Receiver Preamp?

Gotta watch those Fresnel zones!
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ChuckG
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Receiver Preamp?

Post by ChuckG » Fri Aug 28, 2015 12:34 am

I'm working on a STL system (analog, single mono, 950mhz) that is only putting 60uV into the receiver.
Reason why => The transmit dish is pointed into the side of a hill less than 1/2 mile away, the receiver is 15 miles the other side of it.
Not enough tower height on either end to get over the top and a second link isn't going to happen, so I'm looking for alternatives to make whatever improvement I can. Already have 4" parabolic dishes at both ends.
It's been this way since the studio was built long ago, it's hissy if you listen carefully some days, but otherwise surprisingly reliable.

What's the consensus on adding a RF preamp to the receiver to boost the signal level? Something like this:
http://www.advancedreceiver.com/page4.html

Worthwhile, or waste of time?
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Chuck Gennaro
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kcbooboo
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Re: Receiver Preamp?

Post by kcbooboo » Fri Aug 28, 2015 3:27 am

The ARR preamps have 16-20dB of gain. Whether your receiver can take this much additional signal is something you'll have to try.

On UHF amateur repeater equipment, this is way too much gain. While the meters indicate a huge improvement, all the signals are noisy. I've had to add a 10dB pad between the preamp and receiver input to reduce the overall gain to 6-7dB, which is enough to offset cable losses and not overload the receiver. Back in the 70s, Motorola offered preamps for their mobile radios that had 6dB of gain and they worked real good.

If the preamp is feeding a multicoupler/splitter or some other lossy filtering, you probably won't need to attenuate the output.

I'd try to reduce the system losses and increase the signal with shorter runs or lower-loss coax, bigger dishes, or more transmit power if feasible and allowed. This also means fewer connectors and adapters.

Bob M.

KE0MI
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Re: Receiver Preamp?

Post by KE0MI » Fri Aug 28, 2015 11:52 am

My two cents...
If you are trying to improve C/N, the preamp will hurt more than help. If you are trying to correct a lossy feedline, they help. I suspect you are starving for signal at the receive, which the preamp will boost the desired as well as the noise...

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Xenirad
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Re: Receiver Preamp?

Post by Xenirad » Sat Oct 31, 2015 8:54 pm

Be careful adding a preamp. Yes the preamp may increase the receive signal level but it will also increase other signals that might cause interference. A narrow band-pass filter is recommended when using a preamp to avoid this. Also be aware that with a preamp if your STL transmitter goes off air you might have enough receive signal to broadcast someone else's station.
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

TPT
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Re: Receiver Preamp?

Post by TPT » Sat Oct 31, 2015 9:16 pm

We have a 25 mile path--two 6' dishes on transmit, 8' on receive--and using a Hamtronic preamp. Don't think they are made any longer.
Never had a problem with our pair of 606C systems. (Feeds an FM and a booster for another FM).

As I recall, I don't think the ARR preamps were that expensive, so what have you got to lose? Hiss?
You can always pull it back out of the system.

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Xenirad
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Re: Receiver Preamp?

Post by Xenirad » Mon Nov 23, 2015 9:48 pm

TPT wrote:We have a 25 mile path--two 6' dishes on transmit, 8' on receive--and using a Hamtronic preamp. Don't think they are made any longer.
Never had a problem with our pair of 606C systems. (Feeds an FM and a booster for another FM).

As I recall, I don't think the ARR preamps were that expensive, so what have you got to lose? Hiss?
You can always pull it back out of the system.
The 606's had a pretty good front end in the receiver but the newer units do not, this makes interference by adjacent channels more likely with a preamp.
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

radio_guru
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Re: Receiver Preamp?

Post by radio_guru » Tue Nov 24, 2015 7:00 am

Let me add, with broader front ends come off-channel overload and desense. Such as from paging and cellular who might be close by and/or within the main lobe of the RX antenna. I have found adding a good bandpass filter cleans up a lot of perplexing anomalies. Particularly with digital systems.

Case in point, I am using a 9003 system for a 33 mile hop across some densely developed areas. With a 10W amp at the TX end, and a ARR GAsFET pre-amp behind a TXRX optimized bandpass filter, I only experience inversion problems with a normally received -60dBu signal (signal failure occurs at -83dBu). But I don't have IX issues.

As others have opined, the path needs to meet good standards on LoS and Fresnel clearance and the antenna systems reasonably minimized for loss before even getting to that minutia. Adding a pre-amp to a crappy path will only increase the noise floor. Not help the problem.

bmcglynn
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Re: Receiver Preamp?

Post by bmcglynn » Sat May 07, 2016 12:03 pm

I'm running into a similar problem with some nearby ham radio equipment at 440 MHz causing issues with the front end of our STL receiver. Conceptually, a bandpass filter should clean things up.

Can anyone recommend a good supplier for one?

Thanks in advance.

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Re: Receiver Preamp?

Post by TPT » Sat May 07, 2016 5:06 pm

RF specialties handles suppliers that make band=pass filters.

Also look at Microwave filter company.

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