[Repeater-Builder] Re: Preamp and attenuator (follow up)
Gary, Just a follow up to the preamp questions from last month. Your advice was fantastic. We added attenuation right to the point where the sensitivity started dropping off and it was perfect. By putting in the attenuator, it actually increased the receive range of the repeater by about 30%. We're going to do this with every site that we have! Dwayne Kincaid WD8OYG Hook your signal generator up to your system at the antenna port and measure receiver sensitivity with and without the preamp. Then with the preamp in circuit start adding attenuation between the preamp and the receiver. When you just start to loose sensitivity stop adding attenuation. That should give you near optimum sensitivity without excessive gain. Too much gain in the preamp overloads the receiver mixer and front end amp if it has one. For every db of gain you add in front of the receiver you reduce the IM performance of the receiver. You only want enough preamp gain to overcome the noise figure of the receiver. Although the noise figure of the receiver and preamp are cumulative the preamp is the biggest contributor in setting system noise figure. In other words putting a hot preamp on a very hot receiver will give you a better overall noise figure than putting that same preamp on a poor receiver but the difference will not be great. You may not be able to realize the full benefit of the preamp if you have excessive IM. You may have to add more attenuation to where it further reduces receiver sensitivity. When you get down to the point that the sensitivity is the same as it was without the preamp, then throw out the preamp. But you may be able to find a happy medium where the preamp does help some without destroying your IM performance. If you still have excess IM problems you can add attenuation ahead of the preamp by raising the insertion loss of the loops on your band pass filter as others have suggested. By raising the insertion loss on the loops it does the same thing as adding an attenuator ahead of the preamp but with the added benefit of steeper skirts on the band pass filter. By the way don't worry about adding adaptors between the preamp and receiver. After all you are looking to add attenuators anyway. But adaptors really make no measurable difference in attenuation at vhf or uhf. They may give a slight impedance mismatch but you probably don't have anything that will measure the small amount of loss from them. 73 Gary K4FMX
RE: [Repeater-Builder] Re: Preamp and attenuator
-Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of ldgelectronics Sent: Wednesday, March 21, 2007 11:37 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Preamp and attenuator There area also two Sinclair BP/BR cans. One on the TX and one on the RX. The loss is set for 0.5 db on each. The interconnect cable is RG-400, no LMR. Dwayne Kincaid WD8OYG Did you mean to say BP cans and not BP/BR for the extra cans? BP/BR cans provide little pass band attenuation. 73 Gary K4FMX
[Repeater-Builder] Re: Preamp and attenuator
Gary, Yes, Band Pass for the extra cans. Did you mean to say BP cans and not BP/BR for the extra cans? BP/BR cans provide little pass band attenuation. 73 Gary K4FMX
[Repeater-Builder] Re: Preamp and attenuator
Gary, That is very good information. We're going back up to the site today and will try that method. Hook your signal generator up to your system at the antenna port and measure receiver sensitivity with and without the preamp. Then with the preamp in circuit start adding attenuation between the preamp and the receiver. When you just start to loose sensitivity stop adding attenuation. That should give you near optimum sensitivity without excessive gain. Too much gain in the preamp overloads the receiver mixer and front end amp if it has one. For every db of gain you add in front of the receiver you reduce the IM performance of the receiver. You only want enough preamp gain to overcome the noise figure of the receiver. Although the noise figure of the receiver and preamp are cumulative the preamp is the biggest contributor in setting system noise figure. In other words putting a hot preamp on a very hot receiver will give you a better overall noise figure than putting that same preamp on a poor receiver but the difference will not be great. You may not be able to realize the full benefit of the preamp if you have excessive IM. You may have to add more attenuation to where it further reduces receiver sensitivity. When you get down to the point that the sensitivity is the same as it was without the preamp, then throw out the preamp. But you may be able to find a happy medium where the preamp does help some without destroying your IM performance. If you still have excess IM problems you can add attenuation ahead of the preamp by raising the insertion loss of the loops on your band pass filter as others have suggested. By raising the insertion loss on the loops it does the same thing as adding an attenuator ahead of the preamp but with the added benefit of steeper skirts on the band pass filter. By the way don't worry about adding adaptors between the preamp and receiver. After all you are looking to add attenuators anyway. But adaptors really make no measurable difference in attenuation at vhf or uhf. They may give a slight impedance mismatch but you probably don't have anything that will measure the small amount of loss from them. 73 Gary K4FMX
