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Reducing RF power of video transmitter

  1. Jun 10, 2010 #1

    I have a tiny (.6"x.6"x.125") 2.4ghz video transmitter that puts out about 1mw or rf power. I want to severely reduce the output power in order to reduce the range of this transmitter. instead of a few hundred feet line of sight range I would prefer no more than 10ft, and as little as 2 ft would work.

    I am considering creating an enclosure from a conductive material such as copper, aluminum, or even graphite if that would work. Any ideas on what material would be most appropriate, the thickness required, or whether this would work at all?

    Thanks for any suggestions.

  2. jcsd
  3. Jun 10, 2010 #2
    Trying cutting off the antenna to less than an inch in length or completely remove the antenna.
  4. Jun 10, 2010 #3


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    Only need to go through a wall, eh? What's the problem with the standard power and range?
  5. Jun 10, 2010 #4
    Try layers of thin aluminum foil.

    Bob S
  6. Jun 10, 2010 #5
    I have already cut the antenna to about 2-3mm (from the standard 1"), and it still easily transmits the 30ft or so I tested it at. I'll find out the max range later.

    Would thin layers of aluminum foil work better than solid aluminum, or a material like this:

    http://www.blockemf.com/catalog/product_info.php?cPath=747&products_id=5389 [Broken]

    I want to prevent monitoring of the signal by nearby receivers operating on the same frequency. The transmitter is body worn, sending the signal only a few feet to a relaying transmitter that sends an encrypted digital signal.

    Thanks for any help.

    Last edited by a moderator: May 4, 2017
  7. Jun 10, 2010 #6


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    Under ~3ft you'll start getting into http://en.wikipedia.org/wiki/Near_and_far_field" [Broken] propagation at that frequency.

    You might consider a standard RF attenuator. 30db should do it. Don't to stack any more as they'll simply leak. In fact your transmitter housing is probably already going to leak enough power to propagate above the noise floor at 10', so shield it carefully.
    Last edited by a moderator: May 4, 2017
  8. Jun 11, 2010 #7
    I will look in to those attenuators.. The problem is the antenna is just alittle wire soldered on to the tiny circuit board, and there is no sma connector or otherwise to connect the antenna. Not really sure how I would get one of those attenuators connected... ideas?
  9. Jun 11, 2010 #8
    Also, since I'm not familiar with attenuators, when the frequency is specified for a certai attenuator (i.e. 5mhz, or "dc-1500mc" for some) does that mean the frequency must be right around that level, or that is the max frequency, etc... My frequency is around 2.4ghz but most of those attenuators are not rated at 2.4. Are there smaller attenuators that I could solder in line with the antenna, as this video transmitter is very tiny, smaller than most of the attenuators on the page linked above.

  10. Jun 11, 2010 #9


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    Why don't you just use coax, or a wired video camera? What is the need for wireless?
    Last edited by a moderator: May 4, 2017
  11. Jun 11, 2010 #10
    Is this unit stand alone or on a PC board? Might it be radiating from the power supply leads or the cable to the camera? It might be radiating from the PC board itself. Would it be possible to put in one of those anti-static bags used for shipping circuit boards?
  12. Jun 11, 2010 #11


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    Conductive bag around powered circuit board? Hello smoke.
  13. Jun 11, 2010 #12
    Are you going to eavesdrop on other people without their consent? If so then there could be legal issues depending on where you live.
  14. Jun 11, 2010 #13
    The transmitter is a tiny little .5" x .5" circuit board. So you're thinking even though the antenna is clipped the signal might still be strong because its leaking out the power, signal, or ground wire? I can put the whole thing in an antistatic bag to test that... will those block rf signals?

    If that is the case is there any way I could filter that out of the affected wire?

