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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 5:09 am 
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Location: Colorado Springs, CO 80917
I just completed a build of the BCB transmitter following the attached design - but running off a 9v transistor radio battery connected via the LED for now.

Another difference is that I had a few 614.4 KC oscillator modules and there was a clear spot on the band there. I had to change the antenna tuning elements, of course. I ended up with a little toroid of ~450 uH and a Mitsumi PVC-2X ("polyvaricon") tuning capacitor of ~200 pF and the LED peaked up as expected. Antenna wire is about 8 feet long.

A couple of questions, though...

The whole thing draws about 7 or 8 mA when peaked. Does that sound right? I thought some folks were getting more current draw that that.

I also find the effective range to be perhaps less than the width of a living room (assessed with a transistor radio walking around the room) which also seems weaker the performance of other builders. Perhaps that is a consequence of the longer wavelength I chose and I need to some work on making the antenna more efficient. Any suggestions?


Attachments:
lm386n101112a (downsized image).jpg
lm386n101112a (downsized image).jpg [ 170.26 KiB | Viewed 446 times ]
My LM386N AM Broadcaster.jpg
My LM386N AM Broadcaster.jpg [ 532.16 KiB | Viewed 327 times ]


Last edited by hwhall on Oct Sun 25, 2020 9:14 pm, edited 1 time in total.
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 5:29 am 
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I would delete the led and R4 using a new 9V and expect 50'.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 6:06 am 
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Location: Colorado Springs, CO 80917
I didn't use R4, just connected a fresh battery to the LED. I didn't try jumpering out the LED after peaking the antenna, though. I'll do that next time. The museum I made it for now has it in use so I need to build another one for tinkering around with.
Thanks for the reply!


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 6:17 pm 
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What you could do is use a milliammeter instead of the LED.

That way there's nearly no voltage drop.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 8:11 pm 
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Hello hwhall,
Can't wait to see your out come of your next one,

Sincerely Rich


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 11, 2020 9:30 pm 
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Tube Radio wrote:
What you could do is use a milliammeter instead of the LED.

That way there's nearly no voltage drop.

Dependant on P/S voltage, drop can be desirable, approx 8.5v chip supply is max for good audio linearity. I have a meter on my unit but a LED can work just as well.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Tue 13, 2020 12:38 pm 
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True, although with using the 9 volt battery there would be more than a .5 volt drop in the LED.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 25, 2020 9:48 pm 
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>What you could do is use a milliammeter instead of the LED.

I did grab a few little VU meters, 150uA full scale, to try out. Needed a 10 ohm shunt but worked nice & there's less than 0.1 volt drop thru the shunt.

I suspect now that the toroid may be too lossy. It was a surplus find & on unwinding one I was lucky to find the core was actually marked. It's a Magnetics product 55127A2, Kool Mu powder core. That stuff was designed for power supplies & filters, not for RF, though it was described as "low loss" for the intended application.

I added a picture of the build to my 10 Oct post. Housed on a Tupperware soap dish tray for now. If my figures are right, the little variable cap will tune the whole AM band with the same inductor. Most of those caps don't have the longish 1/4 inch shaft like the ones I found.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sun 25, 2020 10:46 pm 
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I'm glad I didn't settle for my toroids wound on iron PS toroids. They barely worked in my testing.

For the AM band you want -61 mix. I have tried the -43 mix that is said to be equal but my side by side comparison the -61 works quite a bit better.


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Toroid Freq.jpg
Toroid Freq.jpg [ 70.58 KiB | Viewed 323 times ]
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Mon 26, 2020 3:18 am 
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Well, that's me... always thinking I'll make a silk purse from a sow's ear.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Mon 26, 2020 1:44 pm 
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Some sort of a counterpoise or ground is needed to form part of the antenna.

