Hey guys, I have a custom notch filter currently connected between my coax and my scanner, at the scanner end. I have an FM notch filter and a preamp from Stridsberg on the way as well. I know the preamp's work best when installed at the antenna end. I can do this, but I'm wondering if it would be better to put the filters before the preamp, otherwise the amp with be amplifying the signals that the filters need to deal with. How would you experts install two filters and a preamp in your line? Can I put the two filters in line between the antenna and the scanner, then the preamp at the scanner end? Can I connect the two filters directly to each other in series? If so, I can put them in my feed line halfway between the amp and scanner, then put the preamp at the scanner end and see how it works. I have a waterproof box as well that I can mount at the antenna end and put all three components in it. If I do that, can all three components be connected to each other in series? Any advice would be great. Thanks!
Several Filters in Series?
- Thread starter emsflyer84
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prcguy
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You always want to put any filters before the preamp or multicoupler to protect them from overload. Some FM trap filters are not very good while a few are great and they all have insertion loss in frequency ranges you don't want to have attenuation. Its best to sweep them and only use one that has adequate attenuation of the things you want to get rid of without affecting what you want to keep, otherwise insertion loss will add up and affect things you want to hear.
Also, putting the filter after the amplifier could make the unwanted signal larger than the filter will stop.
Make sure you test for the reduction or elimination of the overload or desense issues you are fighting using just the new FM trap before you put the preamp inline!
Even though you are getting an FM trap to deal with your reception issues in your other thread, the use of a preamp could very well make your FM issues even worse than they are now even when using the FM trap.
Which Stridsberg preamp did you buy? If it's one of the models they sell for say just the 7/800 MHz bands, those do a pretty good job at not amplifying other bands but if this preamp is designed for the VHF band that I think you are interested in, it may make things much worse than they are now by re-amplifying the same FM radio signals the filter is trying to eliminate. The preamps noise will also be added into the signal.
Even though you are getting an FM trap to deal with your reception issues in your other thread, the use of a preamp could very well make your FM issues even worse than they are now even when using the FM trap.
Which Stridsberg preamp did you buy? If it's one of the models they sell for say just the 7/800 MHz bands, those do a pretty good job at not amplifying other bands but if this preamp is designed for the VHF band that I think you are interested in, it may make things much worse than they are now by re-amplifying the same FM radio signals the filter is trying to eliminate. The preamps noise will also be added into the signal.
The preamp is a VHF/UHF version, so I may not even need it or be able to use it when the filters are in. I’ll try everything with just the filters first. Thanks.
Ubbe
Member
If you look at the Stridsberger $170 preamplifier specs you'll see that they are not that good.
Gain: 18 dB (+/- 2 dB in Passband)
Frequency Range: 40 MHz to 1 GHz (nominal)
Noise Figure: 3.5 dB (nominal)
P1dB: +12 dBm (output) OIP3: +24.5 dBm
If you look at Minicircuits $90 ZX60-P103LN+ preamp:
Gain 23.47
Frequency 50 - 3000 MHz
Noise Figure 0.79
Output IP-3 40.67
1dB Comp. Output 22.70
In most cases a P103+ based amplifier can handle a high level of FM broadcast and other signals without a filter.
So you can probably put any filters after the preamp and not worry about any losses in the filters that would increase the NF noise levels.
/Ubbe
Gain: 18 dB (+/- 2 dB in Passband)
Frequency Range: 40 MHz to 1 GHz (nominal)
Noise Figure: 3.5 dB (nominal)
P1dB: +12 dBm (output) OIP3: +24.5 dBm
If you look at Minicircuits $90 ZX60-P103LN+ preamp:
Gain 23.47
Frequency 50 - 3000 MHz
Noise Figure 0.79
Output IP-3 40.67
1dB Comp. Output 22.70
In most cases a P103+ based amplifier can handle a high level of FM broadcast and other signals without a filter.
So you can probably put any filters after the preamp and not worry about any losses in the filters that would increase the NF noise levels.
