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 Post subject: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 4:24 am 
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Why try to squeeze the last drop of a low powered design, then to go crazy trying to optimize it?
Wouldn't it be easier to go a little overboard then throttle it back? Like a hot rodder's discretion on the throttle to stay within the speed limit?

I found this basic IRF 510 AM transmitter. Could a potentiometer (somewhere) be used to adjust to the "legal output"? This leaving the output level in the hands of the final output, NOT in the limiting design? Part 15 compliant of course.
https://makerf.com/posts/a-better-ampli ... ransmitter


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My proposal block diagram. ---------------------------
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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 7:58 am 
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Although the author appears to have biased the MOSFET output amplifier so that it is drawing a quiescent current and he states that more power can be obtained by using more oscillator output which implies that the output amplifier might respond linearly to the modulated carrier, the linearity of the first circuit would have to be investigated when used with a modulated input.

Using the LM386 modulating the oscillator in the second circuit shown would require a linear Class A output amplifier or the output stage would have to be modulated.


Last edited by LM386 on Sep Mon 03, 2018 8:17 am, edited 1 time in total.

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 8:15 am 
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Why hold back, Mike? Burbank would LOVE to hear your oldies wax! :wink:

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 5:49 pm 
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See the note: "Insert Low Pass Filter here."

Without a filter, you will be filling the airwaves with harmonic radiation. As-is, the output amplifier is un-tuned.

The power output can be controlled by reducing the Vcc voltage to the IRF510. 12 Volts is not a magic number.

The original design was a CW (Morse code) ham transmitter, not AM.

Rich


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 7:17 pm 
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IRF510 has high gate capacitance and would require more drive. Use a 2n7000 instead and biase it in the linear region... I mean a standing current of 5 ma. U can use a transistor like 2n3904 too biased for class AB linear to amplify the AM signal from the oscillator.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Mon 03, 2018 7:37 pm 
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Try this https://ibb.co/fGgMNz

The fet is a 2n7000 and it will offer very high gain. You can vary the 2.2K input resistor as to not overdrive the fet and for output power control.

Set the 10k biasing pot for 10ma of standing current through the 2n7000 with NO signal input and you have got a nice linear amplifier here. Using IRF510 wouldn't give you any significant gain. 2n7000 is the best option here due to its low gate capacitance. The output from the TTL can oscillator is very weak anyway to begin with so 2n7000 is a better amplifier for it.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 04, 2018 3:31 am 
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Thanks Dare. 2n7000s ordered. I don't know how to design. You draw it and I'll attempt to build it.
So long as it has the magic pot to adjust it to legal output to operate and discuss here. :roll:


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 04, 2018 3:49 am 
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I still have the Drive In theater xmitter that has a pair of 5881 modulating a 7868. While it was OK, didn't sound as good as a 6888. Neither did the supposedly two watt Tx that uses pair of 6AQ5, that one quickly morphed into a 6GY6 Tx.

Last time I had the Drive In xmit on air Curt Reed was still with us.

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 04, 2018 2:25 pm 
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You can replace the 2.2k resistor with a 4.7k or 10k pot to adjust drive level to the 2n7000.

The other 10k pot from V+ to gate of 2n7000 will adjust the standing current. Designing is simple. The 2n7000 starts conducting when gate voltage is about 2V or so. Check the data sheet. As the gate voltage increases it conducts more and so does the standing current. 10-15 ma is okay for linear operation. Check its datasheet. It mentions the input capacitance of 2n7000. You can calculate input impedance by googling capacitive reactance calculator and entering the 1MHz operating frequency. Since the input gate capacitance is significantly lower than IRF510 mosfet, it will have high impedance and will require less power or current to drive it. Let me know how it goes. Keep all wiring and leads short though. It is goof RF practice.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 04, 2018 3:27 pm 
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Why throttle anything? Just terminate it into a dummy load and use the leakage. Its like a hot rod car on a dyno. Ive known of hams that made contact with each other a few doors away running dummy loads just for fun. Some loads are well shielded and some not so much.

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 04, 2018 6:25 pm 
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Exactly Jeff! So even if the input power to the final exceeds 100 mW , the signal won't be heard beyond your living room.

One more thing mike, the final current to the 2n7000 must not exceed 8 ma at 12V to comply with 100 milliwatts input rule. So you can set the standing current 3-5 ma and then connect the oscillator output to its gate. The drain of 2n7000 has a 1 mH choke. U can connect a 10 feet of wire antenna directly to the drain via a 10n or 100n capacitor. Let me know what range you get and the sound quality.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Wed 05, 2018 3:40 pm 
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Mike Toon wrote:
Could a potentiometer (somewhere) be used to adjust to the "legal output"?

No, because this is not a Part 15 compliant transmitter to begin with.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Wed 05, 2018 7:03 pm 
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That's the whole idea behind the leaky coax antenna; the end of the coax is terminated in a dummy load, and the coax radiates the signal in a controlled amount. This was popularized by LPB, an equipment manufacturer, for drive-in theatres. Now that they are out of business, I don't know who else would carry it.

Tom, I will be sending you a PM about that transmitter.

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Wed 05, 2018 7:34 pm 
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Tim Tress wrote:
That's the whole idea behind the leaky coax antenna; the end of the coax is terminated in a dummy load, and the coax radiates the signal in a controlled amount. This was popularized by LPB, an equipment manufacturer, for drive-in theatres. Now that they are out of business, I don't know who else would carry it.

Tom, I will be sending you a PM about that transmitter.



OK, I'm thinking about it.

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Wed 05, 2018 8:58 pm 
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The leaky coax antenna reminded me that long ago as a teenager I had a ton of wire laying around. I wanted wireless headphones for my am/fm receiver. So I wound maybe 20 turns of wire in the upper corners around the room. Then I wound another coil using a baseball cap as a form. Connected that to a battery powered amp with headphones. Connected the big room coil to the speaker out of my receiver. It worked as long as I was within maybe 3-4 feet from the walls.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Wed 05, 2018 10:29 pm 
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forumuser wrote:
The leaky coax antenna reminded me that long ago as a teenager I had a ton of wire laying around. I wanted wireless headphones for my am/fm receiver. So I wound maybe 20 turns of wire in the upper corners around the room. Then I wound another coil using a baseball cap as a form. Connected that to a battery powered amp with headphones. Connected the big room coil to the speaker out of my receiver. It worked as long as I was within maybe 3-4 feet from the walls.

Early Bluetooth... :wink:

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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 18, 2018 11:53 am 
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Dale H. Cook wrote:
Mike Toon wrote:
Could a potentiometer (somewhere) be used to adjust to the "legal output"?

No, because this is not a Part 15 compliant transmitter to begin with.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


If the circuit is adjusted so that it does not violate part 15 would it then be legal or would the fact that it is capable of more power make it illegal?


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 18, 2018 5:09 pm 
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Tube Radio wrote:
Dale H. Cook wrote:
Mike Toon wrote:
Could a potentiometer (somewhere) be used to adjust to the "legal output"?

No, because this is not a Part 15 compliant transmitter to begin with.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


If the circuit is adjusted so that it does not violate part 15 would it then be legal or would the fact that it is capable of more power make it illegal?


Like a car capable of breaking the speed limit.


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 18, 2018 6:25 pm 
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Exactly, but how would it be viewed per part 15?


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 Post subject: Re: 5 Watt AM Transmitter Throttled down
PostPosted: Sep Tue 18, 2018 8:36 pm 
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If a resistor was used instead of a pot it would meet part 15 specs.

Dave


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