Mow Cop 3cm receiver

Latest news (25.05.15) - Head end failure.

antenna/lnb        antenna on-site
Although failure was caused by water ingress into the LNB via its
waveguide feed, the sealing of the overall assembly was found to be
generally in remarkable condition, given that the assembly is
about seven years old. Plenty of drain holes had been
provided to allow any water that had found its way in to be
safely guided out of the bottom of the 110mm plastic tube mount
- except(!), at the last minute, a covering tube was added
(see arrow opposite) to provide protection of the antenna to LNB
waveguide flange interface, and in the rush,  no drain hole was
added inside, so that water built up and eventually flooded the
flange coupling.

The top mounting tube disc is 10mm pvc, and as can be seen
below, the silicon rubber sealing to this has held well, with no sign of
water having made its way across any of it - you can also see where
the protective tube mentioned earlier, has formed a good seal with the
silicon rubber too:
flange coupling


draining hole 


A drain hole has now been added, and since it is directly above the LNB, a tube has been added on the lower side to sweep the water
off to the inner edge of the 110mm mounting tube.

drainage tube

The replacement LNB is the same model as per the original, and the
noise factor into it is pretty much the same, or slightly better, and is
less than 1 dB (the WR-90 to sma adapter visible above was fitted
to enable this measurement).






Weathering within the cavity holding the LNB seemed non-existent as the following photo shows:

LNB cavity

The white particles are pvc cuttings as a result of having to cut through the 110 outer plastic tubing (super glue having being used as a sealant
between the pvc disc and the mounting tube).


Despite the previous comments, the LNB pcb is very badly damaged:

warer damage

The LHS of the pcb is at the lowest point, so water will have collected there. The dc supply comes in at the left also, and effectively is not current limited, so most of the damage will have been caused by electrolysis.

antenna rust
The waveguide antenna looked in excellent condition,
and the only sign of a leak was some rust staining at
one corner, presumably from the tin plate wing. Two
days with the antenna assembly stood with mildly dyed
water filling the waveguide up to flange showed no sign
of leakage from the upturned antenna top (lid) or side,
though there was slight leakage from the rust stained
section. In view of this, a 10mm hole was drilled into
the base fibre-glass so that any water that did enter
the assembly would be drained (into the protective
tube section for the flange joint, which had earlier had
drainage added).


LNB damage
Corrosion of the LNB feed waveguide can be seen on this picture, though it's not as much
as one might have expected. I'm not sure what has caused the white marking of the red
resin that holds the 22mm straight copper pipe joint in place within the LNB choke rings.

The active antenna probe is the one closest to the camera. There is no dc blocking
capacitor between the probe and the input HEMPT device, and if the IC bias circuitry is
the same as the older discreet circuit versions, a short to ground of the gate due to the
water ingress will destroy the device on over-current. Strange that no current limit circuitry
seems to used for the drain supply, but I suppose any short of the gate to ground implies a
fairly serious LNB fault that is going to require total replacement anyway.

Re-assembled and final weather-proofing/sealing:

initial rebuild
weather-proofing
final sealing