Optimizing Antenna Radiating Power

Optimizing Antenna Radiating Power
Logan Storehouse ERC Training Topic August 19, 2012
Rick Hughes, K7YPR

The goal of this training topic is to offer some suggestions for maximizing the available transmitter power to your antenna load and more specifically the selection and proper use of transmission lines. In addition to the benefits of higher net operating power, improving transmission line performance will also benefit receive signals and particularly those that are weaker.  For the purpose of this brief discussion, it is also assumed that your antenna is properly matched.  An improper match could result in high VSWR with significantly reduced power radiated by the antenna and potential transmitter overheating and failure.

This discussion is likely a review for many.

There are three basic considerations for minimizing power losses in transmission lines while maximizing power radiated by the antenna:
1. Route transmission lines for the shortest path to the antenna feed point.  Longer cable runs will likely require a lower loss cable especially for VHF use.  Use RG-58 cable only for very short cables where applications limit cable size such as mobile, mag mounts or equipment interconnects, etc.  High loss cable lengths are best limited to 10 Feet or less.

2. Select the best transmission line cable for your particular application.  RG-213 and LMR-400 work well in most applications.  Larger diameter cables have less loss, but are more expensive and difficult to work with in some cases.  Shoot for lengths less than 100 Ft. when connecting the antenna to the transceiver.  Cable losses are compared in the table below:

Chart showing various cable losses at 150 MHz.

(1) Cable Type
(2)  Loss per 100 Feet at 150 MHz.
(3)  Power Loss per 100 Ft Given 100 Watt Transmitter Power                
(4)  Net Antenna Radiating Power

          (1)                            (2)                                   (3)                                        (4)                      
Belden RG-58A        -5.2 dB                      70 Watts                           30 Watts
Belden RG-213A        -2.3 dB                      41 Watts                           59 Watts
Times Micro LMR-400   -1.5 dB                      29 Watts                           71 Watts
Andrew LDF-4          -.815 dB                      17 Watts                           83 Watts
Andrew AVA5-50A         -.437 dB                      10 Watts                           90 Watts

3. Proper antenna impedance matching is critical to overall performance.  Many antenna manufacturers’ publish VSWR values of 1.5:1 or less.  A VSWR of 1.5:1 represents about 4% reflected power.  Assuming no other losses, a 100 Watt transmitter would deliver a reduced power of 96 Watts to the antenna given a match of 1.5:1.  Four Watts of power is thus lost in the form heat which is dissipated by the transmission line.

Consider the same case where an antenna is connected to the transmitter with 100 Feet of LMR-400 coax cable.  The transmission line results in another 29 Watts of loss with only 68 Watts reaching the antenna because of combined effect of the VSWR.  It should also be noted that transmission line loss will result in some degree of error in reading reflected power at the transmitter end.

Typically most commercially manufactured antennas have far better performance than 1.5:1.  Antenna matchers are an option and provide a quick fix for situations with higher VSWR.  Fine tuning of the antenna match can also be accomplished by adjusting the transmission line length.  Tuning the antenna in this manner is a little trickier but is effective for a given frequency.

“It has often been suggested that where all elements of a system are 50 ohms, one can use any length of 50 Ω cable and a “perfect match” will result. This can only be true where all of the system elements have a purely resistive 50Ω characteristic, exhibiting no inductive or capacitive reactance”.

In Summary, keep cables short,  use low loss cable where practical and particularly for longer lengths and verify the antenna match giving consideration to an inline matcher or fine tuning cable lengths.