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Radio Antennas Q & A
1 March 2016

What is "Gain"?

Gain measures how much of the energy sent from the radio to the booster antenna is concentrated in a specific direction, and in which direction the booster antenna has the greatest "reach" when receiving a signal. Gain is measured in decibels (dB), related to a known standard reference, usually an ''isotropic'' antenna. Gain figures which show the comparison with an isotropic antenna are written dBi.

So, 2.5 metre antennas are high-gain, right?

Only if they have two or more radiating elements.


If booster antennas can only transmit and receive on a "line of sight" basis, how is it I can talk to radios which are miles away?

You are probably using a "repeater" to do this. The repeater radio sits high on a hill, and re-broadcasts your transmission. Because of the height of the repeater radio, both your booster antenna and the antenna you are talking to have an effective line of sight through the repeater radio.

My booster antenna is receiving but not transmitting. What's wrong?

Probably nothing with the antenna. Check that the battery is fully charged and the connections to the radio are sound. Receiving only takes a little power; transmitting takes a lot and a low battery or bad connections will fail on transmit. Check that your radio is set to the correct channel plan.

My booster antenna’s performance was fine when it was first installed, but is now
marginal. What’s happened?

Most likely, a problem with the coaxial cable. Check that there are no splits in the jacket for water to get in, and that the cable hasn’t been crushed or tightly kinked during the installation. Check that all connectors are fitted properly.


Can I shorten or lengthen the coaxial cable on the antenna?

Sure, but think before you cut. You will have to fit a new connector, and that's not always easy. It might be better to leave the surplus length in a loose coil out of sight. If you want to lengthen the cable.

How close together can booster antennas be installed?

The rule of thumb is to install all booster antennas an antenna's length away from other booster antennas. If you can't get that separation, just as far apart as possible. If transmitting on your UHF causes interference on other electronic equipment, consider installing ferrite clamps around the equipment's battery and connecting cables.

Is it okay to rake my booster antennas back on an angle?

You can, but every degree that your antenna is off vertical decreases the performance of the booster antenna by 1%. Communications antennas should be vertical.


What is the difference between fibreglass and stainless steel antennas?

There is no difference in electrical performance between the fibreglass and stainless steel booster antennas. Both are halfwave antennas. The stainless steel whip antennas are light, low windage antennas for installation on a mast or tower, which is usually the highest point. The fibreglass booster antennas have the radiating element at the top of the radome, which gets the antenna as high as possible. Fibreglass booster antennas may be either halfwave or colinear antennas.

Why are there generally both 1.8 metre and 2.5 metre booster antennas?

In an ideal world everyone would fit longer booster antennas, because longer antennas can "see" further than shorter antennas. In the real world many radio owners find the 2.5 metre booster antenna just a bit too long; the 1.8 metre antenna seems to be the right size for most vehicles. Colinear antennas are recommended for larger more stable sites which may want communications at maximum range. VHF, AM/FM and Cellular antennas are all available in 1.8 metre and 2.5 metre sizes.


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