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This month, we look at the most important part of your GMRS system, the repeater antenna. Not only does the repeater need to cover the greatest area possible while transmitting, but also, it needs to have sensitive receive abilities in order to make reliable communications possible.

First, what is antenna gain? Picture a donut sitting on the counter. Pretend that the antenna is in the center of the donut vertically. The donut represents the transmitted radiation coming off of the antenna. The donut shaped radiation is similar to that of a half wave dipole antenna. The half wave dipole is used as a reference antenna in almost all commercial antenna comparisons, and its pattern does in fact look like the donut. Now, if you were to smash the donut down on the counter, as you did, the donut would expand further away from the center as it became thinner. Exactly the same thing happens with a gain antenna. The more gain, the flatter the donut, or signal, and the further it will go on the horizon

A half wave dipole antenna will work as a repeater antenna, but it will be deaf! In order for an antenna to have good ‘ears’, it must have multiple elements, and gain. In order for you to use the repeater correctly, it needs to be able to not only get to you with its transmitter, but it needs to hear you with its receiver too. This is why you can’t cut corners on your repeater antenna.

Gain is measured in decibels, or db. Each 3 db of gain doubles the signal on the horizon. An antenna with gain has strong signal at the horizon, then as you go higher above and below the horizon, the signal strength slowly drops too. At the point which the signal is half the strength of the signal on the horizon, this is called the ‘half power point’. (See figure 1)

The reason we care about the half power point is, with very high gain antennas, such as a 10 db gain antenna, the half power point is at 6 degrees above and below the horizon. If you used a 10 db gain antenna on a very high mountain, until you got far enough away from the antenna site, the signal would sort of go right over your head and miss you! 10 db gain antennas work great from low level sites, or high elevation sites far away from the targeted use area. For high sites near the targeted use area, we use a slightly lower gain antenna, with a 6 or 3 db gain.

Gain antennas fall under two categories: those with stacked element arrays and those with stacked collinear elements (also called a collinear array). A collinear array antenna has resonant length elements separated by resonant coils in between. (See figure 2).

A stacked element array has separate elements, with centers spaced about a wavelength apart, each fed with its own feedline which is at a resonant point, connected with the others. (See figure 3).

The more elements you stack vertically, the higher the antenna gain.

During transmit, the stacked element array in figure 3 acts like very separate antennas, all fed in phase (meaning at the same exact time), which gives the antenna gain on the horizon. During receive, each element acts separate as well, each catching its share of signal and then adding it to the feedpoint. In other words, if you presented a signal strength of 1 microvolt at the antenna with an eight separate element array, each element picks up the I microvolt and sends it down its coax to the feedpoint. At the feedpoint (minus some cable and combining loss) you would have eight microvolts! This is why I always suggest stacked element array type antennas for repeaters, they have good ‘ears’.

During transmit, the collinear antenna has enough current over its elements and phasing coils that the array acts like distinctly different elements, much like the stacked element array does. However, on receive the collinear acts more like a resonant ‘long wire’ type antenna that one with several elements all adding to the signal at the receiver. Elements closer to the feedpoint lose a lot of the signal coming from the elements above them also, which can result in a signal at the feedpoint, using our 1 microvolt test above, of about four microvolts from an eight element collinear antenna (minus similar losses).

Here’s the actual results from my last test: I used an IFR 1500 service monitor to generate a signal on 460 MHz using a half wave antenna and enough power output to give me approximately 1 microvolt with another half wave antenna on an IFR 1200 service monitor at 100 feet away. I connected the Antenna Specialists ASP-700 collinear 7db gain fiberglass antenna (See figure 4) to the receiving service monitor in place of the half wave and had a signal level of approximately 4 microvolts. Next, I substituted the Decibel 408 stacked dipole 6.6db gain antenna configured for omnidirectional pattern (See figure 5). The signal strength at the receiver was now just over 6 microvolts!

FIGURE 4

Here’s the short of it: for repeaters, the open, multi element, multi fed antennas work best. The more stacked elements, the better the ‘ears’, and also the higher the transmit gain. This was not a very scientific or difficult test to conduct, and is only a reference. I have really over simplified the math and design that goes into antennas, but for the sake of this forum, I threw enough at you to get the general picture. In the future, get ready for the second antenna installment when we will dig into antenna theory a little deeper as we discuss installing your repeater system at a populated radio site. There’s even more to know when you have lots of neighbors to worry about! Next month, landing a good site for your repeater, and what a professional radio site landlord expects form tenants.

FIGURE 5