Ok next question. I have the RTL-SDR FIlter/LNA. I know the “best” thing to do is have it directly connected to the antenna. But what is the best way to accomplish that? I don’t want to strain the SMA connectors on the LNA. Is there a good way to have the LNA connected directly to the antenna and still have a thick feed cable down to the Pi? Or should I just use a NEMA enclosure and a couple 6 inch bulkhead pigtails, and then run a short cable from the antenna to the box and then a longer line down to the RPi?
If you’re going to mount the LNA right at the masthead, I’d use a very short pigtail made from high quality coaxial cable from the bottom of the aerial to the LNA. I’d never connect the LNA directly to the aerial.
I’ve done similar on 2m, very short cable attached to the aerial which goes to the mast head preamp and then thicker coax down to the shack.
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You don’t need to have a thick feed down to the Pi if you have the LNA at the antenna.
The RTL-SDR LNA has a gain of 27dB and a Noise Figure of about 1dB. This will completely dominate the overall system Noise Figure. For example SDR receivers typically have a noise figure of 6dB. With the LNA directly feeding the SDR, with no coax in between, this would give an overall System Noise Figure of 1.02dB. Even with 10dB loss in the coax between the LNA and the SDR the overall System Noise Figure will only drop to 1.26dB.
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I will have in the neighborhood of 50-60 feet of feedline to the RPi. I was thinking of using LMR240. Is that appropriate? Should I use LMR400?
For the connection from the antenna to the LNA my options are RG174, RG316, or LMR-195. This cable will likely be 3-5 feet to the enclosure with the LNA.
For the enclosure I’m planning on using a couple 6 inch RG316 N to SMA pigtails with female N bulkhead connections to the outside.
Does all this look reasonable? Is it too many connections or will the LNA make up for it?
LMR240 should be fine. 60ft of LMR240 will be about 5dB loss and will make negligible difference to the overall Noise Figure as it is on the output side of the LNA.
Losses from the connection between the antenna and the input of the LNA will directly add to the noise figure so you want to keep these as low as possible. Therefore I would go with the LMR-195. 5 feet of LMR-195 is about 0.6dB loss. 5ft of RG316 is about 1.8dB and 5ft of RG174 is about 1.7dB. The RG174 N to sma pigtail and connectors on the input will be about another 0.5dB.
So overall your system noise figure with 60ft of LMR240 down and using each of your cable options between the antenna and LNA would be:
RG174 - 2.9dB
RG316 - 2.9dB
LMR-195 - 2.2dB
What dongle are you thinking of using?
Well I have many in my collection! I was using the RTL-SDR V3 previously, but I have never been a fan of the software selectable bias-tee. That’s a bit of a long story which I won’t go in to unless someone is interested. Anyway I still have that dongle, but I also bought and am planning to use the Nooelec NESDR SMArTee v2. I just prefer having the always on bias-tee, and I’m thinking that for this application the performance of these dongles should be similar. I’ll probably connect the dongle directly to the feedline and then use a short USB extension with ferrite beads to connect to the RPi 3 B+.
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I haven’t tried running PoE and setting up a remote pi box mainly due to the stupid high temps that I can get, and not taking the time to experiment with a test rig to figure out if the temps would be workable.
I use coax for other radio stuff, so went with that.
I have a DPD 1090 antenna on a 20ft guyed mast on the roof,
with 25ft LMR400 to a Kuhne 1090 LNA in a waterproof box with breathers outside under the eave,
to 30ft of LMR400 into grounded lightning arrestor,
to a waterproof junction box for all my cables before entering the house,
to a biasT power injector, LMR400 1.5ft jumper,
InStockWireless PD5122 low-loss, 30db isolation, splitter/combiner,
LMR240 jumpers to Airspy R2s,
to a RPi4 and a RockPi, so I can compare and test different tweaks.
I use LMR400 for most of my antenna runs, since it is very durable and works well at Sat freqs.
I’m happy with my setup.
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Drool… How do you like that bad boy?
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I know I paid less than that, I imagine covid-19 is a factor or something.
Its a great LNA, well built, working very well.
Easily overcomes my cable loss, and works great with the Airspy.
I posted about it before, though some of my setup has changed wince then… Its here
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Last question is about LMR cable. I was looking at the Ultraflex LMR400 expecting it to have a smaller bend radius than thane standard LMR400. But it does not, they are both a 1 inch bend radius. So what is ultraflex for? Applications where it has to flex over and over? If I am doing a normal fixed installation does ultraflex do anything for me?
Yes, it is for applications where it will be repeatedly flexed, such as a tail from an antenna on a rotator.
Note that the minimum repeated flexing bend radius for ultra flex is actually 4 inches. The 1inch bend radius is a one off figure. Loss is also a bit higher on the ultra flex compared to the standard version.
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Ya it sounds like for my application regular LMR400 is fine. Ultraflex is also about 50% more expensive than standard LMR.
I don’t use LMR-400 Utraflex, just the regular stuff for my ADSB and UAT antennas.
Unless you need to make some really tight bends LMR400 should be fine, its great cable and flexible enough, just allow for any corners carefully(make sure you have a about 3-5 inches more per bend).
Though not really required for LMR-400, I’m sure there are some tips illustrated online, for when you have to go around a 90 degree wall/roof bend, where you don’t take a straight path, but more of a curve and funky wave to keep the cable radius greater than spec.
As LawrenceHill noted, the flex stuff is usually for a pigtail for something you expect to move or reposition/tune.
For my Sat antenna I use 1/2 Heliax LDF4-50A, with a superflex FSJ4-50B pigtail to the antenna for the rotator.
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Ok and what’s the best way to make entry into a house these days? Normal Canadian construction. Vinyl siding. I’m using LMR400 for the feedline to a lightning arrestor but I’ll probably use a short run of LMR240 or smaller to make entry and get to my RPi.
Thoughts?
In the past I have basically just drilled a hole, made a drip loop, and then sealed around the cable with that grey sealing putty stuff. Seemed to work OK.
That works fine if you just need to run 1 cable.
If you need to be subtle, you can run under the siding lip, or along window trim and paint to match the house.
If you are going to get “into” this hobby and put up more antennas, you may want a larger tunnel hole covered by an outdoor waterproof box.
In that case, something like this,
and a box for the outside (with your desired dimensions) works well
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I think I’ll just use a single hole. I don’t have any interest in any other radio stuff. I got into this due to aviation, not an interest in radios.
Don’t want to receive VHF aircraft transmissions and listen in on airtraffic too? 
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Friend, my day job is flying large airplanes. The last thing I want to do at home is listen to more stupid pilots!
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What kind of rf surge protectors are everyone using? I have been looking at some Polyphaser units but they are not cheap!
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