Larsen Magnet Base Antenna - Quarter Wave
This is a very nice magnet base antenna for use on your car, truck or RV.  The antenna is tuned to have optimum performance at 123.3 and 123.5 MHz but works well over the entire aviation radio band of 118 to 136 MHz.  It includes an 11.8 foot long cable with BNC connector for connection to your handheld aviation radio or base station radio.

This antenna requires a ground plane such as the roof of a car or RV.

Dimensions of Base
Length: 3.54 inches (90 mm)
Width: 3.0 inches (77 mm)
Thickness: 1.1 inches (28 mm)

Length of antenna rod: 22 7/16 inches (570 mm)

The antenna length was optimized using an MFJ-259B from MFJ Enterprises, Incorporated.  It allowed me to quickly measure the antenna's VSWR across a range of frequencies and find the frequency that works the best at the given length.  Below are the test results.  The testing was done with the magnet base mounted to the top of my sedan.

Summary of the Test Results

The test results show that the 1/4 wave antenna is not quite as good as the 5/8 wave antenna at the frequencies for which it was optimized (123.4 MHz), but it is close.  The 5/8 wave antenna has a slightly lower SWR at the frequency for which it was optimized (123.4 MHz).  However, the 5/8 wave antenna has a narrower frequency band than the 1/4 wave antenna.  It is still excellent at 122.8 MHz and not bad at 121.5 MHz, but it is not great at the extremes of the aviation radio band (118 and 136 MHz).  That is fine since most gliders operate in the 122.8 to 123.5 range.  The 1/4 wave antenna is not quite as good as the 5/8 wave antenna at 123.4 MHz, but it is still excellent with an SWR of 1.2.  The 1/4 wave antenna is still quite good at 121.5 MHz and not too bad at 118 and 136 MHz.  The 1/4 wave antenna is much better at 118 and 136 MHz than the 5/8 wave antenna, but I don't think I've ever used those frequencies.  One other consideration is that the 1/4 wave antenna is quite stiff so that it will remain nearly straight when on the roof of your car or RV at highway speeds.  The 5/8 wave antenna is much longer an much more flexible (bendy).  It will be bent back dramatically at highway speeds.  The bending will not damage the antenna and probably won't hurt its performance very much, but it is safe to say that the 5/8 wave antenna is better for stationary applications while the 1/4 wave antenna is (perhaps) better for use at highway speeds.