Re: [Repeater-Builder] Re: Preamp and attenuator
At 3/21/2007 20:46, you wrote: The garbage was pops, static, crackling on weak user signals. There Sounds like you may have an antenna/loose hardware in near field of antenna problem, which could also generate passive IMD. Does it get better when it's raining? Bob NO6B
Re: [Repeater-Builder] Re: Preamp and attenuator
At 3/21/2007 18:24, you wrote: Steve, It's on 2M, single Station Master antenna with about 400 feet of 7/8 feedline. The duplexer is a BP/BR followed by a band pass can, then the pre, then the radio side. All cables are RG-400. The radio is GE Exec II. The sensitivity without the pre is about 0.35 uV. The noise floor is pretty low, but the ARR pre was picking up a bunch of garbage without the attenuator. If the ARR preamp is nonlinear with a pass can ahead of it, it's probably oscillating. I recommend replacing it with an Angle Linear preamp (http://www.anglelinear.com). Chip's preamps are guaranteed to be unconditionally stable, meaning it won't oscillate just because there's a pass can ahead of it. And without that 6 dB pad in front of it, you'll reduce your system noise figure by over 5 dB! Bob NO6B
RE: [Repeater-Builder] Re: Preamp and attenuator
Just a further note on this: When measuring sensitivity as below don't do it with an isolation T and the antenna connected. The antenna noise will interfere with your measurements. Do it with the antenna disconnected and the signal generator connected where the antenna would normally connect to the duplexer. Be sure to disable the repeater!! When you are finished making sensitivity measurements THEN place an isolated T in the antenna line and measure the sensitivity with and with out the transmitter on to see if you have any de-sense. Actually you might want to make a de-sense measurement before you put the preamp in to verify things are as they should be so you don't end up chasing two or three problems at the end. Then make a final de-sense measurement when all done. 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of ldgelectronics Sent: Thursday, March 22, 2007 11:18 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Preamp and attenuator Gary, That is very good information. We're going back up to the site today and will try that method. Hook your signal generator up to your system at the antenna port and measure receiver sensitivity with and without the preamp. Then with the preamp in circuit start adding attenuation between the preamp and the receiver. When you just start to loose sensitivity stop adding attenuation. That should give you near optimum sensitivity without excessive gain. Too much gain in the preamp overloads the receiver mixer and front end amp if it has one. For every db of gain you add in front of the receiver you reduce the IM performance of the receiver. You only want enough preamp gain to overcome the noise figure of the receiver. Although the noise figure of the receiver and preamp are cumulative the preamp is the biggest contributor in setting system noise figure. In other words putting a hot preamp on a very hot receiver will give you a better overall noise figure than putting that same preamp on a poor receiver but the difference will not be great. You may not be able to realize the full benefit of the preamp if you have excessive IM. You may have to add more attenuation to where it further reduces receiver sensitivity. When you get down to the point that the sensitivity is the same as it was without the preamp, then throw out the preamp. But you may be able to find a happy medium where the preamp does help some without destroying your IM performance. If you still have excess IM problems you can add attenuation ahead of the preamp by raising the insertion loss of the loops on your band pass filter as others have suggested. By raising the insertion loss on the loops it does the same thing as adding an attenuator ahead of the preamp but with the added benefit of steeper skirts on the band pass filter. By the way don't worry about adding adaptors between the preamp and receiver. After all you are looking to add attenuators anyway. But adaptors really make no measurable difference in attenuation at vhf or uhf. They may give a slight impedance mismatch but you probably don't have anything that will measure the small amount of loss from them. 73 Gary K4FMX Yahoo! Groups Links