/Ubbe
prcguy
Member
Not only are the specs lousy even for a 1970s vintage device, its probably the same preamp that's inside their multicouplers. That wouldn't last one minute of monitoring here without getting trashed by all the local high power transmitters.
Ok, so who wants to buy the Stridsberg?
. So, another question. Can my Stridsberg bias-t power the minicircuits unit or another preamp?
Leave the bias-t and preamp both out of the circuit completely while you test the FM Trap along with the pager filter!
You may very well find that stations that were weak before you had the FM trap now come in just fine and you don't need a preamp. Honestly, if the FM station is causing your issues, the last thing you should be adding is a preamp! Even a quality preamp more like what Ubbe mentioned from Mini-Circuits or even higher quality can still cause problems because scanners are not known to have the best front ends in them.
Preamps often cause way more problems than they are worth for many users that live in urban areas with lots of RF around. It's not a problem you can see with your eyes so many users don't realize a preamp may be making things much worse.
Now if you live out in the sticks, by all means, give preamps a try and experiment!
In your case, I'd still play with the preamp you just ordered but only after you see what just having the pager and FM filters inline do. For playing, you can just put the preamp at the scanner for now so you would not even need the bias-t. That would be good enough to test with and see if it seems to help or not.
I'd probably not plan on keeping it though after seeing its specs that Ubbe posted. I had no idea Stridsberg's wideband preamp had such poor specs. If you have it coming though, you may as well play with it and see what it does!
Unless Stridsberg changed their design, their preamps are not weather sealed devices. You will need to get creative in mounting it in an watertight enclosure if it does help your signal and you mount it at the antenna and use the bias-t to pump power up the coax.
As to your question, I can't think of a Minicircuits preamp that would benefit scanner users that can be powered over the coax. I can't say I really looked for that feature though so them may have something. As to other filters, todays world of cheap SDR based tv tuner sticks have a bunch of accessory preamps made for them that can be powered by your bias-t. With an SDR receiver, those preamps would usually be powered by the same 5 volts from the USB bus that powers the SDR stick but there is nothing that says you need an SDR stick to use them. You would just send 5 volts or whatever they are designed for up the coax instead of whatever the stridsberg preamp needed. Adapters to get from SMA connectors which are common on SDR devices would also be needed to connect with the preamps made for the SDR world and your existing coax fittings. Like the stridsberg preamp though, many of the preamps and filters made for SDR users world are not in any case or housing at all so you need to be creative again and waterseal them in the least!
I think they sell preamps made for SDR users that use the same P103 RF amp chip that the Minicircuits preamp uses that @Ubbe mentioned. That could save money as long as you have a way to weatherproof the thing. It would basically have the same specs as Ubbe listed for the Minicircuits preamp as long as its based on the P103 RF amp.
You may very well find that stations that were weak before you had the FM trap now come in just fine and you don't need a preamp. Honestly, if the FM station is causing your issues, the last thing you should be adding is a preamp! Even a quality preamp more like what Ubbe mentioned from Mini-Circuits or even higher quality can still cause problems because scanners are not known to have the best front ends in them.
Preamps often cause way more problems than they are worth for many users that live in urban areas with lots of RF around. It's not a problem you can see with your eyes so many users don't realize a preamp may be making things much worse.
Now if you live out in the sticks, by all means, give preamps a try and experiment!
In your case, I'd still play with the preamp you just ordered but only after you see what just having the pager and FM filters inline do. For playing, you can just put the preamp at the scanner for now so you would not even need the bias-t. That would be good enough to test with and see if it seems to help or not.
I'd probably not plan on keeping it though after seeing its specs that Ubbe posted. I had no idea Stridsberg's wideband preamp had such poor specs. If you have it coming though, you may as well play with it and see what it does!
Unless Stridsberg changed their design, their preamps are not weather sealed devices. You will need to get creative in mounting it in an watertight enclosure if it does help your signal and you mount it at the antenna and use the bias-t to pump power up the coax.
