Amplifier question - Is it actually helping?

[andy404ns]

I'm looking for some help in understanding what an amplifier is doing. Here is my situation:

Years ago when I did first moved into my house I had an antenna mounted in the attic. From there I had RG-6 running down the exterior of the house, in to the basement, and then I connected it into the existing cable lines so I could plug my Electroline splitter into the coax jack in my upstairs office. Not ideal for many reasons. This was probably 250-300 feet of RG-6 with numerous connectors.

Well, in the basement, I ran the RG-6 into an amplifier that I think the Comcast guys had left at the house. It's a Channel Master 3044 (I'll try to attach a picture of it). I have no real knowledge of amplifiers but it was there and it was F-connectors so yeah... I used it. And I got decent reception.

The good news is I've reworked everything as of a week ago. The attic antenna now drops through the wall for a grand total of 25 feet and it's now LMR-400 (a bit overkill I guess). But here's the not fun thing. When I did all that, the reception was pretty much the same, maybe even slightly worse on some channels. I figured I would have seen a big improvement. I got to thinking and wondered if the amplifier really did that much. So to test I added it back into the system. It's now sitting right off the antenna prior to the 25 foot LMR run. So I now have antenna --> RG-6 --> amplifier --> LMR (with adapters along the way). And my reception is much better. And that's across the spectrum.

Here's what I don't get. I've searched and read through dozens of threads here and the consensus seems to be that amplifiers are the last thing you want to add as you're trying things to improve reception and that a lot of times they can make things worse. I've always simply thought of it as "it will amplify everything - including the noise". That's my super scientific understanding. So I'm really confused as to why my experience is so opposite that, especially since this particular amplifier isn't designed for this use. And that begs the question - if an amplifier really is benefiting me, is there a better one I can use?

I started another thread outlining my antenna upgrade experience. I mention it here because I confirmed that reception is greatly improved with both antennas when that amplifier is attached. So it's not related to the antenna in case that's a concern. You can find that thread over here:

New DPD OmniX vs old RS 20-176 -- Findings

I have been using a Radio Shack 20-176 for about 15 years now and wanted to try an upgrade. I purchased a DPD OmniX based in large part on reviews here and also to support a small business with quality customer service. I received the OmniX a few days ago and wanted to share my findings. First...

forums.radioreference.com

Anyway, I'm just looking for some guidance on why this amplifier seems to be helping so much and if there's anything else I can / should do here. All the reading I've done says I shouldn't use it but it sure does seem to make things better.

Thanks so much!

 

[iMONITOR]

To begin with 250-300 feet of RG-6 with numerous connectors would have a fair amount of loss.

An amplifier can help or they can often overload your radios depending on a lot of factors.

25 feet of LMR 400 would have minimal, hardly noticeable loss. Were the connectors professionally installed?

Have you tried just the amplifier and the LMR 400?

What radio and what bands/frequencies are you monitoring?

 

[dlwtrunked]

The answer is simple. A good amplifier in some situations does help. The gain equations show that if has a lower noise figure than your receiver and enough gain, it will help if it does not overload the receiver.

 

[paulears]

The thing with these amplifiers is that they are designed to combat cable loss and give multiple outputs. They are not designed to pull out weak signals and amplify them. Traditional theory is that if you are in a weak signal area, then what you do is put a low noise preamp as close to the antenna as possible, then feeder length doesn’t matter too much. These things boost, split, then divide. So if signal levels are decent, the splitting process works. That is what it is designed for. It’s not a product for doing low noise, wesk station boosting. Basically, if the signal is not there at the antenna, they wont amplify it. Different job.

 

[iMONITOR]

The thing with these amplifiers is that they are designed to combat cable loss and give multiple outputs. They are not designed to pull out weak signals and amplify them. Traditional theory is that if you are in a weak signal area, then what you do is put a low noise preamp as close to the antenna as possible, then feeder length doesn’t matter too much. These things boost, split, then divide. So if signal levels are decent, the splitting process works. That is what it is designed for. It’s not a product for doing low noise, wesk station boosting. Basically, if the signal is not there at the antenna, they wont amplify it. Different job.

