EAS 8.2 MHz antenna design help needed

[Drprez]

Hello

I am trying to design a unique EAS system and need some help with the antenna design

I have a rx and tx PCB boards. On the 8.2MHz I am using soft tags that are flat

What I want to do is very specifically control how wide the antenna detection area is.

I want it to be a very narrow plain yet cover a specific size.

More specifically I have an opening that is 6 feet wide by 4 feet tall. The wall is wood wall however the opening is made of 2 inch steel square tube and I basically want the antenna signal to work inside this steel frame area

As our products pass through this I want the system to activate. But because our system uses a production line it stops frequently. Which is the problem. Only the red area in the picture below I want to be the detection area.


I want it to go off only when the device crosses this line at the 6x4 opening. I want it as exact as possible. But not wider. Because if a device stops say 2 inches before this line I don’t want it to activate.

So my question is
-how would I make this antenna design to work?
-what gauge and type of wire?
- what can I do so the detection is only inside this square area and not on the sides but directed only in the box?
-do I shield the antenna on the side I don’t want it?

I attached my EAS PCB boards as well as a basic drawing of what I am trying to do.



If you can help with specifics to my design or what to do I would appreciate it.

​

• 

[Mod Note: - as per Harald's comments, it is very important to always use the correct notation ... I did some chances for you ]

 

[Harald Kapp]

Please note:
mhz = milli Hertz -> useless for this
What you want is
MHz -> Mega Hertz

Using correct units helps immensely.

 

[kellys_eye]

The lower the frequency the longer the antenna elements have to be to achieve resonance. To 'focus' radio waves into a particular pattern (that pattern is the same regardless of whether you are transmitting or receiving) requires phasing of the elements to 'mathematically add' the various nodes that the elements produce and get the focus point.

In terms of radio transmission we would use a 'yagi' type antenna to do the focusing - a design that is well documented - but at such a low frequency the elements would be 'enormous' which is why signal focusing (or beam width) is most often achieved at very much higher frequencies, in the GHz region.

Your only recourse would be to test for signal absorption as the items pass and set a level at which that absorption triggers a detecting signal. Alternatively you could test for frequency change as the item enters the magnetic field.

This is one case where an optical solution would be far easier to adopt.

 

[davenn]

Please note:
mhz = milli Hertz -> useless for this
What you want is
MHz -> Mega Hertz

Using correct units helps immensely.

I have edited his post

 

[davenn]

In terms of radio transmission we would use a 'yagi' type antenna to do the focusing - a design that is well documented - but at such a low frequency the elements would be 'enormous' which is why signal focusing (or beam width) is most often achieved at very much higher frequencies, in the GHz region.

indeed

So my question is

-how would I make this antenna design to work?
-what gauge and type of wire?
- what can I do so the detection is only inside this square area and not on the sides but directed only in the box?
-do I shield the antenna on the side I don’t want it?

An Antenna for 8MHz is huge in comparison to what you are wanting to do
It would be virtually impossible to focus 8MHz into such a small area. It will radiate everywhere in the room and beyond

your choices are
1) RFID
2) As Kellys_eye suggested magnetic or
3) As he also suggested, optical which would be, by far, the best

 

[bertus]

Hello,

As said, It looks like an RFID project.
For the space you want to cover, you will need several antennas and multiplex the signals.

Multi-loop antenna for high frequency RFID smart shelf application | Semantic Scholar

This paper has presented a patent-pending multi-loop antenna for HF (13.56MHz) RFID smart shelf applications. The proposed antenna prototype has been able to generate magnetic field with uniform magnitude for a larger interrogation region. In addition, the low profile structure makes it very...

www.semanticscholar.org

This was found using this google query:

rfid antenna 8.2 MHz - Google Search

Bertus

 

[kellys_eye]

RFID implies the items passing the antenna will re-radiate a signal as prompted by the RFID transmitter. If these 'conveyored' devices are NOT fitted with the appropriate RFID (trans)responder then your methodology is completely wrong.

 

[Drprez]

They will not have transmitters. They have soft tags in them. No transmitting device. So how can I direct the signal in one direction at least so it only goes toward my openin only and not to the other side? Can I shield one side of the antenna? Do I use a single wire antenna or what ?

 

[Drprez]

indeed

So my question is


An Antenna for 8MHz is huge in comparison to what you are wanting to do
It would be virtually impossible to focus 8MHz into such a small area. It will radiate everywhere in the room and beyond

your choices are
1) RFID
2) As Kellys_eye suggested magnetic or
3) As he also suggested, optical which would be, by far, the best

The antenna can’t be that big they are used in thousands of stores security systems. So how can I make an arena to focus to the opening

 

[kellys_eye]

The antenna can’t be that big they are used in thousands of stores security systems.

