FlightAware Discussions

RTL-SDR Blog LNA: will I need heat sinks?

I’m getting an RTL-SDR triple-filtered ADS-B LNA to connect directly to my antenna, possibly with a SAW filter that I already have in between them, which will all be mounted atop a mast that will be almost 9 meters tall and exposed to the elements. Per a review that I read on the Radio for Everyone blog these LNAs run rather warm if not hot. Also I live in the tropics and it’s already getting pretty hot outside and local summer (March, April and May) is fast approaching, and the LNA will be in direct sunlight for around 12 hours a day (unless it rains.) So, would you recommend adding heat sinks to it? If you have heat sinks on yours how do you keep them from coming loose and falling off when the thermal pads get hot, zip ties or something?

Mine runs warm, but not hot. Heat sinks won’t help because the aluminum box has a bunch of air space inside. The box itself is a heatsink for that air.
The triple filtered LNA box is not weather proof, so you need to use a weather proof box to enclose it.
Use one that is painted white, or with something reflective, like a aluminum foil, to reflect the solar infrared radiation.
Better even, if you can, put a bit of shade above (I know that the sun is directly above in hot season).

I planned to give it a couple of coats of clear acrylic spray paint to seal it (leaving the two rubber caps on the SMA connectors so as not to add a layer of insulation to them) before I deploy it on the mast. Same with the SAW filter if I decide to also use it. I can’t put it in shade as whatever object is causing the shade would also block ADS-B signals. The whole point of the mast is to get the antenna above those obstructions as much as possible.

Acrylic might disintegrate in UV light and leave water get inside.
Get a plastic box or at last cut the top of a 0.5-1 L plastic bottle of Cola and use it upside-down. Both cables exit from bottom, so they won’t drip inside.
Wrap everything in Al foil, to reflect the Sun heat.

The LNA can be like 1 meter below the antenna… 1 meter of cable before LNA is fine, I use 2 meters on mine.

Someone in Norway mentioned in another topic that his RTL-SDR filtered LNA has happily survived more than one winter which I assume includes rain and not just snow (I’ve never been to Norway.) For all I know RTL-SDR may have included gaskets for the screwed-on end plates. Maybe someone’s taken their LNA apart and can verify if that’s so?

I did took mine apart. There is no gasket anywhere. If you have one you can see that there are just four screws on each side.
This is out of the box, I didn’t took pic of the right side end cap, it’s just an Al plate with holes in it - four small ones in corners and one big in middle.

Maybe something like Form-a-Gasket sealant or glue from a hot glue gun can be used to seal the end plates around the edges.

Get an enclosure, like I suggested above. Or a cola PET bottle.

Anything else you try to use as sealant will just disintegrate in UV light and in the heat/cold daily cycles. And it’s not just the edges, the connectors are not sealed either.

How do you seat the holes where the coax enters and exits the water-tight box? Won’t whatever that sealant is also degrade in UV light?

They are on bottom, very small.
No need to seal, water doesn’t go up. If that’s a high humidity area, you can seal them inside with foam, that foam will be out of UV light.

Seems you are trying to just attach it at the bottom of the antenna.
Wrap with water tight tape from the antenna bottom covering it and the LNA and the first part of coax.

Not sure how water tight it will be, but not impossible?

In tropics the tape just disintegrates, the UV are so strong.
I worked a bit in Africa and we need to shade our electronic controls. Heck, in some places they even actively cool (like AC cooling) the critical infrastructure electronics.

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It’s not Africa hot here. We’re 10 degrees or so north of the equator. We’re in a tropical marine climate on an island archipelago so it’s moderated by the ocean.

Type III where I live.It’s not even as rainy as the Pacific NW of the US where I’m from, actually. Except for actual storm systems it’s more a matter of brief showers.

I used a small electrical junction box to enclose the LNA:

image

I used the supplied cable gland for the top to keep water out there, and removed the cable gland from the bottom as it wasn’t quite large enough to let the connector through. I wrapped the lower connector in waterproof tape and positioned the LNA as far up in the box as possible. It’s been up on the mast for 2 years with no issues so far.

image

One thing to be really cautious of if you’re using stick on heatsinks is that they can slip.

If you have a device with heatsinks mounted on the side and it’s vertically above a bare circuit board, what happens if the heatsink slips?

For example, this RPi was mounted vertically inside a computer rack for a couple of years.

slip

I think we had a lucky escape.

Where I go in Africa it’s 11 degrees North. And it’s pretty humid too, being on the coast.

There, about twice a year, the Sun at noon is straight-up above the head.
That Sun straight-up it’s pretty unsettling for someone like me that grew up at about 45 degree latitudes and is used to see the Sun around all the time, without tilting the head too much.
Also, the way it raises (or sets) straight-up, with short twilight time is also un-natural for me. Like somebody turns on a switch in a bake oven.

Yeah, I miss the long summer nights where there’s still enough twilight to see by at 10PM. People would be out working in their gardens at 8 or 9. I don’t miss the sun starting to go down at 4 in the afternoon and it being dark by 4:30 during the winter so much though. It’s sunset at just after 5 in December and before 6:30 in June here. It’s like the heat and everything else though, you get used to it.

Being surrounded by ocean on all sides the climate here’s a bit different than in your part of Africa, which by looking at Google maps would be one of about six or seven countries all of which have a climate affected by a large continental land mass that I don’t have here. You can’t really compare SE Asia to Africa re: climate.

Anyway, back on topic: so I need to worry more about moisture ingress than about heat, is that right?

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I would guess so. Every installation that I saw here is in some enclosure.

Is this a recent version? They might have changed components due to the global chip shortage. Are you able to tell the part numbers used, I’m unable to read them.

Hensel KF 1000G is a wonderful box IP66/IP67 rated, and available in different sizes.

For my Pi (not outside) I use this housings, they can absorb a lot of heat.
image

Because my antenna is in the burning sun during the summer I added some simple heat sinks. Here and Here

I use cable clands to keep the moisture out of the box, where the cables enter the box. We have quite wet winters overhere, don’t want to get moist in. First year, so I don’t know if the heatsink is needed :relieved:

It’s pointless to put any heat sinks on the LNA. Especially when you enclose it after that! Where would that heat go???
I even took a measurement of the LNA case temperature right now, in my garage, for reference.
20.3 degree C!