Technical information 2m band

 

The general set up for 2m is the same as the HF bands. Antenna, cable, filter, SDRPlay1a receiver, computer, internet.

But there is a lot to say about the differences.

 

Antenna 

We use two antennas on the 2m band. One is used for horizontal polarization, the Big Wheel. And one for vertical polarization, a Diamond.  The horizontal antenna is connected to a separate receiver, which is set up for the lower 1MHz of the band. The vertical antenna is also connected to a separate receiver, which is set up for the upper 1MHz part of the band.

There are two reasons for this:

- The lower band is more used for horizontally polarized signals like DX QSO's, while the upper part is more used for vertically polarized signals, such as local QSO's, meetings and repeaters.

- a second reason for splitting up the band in two 1MHz parts is the fact that the receiver is not working very well using a sample rate of 2.880.000 samples per second. We come back on the technical reason for this.

 

Here we show the pictures of both antennas. The vertical is on top of the right mast. The horizontal is the clover with three leafs.

 

The cable we use is H100. The length is about 20 meter.

 

Filter

Between the coax cable and the receiver we have placed a 5 pole high pass filter. All signals under 70MHz are attenuated about 50dB. The pass through attenuation is less then 1 dB. The filter is placed after the vertical antenna. We have to make a second one for the horizontal antenna,.....under construction.

 

Photo of the filter:

 to come

 

 

Receivers

Both receivers are the SDRPlay1a. Both are set up the same for now. The difference in polarization can give us a reason to optimize the configuration. The config file looks like:

 

#######################################################
# 2m RSP1a nr 1
#######################################################
band low
device !rtlsdr 127.0.0.1:1234
samplerate 1536000
centerfreq 144500
gain -33

antenna Horizontal Big Wheel

#######################################################
# 2m RSP1a nr 2
#######################################################
band high
device !rtlsdr 127.0.0.1:1235
samplerate 1536000
centerfreq 145500
gain -33
antenna VerticalDiamant300


############################################################################

 

As you can see, the receivers are set up for a sample rate of 1536000 samples per second. So we cover the whole 1MHz with a little overlap between the lower and upper 1MHz.

 

 

Computer

Initially we used a very old laptop computer, but that did not work very well. We did some experiments with a Pi4 minicomputer, running Raspbian Operating system.

So, we were forced to use the websdr-rpi software. And luckily it worked OK, as long as we did not go higher than 1536 ks/s.

So, two receivers go into the Pi4, so two rsp_tcp processes are running on the Pi4 and one websdr-rpi process. Our start up file (start-websdr.sh) looks like:

-----------------------------------------------------------------------------------------------------------------------

## put all cores in performance mode
echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

killall -s 9 websdr-rpi
killall -s 9 rsp_tcp

sleep 1

SoapySDRUtil --find

sleep 1

screen -L -Logfile ../pub/2mlow.txt -dmS rsp_tcp1 taskset -c 0 rsp_tcp -d1 -p1234 -a 127.0.0.1 -r53 -L0 -G0 -A-50 -W -n16384
screen -L -Logfile ../pub/2mhigh.txt -dmS rsp_tcp2 taskset -c 1 rsp_tcp -d2 -p1235 -a 127.0.0.1 -r53 -L0 -G0 -A-50 -W -n16384

sleep 3

screen -dmS websdr taskset -c 2 ./websdr-rpi

exit

---------------------------------------------------------------------------------------------------------------------

 

The CPU of the Pi is quite busy, but no problems with heating up to much.

 

Latency

We were a little concerned about the performance of the Pi4, but all goes well. The rsp_tcp processes are running each on a separate core ( -c 0 and -c 1 ) and the websdr-rsp process on the third core (-c 2). In the TOP command we get the following output:

 

top - 21:57:16 up 70 days, 10:44, 1 user, load average: 3.13, 3.03, 2.98
Tasks: 122 total, 2 running, 120 sleeping, 0 stopped, 0 zombie
%Cpu(s): 60.1 us, 7.5 sy, 0.0 ni, 31.6 id, 0.1 wa, 0.0 hi, 0.7 si, 0.0 st
MiB Mem : 3827.9 total, 3372.8 free, 153.4 used, 301.7 buff/cache
MiB Swap: 100.0 total, 100.0 free, 0.0 used. 3498.1 avail Mem

PID    USER     PR    NI    VIRT    RES      SHR   S     %CPU   %MEM    TIME+      COMMAND
474   root      20      0   76432  30116  1868   S     98.0        0.8        1345:26    rsp_tcp
478   root      20      0   76432  29872  1876  S      96.0        0.8        1324:30    rsp_tcp
489   root      20      0   52184  29384  3992  R      77.2        0.7    590:34.58    websdr-r+

With these settings we can keep the latency around 0.5 sec.

 

 

Power consumption

The power consumption is due to the Pi4 very low. Receivers plus Pi4 is around 15Watt.

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