IMPROVING RADIO MIC PERFORMANCE: THE ANTENNA FACTOR
by Edwin J. Somers, CAS

	AGPU Ground Plane Antenna Shown here for illustration purposes only; item has been discontinued.
	SNA6900 Di-Pole Antenna
	ALP700 Log Periodic Antenna

Let's talk about ways to improve radio mic performance. Let's start by talking about antennas. You already know that high gain antennas increase range. Well, how much better do they work? Let's examine the facts. It would be difficult to put a high gain antenna on the transmitter in most cases, so we will look at receiving antennas. The whip antenna that comes with the receiver has a gain of 1 dBi (dBi stands for isotropic, and is used for the measurement of antenna gain). Ground plane and di-pole antennas have a gain of 3 dBi.

When you increase the received signal by 3 dBm, it is the same as doubling your transmitter power (dBm is used for the measurement of power). When you increase the received signal by 6 dBm, it is the same as quadrupling your transmitter power. Log Periodic antennas have 6 dBi or more gain.

Due to the losses involved in transmitting radio signals, you will get approximately a 20 to 25% increase in range when you double the power, or increase the antenna gain by 3 dBi. In physics, it is called the Inverse Square Law. You don't need to understand the laws of physics; you just want to know how the performance will improve. The use of gain antennas is a very inexpensive way to increase range. In the case of beam antennas (Yagi and Log Periodic), you will have the added advantage of a directional antenna that can be positioned to ignore identifiable interference.

There is one instance where you can increase the gain of a transmitting antenna, and that is when using a VHF plug-on transmitter first developed by Lectrosonics. We have found significant improvement in range by adding a counter-poise antenna to the H series transmitters. This is nothing more than a thin 17" stainless steel wire attached to a copper clad, one sided piece of circuit board material, strapped to the side of the transmitter (there is no direct connection, it is capacitively coupled).

Another very important rule is antenna polarization. If the transmitting antenna is vertical, then the receiving antenna should also be vertical. This is called vertical polarization. If the antenna alignment is off by 90°, the loss is infinite. You will only be receiving reflected signals. Obviously, antenna alignment will always be a compromise, so you should pay close attention to it. Receiving antenna height is almost always an advantage; the higher the better.

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