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ThomasScherrer

40W RC control system

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Got a little toy from eBay today too. It is a 0-10db attenuator. It was real ugly when I got it, but I am starting to clean up the corrosion. The BNCs are clean now at least. :) So, opinion please, should I reduce the power of the amp by attenuating the input signal with my nifty new attenuator or should I build something to change the bias voltage? I was thinking I could have a similar rotary switch and change from 5v, 6v, 7v, 9v. Right now I am at 9v regulated.

-dave

post-6-1193962269_thumb.jpg

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If you use an adjustable regulator you can set any output you want but in practice 2 or 3 settings is enough. Try 1W, 4W and 12W but you will spend most of your time on 1W ;)

Terry

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you can use the attenuator if you like, but it is much more efficient to lower the bias, you have seen the 40W page I made ?

all my gain stages are made exactly like this, controlling bias on first stages.

ok I have a REGULATED output power since my PA stage provide a DC voltage

that is propotional with output power.

it is easy to make a diode sensor so you can regulate and measure your TX power.

I use 0.1 W at ground and when over 1km away I go up to 1W

I get noise on video receivers when I tx at 10W or more,

so I need to fix that before I fly over 10km away :-)

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I was modifying my amp today, just adding a power LED so that I could be reminded when it is on when I decided to hook it up with the 10db attenuator I bought from eBay. Today it seems I have more power. Check out the vid. As you may remember, when I tested it on the spectrum analyzer last week I measured 40db or about 10w. I start the video with the attenuator set as a 50 ohm load with no power passing to the amp. The amp at idle is pulling about 0.28 amps. At -10db the amp pull 2.19 amps and pushing out about 8 watts rf. At 0db attenuation the amp is pulling max 4.23 amps and pushing out just under 25w.

Thoughts and opinions welcomed!

RF Amp Test

-dave

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I use 0.1 W at ground and when over 1km away I go up to 1W

I get noise on video receivers when I tx at 10W or more,

so I need to fix that before I fly over 10km away :-)

Put your RC transmitter antenna high up and use an antenna that radiates mostly to the sides. My 6 meter antenna is 12 feet off the ground at its base, and I dont get RF into the video at all. At moderate power levels its perfect, A little RF starts to get into my goggles at 170 watts, but its still flyable.

JettPilot

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I am about to test my system, however, I have a rudimentary question. I was planning on using a 6m magnetic mount 1/4 wave antenna mounted to my car. But then I was thinking that the antenna would be verticaly polorized and the antenna in the airplane which goes down one wing, would be horizontaly poloarized. One person I spoke to said at low frequencies it does not matter much. Is that true? Need I only be concerned with polarization at higher frequencies? If so, what is the 'cut off' where one would be concerned? If I do need to consider it, would you happen to have any opinion on this antenna that I recently saw in an MFJ catalog? http://www.mfjenterprises.com/products.php...prodid=MFJ-2206 I realize I would need a balun or coil up so coax, but was curious if any of you wise sages had a word of advice. Thank you.

-dave

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No cut off Dave, it just gets more important to get it right as frequency goes up. It would be good to match the aerials but with the power you have availible I would not worry too much just now.

Terry

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Here is a pic of my new 6m antenna!

I still need to add a balun and tune it as it currently resonates at 35Mhz. This is the MFJ antenna mentioned a few posts up. It just took me a while to get it and a tripod/mast. The holidays sucked up all my spending cash...

DSCN4657.jpg

Edited by dalbert02

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Here is a pic of my new 6m antenna!

I still need to add a balun and tune it as it currently resonates at 35Mhz. This is the MFJ antenna mentioned a few posts up. It just took me a while to get it and a tripod/mast. The holidays sucked up all my spending cash...

DSCN4657.jpg

Hi Dalbert,

Would you mind sharing some details on your connections from your Tx to the MJF antenna?

Thanks,

Fionn.

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matching and polarisation is exactly as important no matter what frequency you use.

but it is true that long wave antennas mounted relative close to ground compared to the wavelength

with get a blured or jammed radiation pattern, and also its polarisation will not be clean,

this is actually to your benefit :-)

so you get less dead spots

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I am very interested in reproducing what you have created for my own use, however, so many modifications have been made to the original design you posted that it is very hard to follow. Perhaps you could post the final design including a parts list to make it easier to follow in your footsteps.

Thank you,

James

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Should be able to use a pair of back to back shockley diodes on the rcvr input to protect it..

Edited by W3FJW-Ron

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i would like to do the same thing with my 9cap 72mhz. i know i need a module m67742 and a printed rf board. but can anyone give me a parts list of the rest of the components to complete this system? regulator, diodes, etc. etc

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It has been a little while since I posted here (or anywhere for that matter) but I thought I would offer some guidance to those that are just starting out building an rf amp. I am by no means a radio expert or even an electronics expert. I just read what the smart people on this (and other) forums have to say and figure things out.

1- Select the rf module that corresponds to your frequency. I bought mine at www.rfparts.com along with a pre-made circuit board.

2- Once you have the module and board for your frequency, there will be a parts list included. I bought my parts from Digi Key and Mouser. Basically, you need a few capacitors and a voltage regulator. You will also need some coax cable, some rf connectors of your choice (BNC, TNC, UHF, etc.), an enclosure box and a heat sink. The value of the capacitors is dependent on the frequency you choose so I can not tell you in advance what caps you need to purchase. I use a smiley rubber duck antenna when I fly my park flyers. When I needed added range, I diconnect my smiley antenna and connect my amp using BNC connectors. It is fast and easy. I would love to go to TNC or SMA or even UHF but I have not found a way to interface it to my Tx as elegantly as the premade BNC connector that comes with the smiley antennas.

3- The advice on this board suggests keeping all cables as short as possible and the distance from the ground shielding and the center conductor as short as possible. You may want to modify your circuit board to accomplish this. I used a dremel tool with a sanding bit to remove the traces on the board that were not needed. Leaving the traces in place may act like a tuning stub and alter the operation of your amp.

4- Unless you make some sort of attenuator, you will be transmitting at full power which is not needed and not legal. I bought an rf attenuator off ebay so that I can operate at 1 watt most of the time and just "crank up the power" when I need to. There is no reason to be pumping out 25w when you are flying only a mile away.

5- Ideally you would want to connect your amp to a spectrum analyzer to see what is going on before you blast the airwaves indiscriminately. Luckily I have access to one at work. I still have a small 2nd order harmonic, but it is not too bad. A cheap way to see how bad your second order harmonics are is to use a simply handheld radio and tune to exactly double the input frequency. I used a Yaseu VX7R and tuned to 50.860x2=101.72Mhz. I could then walk or drive to see how far my second order harmonic traveled. You could also adjust the input frequency to exactly half of an unused FM radio frequency such as say 104.4 = 52.2Mhz and just use a common FM radio. If you can still tuen into your second order harmonic more then a few hundred feet, you have some additional work to do. Additionaly, I think it needs to be said that all this experimentation requires an Amature Radio License. The license is easy to obtain and keeps you legal. It is also required for any video transmission you may want to do from your aircraft. Goto QRZ.com for the practice tests and to find a location near you to take the test.

Good luck and keep us posted on your success!

-dave (FCC license KI4HVT)

Edited by dalbert02

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