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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Oct Sat 31, 2020 5:56 am 
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Location: Beautiful Downtown Burbank CA
The 90 mile away KCEO Vista CA station I can hear a little in the daytime but as soon as I turn on my transmitter it disappears.
1500 is what my SSTrans is set for but it remains in the box. I kinda like the sounds of my own creations and they have built-in mp3 players.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Nov Thu 26, 2020 5:41 pm 
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Posts: 138
Location: Texas, U.S.A.
Studying the MK-XI transmitter circuit some more, I've determined the following:

The schematic diagram shows that the source of power for the RF output coupling network and antenna is the oscillator output, pin 8. ( ref. https://antiqueradios.com/gallery/main. ... ewsIndex=1 )

The transistor(s) within the oscillator package that drive pin 8 comprise the final RF stage. Power is supplied to the output coupling network L2 C6 when pin 8 is in the high state (sourcing current). The high state output of the ECS-100 oscillator is specified to be 2.4 V when sourcing 1 milliamp (mA). (ref. https://ecsxtal.com/store/pdf/ecs_100.pdf )

2.4 V x 1 mA x 50% duty cycle gives 1.2 milliwatts (mW) power output from pin 8 and the required load resistance on pin 8 would be 2.4 V / 1 mA = 2400 ohms. The final RF stage input power would be 4.5 V x 1 mA x 50% = 2.25 mW. (The unmodulated supply voltage to the final RF stage is used to calculate power input).

In the U.S., the requirement that the antenna used with this transmitter be very short compared to the wavelength means that the antenna will exhibit capacitive reactance and very low radiation resistance.

If there was no loss in the antenna coupling network L2 - C6, the power input to the antenna would be 1.2 mW. Then for example if the antenna radiation resistance was 2.5 ohms, the current into the antenna would be the square root of (1.2 mW / 2.5 ohms) = 21.9 mA.

Given the 2400 ohms needed by the oscillator and the much lower antenna radiation resistance, a coupling network that can transform the low radiation resistance up to the load resistance needed by the oscillator is required.

The arrangement and values of L2 and C6 shown in the MK-XI schematic diagram amounts to a series resonant circuit on the output of the oscillator, causing the final RF stage power to be dissipated as heat with almost zero power delivered to the antenna.

Moving the L2 end of C6 to the C5 end of L2 and changing the values of C6 and L2 is one way of obtaining the needed impedance transformation and delivery of power to the load resistance (antenna radiation resistance). This L-network arrangement will have high Q (around 30 - 35) but should be usable.
If the oscillator can use a lower resistance load, around 130 - 200 ohms, the Q will be around 10. ( J. Stiles, Impedance Matching and Tuning, pp. 24 - 27. http://www.ittc.ku.edu/~jstiles/723/han ... ackage.pdf )

To calculate L2 and C6, the resistance and capacitive reactance of the antenna must be known or estimated. An RF bridge or antenna analyzer can be used to make measurements. For estimates, EZNEC or a similar program can be used ( https://www.eznec.com/demoinfo.htm ), or the capacitance can be estimated (15 - 30 pF for 3 meter length) and radiation resistance calculated referring to antenna handbooks or web sites. The antenna impedance and 2400 ohms resistance for the oscillator load with the reactance of C5 (around -10 ohms) can be plugged into an online calculator, for example http://leleivre.com/rf_lcmatch.html.

An oscillator frequency of 1.2 MHz or greater will facilitate practical values for L2 inductance and C6 capacitance. For example at 1.544 MHz with radiation resistance of 2.5 ohms and reactance of -5727 ohms, C6 would need to be around 1330 pF and L2 around 598 uH.

L2 needs to have very low dc resistance and self resonant frequency greater than around 4 MHz. L2 can be 22 AWG enameled, close wound and centered on a ferrite 33 material rod 0.5 inch diameter by 4 inches long. L2 can be made greater inductance than needed and adjusted by sliding the coil on the core. https://coil32.net/online-calculators/f ... lator.html

C6 can be made of two or three C0G ceramic capacitors in parallel to get close to the required capacitance, along with a 365 pF or greater variable also in parallel. 25 volt rating is sufficient for the capacitors.

L1 needs to be changed to 330 uH with low dc resistance and self resonant frequency greater than 2 MHz, such as Coilcraft RFC0807B-334KE or DR0608-334 or similar.

Addendum: If pin 8 of the oscillator can source 15 mA at 2 V, like a TTL nand gate is able to, then the power output of the oscillator would be 15 mW and the required oscillator load resistance would be 133 ohms. To test the oscillator output capability, known resistances can be used to load pin 8 (L2 disconnected), the output voltage measured with an oscilloscope and the current and power then calculated.

Hopefully I haven't made any substantial errors.

