Tutorial: DIY RF Amplifier

About a year Emmanuel Ombilod, a sound mixer in Singapore, posted a video and a few photos of a “homemade” RF Amp he was plugging a G2 TX into…. and getting 1600ft range for car to car monitoring.

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The original RF Booster was an LNA 20dB circuit and was used with a SMA modded G2. He was able to get 1600ft range and still had 3 bars on his RX.

The whole sound mixer world went crazy! But within a month the world stopped caring about his invention… I didn’t. I bugged the snot out of him for all the details and plans. 

Over the course of the past 6 months in sourcing and learning all about these RF Amps it came to light that a audio equipment maker was going to be bringing this very product to market to boost 2.4Ghz transmitters. And while at NAB last month I spoke the owner of this audio equipment company and asked about the new booster. It was explained me this way…

(Fake Quotes) “We aren’t legally allowed to sell it because Zaxcom 2.4Ghz gear is bidirectional and even the receiver is also transmitting. If we labeled it as a WiFi Booster it would have been legal but we can’t even do that because the FCC has labeled us an audio company. None of our audio dealers would be legally allowed to sell it or any other WiFi boosters, its a real mess. That project is scrapped” (End Fake Quotes)

That’s right. According to the FCC it is illegal to make and sell a RF transmitter booster for any frequency: VHF, UHF, 2.4Ghz and market it towards wireless microphones but if you were to be a separate company and sell the EXACT SAME CIRCUIT and call it a WiFi booster or UHF DTV RF booster for better TV signals… totally legal.

Here is a link to FCC Part 74 – US Government Publishing Office

https://www.gpo.gov/fdsys/pkg/CFR-2009-title47-vol4/pdf/CFR-2009-title47-vol4-part74.pdf
BUT!

If you were to put that circuit inline with an antenna and a receiver (exact same circuit) you would have a battery powered inline RF booster. The same kind of RF booster we see in active antennas but now separate, mobile, and battery powered!

So… If you are based in the US the following photos and tutorial can only be used INLINE with your antenna and receivers for purposes of overcoming RF lose in lone cable runs. For those outside of the US and FCC control…. go nuts!

Normally I don’t like doing DIY projects that would 100% clone or “knock off” a manufactures product using the same suppliers they use. Our world of production sound is small, our local dealers are all great people, and our manufactures are for the most part family run companies who care about the end users. But when I found out this invention really doesn’t have a chance to come to market, well… game on!

ITEMS NEEDED:

***NOTE DO NOT PLUG A TRANSMITTER INTO THIS RF AMP INSIDE THE USA! IT IS ILLEGAL UNDER FCC Part 74! AND WE RESPECT THE FCC. WE WAKE UP AND WISH THEY WERE IN OUR KITCHEN SO WE COULD MAKE THEM PANCAKES***

    OUR CIRCUIT:

    18015683_120300003219604960_2070713287_o

     Photo by Emmanuel Ombilod

    This is pretty basic signal flow but for power. And you are dealing with 12v instead of 1v Line Level audio. You will solder the In/Outs to each other so that the DC Input feeds the Voltage Regulator. The Output of the Voltage regulator can be soldered to a switch or directly to the RF Amp circuit. Match your +/- and you are done! Right… thats not hard. And if you buy all the parts in the above list, well… Emmanuel claims he gets about a 1/2 mile range with this second generation RF Amp he built.

    To make it look clean you can add some SMA extensions in case your project box is too big. Drill a 7mm hole in your box for your SMA female ports to stick out of and use the lock washer to hold them in place. It’s that simple!

    But remember we are in the United States, we can’t buy the above circuit legally for use with our wireless microphones. You can buy it but just aren’t allowed to let those SMA ports touch your um400a. The above circuit has a heatsink for taking a powerful signal from a transmitter. We are limited to making inline RF boosters remember, so we are legally limited to buying and using LNA RF Amp circuits that don’t have the heatsinks. Sidenote, Emmanuel’s original invention used a +20dB LNA RF amp to get 1600ft from his SMA modded G2 TX. We can use that same +20dB pcb circuit to overcome the RF loss in a 200ft BNC cable. Here is the RF booster I built to work with my passive PSC shark fin antenna.

    17948632_10158391896850214_964721858_o

    The LNA circuit I purchased is 5Mhz-6000Mhz and only needed 5V of power at the lowest setting. In searching Aliexpress I found some that had a USB port for a power hookup. Here I am using all the same parts but I am using a RF shielded LNA circuit. 

    For the LNA boards here is 5 tips about them from alot of their “makers.”

    1.When working frequency is less than 500 MHZ,it get well gain flatness, can make it less than 1dB after careful adjustment. The lower frequency the higher gain consistency.

    2.Amplifier working frequency of the lower limit is subject to input and output capacitor, the default value is 0.1 uF, working to 0.1 MHz. Increase the input and output capacitance appropriately,can extend the cut-off frequency, such as 10uF capacitance can work to 5KHz.


    3.When the power supply voltage changes in 5-8 v, it can be used as a variable gain amplifier, gain increases with the increase of the power supply voltage, which suitable for radio frequency receive front-end circuit, using DA control power supply voltage, to control the gain of the amplifier, automatic gain control


    4.When the power supply voltage in the 8-10 v, the low frequency end gain up to 30 db, at this time the amplifier has a low noise coefficient and good stability.


    5.When the voltage is 12 v, reach maximum gain, the low frequency end gain of 32.5 dB 

    There you have it, everything you need to know and where to buy parts needed to build an RF Booster for your passive antennas. 

    If you have a DIY tutorial you did and want highlighted leave a comment below.


    About the Author

    Andrew Jones is a location sound mixer based in Los Angeles. He started in the TV and Film industry in 2004. You can email him at  Andrew@HoldForSound.com

    www.HoldForSound.com

    4 thoughts on “Tutorial: DIY RF Amplifier

    1. You wrote:
      “The above circuit has a heatsink for taking a powerful signal from a transmitter. We are limited to making inline RF boosters”
      Do you think that removing the heatsink to use it with a 100mW signal (like an UM400? for example) will be advisable?
      Your first picture is a RF amp and the second picture is an in-line RF booster used to make up for the lost on coax cable, right? Thus the second doesn’t requires a heat sink?

      Like

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