SDRTrunk…

So, I sold my scanner. I needed something that could monitor P25 Phase 2, in addition to Phase I and analog signals.

I could have spent $600 or more on an SDS100 or SDS200 – and some day, I might… but looking at what I had on-hand, I wanted to see what I could do with some RTL-SDR dongles. I tried unitrunker once upon a time, and found it to be cumbersome and not very user friendly. Looking through rtl-sdr.com, I found a couple threads on SDRTrunk which looked quite promising. The setup guide I followed was here: http://www.hagensieker.com/blog/page/?post_id=69&title=create-a-trunking-scanner-for-$25-(okay-$50

After a lot of digging around, i found my F to SMA adaptors, as well as the SMA to MCX adaptors, so i could connect both SDRs to a cable TV splitter, so they could share an antenna. Add a USB hub, and it was a fairly compact setup.

I opted to become a “premium” member of radioreference.com to better allow for software to automagically pull data, and was impressed with how well it pulled the system. I think the only challenge i had was selecting the proper control channel.

I did notice that the auto-correct PPM is pretty good, BUT can be fine tuned by observing how well the control channel is centered in the highlighted column.

Now – one thing I DID Notice was that the loss through the TV splitter, and a non-resonant antenna, is that I needed to turn the gain pretty high (400) on the dongles to REALLY see the control channel pop out of the noise floor. I am not sure how much dynamic range the software needs. The problem with this is that the smaller dongles get HOT – hot enough to smell. I have not noticed this as much with other software…. but maybe i haven’t had the gain this consistently high before… I haven’t seen an AGC setting.

Going forward, in order to reduce the necessary gain, I have 4 RTL-SDRs on order, as well as 4 F to MCX cables, and a 5VDC preamp. I’m thinking a 4 port USB hub should be the trick to combine the SDRs, and power the LNA.

I’m planning on installing all of this on a heatsink, so that’s in the works, when the parts show up in the mail in a month or so. I’ve even been toying with the idea of removing the connectors, and direct cabling the hub to the receivers… this is going to be a bit bigger project, just because of the component removal and cabling.

On a couple of final notes for my local system…. Im listening to a simulcast system – by checking the “LSM” button instead of C4FM, reception improves significantly. Finally… before you start running out and buying parts or buying a scanner…. be sure what you want to listen to is in the clear. All Law Enforcement in my area is encrypted by default, so I dont hear much, unless fire and EMS Are busy, of the state agencies are doing their thing.

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