Hi, I have an application where the motino will by in a metal box. So I need an external ant. How do I connect that to the motino ?
Sid
RF coax to moteino board (GND + ANT port), out through a hole to your antenna. Depending on antenna you will need a certain length on your coax to keep the signals from bouncing in the cable.
See this thread (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/easy-pcb-dipoles-for-433-868-and-915mhz/) for how to to it with dipoles.
I've read the tread a few times. and for the life of me I can't figure out what the length of the coax changes.
I was my understanding that the coax isolates the signal between the output and the antenna.
Sid
It does not isolate at all, that would be very bad. It transports the signal, ideally without loss. But there's always loss. The length matters, that thread goes into more detail on that.
That's what I meant. transport. Thee is loss but I assume that the loss becomes an issue with long runs of several feet. I would have at most 6" probably less than that.
Sid
Have you already determined what kind of antenna you need? And for what frequency?
The frequency is 915 mHz,one side will have an external antenna since every this is metal and the other is a hand held plastic box with a sticky printed circuit antenna. Both antennas have a small lead with a uFl connector to connect to the board.
https://abra-electronics.com/wireless/wireless-antennas-en/1991-ada-slim-sticker-type-gsm-cellular-quad-band-antenna-3dbi-ufl.html
https://abra-electronics.com/wireless/wireless-antennas-en/ant-17-gsm-u.fl-connector-to-sma-antenna.html
The monopole antenna from your link only seems to have an SMA port. Just make sure the monopole is conntected to a metal ground plane. I'm assuming you're going to use an SMA-to-U.FL connector cable to the Moteino in a metal case. Then, both antennas will have an U.FL connector going to the Moteino. I guess the easiest would be to get some U.FL matching connectors and solder them to the Moteino as per Felix' post.
QuoteI've read the tread a few times. and for the life of me I can't figure out what the length of the coax changes.
Besides losses purely due to resistive losses (i.e. a long cable), the length of the coax becomes very important if the antenna does not have a 50 ohms feedpoint impedance (we call this a mismatch). Looking at the pages from your links it's impossible to tell whether these antennas are indeed that. What happens is that with a mismatch at the antenna feed point, some of the power going to the antenna is reflected back to the radio. Because the power through the feedline is AC (alternating current), the forward and reflected waveforms can either add or subtract. Imagine a point where the forward wave (from the radio) is at its highest, right at the point where a reflected wave is also at its highest. At that specific point the voltages add up. If that specific point is close the radio, it could damage it. This is why I recommend the use feedline of half-wave multiples so that the peak of the reflected waves are minimal.
What's working in your favour though, is that the coax that U.FL uses is pretty poor at handling high SWR and will result in resistive losses (warming up the coax). So in theory you could run a slightly higher SWR and get away with it.
If you're trying to get the furthest range and maximum efficiency out of your system you probably want to use different coax and antennas but I would just give it a try and see what it does. The radios on the Moteinos handle up to 3:1 SWR which is fairly robust but not unlimited. Never ever transmit without an antenna.
The PCB (sticker) antenna has a datasheet and on one of the graphs it shows that the SWR for 915MHz is roughly 2:1. For short runs I'd say that's acceptable.
Thanks @captcha for the clarification. I understand better, not fully, but I get the point.
The monopole antenna is has a S.W.R <=2.0 and Output Impedance is 50Ω (stated in the specs section). it comes with a short cable that has a uFl connector and a SMA connector. The range is going to be short but fast. This is a wireless remote control setup for machinery. On the machine, the motino is going to be inside an aluminum box and the SMA port should have a good ground plane from the Al box.
Since both antenna's are 50Ω as the RFM69 radio and both have a S.W.R of around 2, I assume there should not be any resonance issue in the coax ?
Quoteboth have a S.W.R of around 2
SWR is also a function of frequency and not just a property of the antenna.
An SWR of 2 (2 to 1, or usually written as 2:1) means that within a certain frequency range (they state from 824MHz to 960MHz) the SWR will be less than 2:1 (or so they claim). So just take the worst case scenario and assume it's 2:1. That should not give you any grief if those numbers are correct.
QuoteThe range is going to be short but fast.
That's very open to interpretation. Are we talking cm's, metres? Same goes for 'fast', although you'll probably end up somewhere close to the speed of light.. :)
QuoteI assume there should not be any resonance issue in the coax ?
