Co-linear antenna modeling2-1.pdf (700.0 KB)
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Thanks @kylix, if the black device was not a balun but a piece of plastic I wonder how the IRNAS gateway performed as the VSWR without a groundplane is high. 4:1 @LoRaTracker tested these antennas some time ago and reported the results to be something like 4:1
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@dicktonyboy, thanks for the pdf. Will test and see how close the predicted VSWR curve is to my build. Wasnāt able to reply earlier, was attending our local TTN/IOT Meetup.
Build it and check the sensitivity of each element to change. Try to only change one parameter at a time to isolate performance changes and then see if you can scale to move the VSWR to the frequency of your determination but use a ground plane everywhere and observe the 10 lambda rule. - easy eh? see my time sump loom!
thanks @dicktonyboy, will do. Before the 915 version was published I scaled the 868 version to 915MHz. Iāll go back and build the 868MHz version and compare to the modelled performance. Iāll then start moving the frequency. interesting the published 915 version has significantly smaller diameter loops, never understood how they came about. Far more than simple adjusting for different frequency, almost designed to a different concept. Anyway, thanks again for the document, iāll give it a try.
The ātouchy-feelyā argument is to consider the wave travelling straight up the plain wire aerial and then look at the delay incurred as it has to travel around the loop - which is āorthogonalā to the main wire. What happens if the loop is one quarter, one half and one wavelength long? c = f Ī» (remembering that c is slower in cable by the velocity factor).
Today I tested this https://www.aliexpress.com/store/product/868MHz-high-gain15dBi-omni-base-fiberglass-antenna-GSM-900M-outdoor-roof-monitor-antenna-N-female/413214_32772798538.html antenna for a friend who bought it via Aliexpress.
The result is terrible, gain performance is not 15dBi but 1 to 2 dB less than a simple 3 dollar dipole . Noone should ever buy a āglide baseā antenna, it has a totally fake gain figure.
I must say that the VSWR was good, better as specified. Almost as good as a dummy load 
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@dicktonyboy, understand about the phasing loops between the antenna sections so I rescaled the 28mm OD loop in the 868MHz design to 26.5mm in my 915MHz adaption. Then when Fabienās 915MHz design was published it had 13mm OD loops!!! Could not reconcile such small loops. Would mean the velocity factor > 1 and that canāt happen.
So i got myself a new gateway, a Laird Sentrius RG186, but the range is a bit disappointing. Could be my lora test device of course, it has a 1/4 wire antenna. But about 500 meters is max (@ SF12).
I want to get an external antenna and think i have to decide between these 2:
https://www.truckerswereld.nl/sirio-gp-868c-gsm-antenne/
https://www.truckerswereld.nl/sirio-omni-900/
Any other recommendations? Will these antennas increase range significantly?
that depends on where and how you place them.
in general ā¦ donāt use a very long cable and use a good quality cableā¦ with low loss @ 900 mhz.
the antenna should be place on a spot with no other object within1 mtr or so
ideetron is a lora specialist shop so I think they can advise you about the cable type to
Also depends on what antennas yoyu are currently using and where they are located.
Iām using the default antenna that came with the gateway, itās inside about 4 meters high under the rooftop.
Are you getting good coverage from this? I have several roof-space mounted GWās around the country (including a bunch of the Lairds) and in each case I have had to be careful with placement due to various roof constructions, especially in modern or recently refurbished buildings.
Often the roof itself will be heavily lined with thermal insulation that these days is increasingly made with an additional layer of metal foil - a great RF blocker!..sometimes several layers. (Foil backed plaster-board/dry-lining increasingly common also). The foil doesnāt just āblockā but if too close to ant seems to help ādetuneā also. In other instances have discovered a layer of mineral loaded water resistant āfeltā to ensure good weather proofing - again some of these materials appear to badly affect rf propagation. In these cases have found that rather than going to the highest roof point assuming height = better range, have found dropping to attic floor level has actually increased range as signal ingress/egress under insulation better.
Best yet a small hole under the eves, later closed to stop insect ingress, allowing use of external antenna on short run of ultra low loss feed cable usually works wonders (typically 10-30db signal improvement 
) 
Thereās actually no insulation besides the asbestos roofingā¦ Donāt know if that blocks the signal.
Iām going to put an antenna on the roof anyway, but if iām going to spend money on that i 'd rather buy a good one.
With these 2 options:
I think the GP901C is the better one? Thereās also a GP868C, wouldnāt that outperform the GP901C?
The GP901 C has a Frequency Band of 870-960 MHz where the GP868 C has 835-900 MHz.
I think the GP868 is better suited for Lora in Europe.
Yes, if you have managed to increase c - youād better prepare the universe What link of Fabiens are you referring to exactly? Iām trying to gain access to a radio lab here over the student holiday period. Annoyingly we did just build an array which was incredible at about 550M and then we scaled it to 868MHz - Iāve lost the paperwork but the unit is up on the roof. @loratracker has achieved some remarkable results too (but is currently side-tacked, deep in a local, (unused), railway tunnel proving that the Semtech chip has a noise issue).
Here is the link to the two different designs. its on the IRNAS GitHub site https://github.com/IRNAS/ttn-irnas-gw
@dicktonyboy, hope you can find the paperwork for the converted 550M antenna. Guess youāll be shimmying up a drain pipe to measure. 
Over the weekend I will also try a Coaxial Colinear, bit fiddly to build but hope the VSWR match is better than the 2:1 in Fabienās modelling work. If it looks promising, Iāll test the gain and pattern.
So a question for @BoRRoZ and @Charles to see how you guys found the Rak antenna, the fiberglass one you both ordered and discussed in the bargain basement thread?
I hope to deploy some more gateways using @Charlesās pcb to integrate Rak831ās with Pi Zeros. I ordered an antenna with some Rak831ās so would be interested in data on tuning or performance?
Thanks gentlemen!
Garry
https://m.aliexpress.com/item/32852314761.html?trace=storeDetail2msiteDetail
I just received one last week but I havenāt done any testing yetā¦
My gateways have SMA pigtails and the RAK antenna is delivered with an RP-SMA connector, so I need some additional bits to swap easilyā¦