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STL paths

I learned this one the hard way, all climates and terrain are not equal.   An important detail when planning a Studio to Transmitter Link.  The RF STL  is usually in the 950 MHz band, although lately people have been using 2.4 and 5.8 GHz unlicensed systems with good results.  What works well in the northeast, for example, might not work that great in Florida, where tropospheric ducting and multi path can create reception problems.

One example of this happened in Gainesville, Florida.  A station there had a 15 mile path over flat ground with tall towers on either end.  It had full line of sight and Fresnel zone clearance.  Ordinarily the signal strength was -65 dB, which is about 25-30 dB of head room for the equipment being used.  However, in the mornings, most often in the late summer early autumn, there would be brief drop outs of a few seconds.  After two years of suffering through the mysterious morning drop outs, we finally rented a plane and flew the STL path, only to discover there was a swamp right in the middle that was not on the topographical map.  On those mornings when drop outs occurred, it was surmised that dense fog would rise up, causing the RF path to bend and creating multipath at the receive antenna.  Since it was a Moseley Starlink, the digital demodulator would unlock due to high BER.  The signal strength never moved off of -65 dB.

Of course, had this been an analog STL, it would not have dropped out, although it may have gotten a little noisy for a few minutes.

950 MHz STL path study

950 MHz STL path study

I have learned to be very conservative with my STL path analysis, using software tools like RF Profiler to look at the theoretical path, but also surveying ground obstacles like trees and building, which are not accounted for in the USGS terrain database.  There are several RF software programs out there that will do the same thing.

Last week, when a station manager insisted that an STL path was possible from a proposed new studio location, I deferred to the path study, which showed only about 50% Fresnel zone clearance.  While it was true that the path is less than a mile, and it is also true that one can see the top of the transmitting tower from the roof; trees, buildings and even an access road create problems which could potentially cause STL drop outs.  We are not going down that road again.  The station manager, who’s background is in sales, was told to find another location or order a TELCO T-1.

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9 comments to STL paths

  • I read your entries regularly and have found them interesting and informative.

    Note that there is a typo in this one:

    “…. we finally rented a plane a flew the STL path,….”
    should probably be:

    “… we finally rented a plane aND flew the STL path, ….”

    Keep it up.
    Thanks,
    -Dale

  • Paul Thurst

    Doh! Thanks, Dale.

  • J. Aegerter

    A -65 dBm signal is inadequate for digital broadcasting. My experience is that -40 dBm is the minimum level for a quality digital link.

  • Paul Thurst

    -40 dBm is a huge signal, I don’t think we have an STL system that has that kind of signal. The Moseley Starlink systems will work down to about -80 dBm, as measured with a Rhode Schartz spectrum analyser. The Moseley units will obviously work better with a little head room, but I have seen systems with -70 dBm run reliably over a good path.

  • The absolute signal level is not important; what is important for a digital signal is the Eb/N0 (typically pronounced “Ee-bee over en naught”). Eb/N0, in turn, is the C/N (carrier to noise ratio) measured in a bandwidth equal to the bit rate. Once you know the Eb/N0 and the modulation, you can get an idea of the type of BER you’ll see on a perfect (read “no ISI” or “multipath”) link.
    That’s my $0.02.

  • J. Aegerter

    Digital packets even with forward error correction behave quite differently in practice on wireless links than on wired or fiber-optic links. It gets worse as bandwidth of the signal increases. We ran quantitive testing back in 1989 comparing two competing digital paging formats, and Golay Sequential Code won out every time over 512 baud POCSAG. As the baud rate increased above 1200, the range dropped significantly. At 6400 baud, National Satellite Paging became a service good only around the airports. Further testing with our mobile data systems revealed similar overall quality issues with baud rates above 4800, using 1/4 wave whip antennas on the rooftops of vehicles. You need a lot of signal for 99.9% reliability in the digital world. AT&T upgraded many paths for their DR135-6 6 GHz. digital microwave systems in the late ’80’s by grossly increasing tower heights to maintain the same quality as their older TH-1, and AR-6A analog systems.

  • I can imagine how this can drive you nuts especially at that frequency.New Jersey ocean front towns with 2-way radio users with police/fire allocations on VHF Hi Band 150-174 MHz would get ducting from stations in Long Island and from the south. This would result in a service call where there was nothing you could do about it.It was usually at its worst in the mornings. At least in this case.

  • Hello,
    My organization has no enogh fund to buy the STL Link for its Radio Station Mazingira fm,

    Therefore I hereby request you to find us the Stl link wth the cost range of 400-600 USD, With frequency range of 290-312 Mhz

    The distance from the studio to Transmitter sigh is 15km.

    Regards

    Mramba Simba
    Mazingira fm
    Tanzania

  • Paul Thurst

    Sorry, I am not an equipment dealer, even if I were, we’d not have a STL radio on that frequency for that price. Good Luck

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