[Repeater-Builder] Re: Preamp and attenuator
You mentioned that the site's noise floor is pretty low. Could you be more specific about that and how it was measured? The usual method of determination of the site noise floor is to measure the receiver's effective sensitivity using an RF coupler such as the Microlab FXR or Bird devices that have been mentioned recently [or one you have constructed] along with a signal generator and a dummy load. The signal generator is connected to the isolated port , a reference level is established for 20 dBq or 12 dBs with the antenna replaced by a 50 ohm load, and then the antenna is connected in place of the load. It is often [especially at VHF] necessary to increase the generator level by several dB to re-establish the same quieting level that was measured with the load in place. This number of dB is the degradation of your receiver due to site noise. Along with the receiver's basic [static] sensitivity measurement [made by connecting the generator directly rather than through the coupler] - you have sufficient information to state the receiver's effective sensitivity and also to compute the potential improvement that a pre-amp may be able to offer. -- --- In Repeater-Builder@yahoogroups.com, ldgelectronics [EMAIL PROTECTED] wrote: ... The sensitivity without the pre is about 0.35 uV. The noise floor is pretty low, but the ARR pre was picking up a bunch of garbage without the attenuator
[Repeater-Builder] Re: Preamp and attenuator
Never noticed. We're supposed to get some rain later, I'll check then. Sounds like you may have an antenna/loose hardware in near field of antenna problem, which could also generate passive IMD. Does it get better when it's raining? Bob NO6B
[Repeater-Builder] Re: Preamp and attenuator
Bob, How would we know if the preamp is nonlinear or not? If the ARR preamp is nonlinear with a pass can ahead of it, it's probably oscillating. I recommend replacing it with an Angle Linear preamp (http://www.anglelinear.com). Chip's preamps are guaranteed to be unconditionally stable, meaning it won't oscillate just because there's a pass can ahead of it. And without that 6 dB pad in front of it, you'll reduce your system noise figure by over 5 dB! Bob NO6B
Re: [Repeater-Builder] Re: Preamp and attenuator
At 3/22/2007 09:34 AM, you wrote: Bob, How would we know if the preamp is nonlinear or not? Only way to know for sure is to connect a spectrum analyzer to the output of the preamp look for anything greater than, say -15 dBm. The advertised 1 dB gain compression point (P1dB) for these preamps is +12 dBm, though other brands I've measured clocked in a bit lower than this (+7 dBm) so unless you measure it I'd assume the worst. If it's oscillating you'll see a very strong continuous signal, possibly near the 100 mW level so start the spectrum analyzer on a high reference level with some internal attenuation. Bob NO6B
[Repeater-Builder] Re: Preamp and attenuator
Bob, Great info. I'll check that too on the next trip. For right now, we are using a stock GE pre with about 10 db gain and no attenuators. There has been enough great feedback from the group that will give me several hours of testing on the next trip. There are all kinds of things to try. I was hoping for a quick - just do this - answer, but it looks like it doesn't work that way. With all this new info, we'll be ready to optimize the receive system at any of our sites. Thanks guys, Dwayne Kincaid At 3/22/2007 09:34 AM, you wrote: Bob, How would we know if the preamp is nonlinear or not? Only way to know for sure is to connect a spectrum analyzer to the output of the preamp look for anything greater than, say -15 dBm. The advertised 1 dB gain compression point (P1dB) for these preamps is +12 dBm, though other brands I've measured clocked in a bit lower than this (+7 dBm) so unless you measure it I'd assume the worst. If it's oscillating you'll see a very strong continuous signal, possibly near the 100 mW level so start the spectrum analyzer on a high reference level with some internal attenuation. Bob NO6B
[Repeater-Builder] Re: Preamp and attenuator