As to your question, I can't think of a Minicircuits preamp that would benefit scanner users that can be powered over the coax. I can't say I really looked for that feature though so them may have something. As to other filters, todays world of cheap SDR based tv tuner sticks have a bunch of accessory preamps made for them that can be powered by your bias-t. With an SDR receiver, those preamps would usually be powered by the same 5 volts from the USB bus that powers the SDR stick but there is nothing that says you need an SDR stick to use them. You would just send 5 volts or whatever they are designed for up the coax instead of whatever the stridsberg preamp needed. Adapters to get from SMA connectors which are common on SDR devices would also be needed to connect with the preamps made for the SDR world and your existing coax fittings. Like the stridsberg preamp though, many of the preamps and filters made for SDR users world are not in any case or housing at all so you need to be creative again and waterseal them in the least!
I think they sell preamps made for SDR users that use the same P103 RF amp chip that the Minicircuits preamp uses that @Ubbe mentioned. That could save money as long as you have a way to weatherproof the thing. It would basically have the same specs as Ubbe listed for the Minicircuits preamp as long as its based on the P103 RF amp.
Ubbe
Member
You can use any type of standard $5 watertight junction box.
Can't you send that Stridsberg back for a refound?
Most preamps can be powered from the coax. It's just an added $0.25 choke coil on the preamp that goes from the coax to the 5 volt on the circuit board. The bias-T power are also just a choke coil and a coupling capacitor and a couple of decoupling capacitors as standard practice that often isn't needed. There are even $5 ones that are used for powering TV antennas that can be used if they supply 5volt.
/Ubbe
Can't you send that Stridsberg back for a refound?
Most preamps can be powered from the coax. It's just an added $0.25 choke coil on the preamp that goes from the coax to the 5 volt on the circuit board. The bias-T power are also just a choke coil and a coupling capacitor and a couple of decoupling capacitors as standard practice that often isn't needed. There are even $5 ones that are used for powering TV antennas that can be used if they supply 5volt.
/Ubbe
Thanks for all the help. I’ll try the FM and pager filter first with no preamp. If I want to try the preamp after, I’ll just mess with it at the scanner end a see what happens.
I actually have a waterproof box I was going to put everything in at the antenna end, but if I don’t end up using the preamp, I’ll scrap that idea and keep the filters inside in the feed line.
Thanks again.
I actually have a waterproof box I was going to put everything in at the antenna end, but if I don’t end up using the preamp, I’ll scrap that idea and keep the filters inside in the feed line.
Thanks again.
Ubbe
Member
That's usually what I'm doing. Remember to attenuate enough of the signal so it doesn't overload the scanner. No scanner can take the full gain of a preamp without running into severe problems.
/Ubbe
Yea, I figured I’d start on the lowest setting and just work up from there.
Ubbe
Member
Just try and monitor an analog signal thats weak enough to produce some background noise, maybe one of the NOAA channels, and then adjust gain for lowest possible noise. If the gain starts to be too much the noise level will start to increase again.
/Ubbe
/Ubbe
I've found preamps to be useful if you can put them before a very LONG coax run if you have one, if you are splitting the signal with a multicoupler, or trying to receive a VERY weak signal, otherwise they can certainly cause more problems than they can fix.
Ubbe
Member
A good preamp with low noise like NF1dB will improve a scanners bad NF4-6dB if the scanner doesn't overload and run into other problems.
I've tested different amplifiers and even the cheap $10 sat dish booster ones that you put inline in the coax and have some NF of 3-4dB and installed directly at the scanner and a variable attenuator has improved reception in all scanners I've tested. But if you have a transmit site close by it might not work using a preamp as the scanner perhaps needs to be used with an attenuated signal already as it is.
/Ubbe
I've tested different amplifiers and even the cheap $10 sat dish booster ones that you put inline in the coax and have some NF of 3-4dB and installed directly at the scanner and a variable attenuator has improved reception in all scanners I've tested. But if you have a transmit site close by it might not work using a preamp as the scanner perhaps needs to be used with an attenuated signal already as it is.
/Ubbe
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