Author Topic: Antenna Length and VSWR effect (reflected power from antenna)  (Read 3521 times)

sparky

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Antenna Length and VSWR effect (reflected power from antenna)
« on: August 22, 2016, 07:38:05 PM »
First I would like to say that I'm very new to all this so forgive me for the noobie questions moving forward.

I purchased a couple RFM69HW 868/915 MHZ Moteino's and I'm just about finished soldering everything up.  I was about ready to solder the antenna's but had a question about the length.  According to the Moteino "pinout" page, the length should be 86mm (868) / 82mm (915).  The antennas that came with mine are both 79mm. 

After reading some posts on damaging the units from either incorrect antenna or wrong settings I wanted to make sure the lengths are correct.

Thanks,
GM 
« Last Edit: August 23, 2016, 11:19:08 PM by Felix »

perky

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Re: Antenna Length
« Reply #1 on: August 22, 2016, 08:25:56 PM »
The lengths quoted are for 1/4 wave wire type antenna, however you can twist or coil the antenna to add inductance and shorten it's length and it will still be 1/4 wave. Does this 79mm antenna have a spiral around the edge?

Mark.

sparky

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Re: Antenna Length
« Reply #2 on: August 22, 2016, 08:31:09 PM »
No, it's just a straight piece of wire that was shipped with the moteino.

Thanks

perky

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Re: Antenna Length
« Reply #3 on: August 22, 2016, 09:05:51 PM »
Might not make a significant difference. There's a theory for dipoles that suggests slightly less than the calculated length results in resonance and practically the same radiation pattern, probably also true for 1/4 wave. There'a also not much of a ground plane either so I'm tempted to say this slight shortening could even be beneficial:
http://www.antenna-theory.com/antennas/halfwave.php
Mark.



sparky

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Re: Antenna Length
« Reply #4 on: August 22, 2016, 09:11:20 PM »
So being these are monopole, would the same be true?

perky

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Re: Antenna Length
« Reply #5 on: August 22, 2016, 09:24:58 PM »
I don't know the answer to that :-(

Felix

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Re: Antenna Length
« Reply #6 on: August 22, 2016, 10:06:29 PM »
Basically the shorter length is to compensate for the PCB traces that lead to the actual matching network on the radio modules.
Either way the manual cutting of these is not an extremely precise process and in practice small differences don't seem to make a difference.

sparky

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Re: Antenna Length
« Reply #7 on: August 23, 2016, 07:08:16 AM »
Basically the shorter length is to compensate for the PCB traces that lead to the actual matching network on the radio modules.
Either way the manual cutting of these is not an extremely precise process and in practice small differences don't seem to make a difference.

Ok, just didn't want to overheat anything.

Thanks

captcha

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Re: Antenna Length
« Reply #8 on: August 23, 2016, 07:16:04 AM »
Hi Felix,

With regards to 'compensate for the PCB traces', I'm curious.. Has anyone ever published any useful data on that?

Felix

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Re: Antenna Length
« Reply #9 on: August 23, 2016, 10:22:52 AM »
I don't think so. Just common sense and practical feel while not drifting too far from the theory. The one way you'd test this is with a VNA.

captcha

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Re: Antenna Length
« Reply #10 on: August 23, 2016, 06:36:04 PM »
I know from experience when tuning my 433Mhz quarter-wave vertical and getting close to resonance I was cutting off bits 1mm at a time to get to the lowest SWR. Things are twice as sensitive on 915MHz. I wouldn't exactly say that it isn't a precise process. Will things work? Probably yes, but you're way off the scientific method.. ;-)



perky

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Re: Antenna Length
« Reply #11 on: August 23, 2016, 07:33:19 PM »
Given there's little ground plane and is pretty sensitive to orientation and other surrounding things, are those few mm really going to make a big difference? I don't think so, I think there are so many other factors that would have an effect. I've tried manually snipping bits of antenna before and quite frankly moving the unit a little throws it right out again.
Mark.

Felix

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Re: Antenna Length
« Reply #12 on: August 23, 2016, 10:21:01 PM »
I know from experience when tuning my 433Mhz quarter-wave vertical and getting close to resonance I was cutting off bits 1mm at a time to get to the lowest SWR. Things are twice as sensitive on 915MHz. I wouldn't exactly say that it isn't a precise process. Will things work? Probably yes, but you're way off the scientific method.. ;-)

I agree and I think I've done that long long ago when I was trying to get an accurate feel for the antenna length that I will ship to thousands of folks. I've arrived at the given length and it has never really been an issue and folks report ranges of hundreds of meters in the open, or even 1km+ with these wires by tweaking the settings. I'm sure things can be improved on a case by case basis but sometimes people bend the antennas and enclose them in cases or wire them in ways that alter the antenna performance way more than a perfect length. Plus the ideal performance is achieved when the antenna is perfectly perpendicular to it's "launch-pad" - the GND plane - which is almost never the case, even in my own projects.

captcha

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Re: Antenna Length
« Reply #13 on: August 23, 2016, 11:03:27 PM »
Real-world scenarios:
Quote
but sometimes people bend the antennas and enclose them in cases or wire them in ways that alter the antenna performance way more than a perfect length

Quite true.. Fortunately, the RFM69HW is tolerant of up to 3:1 SWR* (a reflection of 25%). Pretty amazing unit..

Still, I wouldn't mind having access to that extra 25% of output power. Especially when the module only outputs 100mW.

* Reference: Page 11 of the datasheet: http://www.hoperf.com/upload/rf/RFM69HW-V1.3.pdf

Felix

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Re: Antenna Length
« Reply #14 on: August 23, 2016, 11:18:20 PM »
In practice, sometimes the antenna has to loop around inside a box, or around the enclosure or even around the PCB itself. I expect that to cause gross VSWR (coefficient power reflected back from antenna). Even the projects where I've done this have worked without a problem although with a significant loss in RSSI at the other end.

Whenever possible keeping the antenna perpendicular to GND will boost the RSSI, and allow RFM69_ATC to reduce power and hence save battery on low power nodes.
It's all a tradeoff.

Here's a good read on VSWR: http://www.antenna-theory.com/definitions/vswr.php

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.