I'm not in total agreement with that. I have two BCD996P2 scanners feed by the following: dual-band VHF/UHF aircraft antenna mounted on top of a steel file cabinet in my home office. The antenna coax goes into a FM Notch filter, which in turn goes into a 18 dB preamp, which goes into a 2-port passive splitter. This setup does not overwhelm the scanners. The performance in reception is impressive. I am monitoring much more air traffic than ever before. If I cut the 12VDC power to the preamp, which allows internal bypass relays to drop out and create a minimal loss and currently active signals often drop out. They don't return until I power the preamp again. Everyone's results will vary for many reasons but I'm very happy with the minimal investment and the substantial gain. Increase noise does not to be a problem.

 

[Ubbe]

When I first started using amplifiers it was the inline sat dish type for $5 and $10 and I feed them using a voltage variable power supply. I quickly noticed that reception improved when I lowered the voltage from the supposed 12-18V down to 6V. I didn't really grasp that a receiver could get too much signal and by lowering the gain by reducing the voltage produced a sweet spot where the signal to noise ratio where at an optimum for my scanner receivers.

Later I begun using attenuators to let the amplifiers run at their optimum operating voltage where they had their least noise added. The amount of attenuation needed varies with the RF environment and less attenuation can be used if there are less interfering signals in the air and if the receiver are of a good quality.

I've tested all kind of amplifiers from that $5 one to $300 ones and they all improved reception even in RF hell situations as long as the correct amount of attenuation where used. Even when using amplifiers at the receiver end of a coax will help to improve reception as it's internal noise figure are often less than a scanners.

The biggest improvement will be had if the amplifier are at the antenna, working as an impedance buffer between antenna and coax, and then use a CATV splitter at the receiver that will also work as an impedance buffer to the coax and also as a 20dB isolator between splitter outputs. Having an amplifier at the antenna will also compensate for any coax attenuation. It can also help using filters to get rid of strong signals from broadcast radio and TV or any other strong transmitter that are too close to you.

Many amplifiers have a higher gain at lower frequencies and less at higher frequencies and a coax needs the opposite and often receivers can handle higher signal levels better the higher the frequency. Then inside a splitter a 2pF-4,7pF capacitor can be inserted in the signal path to attenuate lower frequencies more. If you are monitoring VHF-LOW the signal can be attenuated too much and the capacitor must then be bridged by a resistor, perhaps 470-680 Ohm, to limit the attenuation to a suitable level.

/Ubbe

 

[paulears]

I also wonder if it's the maximum signal present at the amplifier input that matters - if you want reception at say marine band in the UK, there are amazingly strong pagers - 100W+ on 155-156, so if you start to try to add gain at 156-157, then that 155 pager is going to really overload the amplifier and distortion products are bound to impact on the band you want. I suppose the best scenario is to check what is local and strong, severely filter them out, and then have a clean band to apply amplification to. These sorts of approaches help and hinder a scanner user, who might want to hear other things - so high or low pass filters are no good, you need notch filters - and probably even very narrow ones. I also wonder if unfiltered, these string local signals also degrade performance enough that things appear weaker than they are when they overload the front end, just a little away from where you wish to listen?

 

[andy404ns]

To begin with 250-300 feet of RG-6 with numerous connectors would have a fair amount of loss.

An amplifier can help or they can often overload your radios depending on a lot of factors.

25 feet of LMR 400 would have minimal, hardly noticeable loss. Were the connectors professionally installed?

Have you tried just the amplifier and the LMR 400?

What radio and what bands/frequencies are you monitoring?

This was my understanding as well. It's exactly why I tossed the amplifier into the corner of the basement when I went to the 25 feet of LMR-400 run. Because why would I need it with such a short run of low loss cable? So that's why I was so surprised when a) my reception did not improve, and b) it improved a lot when I put the amplifier back in!

Have you tried just the amplifier and the LMR 400?

