Repeater PreAmps
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Repeater PreAmps
Interested in the idea of placing a preamp on the Rx side to increase HT coverage. I've read in an article they should not be used with Notch-Reject Duplexers though...we have one. Thoughts were to place it in the line between the duplexer and Rx radio on a duplexed, single antenna repeater setup. Any successes or failures with using a preamp w/ this type of duplexer?
The reason you should not use a pre-amp in line with a notch/reject cavity, is there is little port to port isolation, you'll take out the active device on transmit.
All it takes is a tiny amount of RF to leak into the RX port, and you're done.
Notch/reject cavities are not very good coices for repeater duty either, you are much better off with a band-pass, band-reject(BpBr) duplexer for greater port to port isolation plus you can use a GaAs FET preamp inline with your receiver without worrying about blowing it out.
Telewave makes a great BpBr cavity set, it's the TPRD-4544, it will cover the entire 440 band all the way to 470.
I don't have the specs on hand, but I believe they have a -110dB isolation, and can handle 200 watts.
Don't quote me on this though, but I have several sets of these doing commercial duty and I have yt to blow a preamp if you tune each caivty properly.
From 440 all the way to 467, I'll take these cavities over those chintzy Circle-M cans you see at hamfests.
Just my experience...
Enterprise.....One to beam up!
All it takes is a tiny amount of RF to leak into the RX port, and you're done.
Notch/reject cavities are not very good coices for repeater duty either, you are much better off with a band-pass, band-reject(BpBr) duplexer for greater port to port isolation plus you can use a GaAs FET preamp inline with your receiver without worrying about blowing it out.
Telewave makes a great BpBr cavity set, it's the TPRD-4544, it will cover the entire 440 band all the way to 470.
I don't have the specs on hand, but I believe they have a -110dB isolation, and can handle 200 watts.
Don't quote me on this though, but I have several sets of these doing commercial duty and I have yt to blow a preamp if you tune each caivty properly.
From 440 all the way to 467, I'll take these cavities over those chintzy Circle-M cans you see at hamfests.
Just my experience...
Enterprise.....One to beam up!
When you put a preamp ahead of a base station receiver (particularly high band or UHF) you greatly increase the probability of interference from intermodulation distortion in the receiver. This is because the off channel signals are amplified as well as the desired signal, with the result that the receive mixer overloads with lower interfering signal input.
Looking at it another way, since the interfering signals at a particular site are usually fixed in amplitude, increasing the gain in the receive path makes the IMD worse.
Bandpass type duplexers provide some selectivity to off-channel interfering signals, and so help the IMD situation a bit. Notch type duplexers do not have selectivity, therefore the recommendation against preamps.
Looking at it another way, since the interfering signals at a particular site are usually fixed in amplitude, increasing the gain in the receive path makes the IMD worse.
Bandpass type duplexers provide some selectivity to off-channel interfering signals, and so help the IMD situation a bit. Notch type duplexers do not have selectivity, therefore the recommendation against preamps.
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Birken Vogt
- Posts: 262
- Joined: Thu Nov 10, 2005 7:53 pm
I agree. We recently set up a 100W high-band repeater and the only filtering is one of those Celwave compact duplexers in the side of a Motorola case, and there is zero desense on transmit. We are the only radio on the site at all though is the only reason that works. It all depends on the application though. Any site that has more than one repeater/base station needs, in addition to the duplexer, a band pass cavity on the transmitter and 1-3 on the reciever. The preamp is usually placed after that to regain some of the loss through the filters, though a little extra gain doesn't hurt either.
Obviously a notch duplexer does no filtering of transmitter wideband noise other than what is exactly on the reciever frequency and that is what they would be referring to as far as hitting the preamp. It matters what kind of preamp as well. GaAsFET are more suceptible to this than bipolar and I am kind of partial to bipolar myself because most of our sites are really swamped with crappy transmitters and inadequate filtering.
It's always good to call the company and ask them what they think.
Birken
Obviously a notch duplexer does no filtering of transmitter wideband noise other than what is exactly on the reciever frequency and that is what they would be referring to as far as hitting the preamp. It matters what kind of preamp as well. GaAsFET are more suceptible to this than bipolar and I am kind of partial to bipolar myself because most of our sites are really swamped with crappy transmitters and inadequate filtering.
It's always good to call the company and ask them what they think.
Birken
FYI, I do believe they are only rated for 50 watts maximum input.Birken Vogt wrote:I agree. We recently set up a 100W high-band repeater and the only filtering is one of those Celwave compact duplexers in the side of a Motorola case, and there is zero desense on transmit.
Birken
Todd
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Welcome to the /\/\achine.
