Constructing a pair of transmitter and receiver that operate under 27MHz indoors

[tommyhung]

Hey guys,

I am going to design a 27MHz antenna but small size. So i decided to use ferrite rod antenna. I found that most of the resources indicated that an extra capacitor is required to connect in parallel with the ferrite rod antenna to make it in parallel resonance. However, input impedance will become several thousands.

I guess because the input impedance of most of the system that operate in HF band is in several thousands so that it need to be resonated in parallel instead of in series. If the input impedance of such system is small i.e. 50ohm, it is better to use series resonant instead of parallel. Is this statment correct?

Thanks

 

[biff44]

Re: Ferrite rod antenna

I am not an expert on ferrite rods, but I think they do not work as high at 27 MHz. If you can get one to work, it will be a "special" ferrite, not just one like you find in an AM radio.

Added after 1 hours 12 minutes:

https://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part7/page5.html

 

[FvM]

Re: Ferrite rod antenna

Generally, ferrite rod antennas are discussed among loop antennas in comprehensive antenna theory textbooks, e.g. Ballanis.

Also the "bible" of ferrite applications, Snelling Soft Ferrites has a chapter about ferrite rod antennas. All practical examples are exclusively MW antennas, however. Interestingly it mentions an (execptional) paper A Small Ferroxcube Aerial for VHF Reception. It's also listed in the below chapter on magnetic antennas, but although often quoted, apparently not available in the internet:
http://www.avionics.com/images/jpg/ant eng hdbk ch5.pdf

In a technical view, I think bif44 is right about selection of ferrite material. Usual ferrite rod antennas or optimzed for the LW and MW band and using µr values of at least several 100, as you can see from the manufacturer catalogs. It's the same material as used for 125 kHz RFID applications. For upper SW band, you should choose RF ferrites with µr below 50 to reduce ferrite losses to an acceptable amount. The availability of the stock is possibly limited at most manufacturers.

You also didn't mention, if you intend a receiver or transmitter application. The usual LW/MW ferrite rod designs are noise optimized but not impedance matched, thus not suited for transmitter applications.

 

[tommyhung]

Re: Ferrite rod antenna

Generally, ferrite rod antennas are discussed among loop antennas in comprehensive antenna theory textbooks, e.g. Ballanis.

Also the "bible" of ferrite applications, Snelling Soft Ferrites has a chapter about ferrite rod antennas. All practical examples are exclusively MW antennas, however. Interestingly it mentions an (execptional) paper A Small Ferroxcube Aerial for VHF Reception. It's also listed in the below chapter on magnetic antennas, but although often quoted, apparently not available in the internet:
http://www.avionics.com/images/jpg/ant eng hdbk ch5.pdf

In a technical view, I think bif44 is right about selection of ferrite material. Usual ferrite rod antennas or optimzed for the LW and MW band and using µr values of at least several 100, as you can see from the manufacturer catalogs. It's the same material as used for 125 kHz RFID applications. For upper SW band, you should choose RF ferrites with µr below 50 to reduce ferrite losses to an acceptable amount. The availability of the stock is possibly limited at most manufacturers.

You also didn't mention, if you intend a receiver or transmitter application. The usual LW/MW ferrite rod designs are noise optimized but not impedance matched, thus not suited for transmitter applications.

Thanks for you information.

May be it's better for me to state in clear the objective of my project.
I am going to construct a pair of transmitter and receiver that operate under 27MHz in indoor environment. The communication range of the system is about 10 meter. The transmitter should be as small as possible because it is carried by somebody(better than smaller than a mouse). The receiver will be installed in the corner of the ceiling.

Any suggestion on the antenna?

 

[FvM]

Re: Ferrite rod antenna

For a transmitter, impedance matching is required of course. Some PC interface devices (mouse /keyboard) are transmitting in the SW band. I guess, they are using magnetic antennas, but they have to span 2 or 3 meters at maximum. The "antenna" properties can be described sufficiently by simple magnetic field equations. 10m rather needs EM analysis.

Depending on the form factor of your device, you should also consider a loop antenna without ferrite core, it doesn't depend on the availibility of special parts.

 

[biff44]

Re: Ferrite rod antenna

Well, you chose the wrong frequency.

You will have a lot of cb radio interference. The antennas needed are physically large, and your system range will suffer if you try to use smaller antennas.

I would suggest doing it all at 915 MHz in the US (or 433.92 MHz in europe). You can use tiny chip antennas there.

If you really need to use 27 MHz (like if someone was holding a gun to your head), you should use a really large antenna on the receiver mounted on the ceiling (probably need one that is at least 24" long). Then I would use a loop antenna (which will be too short) or around 1 to 2" length, with reactive series tuning to get it right on frequency. You will probably have to tune the antenna while it is being worn by the subject, as the frequency will tune lower when it is mounted.