Antenna in metal container

A sealed container? No...

Dan - Owner

You don't need much of a crack for the energy to get out of the container. 1/2 wavelength crack that's very thin can get the energy out fairly strong. That's a 2.5 inch crack for your 2.4 GHz and 7" crack for your lower freq.

One assumes you want to radiate out of the sealed container.

Describe your container more if possible.


kch

Thank you for your replies.

About the container. It will be roughly something like this


In the worst case the container will be metallic with no special openings except the doors. Some containers might have a ventilation system in the four upper corners, something like four holes.

Considering this, can I succeed a range of about 300-500m using a dipole-like antenna?

An antenna in a shipping container will radiate somewhat along any gaps in the container that are larger/longer than 1/4 wavelength. This will be especially true of the gaps along the doors for both 860M and 2.4 GHz. Some radiation will occur along the edges of the steel construction where sheets meet but there is not a continuous weld seam - since these gaps are shorter, a 2.4 GHz signal leak-out better here than a 860 MHz signal.

The radiated signal is going to have a unusual pattern, or gain in different directions from the container - probably will radiate better in the directions of the doors.

I assume you're trying to get 300-500meters range regardless of the orientation of the container to the opposite end of the link. Now, ignoring other 'complex system' factors, how much transmitted power you begins to become a factor of how reilably you must communicate, and whats the worst signal gain you might have from the container. Most ISM band devices have somewhat similar receiver sensitivity and noise figures (I won't get into processing gain of digital modulation systems). What it gets to on your link budget is that you have a lot of unpredictable and variable unknowns.

The major factor for success in such a unpredictable link is to depend upon what I called 'complex system' factors. Use of diversity antennas at both ends of the link help to avoid radiation pattern nulls, frequency hopping helps only a slight bit, using major frequency differences (i.e. both 860 and 2.4 GHz) could improve the communication, repeating data packets could help, but a packet based system with handshaking and other complex network system tricks are your best bet. Such systems are usually organized around standards such as 802.15.4, z-wave, zigbee, etc.

Radio systems for shipping containers are already made -some using the techniques indicated above. However, I would not use the 860 Mhz band for a transportable radio as the 868 MHz ISM band is not universal world-wide. In north America and some other countries, this frequency range is used for some cellular services, and the ISM band is at 902-925 Mhz. The 2.4 GHz band is used for ISM use around the world with only minor restrictions in some areas (transmit power must be lower in Europe, some minor restrictions in Japan, etc.

Ahhhh, a cargo container... I was thinking more like an Altoids tin, or a coffee can-sized container ;-) That's a whole 'nother beast.

Dan - Owner

Thanks a lot Comcokid, you have been really helpful! Is there any documentation that you could suggest me? I would like to find something about antenna's performance in real applications like the discussed appl above.

I think this is more of a try it out type assignment. Could be tough finding the answer somewhere in a paper (though still worth a try searching).

What's the experiment used for? Real shipping containers on ships? or some dry land experiment?

kch

It's for real containers! I have to find an antenna that has good performance near metal, and plays at two bands (868/2.4).
For a zigbee communication protocol between two containers for example, what should I look at first to maximize the possibility to make them communicate.
I think that an omni directional antenna could give better chances than a directive one.

You need to place the antenna near a seam in the container door.

Find a patch antenna which uses a metal ground plane, or a pair of patch antennas and place them on the door (inside) if possible. Find the point where the gap might be maximum on the doors (center of doors? edge?). If you want energy to come out of the top of the door, orient the antennas vertically polarized. If the energy comes out the vertical edges of the door, orient the antenna horizontally polarized. Current on the door surface will leak out the door. If there isn't a gap in the doors, make one with a thin dielectric, i.e. glue a sheet of bare FR4 in the door seam.

We've proposed similar antennas for sensors inside thermal chambers.

kch

Thks a lot Higgler !
Another thing I wanted to ask: Is every antenna compatible with zigbee standars? (the only thing that should I care is the operation freq 868 and 2.4 GHz or is there smthing else to consider before I choose an antenna)

Thanks again!

Is this container all by itself in a field, or packed into a huge pile of similar containers in a shipping port?

In a reflective environment, it might be advisable to use two antennas in a spatial diversity arrangement. At two bands, that becomes a wee bit more complicated.

An alternative would be to provide the spatial diversity at the other end of the link.

Antennas have a usable bandwidth (usually a VSWR spec. or a gain curve measured), as long as the antenna bandwidth matches your system communications bandwidth, any and all are ok.
Only UWB (Ultra Wide Bandwidth) antennas need some constant phase versus frequency control. Those are very specialized.

If these antennas get condensation/ice on them, often times they'll shift frequency, so be cautious about that. i.e. your system goes bad on a cold or moist day and is ok on a hot dry day.

Your task is not a simple one, good luck

kch

This container will be stacked with others that will be able to communicate with it and others irrelevant with no communication ability(I consider them like obstacles in the communication link). I want to maximize the possibility of the containers that have a communication system, to communicate with each other.

I believe those containers can have holes drilled in them.
You will probably get a lot longer range by adding a hole and putting the antenna outside. I realize that wouldn't be considered unless it's absolutely needed.

An alternative to a hole is a flex cable antenna, electronics inside, co-planar coax. (or stripline) flex line through the door edge (if possible), and antenna outside. The antena could be a simple notch antenna etched right into the flex line.

Extreme multipath environments can limit range.

kch

Thanks Higgler! I think that this could help me! I will try to get some sleep now and re-think about it (here...in Greece it's a bit late...) thanks again!