Guess I'm just lucky. I have used and use ARR preamps in all sorts of situations with cavities and without, with many random pieces of feedline and cabling, on bands from 29 to 470 mhz. and have never had an oscillation problem. They have been very stable in my experience. (Of course, I never hooked the output directly back into the input.) Al, K9SI Re: Preamp and attenuator Posted by: Bob Dengler [EMAIL PROTECTED] no6b Date: Thu Mar 22, 2007 11:04 am ((PDT)) At 3/22/2007 09:34 AM, you wrote: Bob, How would we know if the preamp is nonlinear or not? Only way to know for sure is to connect a spectrum analyzer to the output of the preamp look for anything greater than, say -15 dBm. The advertised 1 dB gain compression point (P1dB) for these preamps is +12 dBm, though other brands I've measured clocked in a bit lower than this (+7 dBm) so unless you measure it I'd assume the worst. If it's oscillating you'll see a very strong continuous signal, possibly near the 100 mW level so start the spectrum analyzer on a high reference level with some internal attenuation. Bob NO6B
RE: [Repeater-Builder] Re: Preamp and attenuator
I have a ARR On My 440 And 220 Repeaters, No Problems Of course they are both Motorola Micors thanks to this group . So that explains No Problems You can tell I am biased. Happy Repeater Building Don KA9QJG
[Repeater-Builder] Re: Preamp and attenuator
Had a good rain this evening. No change at all in the system. Dwayne WD8OYG Sounds like you may have an antenna/loose hardware in near field of antenna problem, which could also generate passive IMD. Does it get better when it's raining? Bob NO6B
[Repeater-Builder] Re: Preamp and attenuator
Had a good rain this evening. No change at all in the system. Dwayne WD8OYG Sounds like you may have an antenna/loose hardware in near field of antenna problem, which could also generate passive IMD. Does it get better when it's raining? Bob NO6B
[Repeater-Builder] Re: Preamp and attenuator
Steve, It's on 2M, single Station Master antenna with about 400 feet of 7/8 feedline. The duplexer is a BP/BR followed by a band pass can, then the pre, then the radio side. All cables are RG-400. The radio is GE Exec II. The sensitivity without the pre is about 0.35 uV. The noise floor is pretty low, but the ARR pre was picking up a bunch of garbage without the attenuator. Dwayne Kincaid WD8OYG I have added a preamp (ARR GasFet) in front of older GE Pro receivers on UHF and really made an improvement. I am guessing you are using a single antenna and a duplexer and the preamp is between the duplexer and receiver? Are you using a band pass / band reject duplexer? Some notch duplexers on 440 let signals either side of the notch pass through freely. You might change the coupling on the loops in the receive side of the duplexer to increase loss (attenuation) and get steeper skirts. Tell me a little about the system, antenna, feedline, duplexer, jumpers, the repeater itself? Thanks, Steve NU5D On 3/21/07, ldgelectronics [EMAIL PROTECTED] wrote: Hi all, We've added a preamp to our local repeater and found that it had too much gain. It started picking up lots of garbage. Reducing the gain by about 6db seems to put it in a good operating place. Ham Radio Spoken Here.NU5D Visit the Temple Ham Club Website http://www.tarc.org www.yahoogroups.com/group/Temple_arc www.yahoogroups.com/group/60meter
[Repeater-Builder] Re: Preamp and attenuator
Ken, thanks for the input. So if I were to adjust the band pass can to have 3 db loss (it's at 0.5 db right now) and steeper skirts, it would raise the noise figure about 2.5 db as well. That seemed like a good trick at first, but still raises the noise figure. Obviously lower noise figure is better, but is there some place where the trade off would be worth it? Maybe I'm not asking the question properly. The question is how much trouble will be getting into by putting the 6db attenuator on the input side of the preamp? Would it still be better to put it after the preamp even though it would add two adapters? ---You'd be much better placing the attentuator in the OUTPUT of the preamp (between it and the receiver). Remember that every dB of loss you place ahead of the preamp adds 1 dB of noise figure to the receiver. So you'd be adding 6 dB to your overall receiving system if you place the attentuator between the preamp and the duplexer Ken -- President and CTO - Arcom Communications Makers of the world famous RC210 Repeater Controller and accessories. http://www.arcomcontrollers.com/ Coming soon - the most advanced repeater controller EVER. Authorized Dealers for Kenwood and Telewave and we offer complete repeater packages! AH6LE/R - IRLP Node 3000 http://www.irlp.net
Re: [Repeater-Builder] Re: Preamp and attenuator
At 06:31 PM 3/21/2007, you wrote: Ken, thanks for the input. So if I were to adjust the band pass can to have 3 db loss (it's at 0.5 db right now) and steeper skirts, it would raise the noise figure about 2.5 db as well. Yup. *Any* loss, be it in the duplexer, feedline to the antenna, etc. that is ahead of the preamp adds that loss as noise to the system. That seemed like a good trick at first, but still raises the noise figure. Obviously lower noise figure is better, but is there some place where the trade off would be worth it? Maybe I'm not asking the question properly. --What make and model preamp are you running? Or at least, what is it spec'd for gain-wise? Ken -- President and CTO - Arcom Communications Makers of the world famous RC210 Repeater Controller and accessories. http://www.arcomcontrollers.com/ Coming soon - the most advanced repeater controller EVER. Authorized Dealers for Kenwood and Telewave and we offer complete repeater packages! AH6LE/R - IRLP Node 3000 http://www.irlp.net
