Antenna Length


Standard ¼ Wave Whip Formula

The following formula is the industry standard for calculating the correct length of a standard ¼ wave whip antenna, in order to achieve the frequency you require.

Please note:

You need to measure up from the bottom of the body mount/top of car roof to achieve the correct position to cut the antenna, see example below.

Formula:

300

________________ = X / (Divided by) 4 = Cutting Length in metres

Frequency in Megahertz

Example: I want to transmit/receive on 169.51200MHz

300 / 169.512 = 1.769 / 4 = 0.442 metres (442mm).

Place your antenna on the body mount and measure up from the base of the body mount/roof of car until you get to 444mm, this is where you cut the antenna. The antenna is now suitable for 169.512MHz.

Please note that the above assumes you have an adequate ground plane (metal roof) to show at least 1/4 wave (442mm) radius around the bottom of the antenna.

If the ground plane is not at least a 1/4 wave then the antenna will need to be trimmed using a VSWR/power meter. In this case it is advisable to leave 20mm extra length on the antenna so that you can trim it back to resonance.

How do you know if the antenna is too long or too short?

With a VSWR meter in line, set to full scale deflection on the 'set', de key and switch to 'Reflect', then key the radio. Let us say that you are reading 2.5:1 VSWR on the meter.

With your hand open, move your palm, slowly, close to the antenna whilst still transmitting.

Watch the meter and if the reading goes up, then the antenna is too long.

If it goes down, find another whip because you have cut it too short!

If it does not move at all, check the installation, there is something wrong, maybe the ground connection to the body is poor or the coax plug is wrongly wired. (short between centre pin and coax braiding)

Ground Plane

A ground plane is the 'other half' of an antenna. Often ignored, misunderstood and wrongly calculated!

If we accept that an antenna consists of two lengths of metal, each 1/4 wave of the given frequency. (This is known as a 1/2 wave dipole.)

For mobile use we mount the radiating element vertically.

The other side of the antenna is now connected to the earth side of the body mount.

Likewise, it must also be at least a 1/4 wave at the given frequency.

The best way to check if your roof is big enough, lay the antenna on the roof with the bottom cone in the position you plan to have the mount.

The tip of the antenna should, if you are lucky, still remain within the metal roof.

If not, then allow 20mm on the antenna length before trimming to resonance with a VSWR meter.

Artificial Ground Plane

There are some vehicles that have plastic or fibre glass roof which will not work as a ground plane. Therefore we need to install one.

The easiest way I have found is to drop the headlining and stick strips of Aluminium Tape to the underside of the roof.

Again, do not forget the 1/4 wave rule, the tape must stretch a 1/4 wave either side of the mount.

The best mount to use on a installation in a fibre glass or plastic body is the PB050 range.

This mount has a 25mm 'foot print' but only a 7mm hole in the roof. Also it can be used on roof thickness up to 50mm!

Can I use one antenna for several radios?

Precise filtering and isolation of different channels, through one antenna.

There are many applications in which you may wish to use more than one channel for communication, but do not wish to have multiple antennae. Perhaps there might be a space restriction (such as on a motorcycle or small craft), or you do not wish to draw attention to a “forest” of different antennae (maybe on a car being used for covert surveillance).

With our mobile filters, it would be possible to use a single antenna (matching the original vehicle design) on an unmarked Police vehicle ...handling communications that use different channels. This could allow the occupants to combine GSM cellular access with the PMR radio band ...and still tune in to their favorite radio station, meanwhile!

 

This combining of antennae and radios need to be carefully set up as like with everything in this world, nothing comes free!

Because of the nature of these filters there are high losses due to the high Q of the filters.

If you drop us an email with your requirements, we can suggest a suitable setup and give you the expected losses in db i.e. 3db = 50% loss!

 

© XL Systems 2014