Reducing RF power of video transmitter

[larsbentley]

Hi,

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.

Lars

 

[waht]

Trying cutting off the antenna to less than an inch in length or completely remove the antenna.

 

[berkeman]

Hi,

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.

Lars

Only need to go through a wall, eh? What's the problem with the standard power and range?

 

[Bob S]

Try layers of thin aluminum foil.

Bob S

 

[larsbentley]

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

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.

Lars

 

[mheslep]

Under ~3ft you'll start getting into http://en.wikipedia.org/wiki/Near_and_far_field" 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.
http://www.surplussales.com/rf/RFAtten-2.html

 

[larsbentley]

I will look into 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?

 

[larsbentley]

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.

Lars

 

[berkeman]

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

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.

Lars

Why don't you just use coax, or a wired video camera? What is the need for wireless?

 

[skeptic2]

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?

 

[dlgoff]

Would it be possible to put in one of those anti-static bags used for shipping circuit boards?

Conductive bag around powered circuit board? Hello smoke.

 

[waht]

The transmitter is body worn

Are you going to eavesdrop on other people without their consent? If so then there could be legal issues depending on where you live.

 

[larsbentley]

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?

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?

 

[berkeman]

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?

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.

 

[larsbentley]

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.

lars

 

[berkeman]

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.

lars

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.

 

[fizz_it]

Instead of using an attenuator with an sma connector on it use one of these from minicircuits. They are a lot smaller and cheaper

http://www.minicircuits.com/pdfs/PAT-10.pdf

 

[mheslep]

Instead of using an attenuator with an sma connector on it use one of these from minicircuits. They are a lot smaller and cheaper

http://www.minicircuits.com/pdfs/PAT-10.pdf

Yes, but those are likely going to radiate off board enough to be detectable at ~5-10'

 

[larsbentley]

Those minicircuits attenuators look interesting. So if i ran the antenna wire into 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.

Lars

 

[larsbentley]

...and could I stack a few together if needed or is that inadvisable?

Thanks

Lars

 

[fizz_it]

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.

PAT-0+,PAT-1+,PAT-2+,PAT-3+,PAT-4+,PAT-5+,PAT-6+,PAT-7+,PAT-8+,PAT-9+,PAT-10+,PAT-12+,PAT-15+,PAT-20+,PAT-30+

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

 

[sophiecentaur]

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.

 

[larsbentley]

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.

The power supply is a small Lithium polymer battery located a few inches away. Would this still need to be filtered this way?

 

[fizz_it]

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

 

[larsbentley]

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) ?

thanks,

lars

 

[sophiecentaur]

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

The point about the decoupling capacitor is that it can be small and right on the board. The leads from a battery can be quite good radiators in their own right.

Ferrite beads are great as they really behave more like resistors to the RF and dissipate the stray energy rather tna radiating it.

Coax cable is the best way to rout RF signals around the place - even a coax with the braid soldered right onto the circuit board will produce little radiation as it will be terminated with something like 50Ohms.

It does make you think, though. When you look at the domestic UHF booster amps that are available. They have plastic boxes and cheepy cheep wiring but still work tolerably well.

 

[larsbentley]

The point about the decoupling capacitor is that it can be small and right on the board. The leads from a battery can be quite good radiators in their own right.

So will a decoupling capacitor help stop RF from leaking out the battery leads? I thought they just kept noise from a power supply out of the circuit.

Also... so this little copper enclosure is just covering the transmitter itself, not the battery. does it need to be connected to the transmitter ground (battery neg terminal)?

lars

 

[sophiecentaur]

They will make the RF signal level very low at the board end of the battery leads so very little power will 'get to' the leads to radiate. It works both ways!.

If you put the battery inside the enclosure then the decoupling won't be necessary - but there will be a switch? You will need to decouple the leads going out to the switch or you'll have a nice little antenna there!

I should think that every 'earthy' bit should be connected together - including the outer of the coax connector. Braid (coax outer) is very good for this purpose because it is FAT and very low resistance.

 

[larsbentley]

They will make the RF signal level very low at the board end of the battery leads so very little power will 'get to' the leads to radiate. It works both ways!.

Thanks! There is also a video line input heading into the enclosure from the camera. I imagine the RF could leave on this line also... is there a need or a way to filter this? Also, the ferrite beads are available with various impedance and "max DC resistance" specs... any help in figuring out which is appropriate?

thanks again,

Lars

 

[sophiecentaur]

DC resistance for signal leads need not be very low (unlike for power leads).
Low pass filtering on the signal input is a bit more critical as you need to be letting video (up to, say 5MHz) through. I'll bet you can buy something to do the job.

It does strike me that the best solution to your problem might be to use a very low power RF signal source in the first place. For instance, you could do away with the final RF amplification stage of the unit. It's a waste to generate the power then to dissipate it inside the box!

 

[fizz_it]

It does strike me that the best solution to your problem might be to use a very low power RF signal source in the first place. For instance, you could do away with the final RF amplification stage of the unit. It's a waste to generate the power then to dissipate it inside the box!

I second that. Your batteries would last longer as well. If there is an external amp in the circuit, typically there is a resistor that is tied to the output of amp. This is a current limiter that sets the output power. It should have a value from 1 to 300ish ohms. Replace that one with one that is slightly higher and see if the transmission distance drops.

As for ferrites
http://mouser.com/search/ProductDetail.aspx?qs=uSeJpk%2f8In1VmERn9ukNjA%3d%3d

those are rated for 2.4 GHz operation
Murata has information about their ferrites here
http://www2.units.it/carrato/didatt/el_appl_2/doc/filters/murata_sf.pdf

You can read about which ones are suited for use @ 2.4GHz

 

[larsbentley]

This is a current limiter that sets the output power. It should have a value from 1 to 300ish ohms. Replace that one with one that is slightly higher and see if the transmission distance drops.

I'll look into it and see if I can identify it. The transmitter I'm using is a tiny little circuit board 1/2" x 1/2" and covered with some heat shrink plastic that looks to be epoxied at the ends, so its going to be a pain to even uncover the board. Thanks for the tip on those ferrites...ordered some.

Lars

 

[ravioli]

Since it appears that you are looking to make this into a business, let me give some advice. Get the expertise to get the job done right. Contract out. You will save time(and money) for yourself, and come out with a device that performs better.