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SkyBridge Plus · Volume 4

SkyBridge Plus — Vol 4: Reference

BridgeCom commercial DMR hotspot — turnkey appliance

4.1 Tips and tricks

The non-obvious things worth knowing about running a SkyBridge for any length of time:

Figure 1 — Figure — The WPSD Live Caller / Last Heard view, which lists every transmission the hotspot decodes across its subscribed talkgroups with callsign, DMR ID, talkgroup, and duration. Source: mmdvm.com.
Figure 1 — Figure — The WPSD Live Caller / Last Heard view, which lists every transmission the hotspot decodes across its subscribed talkgroups with callsign, DMR ID, talkgroup, and duration. Source: mmdvm.com.

4.1.1 Software updates — the dashboard Update button, or a reimage

BridgeCom’s 2022 blog describes the ordinary Pi-Star Update button on the dashboard as the update path for the Pi-Star-based SkyBridge; a separate BridgeCom update server behind that button could not be confirmed. BridgeCom’s current recovery and upgrade path is a reimage of the microSD card with the WPSD image built for the SkyBridge MAX and SkyBridge Plus (Vol 3), which BridgeCom intends to be done under the guidance of its technical support. If immediacy on a specific upstream feature matters, that is the DIY WPSD’s job — see Vol 22.

4.1.2 Last-heard transmissions across the network

The WPSD dashboard’s “Live Caller” and “Last Heard” panels show every transmission the hotspot has decoded, with caller’s callsign, DMR ID, talkgroup, duration, and RSSI. The view is intoxicating once you realise it — the “is anyone talking on the talkgroups I’m subscribed to” question goes from “press PTT and hope” to “scroll the dashboard panel.” Use it to find active talkgroups before committing them to the static-subscription set; use it to verify that a transmission you keyed was actually heard by the network.

4.1.3 The 2.4-inch screen

The SkyBridge Plus’s 2.4-inch colour screen is driven by MMDVMHost as a Nextion display on the modem port (Display=Nextion, ScreenLayout=3 in BridgeCom’s default configuration). Its most useful job is showing the hotspot’s IP address in the upper-left corner, which saves hunting through the router’s DHCP table. The display type and layout are set on the dashboard’s Configuration page; changing them away from BridgeCom’s values can leave the screen blank, so the backup in Vol 3 is taken first. No BridgeCom screen-layout (.tft) file has been found published.

4.1.4 Ghost activity

When the dashboard or the SkyBridge’s screen displays TX/RX on a talkgroup you’re not listening to, it’s not a malfunction. The hotspot is receiving DMR network traffic from the talkgroup over the internet (because the talkgroup is in the dynamic-subscribe window or there’s a cross-mode bridge active), decoding it, queuing it for RF transmission, and turning on the TX LED. From the user’s perspective the hotspot looks like it’s “doing something” without being asked. This is normal — the dashboard’s “Live Caller” panel will show the inbound transmission. The only operational concern is that ghost activity on a busy talkgroup that’s been bridged onto your slot 2 will block your own transmissions until the inbound finishes; if this happens often, the talkgroup is too busy for static subscription and should be moved to dynamic.

4.1.5 Power supply quality matters more than it should

BridgeCom attributes most SkyBridge “stuck on Initializing” faults to a failing power supply or a corrupted microSD card, and Raspberry Pi boards in general are sensitive to supply undervoltage. The supply BridgeCom ships is the reference; a generic phone charger, or a cable shared with a hub, is sometimes not good enough. The symptom is the WPSD dashboard’s “Under-voltage detected” warning at the top of the page; the on-air symptom is intermittent dropped transmissions and dashboard alerts. Use the BridgeCom-bundled supply or a known-good one of at least the same rating; don’t share the supply with another device through a hub, and don’t power the hotspot from a laptop that may sleep.

4.1.6 Frequency coordination is on you

Pick a hotspot frequency that is not used by local repeaters within roughly a 5-mile radius — the hotspot’s TX is low-power but its RX is wide-open, and a local repeater on the same frequency will desense the hotspot’s receiver into uselessness. Check your local repeater coordinator’s database (e.g. for Michigan, the Michigan Amateur Radio Council; for other regions, the corresponding state or regional coordinator) before assigning the hotspot’s frequency. Most regions publish a “low-power simplex / hotspot” allocation specifically for this case — use that range. The deep regulatory and band-plan treatment is in Vol 4 (Frequency Planning) and Antennas Vol 31 (Regulatory & RF Safety).

4.1.7 Use Ethernet where possible

The SkyBridge Plus has a built-in Ethernet port, and a wired connection removes the most common cause of hotspot dropouts. On Wi-Fi, Pi 3B-based units (all-black USB ports) are limited to 2.4 GHz, which suffers in congested neighbourhoods or far from the access point; Pi 4B-based units (two blue USB ports) can use 5 GHz as well. Symptoms of a weak link are “disconnected” network status on the dashboard and missed talkgroup activity; most clear on the next reconnect. Persistent failures call for Ethernet, a closer access point, or (on a Pi 4B unit) the 5 GHz band. Keep wpa_supplicant.conf / the .nmconnection file backed up — if the SD card has to be reimaged, the Wi-Fi credentials are otherwise lost; the WPSD WiFi Config File Generator recreates the file for the card’s boot drive.

4.1.8 Don’t change colour code from 1 unless you have a reason

The DMR colour code is a per-channel layer-1 squelch — a radio with a different colour code on its channel won’t decode the hotspot’s transmissions even if everything else matches. CC 1 is the convention; the AnyTone D878 defaults to CC 1 on a new channel; every codeplug template uses CC 1; every shared-codeplug file uses CC 1. Don’t change it on the hotspot unless you have a specific reason (e.g. you’re operating two hotspots in close proximity on the same frequency and want to keep them from cross-talking, which is itself an unusual configuration). The hours-of-debugging trap is “I changed the hotspot to CC 5, forgot, then changed the radio codeplug and couldn’t figure out why nothing decoded.”

4.2 Firmware and software on this site

The site lists the software BridgeCom points SkyBridge Plus owners to, with checksums and README files, at https://radios.fubsypoly.com/downloads/#skybridge-plus. The WPSD system image is linked rather than hosted: images near or over 1 GB are linked while the project still offers them.

Table 1 — Firmware and software on this site

ProductRecommended release
SkyBridge MAX/Plus system image (WPSD)Trixie (Debian 13) — via the WPSD project (not hosted)
SkyBridge Plus factory default configuration2021-10-28 — for the Pi-Star-based software

4.3 Resources

Vendor pages:

Upstream open-source projects (the SkyBridge is built on these — useful for the SSH-and-tinker case):

DMR network registration and infrastructure:

Cross-references within this series:

Cross-references into the sibling Antennas project:

Wider amateur DMR references:

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