What is "Gain"?

Gain measures how much of the energy sent from the radio to the antenna is concentrated in a specific direction, and in which direction the 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.

ANTENNA PERFORMANCE ISSUES

If 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 antenna and the antenna you are talking to have an effective line of sight through the repeater radio.

My 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 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. 
 

INSTALLATION ISSUES

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 antennas be installed?

The rule of thumb is to install all antennas an antenna's length away from other 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 OK to rake my antennas back on an angle?

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

Land based ANTENNAS

What is the difference between fibreglass and stainless steel antennas?

There is no difference in electrical performance between the fibreglass and stainless steel 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 antennas have the radiating element at the top of the radome, which gets the antenna as high as possible. Fibreglass antennas may be either halfwave or colinear antennas.

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

In an ideal world everyone would fit longer antennas, because longer antennas can "see" further than shorter antennas. In the real world many radio owners find the 2.5 metre 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.