During the The Things Network (TTN) conference in Amsterdam recently Thomas Telkamp arranged to borrow some time on the maritime satellite NORSAT2 and used it’s SDR to transmit LoRa as the satellite passed over Amsterdam.
The LoRa was transmitted on 162Mhz at 26dBm (400mW) into an approximate 6dB gain steerable yagi which was pointed to the horizon and at Amsterdam during the pass, data rate was 292bps.
The TTN receiver was a standard Microchip LoRa 169Mhz device with a simple antenna that was placed on the roof of a two storey building in the industrial area just to the North West of Amsterdam center. The LoRa was received as soon as the satellite cleared the horizon, a range of 2763km.
There was a challenge (prize) to be the first one to receive the LoRa during the conference. One of my own very simple handheld Arduino Pro Mini receivers fitted with a 433Mhz LoRa module (i.e. the wrong one for the frequency) and a telescopic whip antenna was good enough to receive the first LoRa on 162Mhz. The insertion loss of 162mhz into a 433Mhz receiever had been estimated by Thomas at around 20dB! The receiver was just left propped against the window of my hotel bedroom . I received packets close to the limit of reception with an SNR of -18dB.
The Video of the ‘LoRa from Space’ presentation can be viewed here;
I have been experimenting and testing some simple antennas for 868Mhz, primarily for Internet of Things (IOT) applications.
A dipole used vertically has around the same performance as a 1/4 wave vertical with 4 x 1/4 radials, but the dipole is easier to build. In the test setup shown below I am using a LoRa module as a transmitter and measuring the RF field strength it produces at some distance.
The dipole is built on a BNC chassis socket and connected to the radio module module is mounted direct onto the antenna using some adapter, I did not quite have the right one so there is a BNC plug to plug in there. I will re-test when I have the right adapter, but the objective of the test was really to see what we can do to reduce losses when a co-ax cable is used to connect the antenna.
Mounting the module direct on the antenna like this is often not going to be convenient especially if the antenna is going outside.
So what happens when a length of co-ax is used to connect the antenna as shown in the second picture ?
The co-ax cable (which is 0.5M long) outer now becomes part of the antenna and acts to detune it. Compared to the setup in the first picture the co-ax causes the antenna output to drop by circa 5dBm, a significant loss. Whether this can be mitigated by retuning the antenna will be the subject of another article.
So we are using the co-ax but the antenna performance drops significantly, what do we do about it ? One simple option is to use a clamp on choke of the type you see on monitor cables or power supplies.
The ferrite choke acts to stop RF travelling down the co-ax so it does not detune the antenna so much. The test result was that the transmitter was only putting out 1dBm less than in the first picture, which is a significant improvement over the 5dBm loss without the choke in place.
will be checking some other clamp on chokes to see if the same results occur with low cost chokes and longer cables too.