“No Ground Plane” Antennas

[Over2U]

What is the theory of a ‘no ground plane’ (required) antenna for transmitting on GMRS frequencies (i.e. ‘how does it work’)?  Also, does such an antenna require a special type of cable?

[WRQC527]

21 minutes ago, Over2U said:

What is the theory of a ‘no ground plane’ (required) antenna for transmitting on GMRS frequencies (i.e. ‘how does it work’)?  Also, does such an antenna require a special type of cable?

There's a whole lot of theory behind it, I'm sure someone will try to explain it. Bottom line, it's a half-wave vertical antenna with a matching circuit in the base. It doesn't need a special type of coax. Just 50-ohm coax like RG-58, RG-8x, something like that. 

[Lscott]

19 hours ago, Over2U said:

What is the theory of a ‘no ground plane’ (required) antenna for transmitting on GMRS frequencies (i.e. ‘how does it work’)?  Also, does such an antenna require a special type of cable?

Well it's the difference between a 1/4 wave long antenna verses a 1/2 wave long antenna. The minimum length antenna to radiate RF is a 1/2 wave length long by theory. Since a 1/4 wave length antenna is "missing" the other part, the absent 1/4 wave length, it has to be made up for some how. That's where the ground plane comes in, it doubles the antenna length so it looks like it's a 1/2 wave length long to the transmitter.

To make the idea simple to understand place your finger against a mirror so it's perpendicular to it. You'll see immediately the reflection of your finger touching the end of your real one. That's basically what the ground plane does for a 1/4 antenna, it "mirrors" the other half of the antenna so it looks like it's really a 1/2 wave length long.

With a 1/2 wave length antenna you have a choice where to attach the coax cable, in the middle or at one of the two ends.

If the coax is attached in the middle the impedance is around 70 to 75 ohms more or less. That's a reasonably good match to a 50 ohm coax resulting in an SWR of about 1.5:1, which all most all radios will have no issues with it.

For a vertical antenna the most convenient place is on the end. For some complicated reasons the impedance is far higher than the 50 ohm coax cable. For those antennas a matching section in built into the base. An example is the common "J-Pole" antenna. It's really a 1/2 wave length antenna with a 1/4 wave parallel transmission line matching section on the end. The feed point is selected such that the ratio between the voltage and current is 50, which is the required value for the 50 ohm coax. If the coax feed point was exactly on the shorted end the voltage would be zero, resistance = voltage/current, so the impedance would be zero. At the other end the current is nearly zero, the un-terminated end of the one element, thus the impedance is extremely high. Thus the match point must lie someplace between those two points.

This is about as simple of an explanation without getting deeper into the theory.