I assume you mean with the antenna right? i.e. straight from the LMR into the scanner (vs going through the multicoupler?). If so then yes that was one of my tests and the results were the same. Much better with the amp than without it.

I am primarily monitoring VHF high (150 - 160) with a bit of 450 - 490 as well. Very limited 800.

 

[andy404ns]

When I first started using amplifiers it was the inline sat dish type for $5 and $10 and I feed them using a voltage variable power supply. I quickly noticed that reception improved when I lowered the voltage from the supposed 12-18V down to 6V. I didn't really grasp that a receiver could get too much signal and by lowering the gain by reducing the voltage produced a sweet spot where the signal to noise ratio where at an optimum for my scanner receivers.

Later I begun using attenuators to let the amplifiers run at their optimum operating voltage where they had their least noise added. The amount of attenuation needed varies with the RF environment and less attenuation can be used if there are less interfering signals in the air and if the receiver are of a good quality.

I've tested all kind of amplifiers from that $5 one to $300 ones and they all improved reception even in RF hell situations as long as the correct amount of attenuation where used. Even when using amplifiers at the receiver end of a coax will help to improve reception as it's internal noise figure are often less than a scanners.

The biggest improvement will be had if the amplifier are at the antenna, working as an impedance buffer between antenna and coax, and then use a CATV splitter at the receiver that will also work as an impedance buffer to the coax and also as a 20dB isolator between splitter outputs. Having an amplifier at the antenna will also compensate for any coax attenuation. It can also help using filters to get rid of strong signals from broadcast radio and TV or any other strong transmitter that are too close to you.

Many amplifiers have a higher gain at lower frequencies and less at higher frequencies and a coax needs the opposite and often receivers can handle higher signal levels better the higher the frequency. Then inside a splitter a 2pF-4,7pF capacitor can be inserted in the signal path to attenuate lower frequencies more. If you are monitoring VHF-LOW the signal can be attenuated too much and the capacitor must then be bridged by a resistor, perhaps 470-680 Ohm, to limit the attenuation to a suitable level.

/Ubbe

I 100% knew that I would get a well thought out and intelligent response from @Ubbe when I made this thread and I also knew that I would understand maybe 37% of it

When you ran the attenuators... were you placing them before or after the amp? Or were you using the scanners internal attenuator by channel as needed?

I'm in a fairly rural area so there's not a lot of interference around me EXCEPT for one tall tower that is 1.3 miles from my house. Among other things, it serves as a repeater for my local FD and PD which are right in the 154 - 156 band that I monitor the most heavily. So I don't want to attenuate _all_ of the traffic coming in there. I probably should attenuate the PD and FD at the scanner but I don't know if that will help with a potential overload problem.

 

[andy404ns]

The answer is simple. A good amplifier in some situations does help. The gain equations show that if has a lower noise figure than your receiver and enough gain, it will help if it does not overload the receiver.

Yeah I mean in the end the only question is: does your enjoyment of using the scanner increase or decrease with whatever you're adding to the setup. If it increases, what's the reason to not do it?

 

[andy404ns]

I'm not in total agreement with that. I have two BCD996P2 scanners feed by the following: dual-band VHF/UHF aircraft antenna mounted on top of a steel file cabinet in my home office. The antenna coax goes into a FM Notch filter, which in turn goes into a 18 dB preamp, which goes into a 2-port passive splitter. This setup does not overwhelm the scanners. The performance in reception is impressive. I am monitoring much more air traffic than ever before. If I cut the 12VDC power to the preamp, which allows internal bypass relays to drop out and create a minimal loss and currently active signals often drop out. They don't return until I power the preamp again. Everyone's results will vary for many reasons but I'm very happy with the minimal investment and the substantial gain. Increase noise does not to be a problem.

What FM Notch filter and preamp are you using? I don't know that I am too heavily impacted by FM broadcasts though I can very clearly. As an example, 95.7 is five bars with an RSSI of -30 which is pretty strong I suppose.

