Speaking of Receiver Preamps

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Thread seen on the AR902MHz Yahoo! Group in July 2007

Bill K2WH wrote:

I would love to add a low noise preamp to my 900 MHz repeater, but, since the repeater uses a single antenna and is both transmitting and receiving at the same time, a mast-mounted switchable preamp is out of the question.

An alternative would be to use separate antennas then a mast-mounted preamp would work and would benefit or make up for feed-line losses and that is what I would like to capture. However, since I am not going to use separate antennas, my question is: would using a preamp on the receive side of my duplexer and prior to my MaxTrac 800 MHz receiver, do anything to improve reception or would it just introduce more noise into the receive path? Is there any value to this since the signal is already lost in the feed-line and cannot be made up? Is anyone using a preamp in this configuration? If so, let me know how it works and others thanks for your input.

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Andy KC2GOW wrote:

Yes you will see an improvement "usually" when you add a preamp in line with duplexer in front of it. I only use Advanced Receiver Research (ARR) Pre-Amps, mainly because of low cost, and they work extremely well. You will want to use a GaAsFET type pre-amp in repeater service. Also, sometimes adding a 3 dB attenuator between the output of the amp and the receiver can actually help the overall performance... this is if the level from the amp is overloading the RX the attenuator will cut down the output level that the receiver sees making the front end of the radio happy while keeping performance. You may want to use a signal generator to test this though.

Also, being that the system is duplexed, you need to be sure that you have enough isolation between your RX and TX. Otherwise any minute degradation of the receiver by the transmitter, "desense", will just get worse. If you have just enough isolation in your system where there is currently no degradation of the receiver, some may now appear as the receiver can now hear extremely low-level signals with greater ease.

On a MaxTrac type radio a good pre-amplifier will take the receive sensitivity from around 0.34uV down to around 0.22uV or 0.24uV. This is quite a noticeable difference in RX coverage.

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Mark N9WYS wrote:

So now I'll add my question... since I'm building a 900 MHz repeater, I want it to have the best receive sensitivity possible, without overloading the front end. (I am using a converted 800 MHz MaxTrac with 915 MHz filters.)

Do I want to use something like Angle's single-stage preamp, or the dual-stage unit? I see ARR only has a single-stage unit... Costs are relatively comparable, so I want to know which (practically) is "best".

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Andy KC2GOW wrote:

You can only get so much sensitivity out of a receiver. After a certain point it becomes unusable and hard to run in a duplexed system. In your case as well as Bill's, I doubt that you would see much difference between the dual stage and single stage pre-amplifiers especially in a MaxTrac RX.

The best bet in 902-land would be the ARR with a middle of the road gain spec and the same noise figure as the rest. My opinion against the others on the group, but there it is.

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Lee K6LQT wrote:

OK, I HAVE to weigh in on this one. First, no one solution fits every application, so what may be good at my site may not be the best solution for your site. I suggest you do nothing until you read through the excellent information on Chip Angle's website.

Chip gives some very helpful information, so whichever product you choose, at least you can make an informed choice. BTW: I do not have anything to do with Angle Linear, but have used several of Chip's 440 MHz preamps over the years -- and been impressed with them.

Another subject: I second the thought that a dual preamp is overkill. Preamps are all about noise figure (NF) and gain. The lower the NF, the weaker the signal that will be heard. Regarding gain, most modern preamps have gain on the order of 16dB in a single stage. That's enough to take a signal that's barely above the noise floor and make it very copy-able on a MaxTrac. More gain (dual-stage, for example) often does more harm than good.

If you have a 900 MHz paging transmitter nearby, it can easily overwhelm the preamp unless you protect the preamp well with cavities. It is important to have real steep skirts on your duplexer or put additional cavities before AND after the preamp lest you amplify off-channel signals as well as the desired signal.

Again, one size does not fit all. I have used different solutions for different sites, depending on the circumstances. Do the homework and you will save yourself considerable grief and $$.

And that's my 2-cents' worth.

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This page originally posted on Tuesday 10-Jul-2007

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