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SAM3, the best radio automation software out there!

Preferred type of aerial 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 aerial which transmits equally all ways, while outside your coverage area you can beam the signal in with a directional aerial. Before going on air get a low VSWR by adjusting the position of the aerial and any of it's adjustable pieces. Aim for 2:1 or less. Use low power into the aerial when tuning it up and making adjustments. If you were using 100's of watts and a bit of the aerial 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 aerial 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 aerial type. A dipole shown below should be an open circuit.


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 aerial, 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 (almost) power 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 donít have dummy load, you can build them easily from BNC RF connectors 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 for $5.00 -$25.00

The simplest possible aerial for VHF is known as the Half-wave Dipole:



dipole.gif (1586 bytes)

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 (wives from top) :

radpattern.gif (1542 bytes)

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.

I recommend this aerial to beginners, as your knowledge broadens go for one of the following babies:


GP Antenna

gp_ant.gif (6375 bytes)

Click here for alternative image. Most designs on the web don't compensate for the fact that GP aerials are not wideband aerials. Here is a Freq./element length chart for this simple GP aerial, 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, leave a bit longer radiator and adjust it later by cutting to achieve minimum SWR.


Slim Jim

This is a vertically polarized omnidirectional aerial.

Radiation efficiency 50% better than ground plane aerial, due to low angle radiation
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 one interesting antenna. Check these other cool links:







RPL0205 H

Precise manufacturing, excellent technical characteristics and no-tune wideband operation make this log-periodical aerial a recommended aerial for all FM MAX transmitters. Remember that this is a 6-element directional aerial, meaning it radiates most of the power in one direction, significantly boosting range in that direction.


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 aerials (Comet or J-pole)
Attractive price

Check our antennas, the RLP0205 H or COMET from PCS Electronics (under Products in the left frame).

 Cool link for antennas

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