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Application Notes

Decibels, What Are They?

A decibel is simply an expression of a ratio. A decibel is often used in radio systems because a range of signal values can be quite large. Using decibels makes these numbers much more managable. For example, a range between 100,000,000,000,000 and .000,000,000,000,001 is simply a range between 140dB and -150dB. In addition, decibels are convenient because they are added when multiplication of absolute signals takes place. The formulae for decibels are as follows:

• for power: 10 log(P2/P1)
• for voltage: 20 log(V2/V1)

The dB for a voltage ratio is twice that of a power ratio because power varies with the square of the voltage. Thus ohms law is preserved. If no reference is used, the expression in dB is the change in signal strength from input to output through a system.If dBm is used, the signal is being referred to one milliwatt. Therefore, 0 dBm is one milliwatt and 10 dBm is 10 milliwatts.If dBuV is used, the reference is one microvolt. Therefore, 0 dBuV is one microvolt and 20 dBuV is 10 microvolts.

How To Use Antennas

There are several important factors that telemetry users should be familiar with when using antennas. Familiarity with these will help reduce confusion and increase success in tracking.

1. Keep clear of the antenna when taking a bearing. Do not stand, or have assistants stand, within 1/2 wavelength of the antenna elements (1m at 150MHz).
2. Protect the antenna elements to prevent them from being bent out of shape. Bending of elements will alter the pattern and efficiency of an antenna thereby misdirecting you towards the target. If the antenna elements get bent, carefully bend them back to their original shape.
3. Periodically inspect the antenna feedpoint to ensure that the connections are still tight and reliable. If the feed apparatus has become loose, have the antenna repaired and re-tuned with the proper test equipment.
4. An antenna is polarized in the direction of the elements in the array. i.e. if yagi elements are vertical the antenna is vertically polarized.
5. Keep all metal objects (including the coax feedline) from interfering with the antenna. The metal elements of an antenna will be influenced by metal objects that are physically parallel to it. With horizontally polarized antennas, it is simple to have the mast and feedline extend away from the antenna perpendicularly, however, in vertically polarized antennas, the mast often has to attach to the center of the antenna's boom right in the middle of the antenna elements. In this situation, the mast MUST BE NON-CONDUCTING. The feedline should also loop out behind the antenna and attach to the mast below end of the antenna elements. If it is not practical to bring the feedline down from behind the antenna, tightly fasten it to the boom and mast with cable ties or use a cantilever design.
6. The horizontal beamwidth of a vertically polarized yagi is greater than the horizontal beamwidth of a horizontally polarized yagi.
7. In wildlife tracking, using a yagi antenna horizontally helps isolate the direction of the transmitter more precisely, however, when simply trying to find a transmitter, it is wise to try both vertical and horizontal polarizations since the transmitter may be in either orientation.
8. There is at least a 20dB loss between polarizations. i.e. when listening to a vertically polarized signal with a horizontally polarized antenna.
9. In fisheries tracking, always use the antenna in vertical polarization. Horizontally polarized signals experience much greater loss at the air-water interface.
10. Be aware of the sidelobes and backlobe on your antenna. Don't simply assume the first peak you see in the received signal indicates the antenna is pointed at the transmitter. Do a quick 360 degree scan to ensure you are checking the greatest peak.