Custom Products from Masters Communications

On the surface, adding a preamp to a repeater may seem to be a trivial task. Just buy one, install it, and the repeater will automatically "hear better". Nothing could be further from the truth. There are many things that determine whether a preamp will be beneficial or detrimental.

Some repeater manufacturers purpously limit the receive sensitivity to help insure their repeater will operate without desensitization or "desense" using minimal filtering. Therefore, there are situations where adding a preamp can improve the sensitivity - as long as desense is not experienced. Desense happens when undesired signals appears at the repeater receiver and lowers its baseline (or bench) sensitivity. There are many things that can cause desensitization, and we're not going to cover all of them - just the most common ones.

Some duplexers may not provide enough isolation to add a preamp even if the receiver could benefit from one. If your duplexer or filtering system is inadequate, you may experience desense. This type of desense is usually caused by the repeater's transmitter making the undesired signal(s). If the duplexer isn't providing enough isolation, the repeater's receive sensitivity drops when the repeater's transmitter is activated. If you have this type of desense where a user gets cut off as soon as the repeater transmitter turns on, (cycling over and over) adding a preamp will not fix the problem. In fact - it will make it worse.

Many repeaters will operate fine with a mobile (notch only) duplexer. These duplexers generally provide barely enough isolation to allow repeaters to operate properly (with no desense). When you go adding a preamp - you may not have sufficient reserve isolation, and desense will result. This is exactly opposite of what your wanting to accomplish. In this case - the money would be better spent on a more capable duplexer and get (back) the receive sensitivity that you're currently not seeing. Then - if you have any extra isolation in reserve - a preamp may have a chance to add even more sensitivity.

Other signal emitters can also cause desensitization. If you are at a shared communications site, desensitization can occur when a different transmitter turns on. This type of desense occurs when there isn't enough filtering of signals that occupy frequencies that surround your repeater's frequencies. In this case - adding additional filtering is required - either in the preamp, or before it, or both. One problem is - most preamps that have a really low noise figure specification don't have any filtering in front of them. Losses that occur in adding selectivity increase the noise figure of the system - there is no free lunch. Therefore - if there are other services that are co-located at (or near) your site, they can overload the preamp, and additional filtering or isolation must be added to keep that from happening.

So - how can you tell if you can be successful adding a preamp to your repeater? Without some major evaluation with expensive test equipment, it's a guess at best. If you don't know - I certainly can't know. A communications service monitor is a necessary tool to help determine if a preamp installation is feasible. A service monitor can tell you what the sensitivity of the receiver is currently, and may be able to provide the amount of isolation your filtering has. It's best to do some testing and determine if a preamp even has a possibility of making an improvement. It can be a waste of time and money if you don't.

If you can't do this testing - I will still build a preamp for you - but it's with the understanding that it may or may not work for your situation. There are no guarantees that adding a preamp (no matter the cost or quality) will make your repeater's receiver hear better.

Because our enclosed preamps are custom made to order - there are no refunds if it doesn't work for you. That's why no payment option is given without some consultation. I'm not trying to discourage you - but rather provide you with some expectations of possible improvement or failure.

Preamp Index   Click here to see the choices.
P-Series preamps have add to cart links - enclosed preamps do not, so email me to make the purchase. I may ask you some questions.

For those that want to learn more, I recommend these documents from my other site:

Effective Sensitivity Measurement
Receiver Performance Evaluation

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