Flycom Avionics Ltd

ANTENNA SWR TUNING

The SWR, or Standing Wave Ratio, of an antenna is a measure of how efficiently your radio is radiating the energy it produces when you transmit. There are many good reference books on the subject if you want to know more, but you don't need to know what it actually is in order to measure it.  You just need an SWR meter.

Here is what you need to know:
The scale on an SWR meter reads from 1 to infinity.
The smaller the number, the better the SWR reading.
The scale is not linear, it is logarithmic. From 1 to 3 covers more than half of the scale on an SWR meter. The remainder covers 3 to infinity!
An SWR reading of more than 3 is hazardous to your radio.  (often marked in red on the scale).   Because of the logarithmic scale, you don't have to be far into the red before you are into the big numbers!
For any given frequency there is a corresponding wavelength and one ideal length of antenna whip. (1/4 of the wave length.)
Because we are changing frequencies often, it stands to reason that the actual length of our whip has to be a compromise since we can't keep changing the length of the whip every time we change frequency.
In truth we don't need to.
We pick a frequency in the middle (usually 127 mHz.) and tune to that frequency.
The ideal number to aim for is 1:1 or 'flat line' in the trade.  This means all the energy is being radiated and none being 'reflected' back into the radio. An SWR of 2:1 or less is OK.
Less than 1.5:1 is very good.
In practice the length and shape of the antenna's ground plane can affect the SWR of a given whip. 
ATC radio transmissions are vertically polarised.   This means that your antenna should be more vertical than horizontal when mounted.
How to use the SWR meter
  1. Connect the feeder BNC plug to the right side of the meter marked antenna (ANT).
  2. Connect the radio to the left side of the meter marked transmitter (XMTR)
    In other words the SWR meter is connected between the radio and the antenna.
  3. Rig the aircraft and move it well away from hangars (and hangers-on for that matter)   SWR'ing inside a hanger is a complete waste of time!
  4. During brief transmissions* (2 seconds max) adjust the power needle to the 'set' position (100% full scale deflection) using the knob on the meter.
  5. Observe the corresponding SWR reading on the other dial.  If you have a single dial meter you will have to switch between the two operations and note the readings alternately.  Read the maker's instructions.

    *Note: You don't need to talk to anyone, in fact you don't even need your headset plugged in. You are just measuring the efficiency of the 'carrier wave' not the modulation (voice signal)
How to tune the whip
  1. Insert the whip into the whip adapter as far as it will go while the adapter is screwed fully into the antenna base.  Tighten the grub screw.
  2. Tune your radio to 127mHz.
  3. Press the PTT and note the SWR reading.
  4. Saw off the whip about 5mm at a time, (repeat 3) and note the reducing value of the SWR reading each time.  Take care.  It may not be possible to get 'flat line' in all cases.  If the SWR reading starts to increase again before reaching 1:1 then you have gone too far, so stop cutting.
  5. There is a small amount of adjustment in the whip adapter if you do go too far.
    Tip:  If you want to know if a whip needs to be longer or shorter get a helper to put a hand near the top of the whip when SWR'ing.  If the reading goes up the whip needs to be shorter.  If the reading goes down the whip needs to be longer.  To get a proper reading keep people away from the antenna.
  6. When you you've got the best SWR reading you can, remove the meter and reconnect the feeder directly to the radio.
  7. Go for a fly and get a proper radio check.
    If you don't get 'strength 5' every time using FLYCOM equipment then you have a problem on your aircraft!

 
  WARNING! 
  DO NOT transmit  without an antenna fitted specifically designed for air-band frequencies.
  DO NOT transmit with a damaged coax feeder cable.
  DO NOT transmit with a disconnected coax feeder cable
  DO NOT transmit with the antenna is grossly out of tune i.e. SWR greater than 3:1
 
 


IMPORTANT NOTES:
 If your antenna is supplied over-size it will be grossly out of tune and you will need to tune it.

If your ground plane is not making good electrical contact with the antenna base your antenna will be grossly out of tune.
Transmitting (more than 1-2sec for test purposes) without an antenna connected or with an antenna grossly out of tune could damage the radio. The energy produced by the radio when in transmit mode needs to radiate (and hence dissipate heat). If it is unable to do so (for whatever reason) it will be reflected back into the radio and damage the PA (Power Amplifier) unit.
 

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