Ordered.the rtl-sdr. Have asked them a question though, they like all amp producers, want it as close to the antenna, before the coax, as possible. In powering it from inside via bias t set up or suitable powered dongle for coax work, what the distance limits are before the voltage starts to be affected by distance.
The RTLSDR blog dongle has an integrated bias-tee that can put about 5V onto the coax line. A normal RTL dongle doesn’t put 5V on the coax and the prostick doesn’t. There are some systems that will fail with 5V on the coax and there are some antenna that don’t work with 5V on the antenna input.
There is a program you have to run on PiAware to turn on the bias-tee on. And then it will power the coax with 5V and all the way to the mast head amplifier.
The other solution is to use a power injector for coax. These are fairly common with F connectors but harder to find with sma connectors.
The distance limits for power over coax is hundreds of feet. A good coax cable has a DC resistance of a couple of ohms. The 50ohms and 70ohms rating is based on RF signal reactance.
When you know the dimensions of the cable you can calculate the resistance and using this value the voltage drop.
But before you get into such voltage problems you will have signal problems…
For a voltage drop of about 2.5 V your cable should have about 350 m (considering a center core diameter of 1 mm)
Nominal Conductor DC Resistance = 6.4 Ohm/1000ft
Nominal Outer Shield DC Resistance = 2.7 Ohm/1000ft
Total DC Loop resistance = 6.4 + 2.7 = 9.1 Ω /1000ft
For any appliance to work properly, DC supply voltage at its terminals should not be less than 90% of its rated dc voltage (unless it has a built-in voltage regulator). Hence the Amp should get not less than 4.5V DC at its terminals.
With Pi sending 5V DC, this allows only 0.5V drop in the coax. Assuming Amp consumes 300mA, the total DC resistance of dc loop should not exceed R = Voltade Drop/Current = 0.5 / 0.300 = 1.67 Ω
Max permissible length of coax = 1000 x max permitted R of DC loop /Coax DC loop resistance per 1000 ft
= 1000 x 1.67/9.1 ft = 184 ft
Thanks all for your contributions.My plans,… first some description of my setups. 2 antennas total heights 8.3m off the ground and 6m off the ground. Both next to a flat roof, 1st antenna is about 5m above roof height, 2nd about 2 m above roof height. So to install the amps and PI3’s near the antennas, 1st antenna is a job to drop the mast down to get to the antenna, 2nd one not so bad, a step ladder has you near the antenna.
So to install the PI3’s and amps near the antennas, 1st antenna not easily accessible, 2nd one no problems. So method of working this, using existing coax runs OR putting in network cables (I have about 90 metres of Cat 6 network cable in the shed, so no issue/cost there… lol). So network cable and utilising POE power source, rated to 100 metres, gigabit speed, plug and play with auto determines power requirements. I will be going with the POE option given what that unit can do.
So the 5 metres off the roof problem. Thank heavens for heywhatsthat !!! I have both antennas plotted, with location and heights. I had a look at the lovely dump 1090 maps, showing the theoretical distance of each antenna (which I find very accurate, when observing actual detection’s !!!). The 2 plots, with the difference in heights, in my main detection areas, are virtually the same with heywhatsthat system, so I don’t need to have antenna 1 as high, to get the same results, access to the weatherproof box and Pi3 /amp a lot easier, without have to raise or lower the mast. I am going to get the V3 rtl-sdr dongle and the new pre amp and also the nevis uptronic pre amp filter, and compare the 2.
Now I will firstly have to drop antenna 1 to remove a masthead amp near the antenna that my current amplifier uses (and as well on antenna 2). But then I will be able to do this upgrade in stages. I can trial the new amp similar to the current set up, with the amp/Pi3 being located at the end of the coax run. and see/compare with current setup. And then, with not too much problems, run the network cable, mount the Pi3/amp in the weather proof enclosure, close to the antenna.
The sdr-rtl amp power use is about 170 mA with a 180 mA at startup (checked 3 times just to be sure)
Voltage drop value helps me to chose the input voltage to have at consumer (LNA) the correct voltage.