INTRODUCTION TO THE PR100
This amplifier is a truly necessary piece of equipment for your 2 meter receive applications. With 16dB of gain, you mount this up on your mast on the antenna to boost those weak signals to a receivable level. This results in clear, unbroken reception, and a more pleasant listening experience.
The filtering of the preamplifier is narrow enough only to allow the desired frequency band through, while rejecting all others. This prevents many problems associated with interference such as intermodulation and front end overload. Having such a narrow bandwidth also reduces background noise that would normally be present in wider band amplifiers.
Hookup is very simple in that the unit is powered through the same cable that you receive your signals through. It is possible to ‘feed’ the 12VDC up the coax cable to the preamplifier. No new wires to run and makes for less weatherproofing.
The PC board was designed to fit within a 1 1/2” PVC pipe, so a simple enclosure could be created out of one piece of pipe and two endcaps. Glued together it can make for the perfect weatherproof enclosure.
HOW IT WORKS
There really isn’t much to the preamplifier if you look to the schematic at the right. It mostly consists of filtering, and has only one active part, Q1. We will start from the Antenna end of the preamp, and then on to the receiver end.
J1 and J2 is where the user can connect two different antennas. Some people recommend that you use two turnstile antennas at right angles to each other for good satellite imaging, if you’re using this with a weather satellite receiver. This allows for good coverage of the horizons. Normally you will only use one of these jacks.
From these jacks the unfiltered RF passes through C6, into the bandpass tank circuit consisting of inductor L3 and capacitor C8. This tank circuit and the next two are eventually tuned to be centered at 145 MHz. C4 allows some of the RF from the first tank circuit to be allowed into the next, but is very small in capacitance to offer a large reactance (resistance to AC). This high reactance allows the tank circuits to perform their jobs better by giving them a higher Q factor. This means that the spectrum of RF the tank circuits will allow through becomes narrower due to the high Q.
