I am working on a project that uses a a single ended 868mhz RF ic with a 50ohm microstrip. I want to use a PCB dipole design but have very little experience in this regard. the dipole antenna as I understand it, it has a feed impedance of 73 ohm or close to it. I have looked at balun's but they all are to expensive. I want todo a discrete solution but not sure how to feed a single ended RF line into a dual line dipole antenna and match the impedance. below is a circuit and a PCB layout that I have been playing with as an idea but nothing is set in stone. Any advise on how to do this cheaply is appreciated.
The Feeding line will be microstrip with pi network. the values on the components are just default.
will this type of connection work for what is intended ?
I only have 13 dBm tx power so half wave dipole is my best bet
How big is your PCB.
868Mhz is a wavelength of ~34.5cm.
That calculates to a dipole of ~16cm long.
Gut feeling is that a ~8cm well tuned 1/4wave would perform the same or better as an ~8cm (too short) 1/2wave.
But I think you should ask this on an RF forum, not an Arduino forum.
Leo..
Wawa:
How big is your PCB.
868Mhz is a wavelength of ~34.5cm.
That calculates to a dipole of ~16cm long.
Gut feeling is that a ~8cm well tuned 1/4wave would perform the same or better as an ~8cm (too short) 1/2wave.
But I think you should ask this on an RF forum, not an Arduino forum.
Leo..
Freq 868.6Mhz
Full wave length 345.1444mm
half wave lenght 172.5722mm
This is a half wave length dipole antenna ( each leg is 1/4 wave) as per general rule of thumb 1/4 wave leg is normally slightly shorter then actual wavelength this is calculated but taking the half wave and times it by 0.47
81.10mm per 1/4wave leg
Personally I would not spend too much time trying to calculate and overthink this, the type of module you are using might notionally be 50ohm output, but cheap modules use cheap components for antenna matching so the output impedance might well be different to 50ohm. Thats a guess as you did not reveal which module you are using.
I also would not bother with trying to implement a matching transformer for the dipole, yes in 'theory' it is required but I suspect that you will find that in practice it makes little or no differance as long as you actually tune the lengths of the dipole for maximum radiated power. I have built 1/4 wave verticals with radials (which is a 50ohm antenna) and compared it directly with the simpler dipole (no matching transformer) at 868Mhz, I dont see much difference, the dipole was actually slightly better.
There are many who will use 'reassuringly complex and expensive' equipment, noise sources, reflection bridges, spectrum analysers etc to test or tune antennas in the workshop which is fine if your workshop is an anechoic chamber or the antenna is going to be used in the workshop.
Testing and tuning antennas for the real world is dead easy, find a large open field, write some Arduino software to have one radio module act as a transmitter another module to act as a field strength (RSSI) meter. This setup will measure the change in radiated power as you vary the length of elements etc. Leave the lab equipment in the lab.
You can read a summary of my musings on antennas for 868Mhz here, these tests were on LoRa modules;
ted:
It not necessary must be dipole can be spiral - 3cm long
Those spring antennas are sort of useful if you must have a very compact RF device, but they do not perform well, according to the many reports I have seen.
The small helicals work Ok, but only under fairly ideal conditions , one being no metal anywhere near the helical, as they are high Q and detune easily.
You normally find them in small consumer grade electronics where the case is all plastic and the helical is at one end as far away as possible from the circuitry.
Depends to the range which he need, in my designs I never know or consider it is 50 ohm or 50 k antenna, just turned it for resonance, for 80 MHz 50 cm piece of wire 2 transistors, 9V battery the range = 500m. Spiral anttena 315MHz , 6V battery =150m.
5W RBM1 transmiter (very old) with antenna
I too am going to try a Dipole PCB antenna for 868Mhz too, it would make for a compact low cost and easy build antenna of known performance.
Just waiting for some self adhesive coper tape to arrive so I can jig it up and test\tune its real world performance. Ought then to be easy to transfer to a PCB design.
srnet:
I too am going to try a Dipole PCB antenna for 868Mhz too, it would make for a compact low cost and easy build antenna of known performance.
Just waiting for some self adhesive coper tape to arrive so I can jig it up and test\tune its real world performance. Ought then to be easy to transfer to a PCB design.
They sell such copper tape for greenhouses to stop slugs in fact, and its very useful for such RF/MW experiments.
4nec2 antenna modeler is only modelling free space, but it's a reasonable place to start - in practice you'll have to make up a board and trim it to fit anyway. (copper "slug tape" is good )
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