Unless you create a trap (to create a multi-band antenna) there is no resonance in coax that we should worry about here. I think the term you're after is reflection. Unless the antenna is perfectly matched to the coax and the coax is perfectly matched to the radio there will be reflections. However, if you're trying to communicate with reasonably decent antennas (which these seem to be) over a range of say, 100m you should be fine.
Quote from: captcha on March 29, 2017, 03:31:29 AM
That's very open to interpretation. Are we talking cm's, metres? Same goes for 'fast', although you'll probably end up somewhere close to the speed of light.. :)
lol your right. The distance will be generally within 5 meters and at a rates of 300 kbps.
And you're right I was thinking reflection. So Ideally using a half wave length of coax would help. Since @ 915 mHz is only 6" I can tuck than in my box fairly easily.
I was reading your explanation and re read the thread on the subject and it reminded me of the wave combination effect on water. one thing I'm curious about is if only the half wave length helps. What would a quarter length do? I'm not sure but I think it would also help, but not as much ?!?!?
My plan is to write state machines that will transmit state changes to one another. I plan to have a handshake every 500 ms when no state changes are present. This means that the radios will have a duty cycle that is very low most of the time with short bursts of busy times. With the info you gave me I'm pretty sure I'll be fine.
Thank you.
Ah, data rate, yes.. forgot about that.. ;) Although I've never ran them that fast, 300kbps should be fine. Perhaps check one of the other posts on this forum about higher data rates and what to expect.
Quoteone thing I'm curious about is if only the half wave length helps. What would a quarter length do? I'm not sure but I think it would also help, but not as much ?!?!?
Just one quarter wavelength is probably the worst you can do. Again, if all is perfectly matched, then no issues, but with impedance mismatches you will see them most at the quarter wave. Two quarter waves (half wave) is optimal, but three again is worst. See this post on the matter:
https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/connecting-sma-pigtail-to-moteino/msg15744/#msg15744 (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/connecting-sma-pigtail-to-moteino/msg15744/#msg15744)
The curved lines around the antenna elements and feedline is a visual representation of the current distribution. Alternating
Current in an antenna is what makes it radiate. More current (line further away from the element) equals more radiation.
In the first example you'll see high current at the antenna feed point and almost no current on the feedline. Second picture is worst case scenario. A lot of current is present at the feed point on the outside of the coax. From Ohms law we know that when there's high current, there's a low voltage. Now look at the bottom of the quarter wavelength long feedline. The current line is pretty much 'touching' the feedline to indicate almost no current. This means high voltage.
That can kill your radio.
QuoteMy plan is to write state machines that will transmit state changes to one another.
That sounds like a fun project. Please, do share when there's something to show! :)
Oh I see. you want the reflection to be at the same point as the transmission and not at the opposite range. Basically the difference in potential between the signal and the reflection creates voltage and the voltage can kill the radio.
And I thought (how hard can using FM be ?) LOL turns out there are a lot of beasts all intertwined to make FM work right. This is quite the learning experience.
Quote from: captcha on March 29, 2017, 05:53:14 AM
That sounds like a fun project. Please, do share when there's something to show! :)
I have been taking pictures. I'll be taking some video's soon. My first step is to change the throttle control from manual to electric so I can change the engine speed via remote. I'm in the middle of that at the moment. I will post the project once I'm done.
I find it fun to post some of my projects (here or elsewhere) as a sort of memento as well. Sure, I save pictures locally and have all sorts of datasheets to go with it, but posting something on a forum forces me to be to the point, concise and.. have a little fun with it. See how you go. We'll be here.. :)
Quoteturns out there are a lot of beasts all intertwined to make FM work right.
With most of my answers I actually had HF SSB in mind. That said, what holds true on HF mostly applies to UHF as well.
QuoteThis is quite the learning experience.
Mission accomplished, haha..
Hello,
I have the same situation as topic starter - I need use external antenna for Moteino with LoRa 915Mhz
I've read all the information in this thread, and others related to this question and this is what I have:
- my RF cable is Radiolab RG-58 A/U (1x1.0)
impedance is 50 Ohm
vF is about 1,28 - I also found another values for vF, for example here: http://www.febo.com/reference/cable_data.html says that vF is 73.0
another sources says about 0.66...
I decided to use 1.28.