Welcome to the /\/\achine.
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Birken Vogt
- Posts: 262
- Joined: Thu Nov 10, 2005 7:53 pm
Good point...I'll have to look at it again...we have another identical (I think) one in the shop. I don't know where it came from though. The repeater is old and has been going that way for years. The duty cycle is low enough I guess. It just happened to pop into my brain because we had just moved it to a new location, and when we tested the desense, there wasn't any.
Birken
Birken
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Birken Vogt
- Posts: 262
- Joined: Thu Nov 10, 2005 7:53 pm
My two bits worth:
When to use a preamplifier on your receiver?
In practice it does not matter if this is a repeater or not.
A receiver cannot receive below the site noise level.
The "bench sensativity" is measured using your test equipment with no external signals present.
In practice, when your receiver is connected to the site antenna there will be very many signals of various signal strengths, frequencies determined by the band pass of what ever is in front of the receiver.
A measurement called "effective sensativity" reveals the minimum sensitivity of your receiver on that antenna at that instant.
This is the same thing as "receiver desense", except the whole world is taken into account.
This sort of measurement is necessary on commercial radio sites.
I have hand out describing these measurements in detail if any one is interested.
In short, the receiver sensitivity is measured at the antenna port of your antenna system ahead of any amplifiers.
Note this reading in DBM. I prefer to use 12DB SINAD.
Connect an Iso-Tee to the antenna port, the iso tee being terminated in a known quiet 50Z load. (Loads can be noisy, try several to see which one works best.)
Generate into the isolated port on the IsoTee, note the signal level, about 30 DB more than the basic reading.
Reconnect antenna.
Note how many more DB of signal is required to return to your 12 DB SINAD measurement.
The difference represents site noise, along with your amplifier noise.
In the case of an amplifier attenuators are inserted in the OUTPUT
of the amplifier. (Attenuates amplifier noise along with site noise)
I find that a 3DB noise increase seems to work well on sites.
If a variable attenuator is available the attenuation can be varied until a "sweet spot" is found.
Too much attenuation, poor sensativity, too little and the receiver is overloaded with noise.
An amplifier MUST be preceded by a band pass filter.
The amplifier is broad band in nature, and can be desenitized be signals very far out of band. If one wishes to use an amplifier with the run of the mill notch type duplexer use a pass filter in front of the amplifier.
If size is important helical pre selectors are available.
How about the front end from a Motrac or Micor?
This is a prescription. You must make the measurement on site.
If the receiver has excellent sensativity in the first place an amplifier will actually degrade your system.
I have removed many amplifiers over the years improving overall system performance.
When to use a preamplifier on your receiver?
In practice it does not matter if this is a repeater or not.
A receiver cannot receive below the site noise level.
The "bench sensativity" is measured using your test equipment with no external signals present.
In practice, when your receiver is connected to the site antenna there will be very many signals of various signal strengths, frequencies determined by the band pass of what ever is in front of the receiver.
A measurement called "effective sensativity" reveals the minimum sensitivity of your receiver on that antenna at that instant.
This is the same thing as "receiver desense", except the whole world is taken into account.
This sort of measurement is necessary on commercial radio sites.
I have hand out describing these measurements in detail if any one is interested.
In short, the receiver sensitivity is measured at the antenna port of your antenna system ahead of any amplifiers.
Note this reading in DBM. I prefer to use 12DB SINAD.
Connect an Iso-Tee to the antenna port, the iso tee being terminated in a known quiet 50Z load. (Loads can be noisy, try several to see which one works best.)
Generate into the isolated port on the IsoTee, note the signal level, about 30 DB more than the basic reading.
Reconnect antenna.
Note how many more DB of signal is required to return to your 12 DB SINAD measurement.
The difference represents site noise, along with your amplifier noise.
In the case of an amplifier attenuators are inserted in the OUTPUT
of the amplifier. (Attenuates amplifier noise along with site noise)
I find that a 3DB noise increase seems to work well on sites.
If a variable attenuator is available the attenuation can be varied until a "sweet spot" is found.
Too much attenuation, poor sensativity, too little and the receiver is overloaded with noise.
An amplifier MUST be preceded by a band pass filter.
The amplifier is broad band in nature, and can be desenitized be signals very far out of band. If one wishes to use an amplifier with the run of the mill notch type duplexer use a pass filter in front of the amplifier.
If size is important helical pre selectors are available.
How about the front end from a Motrac or Micor?
This is a prescription. You must make the measurement on site.
If the receiver has excellent sensativity in the first place an amplifier will actually degrade your system.
I have removed many amplifiers over the years improving overall system performance.
Aloha, Bernie