*Very* Small Low Freq Antennas with directivity?

[dnyberg2]

I'm curious about anyone that has concrete hands on experience tinkering with very small low frequency antennas.
My band of interest is 6, 13 or 27 MHz.
I wonder if it is possible to design a very small physical antenna that will radiate with some directivity even if the efficiency is not so good? This is an ISM band investigation. What good is an idea for an ISM band widget if the antenna has to be the size of a house?
I'm not interested in long range communications or high power but the ability to fill a room, rooms or the floor of a house with the maximum permissible SAR allowed by the FCC for tracking objects with transponders attached. Many thanks.

 

[scottdave]

It seems that you are aware that wavelength and frequency are inversely proportional. To achieve direction, the size of the antenna elements will be on the order of the wavelength. For your frequencies, this is on the order of several meters.

 

[dnyberg2]

Yes I am aware of all of the physics that counsel against the possibility, what I am after is a solution that isn't obvious like a low efficiency dielectrically loaded or metamaterial augmentation type of solution. I don't care about communications or efficiency. All I want to do is fill a room with enough power to make the system work.

 

[dnyberg2]

Maybe a short, loaded radiator with a reflector housing around it? Like a tall metal can with the face cut out and a rubber ducky antenna mounted inside??

 

[scottdave]

Maybe a short, loaded radiator with a reflector housing around it? Like a tall metal can with the face cut out and a rubber ducky antenna mounted inside??

That is interesting. I guess you could try it, and see how the results go.

 

[dnyberg2]

Not very scientific granted but even if the SWR's are under 2, it might be practical
.

 

[berkeman]

the ability to fill a room, rooms or the floor of a house with the maximum permissible SAR allowed by the FCC for tracking objects with transponders attached.

So the overall goal is to track objects in your house. I'm not sure using low-frequency ISM bands is the best match for that. Why not just go with traditional RFID tags? Or IR emitters and passive receiver arrays...

 

[jim mcnamara]

I think the OP is only asking how to do - what he had already decided to do. Notwithstanding - giving us a why might help enormously. We have all done 'what if' projects. But simply telling us your goal is not enlightening, IMO.

Want good answers? Give us good questions!

 

[dnyberg2]

One reason is I need the energy at the transponder. According to the FCC OET56 REV E4 bulletin, the lowest frequencies that have the least harmful exposure with the highest energy density allowed are between .3 and 1.34 MHz. An even harder challenge to accomplish. At least the 6, 13 and 27 MHz bands are in the ISM band and I bet this will be forced to be a licensed product so transmitting that much power near the U.S. AM Broadcast band won't be well received. (No pun intended)

 

[berkeman]

According to the FCC OET56 REV E4 bulletin, the lowest frequencies that have the least harmful exposure with the highest energy density allowed are between .3 and 1.34 MHz.

I didn't go to that bulletin, but that sounds strange. The US AM broadcast band is right there around 1MHz...

http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/radio.html

 

[skeptic2]

I'm curious about anyone that has concrete hands on experience tinkering with very small low frequency antennas.
My band of interest is 6, 13 or 27 MHz.
I wonder if it is possible to design a very small physical antenna that will radiate with some directivity even if the efficiency is not so good? This is an ISM band investigation. What good is an idea for an ISM band widget if the antenna has to be the size of a house?
I'm not interested in long range communications or high power but the ability to fill a room, rooms or the floor of a house with the maximum permissible SAR allowed by the FCC for tracking objects with transponders attached. Many thanks.

1. What do you mean by an antenna with some directivity? All antennas are directive to some extent. Can you define how much directivity you need?
2. I presume the objects being tracked will be the transmitters. With what precision do they need to be tracked (angle and range)?
3. How will you distinguish between the objects? Yes you said they have transponders but transponders alone may not be useful in tracking the objects. Are you planning to use a different frequency for each object?
4. Have you considered triangulation to determine the location of the object. I'm thinking of a phase comparison between the signals received by three or more receivers. Knowing the phase difference at two receivers and the wavelength should enable you to calculate the location. Just be sure the wavelength is significantly longer than the range you need.
5. A ferrite rod antenna will probably work pretty well. I have used ferrite rod antennas for both transmitting and receiving from 200 kHz to 27 MHz with very good results. At 250 kHz with ferrite rods for both transmit and receive, I've achieved ranges of up to 150 feet.
6. Here are some unlicensed bands you may consider.
§15.217 Operation in the band 160-190 kHz.
§15.219 Operation in the band 510-1705 kHz.
§15.221 Operation in the band 525-1705 kHz.
§15.223 Operation in the band 1.705-10 MHz.
§15.225 Operation within the band 13.110-14.010 MHz.
§15.227 Operation within the band 26.96-27.28 MHz.
For instance:
§15.217 Operation in the band 160-190 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt.

(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.

