PNWDigital has used repeaters pairs over and over for years and this does not require coverage isolation, though it is a consideration. There are many advantages in making use of the same pairs.
- Very spectrum efficient when used by a linked radio network
- Less potential interference for other repeaters on the same pairs.
- Fewer Channels/Zones that need be programmed
- Very Effective Roaming without the need to program, if even available in the mobile radio
- No need to zone or channel switch, especially while driving
Below is an example along I-90 for our VHF pair (147.4125 -1.00 Mhz) used by 4 repeaters located in Ellensburg, Ephrata, Lind and Spokane. The combined coverage map (below) shows coverages from Easton on the west edge to Coeur ‘d Alene on the east edge. This is about 255 miles or 3:45 hour drive over I-90 without a channel change being needed. This is about the simplest and most effective roaming possible in the ham world on a ham budget.
PNWDigital has more coverage along this corridor using repeaters on different VHF and UHF pairs, so the above is just using a single pair.
So how is it done? The c-Bridge handles the selection the repeaters receiver and the routing of traffic. It sends that traffic to the other 3 repeaters (on FT talkgroups) as well as to the rest of the DMR network as is its nornally processes traffic.
There is a gotcha with this plan and it is known as “capture“. In our I-90 plan shown above, there is some overlap between some repeaters. That overlapping coverage does make it possible for 2 repeaters to both be heard by a mobile radio. When their signal strengths from any 2 repeaters is about the same, it is heard like a high bit error and can be somewhat to completely unintelligible.
We have tested the effects of capture by sharing with overlapping coverage with repeaters as close as 5 miles on UHF and 20 miles on VHF. We have seen that both in testing and/or in actual deployments for years and it works well on our DMR network.
While driving, capture is generally not noticed or a problem and it is not an issue when that mobile is transmitting, only when hearing 2 repeaters on the same pair at about the same signal strength at the mobile radio. For example, there is a 5 mile stretch (I-90, mileposts 181-186 near Atwood WA) that has capture. While 5 miles is significant, it is not overly problematic, more a possible nuisance at most. If you live, work or are stationary in this capture zone, the Moses Lake repeater has HT coverage in that particular capture zone.
PNWDigital currently has 21 repeaters that use shared pairs (more than that including inverted pairs). Most have no overlapping coverage and are selected for that isolation as generally; pairs are used by different organizations or individuals that that “separation” is desirable. But within PNWDigital, separation is only a consideration and re-use of currently used pairs is desirable, more efficient and easier to use by our members.