RF Monitoring in the Ohio Valley

Archive for the ‘Software Defined Radios’ Category

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Whenever you are receiving a simulcast signal, there will be areas where there might be signal cancellation due to the phase difference between one site and another. This is similar to multi-path where one signal arrives directly to the receiver and another arrives later because it has been reflected from the ground or an object like a water tower or building. That phase difference causes the amplitude to drop*. I assume the “Bars” on the G5 respond to signal strength as I describe this. If they respond to bit error rate (BER), then the same multipath effect also results in a high bit error rate either by reduced amplitude, a phase difference in the bitstream or both.

Your radio is not “switching towers, it is responding to an aggregate of multiple signals.

To complicate this, in FM systems, there is a phenomenon called FM capture ratio which is a point where the receiver will favor the stronger signal and ignore the rest. In linear simulcast on narrow band channels, FM capture ratio requires a very large differential in signals, meaning you would need to be very close to a tower.

* In analog systems, while in motion, this multipath exhibits as a “picket fence” popping noise pulse at regular intervals. The higher the frequency (shorter wavelength) , the more frequent the noise.

I expect that when the Uniden SDS100 gets released, dipshits are going to immediately start comparing it to Unications and commercial radios, and somebody is going to find a time when their SDS100 just isn’t copying a simulcast with zero errors. Let this post serve as a reference that even the best radios out will have issues with simulcast. The difference between using a scanner that supports LSM (such as the SDS100) and a scanner that doesn’t (any scanner that came before the SDS100) is that you aren’t going to have to do antenna orientation magic or set up a fixed yagi to null out unwanted sites in the simulcast just to get a good decode — you will simply get a great decode normally. Instead of the norm being a shitty decode with occasional good copy, you’ll normally get great copy with occasional shitty decode.

Written by awadmin

April 9th, 2018 at 8:53 am

SDRPlay RSP – VHF / UHF / Trunking mini review


I recently purchased an SDRPlay RSP. Why? Only to see if the hype, which I did not believe, was true. I’ve got Airspys and R820T tuners already. My main interests are VHF/UHF digital signal decoding / trunking.

If your main interest is HF and you’re reading this mini-review, leave now. I have no use for HF / no desire to monitor HF. I don’t know nor care how the SDRPlay performs as a panadapter or anything else. It might be golden on HF. But that isn’t what this mini-review is about.

Why the SDRPlay is a fail for VHF/UHF digital decoding and trunking

  • Not supported by Unitrunker (as of 4-7-2016)
  • Not supported by DSDPlus (as of 4-7-2016)
  • Not supported by SDRTrunk (as of 4-7-2016)
  • Doesn’t handle adjacent channel interference as well as the Airspy
  • Doesn’t handle strong signal overload as well as the Airspy

For only $50 dollars more you can purchase the Airspy, which is supported by Unitrunker, DSDPlus, SDRTrunk, GNURadio / OP25, and pretty much any SDR software you can obtain on the internet. And if that isn’t enough of a reason, the Airspy simply outperforms the SDRPlay in every configuration / environment that I’ve used both devices.

No, I’m not posting numbers. I don’t have test equipment. I use the equipment and compare the equipment using the same computers, cables, jumpers, coax and antennae. And I’m totally convinced that the Airspy has better performance on VHF/UHF with regard to handling a band full of signals or strong signal overload.

If your opinion is different, oh well — You are entitled to it. If you don’t like my review, you can print it out and then wipe your ass with the paper if it makes you feel better.

Back to using my virtually bulletproof Airspys to trunktrack and record an LSM system while monitoring 6 control channels of a statewide trunking system.

Written by awadmin

April 8th, 2016 at 8:54 pm