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Prodelin OMT Difference

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Ex Member

Jul 9th, 2013 at 10:12pm  
Hello I have a 1.8 m antenna that we have installed pointed to Satmex 6. Over the past few days we experienced a heavy wind and loads of rain.

The are no longer locked into the network and our hub has tried many thing but we are still unable to get into the network. We currently have a signal of 16+ on iSite on our i-direct modem. However, we are not able to transmit.

We have done the follow:

Swapped cables - Same signal strength
increased power to the transmit - No difference in reading
swapped BUC - No difference in readings

The OMT filter on the lens has quite a bit of moisture, so we want to swap this out with a newer one. My antenna came with filter PN: 4080 259 with the smaller filter screen on the front. I have another filter with the same part number however, this filter has a larger screen on the front. Would the filter with the larger screen work on the 1.8m? The larger one i got from a 1.2m Prodelin antenna.

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Eric Johnston
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Reply #1 - Jul 10th, 2013 at 10:55am  
I presume the +16 of iSite is associated with a green iSite display.  This suggests that you are still pointed at the correct satellite and polarisation.  I would make sure you are actually still peaked as the transmit beam is narrower than the receive beam. To prove that it is the correct satellite and carrier the hub could command your modem to do something (e.g. reset) which you might observe, alternatively if you are the only remote site then briefly turning off the carrier would make a clear indication. 
A completely steady readout may indicate that the display is false and that the data flow between your PC and the modem has stopped. Move the antenna slightly to prove that the +16 is a real reading that varies, as expected, with antenna pointing movement.

Regarding failure to transmit, I would take the feed assembly apart and check that there is no water inside. Make sure it is really dry inside. If the LNB and transmit reject filter are uppermost then the receive path may be dry while the lower transmit path is full of water. There must be no visible water condensation on the inside of the feed window.

Regarding Ku band feed horn and its dimensions.  Feed horn diameters are associated with specific reflector F/D ratios (focal length v diameter of the dish), with a view to getting the dish edge illumination level between -10 and -16 dB.  On an offset dish the focal length is from the feed phase centre to the bottom edge of the dish
The larger diameter horn will be associated with a Prodelin dish with an F/D ratio of about 0.8.
There is an older, slightly smaller diameter feed intended for use with very old dishes, F/D approx 0.6.  The older 0.6 dish has not so good cross-pol performance, but that is due to the short 0.6, not the feed.
There is a very small Ku band Prodelin feed intended for receive only applications. It has poor sidelobe performance due to the high edge illumination of the dish and is not suitable for a transmitting VSAT.
I would stick with the correct feed if possible. Make sure it is really dry inside and temporarily seal any damage to the feed window with PTFE self adhesive tape.
There are two policies on sealing.  One is to completely seal the feed system, having filled it with dry air, the other is to permit small ventilation such as pin hole of the top of the feed window and a similarly small air passage at the low point. The problem is that cool damp air is sucked in at night and condenses, then later, the heat of the day causes the upper dryer air to expand and go out.  Over a few days it gradually fills with water, even if there is no rain.  The same applies to air filled cables. Waveguide flanges without silicone lubricated rubber 'O' rings leak air and water vapour.
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« Last Edit: Jul 10th, 2013 at 2:01pm by Eric Johnston »  
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