In the United States, (if I have not made an error in understanding of 47 CFR § 15.219) when using a battery to provide power for the transmitter and not providing an earth ground connection, the antenna wire from the junction of L2 and C6 can be made 1.5 meters (59 inches) long and a counterpoise wire 1.5 meters long can be connected to pin 7 of the oscillator shown and be positioned so as to maximize physical separation from the antenna wire (forming a dipole 3 meters long).

When using the ac power supply with three wire cord, one wire of which is the equipment grounding conductor as shown in the schematic diagram, the antenna wire from the junction of L2 and C6 can be 3 meters long, since a "ground lead" is not needed in that case.
Attachment:
PART15_MODULATED_OSCILLATOR_02.jpg
PART15_MODULATED_OSCILLATOR_02.jpg [ 104.08 KiB | Viewed 300 times ]

............
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Tue 27, 2020 12:05 am 
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Given how short these antennas are compared to wavelength, I'm not sure that treating them as conventional antennas is valid. More like capacity probes, maybe. But I'll give a counterpoise a try next time I'm using it. I don't think I've noticed any other builders using one, though.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Tue 27, 2020 2:02 am 
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I tried my `386/mp3 on a 9V battery out in the yard with 2 - 10' wires laid on the ground dipole style. It worked very well and directional. Rabbit ears dipole works too.
Not nearly as well as powered by a wallwort and 10' vertical ant.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Tue 27, 2020 3:42 am 
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Good to know that even with the antenna lying on the ground that it worked okay, Mike. Did you happen do make an estimate of the distance to the receiver?

Trying to help the OP improve performance, one way of coupling an antenna to a tuned circuit is via a coupling winding on the tank coil (L2 in the schematic), forming the secondary of a transformer.

Here is a procedure I would be inclined to try, the magnitudes given are examples.

For example, if 20 mA lights the LED to full brilliance.

Then assuming for the purpose of illustration that the 20 mA is the same as the RMS output current from pin 8 of the oscillator with the tuned circuit at resonance.

Then assuming for example that the radio frequency RMS voltage at the C5 L2 junction is 2 V.

Then the power input to the tuned circuit at resonance would be 2 V * 0.02 A = 40 milliwatts.

Then the impedance of the tuned circuit would be 2 V / 0.02 A = 100 ohms.

Then taking the radiation resistance of the 3 meter long antenna as around 0.05 ohms.

Then the impedance ratio from tuned circuit to antenna = 100 / 0.05 = 2000.

Using the given 450 uH tank inductance, a CWS Bytemark F-114-61 toroidal core (for example) should need around 77 turns occupying 90% of the circumference to give the 450 uH.

Turns ratio = square root of impedance ratio, so sqrt 2000 = 44.7

77 / 44.7 = 1.7 turns for the antenna coupling winding, I'd try two turns.

Addendum:

Even with the transformer coupling, an impractically large inductance (several millihenries) (loading inductance) will be needed to tune the antenna in the lower end of the medium wave band, such as the given 614 kHz.

Better transmitted signal strength will be obtained if the transmitter is designed to operate in the higher frequency end of the band, for example around 1600 kHz, because the radiation resistance of the antenna will be higher.

For example, with a 3 meter monopole antenna and using the ac power supply shown in the schematic connected to the equipment grounding conductor, the radiation resistance at 1600 kHz could be around 2.5 ohms and the antenna reactance could be around -6000 ohms, if I have not made an error.

The number of turns for the coupling winding (L3) would need to be calculated (as a starting point) for the 2.5 ohms since this makes the impedance ratio different from the 0.05 ohms used earlier.

Around 600 microhenries (+6000 ohms reactance at 1600 kHz) of tuning inductance (L4) would be needed between the coupling winding and the base of the antenna.

Revision: The inductance of L4 needs to be adjustable and be of the minimum practical dc resistance. A coil on a 33 material ferrite rod is suitable.
For this 600 uH example, around 82 turns of 20 AWG magnet or litz wire close wound, centered on the length of a 0.5 inch diameter, 4 inch length 33 rod would provide 620 uH. Tuning is accomplished by pushing turns farther apart to reduce inductance.