(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

 

[dnyberg2]

Useful. The items to be tracked in the home need the power so the emitter in "the corner of the house" is the source of all power. The emitter will also receive but I suspect on a second much higher frequency like VHF ISM band so as not to complicate the emitter design. Bands to transmit on are not the issue, the issue is getting a radiator to radiate as much RF density allowed, generally speaking in a beamwidth of 180 deg, so some sort of method to prevent errant RF from going everywhere is hoped for. I haven't worked out the triangulation aspects yet and its a useless exercise unless I can saturate the dwelling with enough power to satisfy milliamps of DC current needed to run the tracking device. The rules of the game say no batteries but supercaps are OKAY.

The problem is a radiating structure that can radiate into free space that's not 1,000 feet long! The rubber ducky antenna in a can with the long axis cut away seems manageable but not the lower bands of .3 to 1.3 MHz. How the heck do you design a radiator for the band described in §15.219 that is no larger than a lamp siting on a table with some directivity ?? (It doesn't have to be a real lamp)

 

[dnyberg2]

I just looked into some of those rules and there's no way any of them will provide enough RF power density to make this work. The solution will have to be a licensed transmitter with several if not many watts EIRP I suspect.

 

[dnyberg2]

Anyone ever heard of a liquid radiator concept? I wonder what would happen if I filled a sealed hollow plastic tube with some conductive liquid that resulted in some usable impedance at 1 MHz... I wonder if it would radiate in an omni directional manner and then I could just shield the backside...

Just dreaming... Anyone got any dilithium crystals I can use for a few days?

 

[skeptic2]

Let me understand this. You want to transmit a signal from the emitter to the units with enough power that the units power themselves up and transmit a signal to a receiver (perhaps the emitter itself)? I think you need to do a path loss analysis, commonly referred to as a link budget to see if this is practical.

 

[dnyberg2]

Very interesting indeed! Free space loss at 1MHz (isotopic of course) with no receive or transmit antenna gain is on the order of 6dB fro 50 meters! .3dB for 500 KHz!

 

[tech99]

Very interesting indeed! Free space loss at 1MHz (isotopic of course) with no receive or transmit antenna gain is on the order of 6dB fro 50 meters! .3dB for 500 KHz!

A loop antenna that is small compared with the wavelength may be capable of radiating a heart shaped pattern with tolerable efficiency. Maybe you can obtain reasonable efficiency by using a rectangular loop that is a couple of metres tall but only a few cm wide. To obtain the pattern I have mentioned, the antenna needs to combine both loop and monopole type of excitation with a 90 degree phase shift, so you need to refer to textbooks to do this.
To look at the system performance we need to know the type of antenna and power requirements of the receiver device.

 

[dnyberg2]

Ah yes! The same problem will exist in the receiver right? Some low frequency resonated loop of Litz wire perhaps?

 

[tech99]

Ah yes! The same problem will exist in the receiver right? Some low frequency resonated loop of Litz wire perhaps?

How much power do you have to deliver to the receiver?

 

[dnyberg2]

Milliamps at best maybe 20 max?? Not yet determined but its a starting point.

 

[tech99]

Milliamps at best maybe 20 max?? Not yet determined but its a starting point.

At 27MHz over a 5m path the free space loss between half wave dipoles is only about 12dB. If you want 20mA at 3 volts, that is 60mW, or +15dBm. So the transmitter power would be +15+12=27dBm which is 0.5W.
However, you must assume poor efficiency for the two antennas. Maybe -10dB for TX and -40dB for RX. So now you need 50dB more transmitter power, which is +27 + 50 = +77 dBm or 47dBW. This is 50kW, which is clearly absurd.

 

[dnyberg2]

Quite correct but clearly that is absurd however I'm not one to let a few paper napkin calculations ruin any chance of success!

Case in point, The only thing standing in my way of success using your own 27 MHz model is a better radiating antenna and an RX ANT that doesn't suck as bad as -40dB!

I wonder why its more instinctive to tell people why something can't be done than offer advice on solving the problems mother nature hands us every day?

 

[Vitro]

@dnyberg2, why not use near field instead of far field, something like https://en.wikipedia.org/wiki/Resonant_inductive_coupling ?

You could just run a very low frequency loop around the entire area that you want to power, and any receiver inside just needs a coil in the same plane as the outer loop. You can try making the receivers resonant for best efficiency power transfer. If the transmitting loop is sized to be a poor far field emitter at that frequency then you can turn the power up as high as you need.

 

[dnyberg2]

That is a real workable solution but the endgame of this might be a consumer product one day so supplying mom and pop with a roll of wire to string around the walls seems a bit far fetched. The idea of some sort of resonator device that you can pull out of a box, plug in and place in the corner of a room seems more marketable.

 

[berkeman]

I wonder why its more instinctive to tell people why something can't be done than offer advice on solving the problems mother nature hands us every day?

It is poor engineering to keep trying to use a technique which is clearly not going to work.

The idea of some sort of resonator device that you can pull out of a box, plug in and place in the corner of a room seems more marketable.

If you really want to develop a product, it is time to abandon your initial (unworkable) idea, and look more into reasonable solutions (like the RFID tags or IR approaches that I mentioned earlier).

This thread is now on a short leash. We do not want to waste folks' time having to explain a dozen different ways why your approach so far is not workable.

 

[dnyberg2]

As you wish.
Thanks to all for the input. If I craft a working solution I will share it.