More Isn't Always Better: A Guide to Gain, Noise, and Dynamic Range

A Ham Radio Club's Mountaintop Repeater Challenge

Years ago, I was involved with a Ham Radio club that owned a mountaintop repeater in the two-meter (VHF) band. The repeater was located near Los Angeles with many users, making for a rather difficult RF environment.

The club’s directors were not terribly happy with the receiver’s performance on the hilltop. While the transmitter was easily heard (it had a power of 60 watts), many users had trouble “getting in” to the system, with lots of noise and generally poor performance.

So, the technical wizards decided to fix the problem. Their solution? Buy a low-noise amplifier and put it in front of the receiver. Knowing how dense the RF environment was at the site, I suggested that before the club spent any money, some measurements should be made to see if the additional gain was the right answer. Unfortunately, I was a young, fledgling engineer, not on the board of directors, and in no position to press my case.

The club ordered the amplifier, and it was a very nice one for its day, with a good combination of low noise and good gain. However, these desirable characteristics also meant that it was expensive. 

A few weeks later, the amplifier arrived, and the club planned a trip to the radio site. I got permission to go along, mostly because I worked for a two-way radio shop and had access to some very nice test equipment that would validate the hoped-for improvement.

System Chaos

Somehow, things never work out how you expect, especially with RF.

The crew installed the amplifier and fired up the repeater. Instantly there was an ear-splitting noise coming out of the local monitor speaker. The entire system had been reduced to nothing more than a noise-laden jammer. Everyone stared at me as if it was my fault, but the club president made it clear that they were looking for an answer, not a scapegoat.

After removing the amplifier, some measurements with a signal generator confirmed that the existing receiver sensitivity was excellent without the amplifier. Adding the amplifier and testing with the generator made a slight improvement, but not enough to warrant its addition to the system. However, hooking up the system’s antenna combined with the signal generator (using a directional coupler) revealed that all the incoming RF coming down the antenna line was hammering the receiver. Adding the preamplifier to the system only made the problem worse. More gain was NOT better.

Instead, the board and I planned to add a high-selectivity filter to the receiver’s input. At twice the price of the amplifier, it took a bit of convincing that this was the right approach, but eventually, the unit was ordered and made the trip up the hill for installation.

More IS NOT Always Better

Once installed, they repeated the antenna/generator test. This time, the sensitivity was within 2 decibels of the bare receiver without the filter, which made sense, as the filter had about a 2 dB loss. However, the gain in usable sensitivity was close to 20 dB, as the receiver no longer had to cope with all of the RF “schmutz” coming down from the antenna.

So, more is not always better. Adding gain doesn’t always improve performance and, in fact, sometimes degrades performance. Want to know more about decibels? Read our Comprehensive Guide to Decibels!

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