800 mhz patch to vhf?

[ffemtbland]

Madison Fire Dept. Tech Rescue along with other MFD units were called out to Deerfield (Dane co.) to assist Deerfield FD with a silo entrapment. The comm center patched MFD FGND 3 (trunking 800 mhz) up with Deerfield on the county (vhf) frequency B-Baker (FGND-ops). How would MFD be able to use their radios that far out of the city via the trunking system? One would think that 800 mhz use would be confined to the city ... due to the characteristics of the 800-900 mhz spectrum ... unless their are towers out there ...
Thanks,
Lars

 

[MMIC]

Looking at the license:

http://www.radioreference.com/modules.php?name=RR&action=fcc&cs=WNXW548

It appears that they have a tower out on the east side of town at I-94 and I-39. It is kind of far away, but it wouldn't be too much of a stretch to have fair coverage out there. It is also possible that they are using a vehicular repeater system (VRS) that Motorola has developed that will allow the users to use a simplex VHF frequency on the fireground, then have that frequency patched on the fire engine to a trunked 800 MHz talk group, which is then repeated back into the system.

800 MHz isn't terrible, although there are better bands, but it will get out if properly designed.

 

[RevGary]

Compare the Amateur Radio charts available on-line regarding the expected signal distance on frequencies from 28/29 mhz through 1.2 ghz and you may be surprised to find that as the frequency goes higher, much more power is needed to cover the same distance as the next lower frequency spectrum on the chart. In one chart that a member of the ARRL published several years ago, the correlation between distance, power and frequency to achieve the same coverage was logarithmic in nature. As an example they showed that using a 25 watt transmitter and a 1/4 wave VHF ground plane antenna @ 100 feet in height on high band VHF, the sample distance was 18 miles. Using the same power and 1/4 wave UHF configuration, the expected coverage distance went down to 12 miles. On 800mhz using the same 25 watts and 1/4 wave antenna configuration, the example listed approximately 6 miles. With all things being equal, do we see, here, that much more equipment is needed to make 800 operate in the same manner that VHF operates... and that unless the system is designed to take these limitations into consideration, that poor radio system operation may be the result...

 

[ffemtbland]

Compare the Amateur Radio charts available on-line regarding the expected signal distance on frequencies from 28/29 mhz through 1.2 ghz and you may be surprised to find that as the frequency goes higher, much more power is needed to cover the same distance as the next lower frequency spectrum on the chart. In one chart that a member of the ARRL published several years ago, the correlation between distance, power and frequency to achieve the same coverage was logarithmic in nature. As an example they showed that using a 25 watt transmitter and a 1/4 wave VHF ground plane antenna @ 100 feet in height on high band VHF, the sample distance was 18 miles. Using the same power and 1/4 wave UHF configuration, the expected coverage distance went down to 12 miles. On 800mhz using the same 25 watts and 1/4 wave antenna configuration, the example listed approximately 6 miles. With all things being equal, do we see, here, that much more equipment is needed to make 800 operate in the same manner that VHF operates... and that unless the system is designed to take these limitations into consideration, that poor radio system operation may be the result...

RevGary - EXACTLY!!! My question - HOW could they be doing this patch with such a system??

 

[RevGary]

Due to the limitations of 800, my only guess would be that MADISON had VHF equipment with them for mutual aid responses on the Deerfield frequency. The Madison personnel would have been HEARD on the 800 TRS, but were actually operating on VHF. Madison dispatch may have done that so if any other responders from Madison were required, they would be able to hear what had already gone on.

Any other thoughts? I'm well out of that area (north) and do not have any first hand local info about equipment in use by any department in that area.

 

[MMIC]

You'll have to remember that the Madison 800 MHz system is probably running 100 watt stations with anywhere from 9-12 db gain antennas on the sites. This can increase the ERP to somewhere around 300 watts. That same gain figure is probably used on the receive antennas, which, coupled with tower top amplifiers, can increase the talk-in coverage of the system drastically. These sites have coverage patterns that are a lot larger than cell sites, and Deerfield isn't really that far away. I wouldn't put it past them having used 800 MHz equipment out there, but the VHF stuff can't be ruled out, either.

To do a patch between the 800 system and VHF, one scenario would be for Madison to have a radio that is connected to their consoles that they can patch to the 800 MHz talk group and have everything on one system go out over the other. The other scenario is that they have an ACU-1000 (or similar) unit that they have radios connected to and they patch the VHF system to the trunked talk group in the field. Either way, the same thing is accomplished.

 

[biglaz]

The tower is near Cottage Grove Rd and Sprecher Rd, which I believe is around 10mi from Deerfield. The nearest tower with the B-Baker frequency is down hear Rockdale. I was listening to the incident for a short time and I can tell you that there was indeed a patch between the talkgroup and the VHF frequency. I could hear the MFD personnel on both the talkgroup and the VHF frequency. It was an interesting incident nonetheless.

 

[MMIC]

Sounds reasonable.

10 miles isn't very far, so it's not out of the question for Madison fire to have been on 800 MHz and Deerfield to have been patched into the system.

Who knows