Using the calculator here: http://n1gnn.com/webData/CoaxVelocityFactorLength.html
it says:
For coax with Vf = 1.28, 1/2 wavelength = 20.98 cm, 0.688 ft & 8.26 in
so to find length of feedline I need multiple 20.98?
for example 20.98*18 == 377.64 cm will be OK as feedline from Moteino UFL connector to my DIY GP antenna (like here: https://www.thethingsnetwork.org/forum/t/very-simple-homemade-outdoor-868mhz-antenna-groundplane/3160)?
but I use 1/4 wave length wires in my GP antenna (82mm for 915 MHz) - can I use in this case 1/2 wavelength value (20.98) for calculating total length of feedline?
I see that 1/4 value (10.49) give me the same length 377.64 cm.
Please confirm or not my calculations.
The 0.66 and 0.73 is the velocity factor and is greatly dependent on the dielectric material. When you cut a short bit off the outer black insulation and fold back some of the braid, is the material immediately around the centre conductor a hard or soft material? If it's foamy, the velocity factor for that cable could be 0.73. If it's hard-ish semi-opaque material it's probably polyethylene (PE) and your velocity factor will probably close to 0.66.
So for 915MHz a full wavelength is 32.78cm. A half wavelength is 16.39. Compensated for velocity factor you'll arrive at 10.82cm. So use multiples of 10.82cm. If your antenna is perfectly matched to the radio and coax you'll lose about 2dB over a 12 feet length of cable.
Can you reword your last paragraph? Not sure I'm getting it.. ;)
Thank you!
and sorry for my bad English.
Quoteis greatly dependent on the dielectric material.
sure, I've read already about it and seems to understand this fact.
here are couple of photos of my cable.
details of my cable described here (only russian):
http://www.radiolab.ru/ru/3/1106648468/1152529320/1152610930.php
I can see than dielectric material is FTE.
Regarding velocity factor - probably I made a mistake. This document states that parameter with name "Коэффициент укорочения" (Coefficient of shortening) is 1.28.
I googled about this parameter and seems it means velocity factor. So I decided use this value in calculation.
But today I read carefully wiki (in russian again): https://ru.wikipedia.org/wiki/%D0%9A%D0%BE%D1%8D%D1%84%D1%84%D0%B8%D1%86%D0%B8%D0%B5%D0%BD%D1%82_%D1%83%D0%BA%D0%BE%D1%80%D0%BE%D1%87%D0%B5%D0%BD%D0%B8%D1%8F
and there stated this info:
QuoteВ зарубежных источниках вместо коэффициента укорочения используется обратная ему величина — коэффициент замедления (Velocity of Propagation, Velocity Factor), выражаемая в процентах.
Here it is said that
Coefficient of shortening is inverse value of velocity factor.
So seems it is my bad... I should not to use 1.28 value.
Instead I need to use 0.73 as dielectric material is FTE.
so using online calculator
For coax with Vf = 0.73, 1/2 wavelength = 11.97 cm, 0.393 ft & 4.71 in11.97*33 == 395.01 cm
Right?
Also I have to install connectors on this cable.
On one side it should be RP-SMA Male - it connected to such pigtail:
(https://images-na.ssl-images-amazon.com/images/I/71-OPf2mO-L._SL1500_.jpg)
other side of pigtail should be connected to UFL connector on Moteino.
length of pigtail is 26cm from end to end.
On second side will be N-type connector
(http://www.infinitecables.com/images/detailed/1/cn-00-rg59.jpg)
this connector shoul be connected to my GP antenna.
GP antenna also based on N type - (http://www.deepsurplus.com/x_cn429_3_72dpi.jpg)
so finally, I suppose I have to deduct length of pigtail (26cm) from calculated 395cm == 367 cm
and this 367cm - it is length from the end with RP-SMA (which will be connected to pigtail) - to the point on antenna N type connector that should be already connected to N connector on cable on other side.
and if I connect pigtail it should be 395 cm total.
Or 367cm should be without connected antenna connector? I doubt because on this connector there is part where I should solder vertical copper wire (82mm) and this part has length about 5mm.
Also I'm not sure should I consider and add this 82mm to total length of cable.. As far I know length of feedline should measure from start point (in my case it is end of pigtail ) and to end point (I suppose in my case it is end of N type antenna connector which connected to N connector on cable)
Please advice for this situation.
Sorry for long read...
As for my GP antenna: I meant that I build GP antenna, and for it I need 5 pieces of copper wires.
One wire vertical. and 4 wires - on the sides.
And for 915Mhz I need length == 82mm for every wire - it is 1/4 wavelength.
Right?
So my question was - can I use my GP antenna with 82mm wires with cable length of which was calculated with 1/2 wavelengh value. Just want to be sure that it is correct.
