A LoRa 868MHz Collinear antenna

A warning ahead: the following is a YMMV project. Coax collinears have come in very different flavours and it is very hard to find at least two sources which agree on a reproducible design. Ok, they all consist of a number of halve wave segments corrected for the velocity factor of the coax used. But that’s about where the similarity ends.

Variations come in:

  • Top section
    • just end with a halve wave part,
    • left open,
    • shorted or even
    • ended with 50 ohm resistor
    • A quarter wave coax top section with a quater wave whip
  • Bottom section
    • Quarter wave coax section
    • Decoupling with
      • Radials
      • Sleeve
      • Ferrite core

Realizing this could well end in a frustration (i did some collinears for ADSB with mixed results) i started off with what i thought was a proper design. However, while building i didn’t put the bottom part together in the way it was planned.

While measuring the SWR with a N1201SA antenna tester it was impossible to tune the SWR by cutting the whip. Only after removing the complete quarterwave top end, the SWR got right. Tuned it a little more by cutting the top halve wave with a cm or so.

This is what the final design became:

CoaxCollinearLoRa

 

CoaxCollinearBottom

I used a N-connector pigtail cable (so the blue part is a little piece of RG174 coax).

Some pictures:

 

Having having done the tests and shooting the pictures the whole thing was glued together to prevent water from getting in.

Results and field tests

The most exciting part! As mentioned the SWR results were disappointing until the top quarterwave part was taken off (probably as a result of not building the bottom part as designed). When the antenna was properly tuned the SWR was stable even when moving or touching the coax feed: the radials do their job! Also detuning caused by the PVC tube was neglectable.

A temporarely gateway was setup to compare the field strength to two whip antenna’s (which tested to be nicely resonant at 868 MHz). A quick comparison showed the collinear to outperfom the whips. Promising!

20180326_230713

Reference Long and Short whip

Later that day i did some more tests at 530m from my gateway and 7.2km from the gateway at Eindhoven airport (FFFEB827EB75534E). My LoPy LoRa tracker was used as signal source. I used 3 types of antenna’s: Small whip, large whip and collinear. I left the tracker running for a few minutes to have a number of observations). At home the results were analyzed from the datafile Node-Red creates. RSSI values varied +/- 4 dB without touching anything. For each antenna the average of the RSSI values was calculated, resulting in:

Distance to gateway Eindhoven airport: 7.2 km

Short whip    avagage RSSI  -118 dBm   (11 packets)
Long whip    avagage RSSI   -115 dBm  (20 packets)
Collinear       avagage RSSI   -110 dBm  (14 packets)

 

Distance to home gateway: 530.

Short whip    avagage RSSI  -113 dBm   (13 packets)
Long whip    avagage RSSI   -108 dBm  (26 packets)
Collinear       avagage RSSI     -98 dBm  (16 packets)

Still promising and about what to be expected of this configuration! This weekend the gateway and the antenna will move to the rooftop. Results will be shared here!

 

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8 Responses to A LoRa 868MHz Collinear antenna

  1. Rob says:

    Nicely built and congratulations with the results.
    I have built a lot of types of antennas (GP, yagi, J-pole, colinear and others) for different frequencies (144 up to 1800 MHz) in the past 20 or-so years.
    Colinear antennas are not that easy to build, it all comes down to precise measurements and equal spacing between the elements. Wrapping the copper wire around the braided sleeve is a nice way of making this easier.

    Have you ever checked the content of the long and short whip antenna you use?
    I had some short and long whips with the only difference in the plastic housing …

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    • keptenkurk says:

      Rob, thanks for the comment and compliments! I never checked the interior of those whips but was surprised how well they were tuned for 868 MHz (a lot of crap is being sold). I suspect the short one to be a helical and the long one to be a 1/4 wave. I’ll try to find the source of the long one.(it was a gift). My LoPy with this antenne inside the car was picked up by a gateway at 82 km the other day!

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  2. coste says:

    Hi ,
    I believe getting a good VSWR with this type of collineair is very difficult.
    You measured with a N1201SA, what was the VSWR ?
    A screenshot of a sweep of the analyser would be very nice.
    I am thinking of building something like this with Aircell7 coax.
    I must be experimenting with the 1/4 wave at the bottom, as this is the impedance transforming part.

    Costo

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  3. keptenkurk says:

    @Costo. I’m back home again…
    I did measure the VSWR but did not take a picture. It was close to 1.1 after adjusting. But here’s the full story:
    Originally the design was different. I don’t recall the sources (there are so many) but it was with top shorted and with quarter wave whip added. Also the bottom end had the core of the feedline connected to the radials, which in retrospect doesn’t seem to be very clever… When measuring VSWR the resonance was nowhere (600Mhz or so). So i started cutting the whip: surprisingly with hardly any change! Lacking time (and about to give up on yet another coco disaster) i removed the whole top end which showed a big improvement. But it resonated still too low in frequency (820 MHz). I then cut the top halve wave by one or two cm until it was on 868 MHz (the amount can be seen on the picture, taken after modification). By that time i was in doubt if the antenna would work at all as designed but was delighted by the field tests which confirmed the extra gain to be expected.
    I think the radials do a good job and contribute to the good VSWR. Most designs use a copper quarter wave tube (probably to fit it into a tube). while measuring i could not change the VSWR by moving or touching the feedline so it is well decoupled. Another worry was the radiation pattern in the vertical plane. Phasing errors can lead to an elevated main lobe. But up- or down tilting the antenna gave worse results in the field test showing the main lobe is horizontal.
    Lessons learned:
    1. I still don’t understand the coax collinear by theory
    2. Don’t start this if there is there is no VNA (or N1201SA) available.
    3. Wraping copper wire around the shielding ends provides a solid construction and well defined length.
    4. radials provide a good decoupling and don’t get in the way that much.

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    • richardwenner says:

      Radials are a vital element involved in terminating your transmission line. Use 4 at least and watch the stabilising effect on the VNA.

      Liked by 1 person

  4. Geert GF says:

    With these kind of constructions one needs to pay attention: PVC tubes come in flavors and some will actually have a fairly big influence on your antenna. Simply putting a well tuned antenna in free space inside a PVC tube can effect the VSWR dramatically. The effect in itself is to be expected however one wouldn’t expected this effect to be this large! More importantly there seems to be a fairly big difference between the typical grey PVC tubes and less common white versions. ( Don’t know the exact chemical difference between the two) So always tune the antenne inside the housing you’re planning to use. This can explain the difficulties in tuning the antenna in the previous comments

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    • keptenkurk says:

      Geert, that’s absolutely true. I have been playing with the grey type of tubing which contains carbon. Any conductivity of the tubing can be tested by putting it in the microwave. It should not become hot. The white one i used now (brand Martens) did not affect the tuning at all.

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