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- October 21, 2017 at 8:55 pm #11422
I built a class e tx, showed the 2n7000 about 22 ohms and used an L match to transform it to 50, throttled it back to 70mw using a resistor between lm386 output and inductor. Works great, but I’m thinking after punching numbers in the calculator, at 6 volts, at 80 mW, I’m looking at 142 ohms, this next build I’m inclined to lose the L match, and from what I’ve read, higher imp antennas are more efficient. Any thoughts?

October 22, 2017 at 7:38 am #55742StefanGuest

Total posts : 45366Hi,

It is not clearly readable what you are doing here. It sounds like there´s pin 3 from the LM386 then you have a 22 ohms resistor that goes to drain from your mosfet and then you have some kind of PI filter that transforms 22 ohms into 50…

As far as I know the final amp output impendance depends on the coil in collector / drain. So what you could do is look up the value of the coil for your desired transmission frequency. There are calculators on the web wherewith you can calculate the inductive resistance of a coil. Type in your frequency and let it calculate and you will have something like 5…15 µH – depending on what frequency you operate.

Connectinc the coil from drain – to LM386 directly to the chip should get you more output power. On 6V at least 100mW if not even more.

It is important to use a blocking capacitor like .1 µF from pin 3 to ground – otherwise lots of RF will go into the LM386 and will inteffer with it.

on your output you then have a lowpass filter with 50 ohms. Once aggain there are lots of sites where you can calculate filters and coils.

Higher impendance antennas … mhm don´t know about that but I assume as longer the antenne – as better the range, since on small antennas like a 6 ft wire antenna you have to match the antenna to get it tuned on the desired frequency. Then you have loss in the PI filter network.

.1Watts is still cool / “calm” but if you get into the range of 3…4 …5

Watts which can easy be made by a 4060 chip with driver and mosfet (and >18V Modulator so 9V on yor FET) you will get massive loss. I´ve had it that air wire wound coils got hot – with matched antenna – because there is much current in the coil. Small coils like these fixed inductors would go up on smoke. On 1W they get so hot you can´t touch them…

So a long antenna would be good because you wouldn´t need as much matching there.

Also a well matched and big loop antenna in a room is working well. Someone has made tests with an magnetic loop antenna that was placed in his house below the roof. The antenna has been perfectly matched with respect to metal parts in the wall taking away energy because of resonance. He connected a 30mW transmitter to this antenna – matched perfectly – he got a good, nearly static free signal to a sony radio over a distance of 450 meters – or 1350 feet.

I hope that information helped you out a little bit.

October 25, 2017 at 5:07 pm #55751radio8zGuest

Total posts : 45366The unfortunate FCC constraint on input power forces the value of output impedance for a transmitte to be higher than desired for best antenna matching efficiency. For best efficiency, the ouput impedance should be very low and not 50 ohms. The reason 50 ohms is so often used in designs refers to the expected coaxial cable and antenna matching and not to the transmitter output impedance. If the transmitter output impedance is also 50 ohms then half the power will be lostl as heat in the transmitter. Since we are not using coax, forget about 50 ohms.

I found it very difficult to calculate the actual output impedance of my transmitter so I resorted to plotting the measured efficiency (Pout/Pin) for various load resistances and found the best to be around 90+ % for a load of around 100 ohms. I had included a 5 pole LC low pass filter in the design to cut harmonics to less than -35 dBc measured at the dummy load but this may not be necessary as explained later.

The measured feedpoint impedance of my base coil loaded antenna is around 30 ohms so the problem became how to get the transmitter which had highest efficiency around 100 ohms to work with the 30 ohm antenna with best efficiency. The simplest solution was to use a toroid transformer to transform the 30 ohm load up to 100 ohms seen by the transmitter. This cost me about 5 mW of loss and the final result is my transmitter has 85% efficiency when used with my antenna.

PhilB of SSTRAN suggested to me that I could eliminate the output LP filter since the resonant antenna would filter harmonics out to an acceptable level. I did not try his suggestion but he is probably right since the harmonics on my radiated signal are not measurable with a scope FFT which has a noise floor of -50 dBc.

Hope this helps.

Neil

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