Re: [Repeater-Builder] Re: Preamp and attenuator
Sounds like a nice setup Dwayne. I would make sure the duplexer is tuned properly, maybe run an isolated TEE test just to be sure everything is OK. Next I would rotate the coupling loops to increase insertion loss and make the skirts steeper. What Wacom taught me to do was to take a cavity, hook up a sweeper with a TEE fitting between the Gen and Rec and set a reference signal, then hook the TEE to one side of the cavity and rotate the loop for about 8 db of notch at the pass freq, then move the TEE to the other side of the cavity, and again rotate the second loop for the same degree of notch - this makes the cavity symetrical so that both the input and output loops have the same degree of coupling. Next measure the loss through the cavity. Play with the degree of coupling and insertion loss till you get what you are looking for. Best 73, Steve NU5D RG-400 or LMR 400? sb On 3/21/07, ldgelectronics [EMAIL PROTECTED] wrote: Steve, It's on 2M, single Station Master antenna with about 400 feet of 7/8 feedline. The duplexer is a BP/BR followed by a band pass can, then the pre, then the radio side. All cables are RG-400. The radio is GE Exec II. The sensitivity without the pre is about 0.35 uV. The noise floor is pretty low, but the ARR pre was picking up a bunch of garbage without the attenuator. Dwayne Kincaid WD8OYG -- Ham Radio Spoken Here.NU5D Visit the Temple Ham Club Website http://www.tarc.org www.yahoogroups.com/group/Temple_arc www.yahoogroups.com/group/60meter
Re: [Repeater-Builder] Re: Preamp and attenuator
I did mean coupling loops on the single band pass cavity and not the duplexer. SB On 3/21/07, Steve Bosshard (NU5D) [EMAIL PROTECTED] wrote: Sounds like a nice setup Dwayne. I would make sure the duplexer is tuned properly, maybe run an isolated TEE test just to be sure everything is OK. N -- Ham Radio Spoken Here.NU5D Visit the Temple Ham Club Website http://www.tarc.org www.yahoogroups.com/group/Temple_arc www.yahoogroups.com/group/60meter
[Repeater-Builder] Re: Preamp and attenuator
--- In Repeater-Builder@yahoogroups.com, ldgelectronics [EMAIL PROTECTED] wrote: Hi all, We've added a preamp to our local repeater and found that it had too much gain. It started picking up lots of garbage. Elaborate on the term garbage. Intermod? Users from distant repeaters? If intermod, it may be produced in the preamp or receiver if not protected enough with selectivity ahead of the preamp. If users, you've achieved your goal of increasing the sensitivity of your system. Adding attenuation ahead of the preamp very effectively removes much of the sensitivity you've gained with the preamp. I'd deal with that kind of garbage in other ways. Reducing the gain by about 6db seems to put it in a good operating place. We also found that by putting the 6db attenuator on the input of the preamp (right after the band pass can), it's real easy to add in the system. If we mount it after the preamp, we'll need an adapter on each side of the preamp to make it all connect up. Obviously it would be best if we could put the attenuator after the pre and have no adapters, but it's not convenient. Attenuation placed ahead of the preamp is nowhere near the same as placing it behind. Placing it behind reduces the gain of the preamp while retaining the noise figure of the preamp. In my experiments on 2m with an ARR preamp and Motrac receiver, up to about 10db of attenuation after the preamp did not reduce system sensitivity. The question is how much trouble will be getting into by putting the 6db attenuator on the input side of the preamp? Would it still be better to put it after the preamp even though it would add two adapters? In most cases yes. I've read about cases on this group where attenuation ahead of the preamp is the proper thing to do, but the reasons escape me. Laryn K8TVZ Thanks, Dwayne Kincaid WD8OYG
[Repeater-Builder] Re: Preamp and attenuator
Darn, I meant to put that in before. I have severaldifferent units. I'd like to use an ARR with 15 db gain (1 db NF), but we also have a couple of older Lunar with about 15 db gain and some stock GE units with about 10 db. The Angle is on the list to try. One of the systems we just took out the ARR and put the GE back in at the lower gain (higher NF) until we figure out this attenuator thing. Dwayne Kincaid WD8OYG --What make and model preamp are you running? Or at least, what is it spec'd for gain-wise?
