Author Topic: Antenna Length and VSWR effect (reflected power from antenna)  (Read 3588 times)

fgomes

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Re: Antenna Length and VSWR effect (reflected power from antenna)
« Reply #15 on: December 02, 2016, 05:24:40 PM »
Hi Felix

I have the problem you stated, a 1/4 wavelength wire loping around the box, near 2 PCBs, and it is doesn't transmit  unless I put my hand around the wire or if I put the wire straight (but that way it doesn't fit inside the box). I imagine that due to being close to the PCBs at some points, it will have a lower impedance, causing an high VSWR, do you think that the problem could be exceeding the RFM69 VSWR? I didn't find in the documentation (HopeRF or Semtech) any reference to disabling the transmission in the presence of an high VSWR, but of course this could be done to protect the PA.

I posted  the problem in the folowing topic:
https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/rf69hw-and-antenna-compatibility-(receives-but-don't-transmit)/new/?topicseen#new

Best regards

Fernando

Felix

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Re: Antenna Length and VSWR effect (reflected power from antenna)
« Reply #16 on: December 04, 2016, 10:30:20 PM »
I don't think this has to do with VSWR, not as long as your antenna is a quarter wavelength monopole with at least some GND below it (the "launchpad")
Sounds like there may be other coupling issues or EMI.
You will have to start with 2 Moteinos and test that scenario. Then if you change one end with a ESP+RFM69 (and thats when the problem starts) - then you know the problem is on the ESP end.

fgomes

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Re: Antenna Length and VSWR effect (reflected power from antenna)
« Reply #17 on: December 05, 2016, 04:51:48 AM »
Hi Felix

Thanks for you reply!

When i refered before the possibility of being VSWR, it is because if the antenna is near some 'ground' it could represent a much lower impedance to the transmitter and cause an high VSWR. I have no ground plane but have inside a small enclosure the NodeMCU, the RFM69HW, an 16x02LCD + I2C adapter and a protoboard connecting everything, so the antenna will be always near any of these modules.

I'm more inclined to your suggestion of being an EMI problem in the SPI / power connection between the ESP and the RFM, that might be somehow prone to the interference of the antenna signal.

The communication is working well between 2 moteinos, and also between a Moteino and the ESP+RFM, if I use a coil antenna o the ESP+RFM side (this is what was used until now and it was working, but the signal was weak also due to the bad placement of the coil antenna inside the box, so I decided to test alternative approaches and got into this issue). I can observe with a third moteino working as a sniffer that when this problem occurs no message is really transmitted from the ESP+RFM node. I just don't find a reason to it, because even if it is an EMI problem, I think that the RFM only turns on the transmitter after having received all the message in its FIFO, so it should transmit it independently of the SPI communication with the ESP, that will only occur after turning on the transmitter, don't you think? And also it worked well in a breadboard, with much longer wires. Now it was assembled in a protoboard, the wires are now shorter, but there is no ground plane because I haven't made a PCB for this node yet. One thing that I thought trying was to create some ground plane using cooper shielding tape, but didn't tried it yet. I have used tens of RFM69 modules without any issue until now, so I'm still a bit surprised with this issue. It is true that never tried to put the antenna inside the box, I have always used external SMA antennas or simple 1/4 wave straight wires.

Best regards

Fernando

captcha

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Re: Antenna Length and VSWR effect (reflected power from antenna)
« Reply #18 on: December 05, 2016, 06:10:36 AM »
Maybe we can nominate December 5 as International Antenna Day and appreciate what they are all doing for us.. :)

Quote from: fgomes
I have no ground plane

Quote from: fgomes
put the antenna inside the box

What I think is going on here is a typical case of not enough counterpoise. The 1/4 wave vertical NEEDS a ground plane or counterpoise to operate properly, that is just antenna basics. This is not just about getting your signal out to some distance but very much about keeping the radio happy and minimising rf in places where you don't want it.

Let's say for argument's sake that your antenna somehow is still properly impedance matched to the radio and that the radio does go into transmit. By bending the antenna around the box you are effectively radiating a substantial amount of rf back onto all your pcb tracks. It's like shooting yourself in the foot and wondering why you can't walk straight anymore.. :)

If you are that keen on minimising the antenna footprint, please look into pcb trace / loop antennas and work in the 5GHz ISM band.