Several Hams have contacted me asking about the Ramsey Electronics PR40 440 MHz preamp that I use when operating on the satellites. With so much interest, I thought it would be good to make a blog entry dedicated to this preamp.
My daughter Stacie (KI4IAG) did most of the assembly work for me. As you can see, there simply aren't a lot of parts on the circuit board so it didn't take long to put together. The Ramsey PR40 kit comes only with the circuit board and the parts that are on it. The case, connectors and everything else you see in the photo to the right are additions that Stacie and I added.
I power the preamp with two 9 volt batteries connected in series which results in about 18 volts when the batteries are new. The preamp requires 9 - 15 volts to operate. I suspect that it would probably tolerate 18 volts without damaging something, but I thought it would be best to give it a little protection. I added an LM7812 voltage regulator so that a steady 12 volts would be supplied to the circuit board. I thought this would also work out well if I ever want to power the preamp from a cigarette lighter or similar voltage source. The LM7812 is difficult to see in this photo (upper left corner ) because it is mounted upside down and is soldered directly to the power connector jack (the black box with the red dot). Its always nice to know when something is powered on so we added a green LED that we had in our junk drawer to serve as a power indicator.
I can't recall where we purchased the case because it was left over from a previous project. I strongly suspect that it came from Mouser Electronics. It was the perfect size where ever it came from. The BNC connectors and RG-174 coax came from Eagle 1 Communications. By the way, I've been pretty impressed with the Eagle 1 guys. Their prices are very reasonable and the workmanship on some cables they made for me was above and beyond.
The only tuning required after assembling the kit was peaking L1 for maximum signal strength. This involved spreading or compressing the turns slightly to get best results. I didn't spend a lot of time adjusting L1. The preamp seemed to be working fine as it was and lacking RF test instruments, I didn't have an objective way to measure the signal strength anyway.
The big unknown for me was if I needed to switch the preamp out of circuit when transmitting. This preamp has little tolerance for RF applied to its output. My setup uses an MFJ duplexer to connect my Arrow antenna to the radio. I knew that this arrangement would allow some of the transmitted 2 meter RF to "leak" to the 440 MHz receive side of the duplexer and therefore be applied to the output of the preamp. The MFJ specs for the duplexer say it has 60 dB of isolation, so by my calculations that would equate to about 5 micro-watts applied to the preamp when transmitting from the HT at 5 watts. This didn't sound like much, but I really wasn't sure just how much RF applied to the preamp output was enough to fry it.
The documentation that comes with the preamp recommends the Ramsey RF Sensed T-R Relay Kit to keep too much RF from blowing the preamp. However, the description for this RF Relay said that it takes 50 milli-watts of RF to activate it. This is far more power than I expected to apply to the preamp in the first place so I decided to take a gamble and see what would happen without any kind of protection (other than the duplexer) for the preamp. As it turned out, the preamp has given me no problems in this arrangement. I still don't know how much RF can safely be applied to the output side of the preamp. I would assume that since Ramsey recommends a T/R relay that doesn't activate until 50 milli-watts is sensed, then it ought to be safe to apply up to 50 milli-watts to the output of the preamp. However, that is an assumption on my part.
This little preamp has served me very well. Lacking the proper test equipment I can't say how well in any objective sense, but there is definitely a decrease in performance if I remove the preamp from the antenna. At only $14.95 for the kit plus a few extra bucks for the other parts I added to make this a complete product, I feel I have more than gotten my money's worth!
73,
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