Re: [Repeater-Builder] Re: Preamp and attenuator
At 08:21 PM 3/21/2007, you wrote: Darn, I meant to put that in before. I have severaldifferent units. I'd like to use an ARR with 15 db gain (1 db NF), but we also have a couple of older Lunar with about 15 db gain and some stock GE units with about 10 db. The Angle is on the list to try. --I heartily recommend AngleLinear products. You simply can't go wrong! Ken -- President and CTO - Arcom Communications Makers of the world famous RC210 Repeater Controller and accessories. http://www.arcomcontrollers.com/ Coming soon - the most advanced repeater controller EVER. Authorized Dealers for Kenwood and Telewave and we offer complete repeater packages! AH6LE/R - IRLP Node 3000 http://www.irlp.net
[Repeater-Builder] Re: Preamp and attenuator
Steve, That's good info on tuning the BP can. I will put that info in the file. I'm pretty sure the duplexer is tuned properly. The four can BP/BR is a Sinclair 202. It's got right at 1.5 db loss on each side with about 83 db of rejection on each. There area also two Sinclair BP/BR cans. One on the TX and one on the RX. The loss is set for 0.5 db on each. The interconnect cable is RG-400, no LMR. Dwayne Kincaid WD8OYG Sounds like a nice setup Dwayne. I would make sure the duplexer is tuned properly, maybe run an isolated TEE test just to be sure everything is OK. Next I would rotate the coupling loops to increase insertion loss and make the skirts steeper. What Wacom taught me to do was to take a cavity, hook up a sweeper with a TEE fitting between the Gen and Rec and set a reference signal, then hook the TEE to one side of the cavity and rotate the loop for about 8 db of notch at the pass freq, then move the TEE to the other side of the cavity, and again rotate the second loop for the same degree of notch - this makes the cavity symetrical so that both the input and output loops have the same degree of coupling. Next measure the loss through the cavity. Play with the degree of coupling and insertion loss till you get what you are looking for. Best 73, Steve NU5D RG-400 or LMR 400? sb On 3/21/07, ldgelectronics [EMAIL PROTECTED] wrote: Steve, It's on 2M, single Station Master antenna with about 400 feet of 7/8 feedline. The duplexer is a BP/BR followed by a band pass can, then the pre, then the radio side. All cables are RG-400. The radio is GE Exec II. The sensitivity without the pre is about 0.35 uV. The noise floor is pretty low, but the ARR pre was picking up a bunch of garbage without the attenuator. Dwayne Kincaid WD8OYG -- Ham Radio Spoken Here.NU5D Visit the Temple Ham Club Website http://www.tarc.org www.yahoogroups.com/group/Temple_arc www.yahoogroups.com/group/60meter
[Repeater-Builder] Re: Preamp and attenuator
The garbage was pops, static, crackling on weak user signals. There was also some intermod that would drop the receiver on all but the strongest users. The distant users were able to go farther, but the garbage noise made it more difficult to actually use. Dwayne Kincaid WD8OYG Elaborate on the term garbage. Intermod? Users from distant repeaters?
