipole:
Dipole
Both elements can be either aluminum/copper tubes or wire. The lengths of each dipole,
L, is calculated from your transmitting frequency by this formula:
L = 71/F (meters), where F is operating frequency in MHz
A half-wave dipole used vertically is omnidirectional, but when
used horizontally it has a figure of eight coverage like this (view from top):
Note: A dipole needs
a Balanced Feed as it is symmetrical, but a coaxial cable provides
an Unbalanced Feed. What's needed is a Balun (BALance to UNbalance)
transformer. These can be made out of bits of coaxial cable. If
you don't do this power will be radiated from the feeder. You CAN
use it without BALUN, but you might get lower-than-expected performance
and unusual radiation pattern due to interactions with the feeder.
We recommend this antenna to beginners, as your knowledge broadens go for one of the
following babies:
GP Antenna
Click here for alternative image.
Most designs on the web don't compensate
for the fact that GP antennas are not wideband antennas. Here is a
Freq./element length chart for this simple GP antenna, all elements
are in millimeters:
| Frequency |
Radiator - B |
Radials - A |
| 108MHZ |
660 |
693 |
| 104MHz |
684 |
720 |
| 100MHz |
713 |
749 |
| 90MHz |
792 |
819 |
If you have SWR meter (and you SHOULD have one) leave a few inches extra for the radiator and adjust it later by cutting to achieve
minimum SWR.
Slim Jim
This is a vertically polarized omnidirectional antenna.
Radiation efficiency 50% better than ground plane antenna, due
to low angle radiation
Unobtrusive
No ground plane radials, so low wind resistance
Fully weatherproof
50W input impedance
Low VSWR - 1.5 to 1 or better
Integrated balun
Construction Details
J- Pole
We will publish a number of antenna designs here, these have all been
submitted to our forum by our forum members, most notably NormB
in his best days. A number of links is provided at the bottom, leading
to design sources or other interesting resources regarding J-pole
antenna construction. Some of the designs were made for 144MHz (2m
ham band) and need to be scaled down for 100MHz operation. This
usually means increasing element size for roughly 144/100 = 1.44
(44% increase).
Here
are instructions and construction details for another interesting design.
Check these other cool links as well:
http://www.mycal.net/old/projects/mpr/jpole.htm
http://www.hamuniverse.com/jpole.html
http://www.hamuniverse.com/jbeam.html
http://members.tripod.com/~AMN92/cp_ant.htm
http://www.hamuniverse.com/2mladjpole.html
CB antennas
PCS0205
Precise manufacturing,
excellent technical characteristics and no-tune wideband operation
make this log-periodical antenna a recommended antenna for all FM
MAX transmitters. Remember that this is a 6-element directional
antenna, meaning it radiates most of the power in one direction,
significantly boosting range in that direction.
Technical specifications:
- 5 elements
- HIGH GAIN, more than J-pole or Comet
- WIDE BAND no-tune operation
- Easy to assemble/disassemble (15 minutes)
- Better range than omnidirectional antennas (Comet or J-pole)
- Attractive price
Check or purchase PCS0205 and other antennas here.
Discuss this article in our Forum!
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