Simple wire antenna and RF compliance

[nanorobot]

Hi,


I am working on a small project involving wireless communication using short range RF modules.
I am considering selling products based on that project. From the legal standpoint, as a manufacturer, I am obligated to ensure my product complies with regulatory requirements concerning radio emission.

The modules I am using are claimed to be in compliance with regulatory standards (although I am not sure of that because in my opinion one must take RF module and antenna as a whole system into account if that claiming is to be valid).

My problem is choosing the right antenna so that my product can meet radio emission requirements. I am going to use a 1/4 wave dipole type antenna. There are many commercially available antennas of this type, majority of them being helical whips. My initial range tests showed that a simple wire cut to proper length performs far better than those helicals. For this (and of course very low cost) reason I would like to use that simple wire antenna in the product. But I have few concerns.

Well, since I haven't seen many real products on market using just wire as an antenna I have some doubts about that. Can using a simple wire as antenna in a transmitter lead to problems like spurious emissions above permitted levels or are there any additional problems when using such a simplistic antenna?

Thank you for any advice.

 

[jiripolivka]

All your thoughts are correct. If the device was tested for compliance and passed, then the antenna had to be included in testing. Using a different antenna may affect the test results but I doubt it may cause more interference.
There are some classes of devices to be evaluated separately:
- short-range devices for specific frequency bands like garage or car openers. Their range is intentionally limited to say 100 ft by RF power generated and no larger or added antennas are allowed.
- e.g. Bluetooth devices may not be joined with external antennas to extend their range.
- there are "free" frequency bands like FCC No.15 standard in which the devices may cause interference to others and the user is advised to move a device away to resolve the problem, and devices may accept interference while the user may not call authority to resolve the problem.

I agree that your quarter-wave antenna can work better than a commercial antenna, only please use the available connector on your device to be sure no other than the desired signal is radiated. As you intend to sell the device with the new antenna, you will be responsible for the SYSTEM, and according to the particular standard you may be required to have the SYSTEM tested for compliance.

Compliance testing laboratory has to operate under national rules; I would recommend to ask the officials if and how the particular compliance rules hold for your case. Some standards are weak, some tough, depending on system class and frequency band used. Fines for non-compliance are high so it is better to follow the rules.

 

[nanorobot]

Thank you jiripolivka for the reply, although I didn't quite understand the remark about using the available connector.

My RF module ( ) has 50 Ohm RF output to which I connect (through 50 ohm microstrip line) my quarter wave wire antenna - directly soldering to the PCB pad. Normally these types of modules are used with whip antennas with SMA connectors. Simple wire is significantly cheaper so I would like to know if there is any strictly technical reason for using commercial whip antennas rather than simple wires? For example, there are Xbee RF modules using wire antennas and they are fully FCC/ETSI certified.

 

[jiripolivka]

Thank you jiripolivka for the reply, although I didn't quite understand the remark about using the available connector.

My RF module ( ) has 50 Ohm RF output to which I connect (through 50 ohm microstrip line) my quarter wave wire antenna - directly soldering to the PCB pad. Normally these types of modules are used with whip antennas with SMA connectors. Simple wire is significantly cheaper so I would like to know if there is any strictly technical reason for using commercial whip antennas rather than simple wires? For example, there are Xbee RF modules using wire antennas and they are fully FCC/ETSI certified.

Technically any antenna can be used if you have a preference. I mentioned the connector if the device was tested without an antenna or another antenna. For such case the user can use any antenna of his/her choice.
RF devices when connected to a specific antenna are expected that those two parts of the system are matched at the operating frequency. The test result shows that the radiated spectrum fits some template.
Any mismatch may cause that the radiated spectrum changes and may not meet the standard for radiated spectrum. Some devices are sensitive to such mismatch.
In some cases, directional antennas are not permitted for use (like CB transceivers and garage openers).
If your device is FCC sec.15 certified, then any antenna can be used. Other cases may need that tests should be repeated with another antenna type. I would consult the testing office.

- - - Updated - - -

As your RF module is designed for the license-free use, I think you may use any antenna without problems. There may be differences between various countries , so if your device and antenna is to be used outside the U.S., official permit may be required.

 

[vfone]

My initial range tests showed that a simple wire cut to proper length performs far better than those helicals.

Make this antenna and don't tell to anybody about performances. For many years I found the same thing. For small ISM antennas a simple λ/4 wire antenna (well placed in freespace), works better than thousands of antennas published as PhD thesis on the Internet.

 

[WimRFP]

@vfone: I agree from the technical side, however from a regulatory standpoint, I would not take the risk. If they tested the RF module with a small resonant antenna to get better out of band performance, using a quarter wave over ground plane may increase spurious emissions.

@nanorobot: you need to find out how they tested the module (procedure, results and standard). The best is to have the actual report. You may know that ETSI standards can be downloaded for free.