RE: [Repeater-Builder] Re: Preamp and attenuator
Hook your signal generator up to your system at the antenna port and measure receiver sensitivity with and without the preamp. Then with the preamp in circuit start adding attenuation between the preamp and the receiver. When you just start to loose sensitivity stop adding attenuation. That should give you near optimum sensitivity without excessive gain. Too much gain in the preamp overloads the receiver mixer and front end amp if it has one. For every db of gain you add in front of the receiver you reduce the IM performance of the receiver. You only want enough preamp gain to overcome the noise figure of the receiver. Although the noise figure of the receiver and preamp are cumulative the preamp is the biggest contributor in setting system noise figure. In other words putting a hot preamp on a very hot receiver will give you a better overall noise figure than putting that same preamp on a poor receiver but the difference will not be great. You may not be able to realize the full benefit of the preamp if you have excessive IM. You may have to add more attenuation to where it further reduces receiver sensitivity. When you get down to the point that the sensitivity is the same as it was without the preamp, then throw out the preamp. But you may be able to find a happy medium where the preamp does help some without destroying your IM performance. If you still have excess IM problems you can add attenuation ahead of the preamp by raising the insertion loss of the loops on your band pass filter as others have suggested. By raising the insertion loss on the loops it does the same thing as adding an attenuator ahead of the preamp but with the added benefit of steeper skirts on the band pass filter. By the way don't worry about adding adaptors between the preamp and receiver. After all you are looking to add attenuators anyway. But adaptors really make no measurable difference in attenuation at vhf or uhf. They may give a slight impedance mismatch but you probably don't have anything that will measure the small amount of loss from them. 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of ldgelectronics Sent: Wednesday, March 21, 2007 9:32 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Preamp and attenuator Ken, thanks for the input. So if I were to adjust the band pass can to have 3 db loss (it's at 0.5 db right now) and steeper skirts, it would raise the noise figure about 2.5 db as well. That seemed like a good trick at first, but still raises the noise figure. Obviously lower noise figure is better, but is there some place where the trade off would be worth it? Maybe I'm not asking the question properly. The question is how much trouble will be getting into by putting the 6db attenuator on the input side of the preamp? Would it still be better to put it after the preamp even though it would add two adapters? ---You'd be much better placing the attentuator in the OUTPUT of the preamp (between it and the receiver). Remember that every dB of loss you place ahead of the preamp adds 1 dB of noise figure to the receiver. So you'd be adding 6 dB to your overall receiving system if you place the attentuator between the preamp and the duplexer Ken -- President and CTO - Arcom Communications Makers of the world famous RC210 Repeater Controller and accessories. http://www.arcomcontrollers.com/ Coming soon - the most advanced repeater controller EVER. Authorized Dealers for Kenwood and Telewave and we offer complete repeater packages! AH6LE/R - IRLP Node 3000 http://www.irlp.net Yahoo! Groups Links
[Repeater-Builder] Re: Preamp and attenuator
We've added a preamp to our local repeater and found that it had too much gain. It started picking up lots of garbage. I would ask if there is such a thing as too much gain but there can be. Normally I would would not expect gain to be a problem in a well designed - constructed system. But I was not able to get one of Angle's 40 dB gain jobs to work without a whip and a chair. So yes I guess there can be cases of too much preamp gain. But probably not with a typical good repeater receiver preamplifier in many/most common repeater applications. You have a fly in your soup... is it gain fly or something else? Elaborate on the term garbage. Intermod? Users from distant repeaters? If intermod, it may be produced in the preamp or receiver if not protected enough with selectivity ahead of the preamp. A big time Amen... people forget the short distance a preamp travels to become a mixer. We really should know more about his receiver protection as requested below. Adding attenuation ahead of the preamp very effectively removes much of the sensitivity you've gained with the preamp. I'd deal with that kind of garbage in other ways. A much better idea... why point a finger at the preamp when the problem might actually be part of a bigger system picture. Reducing the gain by about 6db seems to put it in a good operating place. We also found that by putting the 6db attenuator on the input of the preamp (right after the band pass can), it's real easy to add in the system. If we mount it after the preamp, we'll need an adapter on each side of the preamp to make it all connect up. Obviously it would be best if we could put the attenuator after the pre and have no adapters, but it's not convenient. Attenuation placed ahead of the preamp is nowhere near the same as placing it behind. Placing it behind reduces the gain of the preamp while retaining the noise figure of the preamp. In my experiments on 2m with an ARR preamp and Motrac receiver, up to about 10db of attenuation after the preamp did not reduce system sensitivity. Some of us are wondering why you need an attenuator in any position? It's not an automatic requirement... The question is how much trouble will be getting into by putting the 6db attenuator on the input side of the preamp? Would it still be better to put it after the preamp even though it would add two adapters? Forget the attenuators and the adapters and give us more information about the system. What type of preamp, bandpass cavity, duplexer and receiver are you using? Gasfet?, Phempt?, Bipolar? Nuvistor? (I said Nuvistor for some of you older radio guys) In most cases yes. I've read about cases on this group where attenuation ahead of the preamp is the proper thing to do, but the reasons escape me. Depends on the equipment but the first answer I would say is for reasons of stability. Some of us actually use more than one series preamp with specific value attenuators in specialized antenna combiner systems... very hot high performance receiver antenna systems. cheers, s.
