Recently I got my hands on a LNA. I have read that LNA should be as close to the antenna as possible to minimize the transmission noise before the LNA. Therefore, it is a good idea to put the LNA in a weatherproof box next to the antenna. I am trying to put together a receiver box and I am looking into what to put in the box.
My current setup is two RTL-SDR sticks on a Raspberry Pi running Power-over-Ethernet. My goal is to maximize the signal-to-noise ratio and hopefully protect the house from lightning strikes.
To achieve that, I have a couple of questions:
Should I put the Raspberry Pi in the box so the signal becomes digital as soon as possible? Or should I run a coax cable to the receiver box to power the LNA instead?
Do I need a DC-block lightning arrestor between the antenna and the LNA or a DC-pass arrestor between the LNA and the SDR receiver?
In the case where the Pi is in the box, does the Ethernet cable require additional surge protection?
If possible, also place the Pi in a metal box on or near the antenna mast. If not, see if you can place the USB stick near the antenna. To do this, use a very good USB cable (thicker copper, robust cable jacket). But from what many people report, this won’t work well beyond 3 meters (10 feet).
If the distance is more than 3 m, you need a coax cable.
Some (or all?) better SDR sticks will not need LNA if using a short cable. Depending on the price/quality/thickness of the cable, you should calculate approx. 6…8 dB signal loss per 10 meters. See cable specification. And even a well-shielded cable can capture interference signals. I’m therefore considering ring ferrite at the bottom of my cable.
So: the longer the cable, the greater the need for an LNA. Not only to compensate for the cable loss, but also to achieve a higher SNR for any noise injection that occurs after the LNA.
About Surge and Lightning… it has many aspects. The GND potential of your USB stick is probably connected to the shield of the coaxial cable and is therefore galvanically connected to the antenna.
If you feed your Raspi via Ethernet, the Ethernet shield is connected to GND / USB and to your router, PC, home automation a.s.o. Surely a reason to think about skipping POE power supply and to use a local PSU and WiFi. For me, it is the decision.
Another aspect of the effects of dangerous inductions is that even a distant thunderstorm can generate powerful charges in your antenna. Especially if it has a very high impedance, such as the ground plane / spider antenna. Voltages then build up in the antenna, which may destroy the subsequent electronics.
For this reason I have currently chosen a CoCo antenna that is shorted at the tip at λ/4. For my other antenna I’m thinking about what D. Balara: ADS-B Antenna Project wrote, see Figure 10, “Lightning Shortcut Stub”.
A 1:1 balun transformer could also provide some level of protection. I’m thinking about combining it with the ferrite ring I’m planning.
Anyway, any antenna mast must be equipped with lightning protection earth. Mine is already, since it got installed with Astra dish years before. But as noted: direct lightning or nearby lightning is not the only source for surge.
EDIT:
Similar discussion has been 10 yrs before here, apparently still valid:
/EDIT
I am looking forward to continue conversation with you, as not all of my thoughts have yet been translated into reality. And maybe some are wrong or inadequate?
I have used a 5 meter USB cable and got very reliable results (including MLAT). As you say it should be a good quality USB cable – like the following Ainope USB 3.0 cable from Amazon:
@foxhunter@jimMerk2 So far I am looking at about 20m from the switch to the antenna mast. From what I checked so far, I think RG-213 has the lowest attenuation of 34.6 dB/100m comparing to other RG cables.
@MaiKeeReis The reason I want to use wired network is that the Raspberry Pi also runs a GPS disciplined NTP. A solar setup and a media converter to optical fiber might provide maximal isolation and minimal latency.
For coax I would recommend the LMR series of coax: LMR400 (0.4 in diameter) or LMR240 (0.24 in diameter). These are double shielded coax versus RG-213 single shield. The loss for LMR240 100 meters is 30 dB @ 1000 MHz.
You should consider a better cable than that - that’s awful.
You need to define “better” amd what it means to you.
I consider the RTL-SDR Blog V3 to be a “better” RTL SDR (TCXO, Clk in/out, Bias-T, filtered USB power, improved heat dissipation), but it has no inbuilt LNA (and greatly benefits from one).
Several purpose built (1090MHz) SDR’s have filter and amp built in, but lack many of the RTL-SDR Blog’s features. These are also "better, but different.
Easy - use UTP, not STP cable.
STP also has the potential for earth-loops (no pun intended)
Right!
With “better” I had in mind the RTL BLOG sticks (V3 and also V4, even if some say V3 is the better one, both can be used without additional LNA at antenna if certain environmental things are given), then I had the blue “SDR ADS-B” in mind, seams to have LNA+filter, but no bias tee.
To differentiate between “better” and “normal” sticks, I would call the DVB-T sticks with a Rafael 820 tuner. I have an example here that is about 10 years old and has achieved interesting results with a good antenna. And with my $3 Homebrew Groundplane and $9 LNA + Bandpass from Aliexpress I actually got very impressive results - connected over 7 meters of that cheap 50 ohm 2.8mm diameter cable that they throw at you at Ali. Of course the environment also counts: I live in a village and the nearest houses are 50…100 meters away, so we have flat horizon and low noise.
The V4 has it’s points, but for ADS-B, performance wise, the V3 is the pick. (plus it doen’t need alternative drivers).
V3 - R820T2
V4 - R828D
Yes, either will work without an amp, but is greatly improved with one.