Or maybe I need create new antenna - with wires with length equal to 1/2 wavelength == 82*2 == 164mm?
Thank you for the extra pictures and information on what you're attempting to do.
It seems you're not just making a connecting feedline from a piece of coax but also making your own 1/4-wave vertical antenna.
Do you have any measuring equipment to test your antenna system when you've built it? If not, it may be easier to just order one online.
Yes, I want to create maximum effective antenna system, because I need transmit/receive data in very difficult conditions on long distances (1km) - mountains relief with a lot of radio shadows.
now I can send/receive data from node on direct view from 5km.
As for measuring equipment - I suppose it will cost a lot of money. My budget not allowing me to buy such devices, unfortunately.
Do I really need it?
I supposed if I create correct feedline (I mean - correct calculate length of cable with all the connectors and pigtail) and create simple GP antenna - it should work better that just piece of wire as antenna.
Interesting challenge! :-)
If you want to create the maximum effective antenna system I would highly recommend you have a look at relatively cheap antennas on eBay. They are by no means perfect, and they would still require some proper setup (ground plane, feedline, orientation) but I think you will have one less thing to worry about. Especially if you don't have a means to test your antenna system once you've built it. I'd like to spare you the frustration in case things don't work as expected.
Btw, that piece of wire can be amazingly effective because there is no feedline that could cause impedance mismatches.
Are the communicating stations fixed or mobile? Is it just two stations or more?
I already buy some from amazon:
https://www.amazon.com/gp/product/B01HHPK7KS/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
inside it looks like this:
(https://ae01.alicdn.com/kf/HTB1Nd7zOpXXXXcuXXXXq6xXFXXXc/868MHZ-915MHZ-GSM-antenna-small-sucker-7dbi-aerial-3meters-SMA-male-SMA-female-RF-connector-straight.jpg)
wire with two springs.
I decided use GP antenna in past, and have several antennas already. So I think I will use them at least till the end of this season.
But anyway - for all the antennas I need feedline - RF cable with connectors and here I want to learn how to calculate the right length of this cable :)
As far I know - it is very important if I want build effective antenna system.
So would you please check my calculations in my previous post?
As for my project - I build my LoRa nodes for measuring different snow conditions in snow cover on mountains slopes.
This info used by avalanche services for forecasting avalaches.
It is non-commercial task - I just helping to friend of mine who work it this avalanche service check his theory about avalanches.
Also it is very interesting for me to learn snow because I like freeride and backcountry in mountains )
So because of all - it is kind of hobby.
Now we have 5 nodes (servers), all of them installed on different slopes and works from November till May.
Man with client device periodically go to safe points to get info from nodes. Usually two times during day.
I'm planning build kind of gateway for nodes that located in the same location and get all data from gateway.
So for these purposes I want build good antenna system.
so...
any updates about my calculations? Im going to make a cable with connectors and would like to know what length to do.
sorry but any updates?
I'm worry if my calculations is correct.
again:
I have cable with VF I decided to use is 1.28 so 1/2 wave length is 20.98 cm.
I need build feed line which contains: UFL connector soldered on PCB Moteino -> pigtail with SMA connector -> RF cable (SMA - N type connector) -> GP antenna based on N type connector.
The main question how to calculate part of length of RF cable?
If I understand all this info correctly (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/connecting-sma-pigtail-to-moteino/msg15744/#msg15744) this length need to be calculated very accurate.
So total length of FEED LINE from UFL to antenna (pigtail + RF cable + N type antenna ) should be multiple of 20.98cm due to "minimize RF going back to the transmitter"
I know length of pigtail - 26cm from end to end and length of N type connector on antenna side is 2 cm so total length of them is 28cm.
Next I need calculate length of RF cable. Let's say I have 400cm of cable.
for example 20.98 multiple of 19 times == 398.62cm
then I need to cut 28cm for pigtail and N type connector lenght so finally I get 370.62cm of RF cable.
and total feed line will be RF cable (370.62cm) + pigtail and N type (28cm) == 398.62cm
Is it correct calculations?
Please take a look and advice.
and second question - can I use such feed line with GP antenna for 915Mhz with 1/4 wave length wires (82mm)?
Thank you!
UPD:
after some investigations I noticed that vF 1.28 which I used in my previous calculations was wrong.
my cable has FPE dielectric so correct vF is 1/(1.28) == 0.78
so:
For coax with Vf = 0.78, 3/4 wavelength = 19.18 cm for freq 915MHz
19.18 instead 20.98 cm.