Whydoes LS300 transmit when door open but not when closed

I have a Sierra Wireless Airlink LS300 AT&T cellular modem in a metal stream gauge house. The airlink modem has a 10 foot long cable antenna with the antenna mounted on the exterior of the roof. The modem is inside the shelter. When I am at the site with the door open, my data transmissions are ok. When I close the door and leave the location, I get no data transmissions. It doesn’t make any sense. Why should it matter if the door is open or closed when the antenna is on the roof?

Maybe there’s a fault in the cable and/or antenna :question:

So, when the door’s open, it’s working by “leakage”; when it’s shut, the “leakage” is blocked.

Can you be inside the shelter with the door shut to observe what’s happening?

Or can you leave a PC or something inside the closed shelter to record what’s going on?

Or maybe the modem is overheating when the door is shut and shutting itself down?

Ciao, Dave

Additionally, the modem is in a location with very low cellular signal (around -101 to -107 signal strength usually).

Two possible reasons for this problem are there is something wrong with the antenna or there is too much loss due to the 10 ft cable length. The antenna is brand new but doesn’t mean it is not broken.

I was able to get the modem to work by attaching the modem directly to the metal roof (inside of the enclosure), drilling a small hole in the roof, and sticking a different short antenna through the hole (so the antenna is on the outside). I used a Sierra Wireless -Penta-Band Monopole Antenna - SMA Male (antenna is roughly 3 inches long). This is not a good installation because the roof gets very hot and I have already seen the modem quit working due to the over heat. I tried to put a small layer of cotton insulation (sock) between the roof and modem but that didn’t work because I lose signal strength. It’s like the shelter provides some signal amplification and works better with the modem directly attached to it.

Anyway, this is not a permanent fix but it is working. I may try a shorter cable length antenna if I can get my hands on one.


If the modem works with the short antenna cable, but not the long makes me suspicious of the quality and type of the long antenna cable. If you’ve got the opportunity, see if you can replace the cable with one with low loss at the dominant frequency that you’re working at. In Oz I would look for something optimized for 900ish MHz for 2G, 850 or 2100 MHz for 3G (depending on network and location). RG58 is not an optimal cable for these situations.

With regards to the temperature issue, are you able to fit a “tropical roof” or hood to your cabinet? Maybe something like what’s on top of this cabinet :http://www.brenclosures.com.au/field.htm.

Ciao, Dave

Actually, I think what he’s saying is that it worked with no antenna cable at all - just the antenna itself poking through a hole in the roof?

Indeed: cable losses at cellular frequencies can be significant - especially over 10 feet :exclamation:

Definitely worth trying at least a much shorter cable and, preferably, a better quality cable …

For what its worth, the 10 foot cable that would not work says “NFC LOW LOSS 100 50 OHM COAXIAL CABEL (sic) 3D” on it. That’s all the information I have for now.

As far as the heating of the metal roof (where the modem is attached to), I spray painted it white and it made a significant difference. Before painting, the roof was 148 Fahrenheit according to an infrared thermometer I used. The LS300 specs say it can operate up to 150 Fahrenheit so it was nearing its operating limit. This was in 80 degree weather. After painting the roof white, the temperature of the roof was 103 Fahrenheit. Wow, big difference! I’m curious what the temperature of the roof will be in 100 degree weather. My guess is 120-130 F, it will probably always be hotter than the outside temperature by some factor.

The roof is beside the point for what this post is really about (proper antennae selection) but thought I’d share.


Thanks for the info about how much difference simply painting the enclosure white made to your internal temperature. Very interesting. Just out of interest, what colour was the enclosure before? I can also recommend putting a ‘tropical roof’ on the enclosure if you can to provide a bit more shade.

If you look very closely at the specs of your device, you may find that it operates in different classes depending on the device temperature. I know the Q26 series drops the transmit power and data rate the hotter the device gets. If you were operating your device near the edge of it’s temperature it may have been in a low(er) power class. This, coupled with the long antenna cable may have been why it was failing on you.

Anyway, good to hear that you’re on the way to a solution.

ciao, Dave

I think everyone calls their cable “low loss” - it’s essentially meaningless.


The fact that they can’t spell “cable” does not inspire confidence :exclamation:


So I think it’d still be worth trying some known high-quality cable from a reputable manufacturer - and keeping it as short as possible.

Here is the link to the antennae specs:
Just based on the amount of effort that they put into the spec sheet, I’d give them some credit. Wow. More than I want to make sense of for the time being.

To answer a previous question, the color of the roof before painting it white was plain old bare steel. I’ve done some other testing of temperatures recently. At another location I have 3" grey conduit (plastic of some sort) that was reading 162 degrees Fahrenheit in 100 degree weather in direct sunlight. Painted it flat white and the temperature dropped to 122.