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Electronics => RF, Microwave, Ham Radio => Topic started by: awef on February 20, 2019, 05:56:39 pm

Title: UHF RFID Antenna Design
Post by: awef on February 20, 2019, 05:56:39 pm
I want to make a UHF antenna with a wide read angle (180 degrees haha), but high performance only in a short range (1m)

Not sure if that's going to violate any laws of physics.

Does anyone have a favorite book, site, resource they use to understand/design/test such UHF antennas?  I've been searching for a while and can't seem to make any headway.  It's like there's a secret UHF antenna society and they removed all the info from my internet.
Title: Re: UHF RFID Antenna Design
Post by: radiolistener on February 21, 2019, 12:12:54 am
1 meter near field zone requires a working frequency much below 149.8 Mhz (2 meters wavelength).
So, UHF RFID is not suitable for your requirements. Probably 13.56 MHz RFID will be ok.
Title: Re: UHF RFID Antenna Design
Post by: awef on February 21, 2019, 01:39:59 am
Hmm, maybe I did violate a law of physics.

In any case are there any UHF Design resources in general I can read up on?  Maybe design a 2 meter read distance antenna?
Title: Re: UHF RFID Antenna Design
Post by: radiolistener on February 21, 2019, 10:17:41 am
You can find a nice book about antenna design: Constantine A. Balanis "Antenna theory Analysis and design"

The problem with low frequency is that it requires much larger size of antenna and more power.
Your RFID can get power supply primary from Near Field region, where RF power density is very high.
It requires low frequency and high power in order to provide power supply for RFID on large distance.
Also it will helps third party to intercept your communication, because Near Field region will be large.

On the other hand, if your RFID has battery, you can use UHF and small directional antenna, but it will leads to another issue. It will be hard to limit distance to 1 meter, because your RFID will works in the Far Field region, where RF power fades with distance not so fast like in the Near Field region. This is not good for RFID, because third party can intercept your communication and even hijack it.

Hmm, maybe I did violate a law of physics.

Yes, if you're trying to use Near Field communication, because antenna radiation pattern exists for the Far Field region only. You cannot create antenna with specific directivity for Near Field communication.

Battery less RFID is designed to work primary in the Near Field region, where you cannot direct the radiation in one direction. Also it helps to receive weak signal, because antenna much more sensitive for weak signal in it's Near Field region. At the same time it minimizes risk of hijack on the long distance.

It will be hard to provide enough power supply in the Far Field region for your RFID with UHF frequency because RF power density will be too small on long distance. So it should use battery and use Far Field region for communication. It means that it will be hard to limit working distance with 1 meter, but on the other hand you will be able to use antenna with directivity, for example Yagi–Uda or parabolic antenna.

Here is picture which shows difference between Far Field and Near Field regions of antenna:

Title: Re: UHF RFID Antenna Design
Post by: awef on February 21, 2019, 04:33:25 pm
Thanks for the book recommendation.  I'm looking it up.

So I don't need to limit to 1 meter (extending beyond that is okay) but my items of interest are right next to/very close to the antenna.

Reading further out is fine, intercept is fine. 

I got the part about NFC RFID and I'm thankfully familiar with a decent amount of it.  That's not my goal.

My goal is UHF RFID, using a radio at around the 920MHz standard.

So at this frequency, I see a lot of very weak ceramic antennas that make no sense to me (designwise, how does it work?)

And the ones that are strong are these patch antennas with fancy looking patterns (multiple PCBs soldered at 90 degrees, each with fancy patterns)

That patch antenna is my goal. (picture below)

Most are very narrow 15-30 degree antennas that are focused on 1-5 meter reading distances. 

I'm more interested in reading very well wide and close.  There are some commercial antennas starting about USD $2000 but maybe I can make one that's decent for less? (excluding time and education cost, just talking part cost)

That's where I'm really short on design resources.

In case that makes you think of an additional book I should read, please let me know :)
Title: Re: UHF RFID Antenna Design
Post by: radiolistener on February 21, 2019, 05:14:14 pm
antenna design is complicated task. It requires a lot of math and physics. It will be easier to assemble already designed construction. You can also use software such as EZNEC or MMANA-GAL for modeling. This software allows you to calculate parameters of antenna and it's radiation pattern.

I never build antenna for so high frequency (920 MHz), so I cannot help with advice about all nuances.
But you can google it, there are a lot of different already calculated antennas for such frequency.
For example, here is one: ( (

I can say, you will be needed for antenna analyzer in order to tune your antenna properly. This is must have.
Without antenna analyzer it will be hard to tune even good VHF antenna.
For 1 GHz it will be even more complicated.

You also should know, that you will need to use proper materials for so high frequency (to minimize losses).
And all sizes should be very precise. Otherwise you will get bad antenna.

how does it work?

Radiating mechanism is very complicated actually  :D
This is animation how Yagi antenna works: (

In short, most of antennas is just a kind of LC circuit with high electromagnetic radiation loss.
This class of antenna is named resonant antenna. But there is also possible another class - traveling-wave antenna.
In short, you're need resonant antenna.
Title: Re: UHF RFID Antenna Design
Post by: awef on February 22, 2019, 08:20:34 am
That's one crazy video.... Thanks for all the info!