The desperate engineer short story 

I was just about to leave the office, thinking about anything other than work, when the phone rang ominously.

It was Mike, a pre-sales engineer I met last March in the Washington Satellite show.

-        "I'm sending the entire batch of BUCs back to Belcom Microwaves" said Mike, "None of them is working"

-        "OK", I replied with my best the-customer-is-always-right voice, "Pack and ship them immediately",

and after a small break, "What is actually wrong with them?"

-        "The BUC just doesn't work" was the answer. "10 MHz is 0 dBm, DC level is 24 Volts, IF level is also within range. Everything is measured on the coax cable end, BUC side. I'm sending the units to you and then catching a plain to Johannesburg. Nothing to do here till the replacements arrive"

Mike was relocated to DRC, leaving his wife and baby in Johannesburg. I could hear the subtext. For Mike, a faulty batch of BUCs means a week with his family…

There was nothing I could tell Mike about testing, but … something was sounding familiar to me. I've been in this movie before …

-        "Mike, do you still have the 24 volt DC inserter we shipped to you last year?"

He said it's in his drawer.

-        "OK, now is the time to try it. Hook it up into the BUC input port to supply the DC externally".

At that point, Mike, as a real professional, started to see the reunion with his family fading out…

We hanged up. Mike had to pull an extension power cable to the roof in order to connect the DC inserter to the BUC input port. This took him twenty minutes, which canceled my plans for that evening as well.

When Mike called again, he was even more desperate than before.

-            "Everything is working just fine." He complained, "We're up in the air again. We're looking for a better coax cable maker"

Mike got the point.

When powering the BUC through the IFL cable, extreme DC resistance of the coax cable will cause a DC voltage drop on the cable itself, leaving too low voltage for the BUC.

Moreover, the voltage drop on the cable is a parasitic power consumer which might, jointly with the BUC, require from the modem more power than the modem can provide.

The DC resistance (per length unit) of the coax cable varies from vendor to vendor and between cable types.

When the distance between the modem and the BUC
is large, make sure to use a low DC resistance cable.


Let's say you have an mBLC-8, 8 Watt Belcom Microwaves BUC with power consumption of as low as 55 Watt. The modem can provide 75 Watts. Coax cable length is 30 meter. The cable DC resistance is 11 Ohm per 1000 feet (average of center conductor and shield, don’t neglect the shielding resistance)

Mathematic calculations will indicate that the DC voltage at the BUC input connector will be 17 Volts, which is OK,  but the power required from the modem is 77 Watts, which is above the modem's ability.

Any connection between the characters to reality is coincidental.

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