Making VHF trunking work
2000: Year of decision
When the dust settled, in mid-2000, two issues were decided: First, operators like Danchik got the coordinators to back off from inflammatory statements that implied that new licenses are not in compliance with the FCC rules.
Second, all the FACs agreed to solve the controversy by treating adjacent-channel frequencies just like co-channel frequencies, which means to get a VHF trunking frequency now, concurrence has to be obtained for a 39dBu (70-mile) contour from co-channel and adjacent-channel incumbents. This will impede, though not entirely restrict, the possibility for new systems in urban areas.
Danchik was already managing about 50 channels before the cow pie hit the fan. He has added 13 more, to date, by accepting difficult coordinations — mixing exclusive channels with shared frequency, non-interference-basis channels. “We take them to fill,” he said.
Operators wanting to deploy new VHF trunking in major urban areas will now have to run a longer gauntlet, but there are frequencies in markets where VHF hasn't caught on yet, and there are still plenty of VHF frequencies available in rural markets.
So, with the initial coordinations, and the fill-in channels set up to monitor and handle any interference, CommNet's framework was in place. But, as with 900MHz, there were technical hurdles.
Fences across the range
Channel pairing was the first stumbling block. With few exceptions, no natural pairings exist in the VHF bands. Danchik asked FIT to try for about 5MHz of separation, transmit to receive, and at least 250kHz between transmits.
The second hurdle was the capability of the newly available equipment. Although all the VHF frequencies are being assigned at the 7.5kHz split, the available manufacturers' receivers are, at a minimum, 7.5kHz narrow. “Sometimes at 15kHz over, there's some 400W and 500W paging channels out there.” Danchik said. “This has caused us a real problem because you do have combining, duplexing, and multicoupling.”
The biggest problem encountered was talkback capability. Talkout was “absolutely wonderful,” but the LTR radios weren't getting the handshake to go ahead. Not only the portables, but even 25W mobiles with 3dB antennas, were being difficult.
“So we asked, ‘What's going on?’” Danchik said. “You go back to the site, get your spectrum analyzer, you look at the noise floor, and you begin to see there are paging channels, there's all sorts of VHF, there's God knows how much unlicensed stuff running around there. They're all over the place, and no one's receivers are tight enough to eliminate this.”
CommNet's repeater supplier, DX Radio Systems, finally came up with the solution: a narrow, crystal receiver filter. “You order it specifically to your exact frequency, you tell them you want it to be a 7.5kHz window, and you put it right on the front end of your receiver. That's the good news,” Danchik said. “The bad news is they cost $400 dollars apiece. So, if you've got a five-channel system, you just spent another $2,000 you weren't planning on spending.”
Delivery time for the precious solution also became problematic. “You can imagine what must have happened at these crystal filter manufacturers,” Danchik said. “They were probably selling 10 a year of these things, and all of a sudden they had orders for hundreds of them.” Four-week delivery time soon became a 16-week backlog, but “We had finally managed to get them and put them on, and that's been a big, big difference,” Danchik said.
One system that was constructed, but not loaded, was used for a customer demonstration just as the solution was being worked out. The prospective client wanted to switch back from ESMR to traditional dispatch, looking for portability in a clear system — that wasn't one-to-one and wasn't cellular — and would cover Dallas. The sale hinged on the demo.
Danchik approved the demo use of one channel. “They're not going to trunk anywhere,” he told his staff, “because there's nobody else on the system anyway. Let's see how that works for them.”
He decided to monitor the demo from his office.
‘They're not going to buy this’
On demo day, the prospective client passed out four or five portables to its field crew, and by mid-morning Danchik was gloomy. “They were talking on it, and I'm listening to them. About half the transmissions were OK, about half were less than OK. Some of them were crappy, quite frankly.” The users were also making unfavorable comments: “I don't think this is going to work for us, you're not too clear.”
“So, I'm sitting there listening to them, and I was shaking my head, saying ‘This isn't going to work. They're not going to buy this. They're not going to be happy,’” Danchik said.
“They'd been demonstrating since 8 o'clock that morning, I'm half-listening to them on the radio while I go about my day's business. About 10 o'clock, my receivables person from the back comes to the door with the inventory list and says ‘We just got in the first five crystal filters.’ I grabbed my tech and said ‘Go put ’em on, go put ’em on. Right now — go put ’em on.’