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 omni-directional 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 realise 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 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 50 ohms). Since all power (virtually 100%) is transferred into heat there won’t be any interference to your neighbours 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 WIRE WOUND 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 centre conductor to outer shell (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 accessories.
The simplest possible antenna for VHF is known as the Half-wave Dipole:
Both elements can be either aluminium/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 omni-directional, 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.
Multi-bay system with multiple dipole antennas
One way to increase range and gain of the antenna system is to place several dipoles one above the other. Quadrupling number of dipoles doubles your range. So going from 2 dipoles to 8 dipoles doubles your range. Same goes from going from 1 to 4 dipoles. These dipoles can either be vertical or circular.
We recommend these antenna designs below as DIY projects for beginners or to improve your range and knowledge :
Click here for alternative image. Most designs on the web don’t compensate for the fact that GP antennas are not wide-band antennas. Here is a Freq./element length chart for this simple GP antenna, all elements are in millimetres:
|Radiator – B
|Radials – A
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.
This is a vertically polarised omni-directional antenna.
Radiation efficiency 50% better than ground plane antenna, due to low angle radiation
No ground plane radials, so low wind resistance
50W input impedance
Low VSWR – 1.5 to 1 or better
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:
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