Making VHF trunking work
Making things XTAL clear
Fortunately, the users from the company trying the demo went to lunch around noon, while Danchik's technician finished the install. “The conversations that I heard the rest of the afternoon — it just cleared up. It was just night and day. They said ‘I don't know what happened. I guess these things have to warm up or something.’ They ended up buying. They have a base, and they have 20 or so Kenwood VHF narrowband trunking hand-helds. They use them all over the Dallas area. They're happy.
“Now, when someone's in my office and I click the radio on, they go ‘Wow, what kind of system is that?’ And I say that's our new VHF narrowband trunking system.”
The addition of the filters also allowed CommNet to reduce the attenuation settings on the site amplifiers. “We're really pleased with where it's headed. All the channels don't have all of the problems. For some of them, the crystal filter helped, but it's not the final solution. There's a lot of things you've got to do to your sites,” Danchik said. That list includes careful antenna matching and using exposed-element antennas.
In addition to using Decibel Products' four-dipole DB224 antennas at its repeater sites, CommNet's VHF narrowband trunking setup includes DX Radio Systems' Millennium series narrowband repeaters and Trident Micro Systems' trunking logic controllers for LTR. The system rings the metroplex with rooftop and tower sites in Dallas, Fort Worth, Richland Hills, Denton, McKinney, Sherman, Greenville, Cedar Creek Lake, and a newly constructed 1,600-foot tower site at Cedar Hill.
For subscriber units, CommNet is exclusively using Kenwood Communications' TK-280 portables and 25W TK-780 mobiles.
“The propagation of VHF is pretty darn good,” Danchik said. As an example, he cited one five-channel system he has placed at a rooftop site managed by Retcom/Trott Communications. The site sits atop Dallas' 550-foot Cityplace building, two miles north of downtown. “I'm operating a VHF trunked system there with 100W power, and typically about a 6dB antenna. I'm also operating several 900MHz systems in downtown Dallas on a 922-foot building, where I'm putting out 150W power per channel. The range is as good or better on the VHF than it is on the 900MHz, even though I'm 400 feet shorter.”
Rounding up customers
“From the perspective of propagation, it's the best we've ever had,” Danchik said. “And I've never had a mobile — that wasn't digital — that sounded this good.”
Danchik said that although VHF trunking only represents about 5% of current loading of all CommNet's radio customers, in the future it will represent about 75% of loading, exceeding the 900MHz commitment.
Keckler is technical editor.
Global VHF trunking
VHF trunking is finding adherents around the world. SmarTrunk Systems, Hayward, CA, has developed a 52-site SmarTrunk II system for the Estonian Railroad, operating throughout Estonia. All 52 VHF repeater sites will be linked so that the dispatch center, located in Tallinn, will be able to communicate with trains anywhere in the country.
Previously, the railroad had been using GSM phones for communications between the trains and the dispatchers. Working with the local Motorola distributor in Tallinn, SmarTrunk and its Argentinian partner, I-SATEL, designed a system using the I-SATEL ISX-510 switch to link the 52 sites together. Now the crews can talk to Tallinn dispatch, any local station on their routes and any track workers along the way. The system also is capable of telephone interconnect.
The network system is in the process of being installed, and there is a great deal of interest from other Eastern European countries in this low-cost solution to a networked communications system, SmarTrunk said.