Selecting An Amplifier To Boost Weak Signals

[adamfancher]

Hi all,

I live just about right on top of a local radio tower that carries, among other things, my state's P25 network.

I currently have an RTL-SDR in conjunction with a 30 dB low-noise amplifier, which is more than enough gain to pull in the signals coming out of the tower.

I would like to monitor the input frequencies as the network passes data packets and can only see the downlink on the repeater's output frequency.

As one might imagine, the radios transmitting to the tower are too weak for me to pick up.

If I'm standing right next to the tower with my scanner, and can hear the input being repeated to the output, the signal is clearly getting to where I am standing.

I figure the big difference is that there is a tower top amplifier to pull them in.

I've looked around a little bit at what's available, I can get a brand new TTA for $1600 (way out of my price range) or a used, discontinued, undocumented one for a few hundred.
I'm not dead set on a TTA if there is another power amplifier available, but I haven't been able to find much and any recommendations or guidance are appreciated.

My target frequency is 806MHz

Thanks!

 

[Ubbe]

A RTL-SDR cannot handle strong signals and will loose a lot of its sensitivity. You make the situation worse by using a preamplifier. If you are that close to a radio tower then you will need a real top notch receiver, not a scanner or SDR dongle but a commercial 2-way radio or a repeaters receiver and will still will need a notch filter for the transmit frequencies coming from that tower. It's a very difficult RF situation to handle.

You can look at a professional duplex filter that have a very narrow bandpass for your receive frequency and a notch for the repeaters transmit frequencies. Maybe PAR electronics have bandpass and a notch filter you can connect in series that will be some $250. To tune a surplus duplexfilter from Ebay you would need some sort of adjustable signal generator to be able to adjust the filter to max notch and max bandpass.

/Ubbe

 

[sonm10]

Maybe try without the amplifier. I bet you are overloading both the amplifier and the rtl-sdr and saturating the signal.

 

[kayn1n32008]

Hi all,

I live just about right on top of a local radio tower that carries, among other things, my state's P25 network.

I currently have an RTL-SDR in conjunction with a 30 dB low-noise amplifier, which is more than enough gain to pull in the signals coming out of the tower.

I would like to monitor the input frequencies as the network passes data packets and can only see the downlink on the repeater's output frequency.

As one might imagine, the radios transmitting to the tower are too weak for me to pick up.

If I'm standing right next to the tower with my scanner, and can hear the input being repeated to the output, the signal is clearly getting to where I am standing.

I figure the big difference is that there is a tower top amplifier to pull them in.

I've looked around a little bit at what's available, I can get a brand new TTA for $1600 (way out of my price range) or a used, discontinued, undocumented one for a few hundred.
I'm not dead set on a TTA if there is another power amplifier available, but I haven't been able to find much and any recommendations or guidance are appreciated.

My target frequency is 806MHz

Thanks!

You need to get some filtering to notch out the downlink band. If you are already using a 30dB(that's that's **** ton of gain) pre-amp, adding another isn't going to do you any good.

 

[dlwtrunked]

Hi all,

I live just about right on top of a local radio tower that carries, among other things, my state's P25 network.

I currently have an RTL-SDR in conjunction with a 30 dB low-noise amplifier, which is more than enough gain to pull in the signals coming out of the tower.

I would like to monitor the input frequencies as the network passes data packets and can only see the downlink on the repeater's output frequency.

As one might imagine, the radios transmitting to the tower are too weak for me to pick up.

If I'm standing right next to the tower with my scanner, and can hear the input being repeated to the output, the signal is clearly getting to where I am standing.

I figure the big difference is that there is a tower top amplifier to pull them in.

I've looked around a little bit at what's available, I can get a brand new TTA for $1600 (way out of my price range) or a used, discontinued, undocumented one for a few hundred.
I'm not dead set on a TTA if there is another power amplifier available, but I haven't been able to find much and any recommendations or guidance are appreciated.

My target frequency is 806MHz

Thanks!

Your first step should be to get an SDR better than the RTL-SDR--something like an Airspy or SDRplay ($150-$200). Adding more gain to what you have is as likely to make things worse with what you have rather than better. And you are not after gain-you are after a better noise figure. Many seem to not understand that. And keep in mind and possibly your real problem, antenna height is the most important thing particularly if trying to see mobiles. A 25 ft tower has a line of about 7 miles to the radio horizon (assuming spherical earth). But a 250 ft tower has a horizontal horizon at 22 miles. (Height even does better in real world due to getting above local obstruction.) No amp/gain or wishful thinking will change that though propagation due to weather or sunspots can.

 

[GTR8000]

I would like to monitor the input frequencies as the network passes data packets and can only see the downlink on the repeater's output frequency.

What exactly are you hoping to accomplish by monitoring the cell's input frequencies? As was explained in your other thread on the subject, the CLMRN does not feature GPS/AVL data, so what do you think you're going to decode from the subscriber's inbound traffic that you can't get from simply monitoring the cell normally? This may all be wasted effort. I get the curiosity aspect, but it seems like you have a bit to learn about the fundamentals of how trunking works before expending time or money on something that will bear very little fruit.

 

[adamfancher]

What exactly are you hoping to accomplish by monitoring the cell's input frequencies? As was explained in your other thread on the subject, the CLMRN does not feature GPS/AVL data, so what do you think you're going to decode from the subscriber's inbound traffic that you can't get from simply monitoring the cell normally? This may all be wasted effort. I get the curiosity aspect, but it seems like you have a bit to learn about the fundamentals of how trunking works before expending time or money on something that will bear very little fruit.

As you mentioned, pure curiosity, more specifically I'm interested in the LRRP data packets that I see:

I spent some time in the thread

DSDPlus - Trying to analyze the Packet Data of P25 in version DSD+ 2.441 (LRRP/Location)

Looking at the Packet Data window and files of the new DSD+ version, one can notice raw LRRP (location) data that is sent in the input frequencies of the P25 system. At least this is the case at my local system. See details below. This is not relevant for the output frequencies of the system...

forums.radioreference.com

and it seems the only way to get the other side of that traffic is to tune into the input frequencies

Understanding now that CLMRN does not use GPS data and knowing from the linked thread above that LRRP doesn't necessarily contain GPS data, the LRRP traffic I am seeing has me intrigued as to how LRRP is used by the network.