Yes you can connect the antenna to the gateway with a coax cable and the length of the cable does has his influence.
The longer the cable the more attenuation to be expected. But also the type of cable has his influence.
For example a consumer cable like the RG-58 has a attenuation of 70 dB for a length of 100m at the EU LoRa frequencies. So a cable with a length of 10m would result in a attenuation of almost 7 dB (a -7dB amplification). Meaning that only 20% of the power put into the cable will be fed into the antenna (and vice versa).
When using a better cable like the AirCell 7 coax cable the attenuation for a 10m length will be just 2 dB (a -2dB amplification). Meaning that 63% of the power put into the cable will be fed into the antenna.
Formulas:
dB (power) = 10log(Pout/Pin)
for example :
Pin = 5W Pout = 1W (very heavy cable loses).
db (power) = 10log(1/5) = -6.989 aprx -7 dB
Pout/Pin = 10^( dB (power) / 10 )
for example : Pout/Pin = 10^(-7 dB/10) = 0.1995 aprx 20%.transfer meaning 80% loss.
It sounds like much even with LoRaWAN who has a good signal to noise ratio a RG-58 could work out.
… I am no RF engineer but for 80mm of copper twisted two-up this works very well, great design.
I have no testing equipment so I can’t say how well it is tuned to 868mhz or how accurately I built this but the difference is clear. I was finding over 400m was hard to achieve, even with the node set to SF12 and the portable gateway placed high inside the roof void like this antenna. RSSI was never lower than -120db but it either worked or it didn’t but the range was disappointing.
The result with the same home made ESP8266 gateway in the same less than ideal position (geographically) but using this ground plane antenna is 4-5km already and I haven’t sough out further suitable places to try. The RSSI is agian in the -114 or -117db range so some room left yet I guess?
I may actually put this antenna 5m above my roof on the old TV antenna pole and try it out on the RAK831 for more testing. I suspect there is plenty more to learn about the node transmit antenna too… wonders if Ryanair will be flying to somewhere near the Belgium/Nederland antenna hangout?
Then with 0.25 wavelength radials, the antenna peaks when the vertical radiator is 0.228 wavelength (average over 3 modules). This is shorter than 0.25, but the diameter of the wire etc does reduce the dimensions a bit.
With the radiator at 0.25 wavelength you might loose 2dB or so.
Antenna tests done at 868Mhz, with a LoRa RFM96 as transmitter, antenna directly attached to module, results at 915Mhz should be similar.
I have 3 such DIY GP antennas based on N-type.
And last one I made based on this tutorial - central wire is 76mm and radial wires - 82mm.
And seems this antenna has worst signal than another 2 antennas which have all wires of 82mm.
But there is also the difference - in this 3rd antenna I use copper wire with 2,5mm diameter which is thicker that on other 2 antennas - they have 1.5mm copper wire.
Is it important to have such thin wires or 2,5-3mm also is ok? I think thicker wire better because it hard to bend.
@lex_ph2lb: the best antenna I have is still a badly-home-made GPA according to your instructions, thanks.
While searching around to understand whether radials length in a GPA need to be shortened according to velocity factor, like the vertical element (warning: I am speaking of things totally outside my expertise), I crossed at least two links that suggested that length of radials should be 0.28•λ•velocity factor:
I found similar hints to longer radials in other places.
Did you ever experiment on this (call open to others too )? Today I have built a new version with 92mm radials, however I have yet to compare (It takes some time, I see different RSSI and SNR depending on time, so I have to collect data to average them).
In ideal situations the radials and radiator would be exactly 0.25 * λ.
But in practice a radial is little larger for example 0.28 * λ instead of 0.25 * λ.
So indeed 0.28 •λ•velocity factor is a good start.
But as you see in the last link you include, the angle of the radials is more of importance then the length (although there’ s a optimum).
The velocity factor depends on the λ and the diameter of the used material. I could go into detail there but i would have to work it out in formulas. But when you can read German see page 85 on the link below (watch it : large download)
Thanks, also for the gigantic book, although I do not read German (but formulas are universal). If possible I would just follow a recipe, but curiosity is always on alert on possible optimizations. In my original GPA I followed the sizes you suggested (@BoRRoZ: yes, the very same sketch ).
Here is mine, built around a SMA connector and installed on top of some sewage pipe couplings to power a ttn mapper device. Performance is quite impressive, we’ve got a 12.4 km sf7 link in a suburban, hilly area. Setup: arduino pro mini+rfm95w+NEO-6M GPS.
Results here: https://ttnmapper.org/special.php?node=testboardrfm95w&date=2019-06-30&gateways=on
Well, that sewage stuff, even if looks shitty, is providing very cheap yet reliable enclosures. I’m using them since years back for all kind of outdoor protos. However, I’m a little bit ashamed to carry it on my car’s roof By the way, it has two HDD magnets on bottom side… Thanks for appreciating and thanks a million for sharing antenna plans.
Very interesting post. I would like to know if it would be better build a bigger antenna. For example, instead of a λ/4 antenna build a 2*λ or bigger to improve reception.
Thank you
Bigger does not always mean better. And even a good SWR or a high gain does not mean that this antenna is suitable for your application. One important thing is the radiation pattern and the requirement that the antenna should not be disturbed by other things nearby.
Look a little bit around in this forum and you will find many ideas.
This will be probably the stupidest wuestion in history of this forum… Lol
Today I made this GP antenna, thanks for this!
I have question. Can I mount it on aluminium pole as it is on the photo (touching the ground)?
Or the antenna should be isolated from the pole? I will be using for LoRa.
Yes, this is one of the best ways to mount an GP- antenna. On top of the pole and as high and as free as possible.
Nothing nearby, this gives you a perfect round- radiation with vertical polarization.
The shield of the coax can and should be, but doesn’t need to be connected to the pole (ground).
)? Today I have built a new version with 92mm radials, however I have yet to compare (It takes some time, I see different RSSI and SNR depending on time, so I have to collect data to average them).
By the way, it has two HDD magnets on bottom side… Thanks for appreciating and thanks a million for sharing antenna plans.