Solar Power for Wireless Bridge Links: What One Pole Carries

Why a Link Pole Is Not a Camera Pole

Solar power for wireless bridge work starts from the device list rather than from the radio’s range, because the two figures that decide the system are watts and hours.

A camera site draws in bursts and idles in between. A link site does neither. The radio holds its transmit power for as long as the link is up, the router answers the carrier around the clock, and the daily figure therefore sits close to the continuous figure multiplied by 24. A load of 20 W that would be a rounding error on a mains-fed pole becomes 480 Wh a day on a battery, every day, including the days the sun does not show.

The second difference is that a link has two ends. Power is usually wanted where the cable cannot go, which is both ends at once, and the far end is often the harder one: a hilltop, a mast on a rented roof, a gate post with no building behind it. One unit per end is the normal layout, so the load list is built twice and the smaller of the two ends does not subsidise the larger.

Neither difference changes the method. Both ends are sized with the same output ceiling and the same daily account, which is the arithmetic in the four-step sizing order.

Test One: The 60 W Ceiling

All four standard systems publish the same output: DC 12 V nominal with a maximum of 16 V, a rated 60 W continuous, and a maximum current of 5 A on a DC5521 socket. Treat 60 W as the load budget and apply it to the total on the output rather than to each device.

A bridge radio at 9 W, a router at 5 W and a camera at 6 W come to 20 W, which leaves 40 W of the budget unspent. That margin is not spare capacity for a second link: the ceiling is a continuous rating, and a radio that draws a burst at start or a camera that switches on infrared adds to the same total. A link pole normally passes this test with room, and the reason it still needs care is the second test.

What the output delivers at the far end of a run is a separate question from what it delivers at the box. The published envelope is set out in the 12 V output at the camera.

Test Two: The Daily Account

Usable energy is quoted on a fixed basis, at 12 V nominal, with the reserve the system keeps in hand and the conversion loss already taken off: about 326 Wh on a 40 Ah battery, 490 Wh on 60 Ah and 979 Wh on 120 Ah. Divide each by the site’s daily figure and the result is the boundary in days of zero charging.

The three-device list lands like this. At 20 W continuous, 480 Wh a day, the boundary is about 0.7 days on the smallest reserve, 1.0 day on the middle one and 2.0 days on the largest. Drop the camera and the link alone draws 14 W, or 336 Wh a day, which moves the boundary to about 1.0, 1.5 and 2.9 days. Drop the router as well and the bridge radio on its own takes 216 Wh a day, giving about 1.5, 2.3 and 4.5 days. Those are worked examples on reference draws; the real list replaces every figure in them.

This is why a link site is usually planned from the battery upwards. The radio is a small number that never stops, so the reserve, not the panel, is what keeps the link up through a long dark stretch.

The Worst Month Sets the Panel

The panel has to put the daily figure back on the shortest day of the worst month, not on an average day. At 4 peak-sun hours and 75 percent overall efficiency, an 80 W panel returns about 240 Wh a day (80 × 4 × 0.75 = 240), a 120 W panel about 360 Wh and a 240 W panel about 720 Wh.

Set those against the three-device list at 480 Wh a day and the arithmetic is blunt. The 80 W and 120 W panels return less than the site spends in the worst month, so the reserve drains a little every day until the season turns. The 240 W panel returns 720 Wh against 480 Wh spent, which leaves 240 Wh a day of headroom for the days that come in under 4 peak-sun hours. The link alone at 216 Wh a day sits inside the 120 W panel’s return with room, which is the difference between powering a link and powering a link plus a camera.

Peak-sun hours are site data, so use the worst month at the actual location rather than a national average. What a run of grey days does to the reserve, and what the site looks like when it empties, is worked in the grey-week arithmetic.

One Unit per End, or One Unit and a Long Run

A 12 V feed loses voltage with distance, so the farther a device sits from the output, the less of the published envelope reaches it. On a bridge link the distance is not a few metres: the two ends are separated by the very gap the link exists to cross, which is measured in hundreds of metres or in kilometres. A single system feeding both ends would need a cable across that gap, which is the trench the link was bought to avoid.

The layout that follows is one self-contained unit at each end. Each unit carries its own panel, its own reserve and the devices at that end, and each end gets its own load list: the near end might be a radio and a camera, the far end a radio, a router and a camera, in which case the far end is the larger system even though the sites look symmetrical on a map.

Where a single unit does serve two devices, keep the run short and check the drop before mounting. Device-side cable and terminations are the installer’s to size, and the connector on the output is a DC5521 socket.

Mounting: Two Things That Share a Pole

A panel and a directional antenna on the same pole compete for the same sky. The antenna needs a clear line to its partner, and the panel needs an unobstructed arc toward the equator, so the usual arrangement puts the array on its own cross-arm below the antenna, where neither shadows the other and the panel’s tilt does not point through the dish.

Two physical details decide more than they look. The first is that the panel’s own shadow falls on the pole and the cabinet, not on the array, so the mounting height has to clear the hardware below it. The second is snow and dust on the glass: a link site is often unmanned, and a panel that is not visited is a panel that is not cleaned. The cleaning interval and the seasonal sweep are covered in the maintenance calendar, and the mounting assumptions in power for a remote wifi hotspot apply to any mast that carries a radio. A mast that carries instruments instead adds a logger and an uplink to the same pole, and the loads on a station mast are listed with their daily figures.

Aim the antenna first and the panel second. Moving a panel a few degrees costs a little yield; moving an antenna a few degrees can cost the link.

When the Link Shares the Mast

Most link poles are not link-only. The same mast usually carries the uplink that feeds the bridge, and often a camera watching the gate or the compound. That is a hotspot site with a radio link attached, and the load arithmetic is the same one worked in how much power does a wifi hotspot use: small devices, present every hour, summed and multiplied by 24.

The uplink is the part buyers forget to count. A router that draws 5 W is 120 Wh a day on its own, which is more than a third of the smallest reserve, before the bridge radio is switched on. The router’s figure is broken down device by device in the router’s share, and a site that needs both a hotspot and a link is planned as one load list rather than two projects.

If the mast also carries cameras, the count question is a separate test on the same budget. Several cameras on one system runs it, and the ceiling applies to every device on the feed, link included.

Night, and What the Reserve Is For

The panel contributes nothing after dark and the reserve carries the whole feed. For a link that means the radio, the router and the camera draw for the longest half of a winter day with no input at all, and the boundary figure calculated above is exactly the number of those days the site can string together.

The practical read is to size the battery on the night figure and the panel on the 24-hour figure, then check both against the worst month. A site that passes the daily account in June and fails it in December has the wrong panel, not the wrong battery. What happens when the reserve does reach zero, and what brings the site back, is described in when the account runs to zero.

Quick Answers

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