Author Topic: LowPowerLab 915MHz dipole antenna looks like 868MHz  (Read 195 times)

gregoireg

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LowPowerLab 915MHz dipole antenna looks like 868MHz
« on: October 24, 2020, 01:52:35 PM »
I have purchased a dipole 915MHz antenna here: https://lowpowerlab.com/shop/product/193

The unmodified antenna looks like if it's optimized for 868MHz. I would like to understand what I'm doing wrong.

I have soldered a u-fl connector and the polarity has been respected.

Now, here is probably an important thing: the cable length is 16.5cm (that's all I have).

On this thread https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/alternative-pcb-dipole-designs-(433-868-and-915mhz)/, it says 915Mhz gives an ideal feedline length of 10.8cm. I don't understand how this number is calculated:

speedoflight / 915MHz = 32.76cm

So the post uses a magical /3 factor while all google links mention more 66% at 50 ohm for the velocity factor.

The length measurement on the post mentioned above doesn't seem to be super precise. And what about the length of the traces on the emitter board?

Anyway, even if the feedline length is not optimized, I would expect a lower deep not a shift.

Has anyone some good explanations?

gregoireg

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #1 on: October 24, 2020, 03:31:37 PM »
Half answering my question:

- the thread mentioned in my original post was probably using λ/2 as the best length of the cable (0.66 = 2/3 which is divided by 2 so 1/3).

- 16.5cm for my cable seems to be the worst as it's close to 3λ/4. That's all I have.

Still, a wrong cable length should inject some noise because of reflection and that would justify a lower deep in the curve. But how to explain the shift from 915MHz to 870MHz?

Felix

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #2 on: October 26, 2020, 09:43:18 AM »
Endless reasons.
Cable length, quality of interconnects, you holding this in your hand with the antenna loosely hanging at some random angle and direction, or close to other objects.
The uFL is not exactly the best way to make measurements. I suggest SMA.

Actually the VSWR at 915Mhz which my guess is its close to the RIGHT in your graph, is not bad given all the reasons above.

Some good articles to complement your quest for antenna tuning and impedance matching:

https://www.baseapp.com/iot/antenna-tuning-for-beginners/
http://colinkarpfinger.com/blog/2010/the-dropouts-guide-to-antenna-design/

gregoireg

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #3 on: October 26, 2020, 01:24:20 PM »
Thanks for those suggestions.

I have done multiple readings including with different angles, without touching the device and so on... The curve definitely varies but the lower peak is rather consistent. Impedance matching is not going to significantly shift the antenna, it's going indeed to change the impedance. That's not my primary issue here.

I'm still confused:

1) At https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/alternative-pcb-dipole-designs-(433-868-and-915mhz)/, the board file for 915MHz is provided in the first post:
https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/alternative-pcb-dipole-designs-(433-868-and-915mhz)/?action=dlattach;attach=2564
The length of each stripe is 60mm from the center.

2) At https://oshpark.com/profiles/captcha, the project "0124-915_v1.0.zip" has also 60mm length trace on each side.

The 915MHz optimized antenna sold by LowPowerLab is 73.5mm on each side and is indicated as optimized for 915MHz.

3) The theory gives c / 4 / 915MHz = 81.91mm for each side.

Can LowPowerLab comment on the physical length of the antenna?

Felix

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #4 on: October 26, 2020, 01:59:26 PM »
Actually even your hand in different positions around an antenna without touching it can detune it, shifting frequency.
You will find that if you just take the theory and make dipoles from different materials, they will vary wildly. Theory is good to start with but c/4 does not include the materials and other physical factors like the copper thickness, soldermask etc, all of which matter.

When I made these dipoles I sampled them at various lengths and widths. I measured on a similar single port vna device, and tuned the length to what showed the lowest VSWR at 915mhz, by cutting away at the length. I managed to place the antenna in a straight up position and capture a shot with very low VSWR and 915 very close to the center. I do not expect that I can easily obtain close to 1:1 every time I try it. It's possible different batches come out different if different materials are used. It's also possible my device differs from yours. I have not repeated the measurements since I first made them. In any case it's generally very hard to get a close to VSWR of 1:1 on such an antenna. Anything below 2 is considered pretty good.
Just look at this table, at VSWR of 1.5 you're still passing through 96% of the emitted energy: https://www.markimicrowave.com/blog/wp-content/uploads/2016/11/return-loss-to-vswr.pdf

gregoireg

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #5 on: October 26, 2020, 02:35:00 PM »
I'm not criticizing the quality of the antenna. I'm trying to understand the length as I would like to design my own (some other constraints apply). There is definitely the problem of the accuracy (and calibration) of those mini VNA analyzers. The length seems to vary greatly between theory (80mm) and reality (73.5mm) and PCB specs (60mm).

Felix

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #6 on: October 26, 2020, 03:05:22 PM »
I think the best way is to start longer and trim it gradually, especially when the materials are not fully identifiable.

gregoireg

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #7 on: October 26, 2020, 03:12:23 PM »
Yes, I think that's makes sense and that's what people recommend on the various blogs. I have done some outdoor tests with the original LowPowerLab dipole antenna and the performance was not bad.

I start to be a little bit doubtful about those mini VNA analyzers (I don't have access to an expensive one...). I'm wondering if a badly calibrated analyzer would shift more (curve left / right) or would be less sensitive (curve up / down) or both...

Felix

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #8 on: October 26, 2020, 03:42:03 PM »
Hard to say without checking against a real lab VNA that you can really trust.

gregoireg

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #9 on: October 26, 2020, 04:04:40 PM »
One more question: how did you calculate the width of the two traces? Captcha had something very narrow, yours is around 5mm.

I think that the width will greatly impact the impedance of the antenna. There are many impedance calculators of a trace on Google but they all consider that there is a ground plane below.

How to calculate the width of a trace for 50 ohm impedance matching without a ground plane beneath?

Felix

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Re: LowPowerLab 915MHz dipole antenna looks like 868MHz
« Reply #10 on: October 27, 2020, 08:59:46 AM »
The dipole arms are not a transmission line. I think I just used the captcha model, I don't remember if I made any changes. The width will influence how wide band the antenna is.