    Another option that would help solve my problem would be some kind of signal scrambling on the video signal line that can be de-scrambled on the receiver side. It wouldn't have to be a full scale video encryption system because I'm only worried about casual observers of the signal. I do have severe size constraints however, and the scrambling device would have to be a small circuit board, around 1"x 1.5" max. Any suggestions?
  15. Jun 11, 2010 #14


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    I agree with waht -- I'm getting uncomfortable about this thread. Please explain exactly what this video camera is for, and why you don't want others to know that you are using it.
  16. Jun 11, 2010 #15
    Oh dear. This transmitter is part of a prototype body worn video surveillance kit to be worn by law enforcement or private investigators/security personnel. It will not be used by me for any nefarious purpose. I can't describe in detail the makeup or exact purpose of why I'm requesting some of these specifications because I have a financial interest in protecting a new product from copycats before it's even introduced. If it makes you uncomfortable then you aren't obliged to help me.

  17. Jun 11, 2010 #16


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    If it's for LEO, then absolutely you need to encrypt the data that is transmitted wirelessly. You definitely don't want a bad guy able to see what an officer is seeing, if they are searching for him or he is barricaded. Just trying to limit the xmit power is not enough. You will need enough xmit power to be able to avoid multi-path nulls between the tx and rx devices, so you can't reduce the power too far.
  18. Jun 14, 2010 #17
  19. Jun 14, 2010 #18


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    Yes, but those are likely going to radiate off board enough to be detectable at ~5-10'
    Last edited: Jun 14, 2010
  20. Jun 15, 2010 #19
    Those minicircuits attenuators look interesting. So if i ran the antenna wire in to the input, the ground pin to the transmitter ground, do I just leave the output pin alone or still put an antenna wire on it? If that limited the signal to 10 feet or so that would be OK.

  21. Jun 15, 2010 #20
    ...and could I stack a few together if needed or is that inadvisable?


  22. Jun 15, 2010 #21
    Lars - You can buy them in a variety of attenuation levels

    The various part numbers are listed below. The number next to PAT- is the number of dB of attenuation. And yes you can chain them serially to create any level of attenuation.


    You would attach one or more of these attenuators in the transmitter or receiver path close to the antenna (make sure you hook up both of the grounds) and then solder the original antenna to the output of the attenuator, the attenuators are not direction specific.

    Ideally, you would keep the antenna that is on the device. An antenna is an impedance matching device that transfers energy from one circuit to another (RF circuit to the air in this case - probably in your case 50 ohms to 377 ohms. 377 being the impedance of free space ). When you simply removed the antenna, the energy that was not radiated off of the board was reflected back into the RF circuit probably distorting the signal you are trying to transmit and perhaps shifting the transmitter off frequency.

    Hope this helps
  23. Jun 15, 2010 #22


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    You need to filter the Power supply with ferrite beads and 0.01muF Capacitors to the case- with very short leads and short connections to the circuit. It would be an idea to mount it all in a copper box with insulating tags. A strong enough copper box for a small circuit would be very easy to cut out and fold using some 2mm copper sheet. All seams should be soldered well and a well fitting lid can be made in the same way. An SMC connector could be fitted directly on the box. It really wouldn't take much effort or money to do this and you'd get very good screening. You could pretty well guarantee that the only signal getting out would be via the 'official' path.
  24. Jun 15, 2010 #23
    The power supply is a small Lithium polymer battery located a few inches away. Would this still need to be filtered this way?
  25. Jun 15, 2010 #24
    Try it without all of extra stuff. If it works, declare victory and move on. If not do all the other stuff. There are ferrite beads that work well in the 2.4ghz band. If you have a battery as a power source, I can't imagine that you need a cap. All of the other suggestions are good design practice
  26. Jun 16, 2010 #25
    Well I've got some attenuators on the way. I have a full machine shop so the metal enclosure won't be a problem. What would be the best way to get the power, ground and signal wires out of the enclosure without letting the signal leak out? should I run the wires out of a path that goes around a few corners before it exits...like a light trap?

    Also, should I ground the metal enclosure to the ground of the power supply (battery) ?


    Last edited: Jun 16, 2010
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