 

[dlwtrunked]

What FM Notch filter and preamp are you using? I don't know that I am too heavily impacted by FM broadcasts though I can very clearly. As an example, 95.7 is five bars with an RSSI of -30 which is pretty strong I suppose.

FM broadcast problems can be very devious. They can cause de-sense. They do this by causing the AGC of the receiver to decrease the gain. Note, you will not hear interference on the frequency you are listening to, only the sensitivity will be lowered. A cheap filter (~$18, rtl-sdr eBay--no reason not to try) will possibly help with more expensive much better ones sometimes needed (Mini-Circuits ~$100 ZBSF-95+ ).
Note thread:

FM bandstop filter comparison

I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...

forums.radioreference.com

Ubbe's above advice, as usual, is good--for really best reception, one has to balance gain/attenuation and the right choice of filters. Depending on how well one is committed to that, it will take time and money and is location/situation dependent. Using an SDR or spectrum analyzer to look at the local RF environment is very very helpful if not almost necessary.

 

[iMONITOR]

What FM Notch filter and preamp are you using? I don't know that I am too heavily impacted by FM broadcasts though I can very clearly. As an example, 95.7 is five bars with an RSSI of -30 which is pretty strong I suppose.

Stridsberg FM-Notch (FLT-201A)

Receive Filters

Stridsberg Pre-amplifier (PRE-20)

Pre-Amplifiers

 

[andy404ns]

FM broadcast problems can be very devious. They can cause de-sense. They do this by causing the AGC of the receiver to decrease the gain. Note, you will not hear interference on the frequency you are listening to, only the sensitivity will be lowered. A cheap filter (~$18, rtl-sdr eBay--no reason not to try) will possibly help with more expensive much better ones sometimes needed (Mini-Circuits ~$100 ZBSF-95+ ).

Awesome thanks for the advice. I suppose there's no point in worrying about the extra connections / adapters needed for the filter causing loss in other bands since I've got the amp right on the other side? I suppose two extra "things" in the line wouldn't be much impact anyway.

Ubbe's above advice, as usual, is good--for really best reception, one has to balance gain/attenuation and the right choice of filters. Depending on how well one is committed to that, it will take time and money and is location/situation dependent. Using an SDR or spectrum analyzer to look at the local RF environment is very very helpful if not almost necessary.

I am happy to spend some money and play around with it to get it the best I can. I will be transitioning to SDR shortly. Gotta put this software engineering background to use somehow. At that point, as you suggest, I can start using some legitimate tools for understanding what's happening.

 

[Ubbe]

When you ran the attenuators... were you placing them before or after the amp? Or were you using the scanners internal attenuator by channel as needed?

I use a variable attenuator at the input of the splitter, but as different scanners have different quality receivers I should ideally have one attenuator at each splitter output. But I adjust to the worst receiver connected to that splitter. I adjust the attenuation back and forth to find where I get the least noise when monitoring a weak analog signal.

I haven't found any amplifier that overloads of the ones I have. It's always the scanner that overloads first. When the signal starts to overload a receiver it's sensitivity starts to get worse and when the signal are strong enough it begins to go into intermodulation where you start to hear signals on frequencies where they do not belong and are only created inside the receiver. You often notice this loss of sensitivity from strong signals when you connect a FM broadcast filter and you get a better sensitivity. As I haven't noticed any amplifier overload from transmitter sites 200 meters away I connect any filters after the amplifier to compensate for their inline losses. But that can easily be tested what happens to reception when a filter are put in front or after the amplifier.

AGC in receivers are only used in AM mode and only Unidens SDS scanners has a high signal AGC protection that goes into action when it sense a big signal inside it's 10MHz bandwidth.

SDS100: - Weird anomaly receiving civil aviation with SDS100

I have only seen this on one frequency so far. Local airport AWOS is 118.175. I normally have any AWOS or ASOS frequencies locked out for scanning. When scanning or sitting on 118.175, the beginning of the AWOS broadcast volume is very low with an obviously incorrect RSSI -60dBm. After...

forums.radioreference.com

/Ubbe