.... yes, as part of the RFID system which will detect the tags on 'stolen' property - but not property that doesn't carry the tags. The range of detection is also relatively wide - in the orders of 'feet', certainly not inches.

If the products on your conveyor passed at a fixed distance at every interval you might have a chance of adjusting sensitivity to achieve a narrower range but that's just a very much hit'n'miss affair and wouldn't be reliable in any way at all.

They have soft tags in them.

Describe your soft tags......

The only way I could see this working is by using TWO antennas and making a constant test for signal strength as the item passes between them and then sensing when the received signals are near-identical in amplitude - at which point the device would be as good as mid-way between the two antennas. Kind of the reverse of a zero-crossing detector.

 

[kellys_eye]

So how can I make an arena to focus to the opening

(my bold in the quoted part)

You can't. End of. The frequency is too low. Google yagi element calculator and research antenna radiation patterns to learn more.

 

[Drprez]

So can’t I shield one side of the antenna where I don’t want the waves to go with some type of material so it’s one active toward my opening? The soft tags using 2 antenna. How would I do this? Can the antenna be just vertical wires or do they need some specific design?
Where I put the tx on one side and the rx on the other side? What type of antenna shape/pattern should it be?

Will something like this work where Half is the tx and half is the rx antennas?

 

[Delta Prime]

Antennas are responsible for emitting and receiving waves that allow us to detect RFID chips. When an RFID chip crosses the antenna field, it is activated & emits a signal. The antennas create different wave fields & cover different distances.
NF(Near Field) antennas are used to read RFID tagswithin a few centimeters.
https://www.dipolerfid.com/short-range-rfid-antennas

 

[kellys_eye]

So can’t I shield one side of the antenna where I don’t want the waves to go with some type of material so it’s one active toward my opening?

No.

Can the antenna be just vertical wires or do they need some specific design?

Told you already - look up antenna radiation patterns. You can't (easily) focus low frequencies and you'll NEVER do it at 8MHz. To do what you want you need to be using 900MHz and above (microwave frequencies) and, even then, it won't be easy. You're talking 'phased array' type antennas.

Stop flogging this horse - it's well past dead and the corpse is rotting. Go to visible light (THz) and use infra-red or lasers (beam break).

 

[hevans1944]

I am trying to design a unique EAS system and need some help with the antenna design

The RF-ID passive soft-tag resonates when exposed to the RF probing field. This resonant energy absorption can be detected and used to signal that the presense of a RF-ID soft-tag has been detected. It does not, in general, provide any location information of the RF-ID soft-tag, other than the fact that the RF-ID soft-tag was detected as it passed through the RF interrogation field. The radio frequency used is too low to allow any sort of "focusing" of the interrogation field by any practical antenna design, as other respondents here have already stated.

It may be possible to design and install a so-called magnetic-loop antenna within the confines of the 2"x2" steel beams that frame the four-foot high by six-foot wide loading opening. Perhaps such a design can be used to sense the presence of the soft-tag if, and only if, the tag is within the loading opening. However, I have some questions...

If your product, while moving along on the conveyor belt and stopping periodically at random intervals, has RF-ID soft-tags embedded, why do you think it appropriate to use these tags to locate your product with respect to the "loading opening" instead of choosing a simpler and more robust solution? Just because your product contains an RF-ID soft-tag doesn't mean you have to try to use it to detect product position with respect to the "loading opening."

A simple optical solution would be to send a collimated, perhaps amplitude modulated, infrared light beam across the six-foot wide opening at a height guaranteed to intercept your product, and detect when this light beam in interrupted. My garage door opener does this anywhere across the width of my two-car garage door opening at a height of about two inches. So the optics and electronics are commercial off-the-shelf items. This type of system is widely used in industry, sometimes with two almost-parallel plane mirrors to create a "beam ladder" across the open area. No need to re-invent the RF-ID "product present" system just because your product contains a soft-tag RF-ID.

However, you could use the RF-ID system to detect when the product is near the loading opening and then use a "smart" digital camera to observe your product as it approaches the "loading opening". This is also a commercial off-the-shelf solution.

Your problem is NOT "How can I identify where an RF-ID soft-tag is?"

Your problem is "How can I identify where my product, that happens to have an embedded RF-ID soft-tag, is?"

 

[Drprez]

The RF-ID passive soft-tag resonates when exposed to the RF probing field. This resonant energy absorption can be detected and used to signal that the presense of a RF-ID soft-tag has been detected. It does not, in general, provide any location information of the RF-ID soft-tag, other than the fact that the RF-ID soft-tag was detected as it passed through the RF interrogation field. The radio frequency used is too low to allow any sort of "focusing" of the interrogation field by any practical antenna design, as other respondents here have already stated.

