The BIG and SMALL ANTENNA topic part 2

Most antenna’s will not have a perfect 50 ohm match and therefor they will return a (smaller or bigger) part of the signal back. This signal will usually also travel on the outside of the feeding coax back to the node. This will make the coax (and the node) radiate and so they will become part of the antenna, disturbing your theoretical radiation pattern. (The pattern may radiate more upward or downwards, leaving less power in the desired horizontal plane)
So comparing different antenna’s with different coax cable lenghts is tricky.

If you want to prevent the feedline and/or node from radiating and disturbing the antenna radiation pattern you should put a ferrite clamp-on over the coax, as close as possible or at 1/2 lambda (17cm) from the feedpoint of the antenna.

Ferrite clamp-on filters are available in many sizes , so you can easy find a size that will exact fit your coax feedline while it’s position on the cable stays fixed.

The placement of a antenna has influence on the radiation pattern. Take a look at the simulations I made for the GPA antenna : Very simple homemade outdoor 868Mhz antenna (groundplane)

Thanks @costo, in this case I have a male to male SMA adapter in one case, and about 20cm of H-155 (with SMA connectors at both ends) in the other. Has ferrite to be isolated from the metal adapters?

Thanks @lex_ph2lb, I crossed your charts time ago (they made me put a single channel gateway just under the roof :slight_smile: ), and I also implemented the GPA following your instructions (with some deviation).
However, in this case there is not much difference in height: 20cm of coax, by the way I also compensated the height in order to have the antenna top more or less at the same height. And RSSI difference is about 5db (-55 vs -50), on 3 alternated sessions. I will have to try with a different node.

1 Like

I do not think it is nescessary to isolate the ferrite clamp electrically from the outher coax.
In most cases the use of several ferriteclamps in series will improve the RF-isolation.

Coincidentally I performed a similar exercise as jpmeijers original catalog, but for the 915MHz band. The apparent poor performance of off-the-shelf antennas caused quite a stir on LinkedIn.

Here are the version one results, with the antenna mounted on the VNA just like jpmejiers did. I believe that’s a good indication of performance in most typical configurations, where you screw the antenna directly to the electronics enclosure.


Here are the updated results with the antenna connected through 50cm of cable, mounted on some non-RF active material. I believe that’s a good indication of what happens if you take the ground plane away, as would be the case when you’re mounting the antenna externally.


Hmm, the previews aren’t very clear. Here’s some links to the originals: Antennas.pdf (2.0 MB) and Antennas with 50cm cable - smaller.pdf (398.0 KB).


I’ve been using this antenna on AU-915 with good results. I’m about to start deploying a lot more gateways, and i’m wondering if i’m better off using osmethign lower gain.

14dBi? Phwoah. Probably a great option if you can also add an additional low gain antenna within the building for reception below the high gain antenna.

Im not sure if i belive that it really is 14dbi

Thanks - this is the serious version of my incidental finding :slight_smile:

@dicktonyboy, early last year you were modelling antennas. I’d appreciate your advice if think the modelling software would be suitable for a Meandering Monopole, something like this one

Interesting antenna (maybe for a node):

interesting yes, and it probably works on a node, but with an VSWR >1.8 @868 there are better (and cheaper ) choices imho.

1 Like

On the large size, maybe useful on the front end of a spectrum analyser.

1 Like

I would have thought an antenna on both a node and a gateways needs to be somewhat frequency selective to filter out unwanted signal and improve SNR. The antenna is the first filter in the receiver chain. A wideband antenna will pass un-necessary RF to the receiver thereby reducing SNR and performance. Seems OK for a spectrum analyser or a broad band receiver but can’t see a benefit for Lora based systems.

1 Like

Hi I’ve built a outdoor gateway using a IC880a and a raspy zero W. Im totally satisfied with the range but I didn’t use the whole potential because currently the Antenna of the gateway is located inside of the case.

Maybe someone made already good experiences with cheap outdoor antennas?

I am working on a future LoRa/LoRaWAN YouTube tutorial (Mobilefish) about Antenna’s. In the EU antennas are used with a freq. of 868 MHz. In the US an antenna freq. of 915 MHz is used. But which antenna freqs are used in other regions? I have searched the web but did not find good answers.
I have included a table with the min and max freq as can be found in the TTN freq plan and also included the average value (fmin+fmax)/2).

My question is:
If you use one of the other TTN freq plans what antenna freq are you using? The last column in the table is MY ASSUMPTION it may be completely wrong.antenna_freq

You should only need 3 different antennas, one for bands in 400Mhz range, another for the 86xMHz bands and a third for bands in the 900MHz range. Have a look at the “The big Antenna S11 catalog” where you can see actual antenna performance displayed on a VNA. You will observe they work across a range of frequencies not just a very narrow band.

Thanks @TonySmith! S11 yet another antenna parameter to study :grinning: and it makes sense.

Is the following statement correct?
Devices (end nodes) using US902, AU915, AS920, AS923 and KR920 freq plan can use an off-the-shelf rubber duck antenna 915MHz but make sure the antenna S11 < -10dB and VSWR < 2 at the required freq.
Devices using EU863 and IN865 freq plan can use an off-the-shelf rubber duck antenna 868MHz but make sure the antenna S11 < -10dB and VSWR < 2 at the required freq.

But for CN470 I am not sure which off-the-shelf rubber duck to use (433 MHz??).

@robertlie, you could make that recommendation. We all have our own “rules of thumb” and I prefer VSWR to be < 1.5.

A far more important message is “BEWARE many antennae do not work as advertised” For example I have a 915MHz antenna which is tuned to 868MHz. It sort of works in the 915MHz band but has severely limited the performance of my nodes. Knowing a lot of antennae are not as advertised I purchased from a reputable supplier and still got caught.
Therefore to have some idea of what you are buying, information sites like “The big Antenna S11 catalog ” are an important reference. Otherwise check with the supplier for the antenna’s data sheets / performance graphs.

1 Like