Mouser also has a good selection of material 61 toroids in stock as of Nov. 2020: https://mou.sr/3mLsqN9
CWS Bytemark has the 33 material rods as of Nov. 2020.

-------------
WB5HDF


Attachments:
ANTENNA_COUPLING_04.jpg
ANTENNA_COUPLING_04.jpg [ 65.32 KiB | Viewed 242 times ]

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Last edited by infzqi on Nov Thu 12, 2020 12:09 am, edited 6 times in total.
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Wed 28, 2020 4:15 am 
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Location: Colorado Springs, CO 80917
I chose 614kc because the other easily available oscillator frequencies seemed to be in already crowded areas of the band, locally.

> assuming.... that the 20 mA is the same as the RMS output current from pin 8
How might we go about measuring the RF current? I suspect it's not anywhere near the total DC current going into the oscillator.

But... I'm not sure I need to have a resonant antenna for this. The radio I need to feed used an external antenna. I can tightly couple a length of wire to the oscillator's short antenna. Or, just skip the niceties & alligator clip the oscillator output directly to the receiver's antenna input.

Still, though, I think it would be generally useful to find a more effective way to radiate energy in the lower end of the AM band. It's always seemed to me that stations sought spots in the upper end & left the lower end usually rather sparsely populated (easier to find a spot for one of these lo-power broadcasters).


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Wed 28, 2020 3:16 pm 
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Location: Texas, U.S.A.
Edited to change secondary load resistance to 10 k ohms.

A toroidal transformer can be arranged for measuring the RF current in the L2 circuit.
Something like a CWS Bytemark F-37-43 or F-37-61 or smaller core could be used.
I'd try starting with 10 turns for the secondary (occupying about 90% of the circumference), with 10 k ohms across it.
This should give 1 V secondary voltage for 1 mA of primary current. (1 V per mA).
The primary consists of the wire connecting C5 and L2, passed through the center of the toroid.

After winding the secondary onto the core, set up a test of the transformer using a signal generator or the like, at the frequency of the transmitter. This is done by passing a known current through the primary of the transformer and measuring the voltage across the 10 k ohm load resistance on the secondary.
To get the known currents, use known load resistances on the signal generator and if the generator voltage and output impedance are known, the current can be calculated. Otherwise measure the voltage applied to the known resistance and calculate the current.
Pass a few different known currents, within the range of current expected in the transmitter circuit, through the primary and use an oscilloscope or other suitable instrument to measure the secondary voltage at each current. This will show the current to voltage relationship of the transformer.

----------
WB5HDF

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Last edited by infzqi on Nov Wed 11, 2020 2:51 pm, edited 3 times in total.

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sat 31, 2020 12:41 am 
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infzqi wrote:
Good to know that even with the antenna lying on the ground that it worked okay, Mike. Did you happen do make an estimate of the distance to the receiver?

Had to retest because I never tried vertical.
With the 2 wires laying on the ground, about 50' to my back door using a transistor radio.
With one wire raised straight up, other on the ground, through my house and out front to the curb...100'.
This was a test as I know the ground and ant combined limit is 10'.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sat 31, 2020 3:20 am 
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Mike Toon wrote:
Had to retest....

Thank you Mike. That reception distance is encouraging. I forgot to ask the frequency.

----------
WB5HDF

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sat 31, 2020 4:00 am 
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The abundant 1,000kHz like most of us, and it is clear for me.
I have 614.400 and 780 and I override the blowtorchs, I prefer 1,00kHz for a better signal.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sat 31, 2020 5:23 am 
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Mike Toon wrote:
The abundant 1,000kHz like most of us, and it is clear for me.
I have 614.400 and 780 and I override the blowtorchs, I prefer 1,00kHz for a better signal.

I'm surprised that KOMO/1000/Seattle doesn't give you interference at night. Out here, it usually comes booming in after dark.
I use 1040 Kc with my SSTrans, and 1306 Kc on my other eBay/Japan purchased kit (fixed Xtal frequency) with NP.

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