It may be possible to design and install a so-called magnetic-loop antenna within the confines of the 2"x2" steel beams that frame the four-foot high by six-foot wide loading opening. Perhaps such a design can be used to sense the presence of the soft-tag if, and only if, the tag is within the loading opening. However, I have some questions...

If your product, while moving along on the conveyor belt and stopping periodically at random intervals, has RF-ID soft-tags embedded, why do you think it appropriate to use these tags to locate your product with respect to the "loading opening" instead of choosing a simpler and more robust solution? Just because your product contains an RF-ID soft-tag doesn't mean you have to try to use it to detect product position with respect to the "loading opening."

A simple optical solution would be to send a collimated, perhaps amplitude modulated, infrared light beam across the six-foot wide opening at a height guaranteed to intercept your product, and detect when this light beam in interrupted. My garage door opener does this anywhere across the width of my two-car garage door opening at a height of about two inches. So the optics and electronics are commercial off-the-shelf items. This type of system is widely used in industry, sometimes with two almost-parallel plane mirrors to create a "beam ladder" across the open area. No need to re-invent the RF-ID "product present" system just because your product contains a soft-tag RF-ID.

However, you could use the RF-ID system to detect when the product is near the loading opening and then use a "smart" digital camera to observe your product as it approaches the "loading opening". This is also a commercial off-the-shelf solution.

Your problem is NOT "How can I identify where an RF-ID soft-tag is?"

Your problem is "How can I identify where my product, that happens to have an embedded RF-ID soft-tag, is?"

The devices can’t have a battery to send a signal. That’s why I was thinking the soft tag would work.

Optical won’t work either due to other limitations we are working with.

Sometimes there are a mix of products with the tangs and some without. I’m only wanting to count the ones with tags not every item that passes the line. And sometimes they are inside or under other items that’s why the soft tag I thought would work best.

Are there any existing systems or devices currently that would do this or am I starting from scratch to build this.

So why can’t I block the side of the antenna I don’t want the signal to go? If the area is between the 2 antennas. Couldn’t I put i don’t know a lead sheiks or similar on the sides I don’t want the waves to go? Couldn’t I do the same to help direct the direction of the waves?

The Yagi antenna won’t work as I need the antenna to fit along the 2 inch beam on either side.
Sorry I’m very green so most of this is over my head and I am learning so I appreciate your help.

 

[Drprez]

No.

Told you already - look up antenna radiation patterns. You can't (easily) focus low frequencies and you'll NEVER do it at 8MHz. To do what you want you need to be using 900MHz and above (microwave frequencies) and, even then, it won't be easy. You're talking 'phased array' type antennas.

Stop flogging this horse - it's well past dead and the corpse is rotting. Go to visible light (THz) and use infra-red or lasers (beam break).

Wow. You couldn’t be more wrong.

I suggest you refrain from making comments unless you actually know what you’re talking about.

The purpose of this form is for people to ask questions and learn from not to knock people down and talk to them like they are idiots.

Your aggressive negative replies clearly are not founded on much science or just ignorance.

If you can’t positively and constructively help people and give them direction, DON’T HELP AT ALL!

For those of you who want to know how it works:

My parts finally arrived. They work exactly as I had suggested. Almost too perfect.

As you see in the video link below when I am inside the opening they work great and down to within 1/8 of an inch of my line.

ANYWHERE IN THE ENVELOPE!

If I’m outside the envelope they don’t work at all. Not even right next to one antenna. It’s flawless. Here is a link to a video of it working.

Video of accuracy of detection opening

The way I got it to work was by adjusting the sensitivity of the rx and tx antennas. With a 180Hz frequency. I run an antenna wire in a parallel loop to itself. With only about 2cm space between the parallel wires


I’m able to very accurately focus the wave on a vertical plain using a simple shield. Shaped in a simple “L”

On the side I don’t want the signal to go I used a simple aluminum foil shielding just like what comes in shielded wire. Then on the side I want to work it is just standard 18awg stranded copper wire and regular wire insulation.


Here is a short video showing how MUCH MORE accurate the signal sensitivity is when it is shielded versus not shielded.

So completely proving you wrong yes you can direct where the waves go and are able to make your detection zone more accurate.

Accuracy of signal sensitive shield vs no shield



Then for the antenna placement i have the RX antenna half way across the top horizontal and the full length of the vertical sides of the opening. The TX is an exact mirror image of this.



The soft tag works at any angle and I have tried it inside plastic dense objects and other materials. ALL WORK FLAWLESS.

 

[davenn]

Wow. You couldn’t be more wrong.

I suggest you refrain from making comments unless you actually know what you’re talking about.

he isnt wrong and he knows what he is talking about as do the rest of us that have tried to guide you

it is you that has no understanding of how an antenna works or how a radio signal from it is propaqgated


Thread closed