Author Topic: Small loop antennas @ 433 Mhz  (Read 34991 times)

joelucid

  • Hero Member
  • *****
  • Posts: 867
Re: Small loop antennas @ 433 Mhz
« Reply #105 on: November 25, 2016, 01:47:38 PM »
While I'm waiting for the new round board with drills for the air trimmers and two independent loops I built a fully assembled version of the first two boards but with the trimmer instead of fixed caps:



This works really well:

I wrote a little utility which sends packets at different frequencies across the spectrum and use the gateway to capture RSSI. That way I can adjust the trimmer without even attaching the VNA and with a fully live setup.

Both boards work well, close to the performance of Moteinos, Tinos etc. The rectangular model has smaller bandwidth than the round design. It might be the vias in the round one that degrade its Q and thus its performance. That had been a problem also when I measured it with the VNA. But it's still as good overall because the larger loop means more radiation resistance.

What's best is - and this is what I wanted to test with these boards - these loops are really incredibly stable to environmental changes. I've had them on wood, on the floor, on the balcony, in hand etc with very, very minor shift in resonant frequency.

For me this proves that these high Q loops are viable without active tuning if you use air trimmers.

Joe
« Last Edit: November 25, 2016, 01:54:11 PM by joelucid »

WhiteHare

  • Hero Member
  • *****
  • Posts: 1296
  • Country: us
Re: Small loop antennas @ 433 Mhz
« Reply #106 on: November 26, 2016, 08:37:32 AM »
@joelucid
nice!  Notionally speaking, could you use something like the following instead of the manual tuning variable capacitor?  http://www.mouser.com/ProductDetail/IXYS/NCD2100MTR/?qs=npTsUczJOtOwQXlqwUd7kA%3D%3D&gclid=Cj0KEQiAguXBBRCE_pbQ5reuq8MBEiQANji2LVV0erIGf3g5t223BxZMYxFCKXWrrXXcT5ZdMpY_Zi4aAoT28P8HAQ

joelucid

  • Hero Member
  • *****
  • Posts: 867
Re: Small loop antennas @ 433 Mhz
« Reply #107 on: November 26, 2016, 08:46:59 AM »
Not really - these digitally adjustable ones have fairly low Q's and are fairly unstable. So efficiency would be worse and you'd have to have some auto-tuning mechanism to keep the resonance right in the face of temp changes.

WhiteHare

  • Hero Member
  • *****
  • Posts: 1296
  • Country: us
Re: Small loop antennas @ 433 Mhz
« Reply #108 on: November 26, 2016, 07:04:38 PM »
After you have a few of the same type assembled and tuned, I'd be curious to know whether the variable capacitors end up having the same settings or not.

Insead of a switched capacitor ladder, what if it was just a capacitor ladder where you cut traces (or perhaps burned out fuses) to zero in on the correct capacitance?  Would that also have inadequate Q?

joelucid

  • Hero Member
  • *****
  • Posts: 867
Re: Small loop antennas @ 433 Mhz
« Reply #109 on: November 27, 2016, 08:46:24 AM »
Unfortunately from what I've read not very high Q either.

perky

  • Hero Member
  • *****
  • Posts: 873
  • Country: gb
Re: Small loop antennas @ 433 Mhz
« Reply #110 on: November 27, 2016, 09:48:25 AM »
@WhiteHare Those digital switch capacitors unfortunately don't work well at UHF, the one you mentioned has large increase in capacitance even at 500MHz. The internals appear to become capacitive at higher frequencies.
Mark.

lemonforest

  • Newbie
  • *
  • Posts: 16
  • Country: us
  • breaking all the things
Re: Small loop antennas @ 433 Mhz
« Reply #111 on: June 06, 2020, 08:39:28 PM »
I found myself enamored by the inductively coupled loop from the beginning of this post. Enough so that, even with my late arrival, I've forgone everything the internet has taught me about resurrecting forum posts.

I'm guessing that you've found better results in a resonant PCB antenna as opposed to the inductively coupled method you began testing?  I'm also as curious as I am ignorant and can only guess that your VNA plot requires you to use a resonant antenna connected to the VNA to receive an actively broadcasting signal. So how is it really that you've measured the coupled loop from your first post?

Thank you!

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6335
  • Country: us
    • LowPowerLab
Re: Small loop antennas @ 433 Mhz
« Reply #112 on: June 08, 2020, 08:42:29 AM »
A VNA is not going to use an antenna for measurement, maybe you're thinking spectrum analyzer.
Unless you just want to analyze an antenna, which can be done on a "single port VNA" which can measure the S11 parameter.

The VNA measures S parameters but it has to be connected to the DUT, to pass a signal into it. It sends a signal and it measures the reflected signal back from the DUT, (and on a 2 port VNA the output through the DUT) to determine things like return signal loss and other values, and overall to determine the quality of the signal path and how well it matches (or is absorbed) to the load (vs reflected back). So on a board like this he probably had the VNA connected directly to where the RFM ANT pin hole is.

Here's an excellent educational video by w2aew which explains the basics of VNAs and how they work and how they are used:


lemonforest

  • Newbie
  • *
  • Posts: 16
  • Country: us
  • breaking all the things
Re: Small loop antennas @ 433 Mhz
« Reply #113 on: June 09, 2020, 03:44:25 PM »
Thanks for the video, watched it and the next one after.  It was the smith chart on the inductively coupled antenna so it sounds like that the internal loop was connected directly to the VNA, yes?

Thanks!

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 6335
  • Country: us
    • LowPowerLab
Re: Small loop antennas @ 433 Mhz
« Reply #114 on: June 09, 2020, 03:47:50 PM »
It was the smith chart on the inductively coupled antenna so it sounds like that the internal loop was connected directly to the VNA, yes?
Most likely.
The internal/coupled loops form the antenna of the device.
So yes I am guessing the VNA was measuring at the point where the radio would connect to the internal loop.