The antenna and cable
Obviously
for adding an external antenna you need an antenna and a cable. An important
question is what gain you want use. The gain is a value that describes the
radiation pattern. If it’s low, the shape of the radiation is like a round
ball. If it’s high the it’s a narrow beam much like the light from a
lighthouse. Keep in mind that antennas do not amplify the signal, so the only
thing they can do is act as a kind of lens to direct the signal in a pattern
that you’d like it to do. In this example the low-gain ball will have a lower
range than the beam from a high gain antenna. But the disadvantage is that if
you put the high gain antenna on a high building you will have a weak spot
right below the antenna which you wouldn’t have if the pattern would be ball
shaped. <o:p></o:p>
The antenna
I use is a GP901 ground plane antenna. (http://www.sirioantenne.it/ru/cellular-base-ru/gp-901-cgp-900-c/).
It has a gain of 5 – 8.15dBi. This is too high, since you’re only allowed to
use 2.15dBi for sending. Because of losses the gain may be slightly higher, say
3dBm or so, but above that you should limit the sending power which can be done in the gateway configuration. This
leaves you with the same high-gain pattern for inbound messages but it will cause
a situation where nodes are out of range to get a reply while they were in
range to successfully send the message itself. <o:p></o:p>
Opinions
whether or not this asynchronous situation should be avoided differ. Some argue
that possible send and receive range should match. Others consider the
additional receive range as a free bonus. You should determine this for
yourself, but if you want to avoid this you should select an antenna with a
lower gain. <o:p></o:p>
For the
coax cable it’s pretty simple; cheap cable has a high internal resistance which
destroys the range of your signal. It’s advisable to use something like aircell
5 or equivalent cable which requires 100 meter or so until you get a
significant loss
Update: following a forum discussion on this subject 3 main types were identified.
1) Aurel GP868 (https://www.conrad.nl/nl/antenne-aurel-gp-868-650200599-190123.html) 35 euro. Cheap but it does have a pretty low gain resulting in a relatively short range. Still they perform better than indoor antenna's.
2) Sirio GP901 (http://webshop.ideetron.nl/GP901C) , about 100 euro. A higher gain resulting in a more narrow beam. Still the black spot while mounted on a 23 storey building was to floor level less than 50 meter. The specs show the average frequency is with 900mhz either a bit too high or a bit too low, but ttnmapper measurements show it performs pretty well on 868mhz.
3) Procom CXL 900-3LW-NB/868MHz, about 250 euro. Top notch stuff. This might be overkill when placed on top of your house but if you manage to get a really good, high spot it may make sense to try to squeeze everything out of the opportunity.
There are two interesting alternatives. The first is an DIY alternative for #1, described here: https://www.thethingsnetwork.org/labs/story/homemade-outdoor-868mhz-antenna
And then there is the Sunhans antenna: https://nl.aliexpress.com/item/2014-Sunhans-790-2700MHz-15dBi-Omnidirectional-Fiberglass-Antenna-For-3G-4G-GSM-CDMA-WCDMA-Cell-Phone/2035550182.html?spm=2114.13010608.0.0.WJfq8s&detailNewVersion=&categoryId=5092201. This was mentioned being the best bang for bucks. I assume it will perform somewhere between #1 and #2, probably closer #2 than to #1. A few community members ordered one and some initial plans are made to keep a test&measure meetup including all of the antenna's mentioned above.