WB5HDF


Attachments:
Antenna_coupling_for_MKXI.jpg
Antenna_coupling_for_MKXI.jpg [ 172.13 KiB | Viewed 523 times ]
Example_leleivre_calculator_02.jpg
Example_leleivre_calculator_02.jpg [ 171.96 KiB | Viewed 523 times ]

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Last edited by infzqi on Dec Thu 03, 2020 1:48 pm, edited 4 times in total.
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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Nov Thu 26, 2020 9:05 pm 
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infzqi wrote:

Hopefully I haven't made any substantial errors.

Looks impressive "on paper", but "the proof is in the pudding". Have you built and tried this, or is it unproven theory?

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Nov Fri 27, 2020 7:00 am 
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Joined: Jan Sat 25, 2020 5:23 am
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Location: Colorado Springs, CO 80917
"The arrangement and values of L2 and C6 shown in the MK-XI schematic diagram amounts to a series resonant circuit on the output of the oscillator, causing the final RF stage power to be dissipated as heat with almost zero power delivered to the antenna."

L2 & C6 look like an L-match which has long been used to feed electrically short antennas. I notice the http://www.ittc.ku.edu/ reference doesn't say anything about L-matches dissipating power as heat.

I suspect that detailed mathematical analyses of these little transmitters is of academic interest only for a couple reasons. One, not all module manufacturers have the same output specs, so if you swap to a different osc module, your numbers may not hold any longer. And two, everyone probably has their antennas in different configurations & even lengths with stray capacitive couplings all over the place & changing every time you move it or move any conducting material objects around it.

That is not to say that different methods of matching and/or loading aren't worth experimenting with!


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Nov Sun 29, 2020 4:50 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
Thank you for your questions and observations.

If I have not made a mistake of some sort, this describes delivery of power to the antenna in original MK-X1 circuit:
For example, say the oscillator output is sourcing 16 mA at 2 V at 1.228 MHz and using the prescribed 220 uH for L2.
Then 16 mA x 50% duty cycle = 8 mA effective current.
If the load on the oscillator output was a resistor, 2 V x 8 mA = 16 mW of power would be supplied.
In the original circuit, L2 and C6 plus the antenna capacitance are a series resonant circuit. The current supplied from the oscillator
is determined by the oscillator source resistance and the resistive component of the L2 - C6 - antenna circuit.
The current through L2 of the original circuit divides among C6 and the antenna according to the reactances.

L2 = 220 uH, gives 1704 ohms Xl, 8 mA x 1704 Xl = 13.6 V
C6 + antenna = 76 pF, gives 1704 ohms Xc, 8 mA x 1704 Xc = 13.6 V
Antenna = 26 pf, gives 4981 ohms Xc, 13.6 V / 4981 ohms = 2.73 mA, 34% of 8 mA.
At 2 ohms radiation resistance (Rr), 2.73 mA gives 15 microwatts (uW) radiated.
C6 = 50 pF, gives 2590 ohms Xc, 13.6 / 2590 = 5.25 mA, 66% of 8 mA. This is not passing through Rr, so 128 uW is wasted.
If the dc resistance of L2 is 5 ohms, 320 uW is dissipated in L2.

(15 uW / 16 mW) x 100 = 0.0937 % efficiency. Almost none of the oscillator power is used to produce radiation.

hwhall wrote:
L2 & C6 look like an L-match which has long been used to feed electrically short antennas. I notice the http://www.ittc.ku.edu/ reference doesn't say anything about L-matches dissipating power as heat.

The original L2 - C6 circuit does not act as an impedance matching L network because, in the 1.228 MHz case, to do so would require that the
oscillator output have 3500 ohms of inductive reactance and much less resistance than the antenna Rr, and the antenna would need to have
100 ohms Rr and 15,000 ohms inductive reactance.

Yes, the reactive components of the output network, for the most part, do not dissipate power.
I was thinking that more power would be dissipated in the oscillator final RF stage transistors with the original antenna coupling circuit
than with the proposed L network but that is not the case.

If the proposed L network with C6 on the input end of L2 is used, much more power will be delivered to the radiation resistance
because the reactance of the antenna is counteracted and the antenna is the only thing that the current from the network can pass through.
The oscillator will be driving a resistive load (current in phase with voltage) of much higher resistance than with the original circuit.

As in any other tuning circuit, the variable capacitance C6 and inductance L2 are intended to compensate for some variation in antenna characteristics.

No, I have not wired up the suggested antenna coupling circuit but it is nothing new.
I don't have facilities to try wiring up circuits at this time.

Addendum: With the original L2 - C6 - antenna output network, the oscillator output must be acting substantially as an
amplitude modulated current source because the output network L2 - C6 - antenna is resonant and has very little dc resistance.
I think that the current from the oscillator is determined by the modulated power supply voltage, the collector - emitter
resistance of the output transistor(s) and the resistance within the oscillator in series with the collector of the output transistor(s).
---------
WB5HDF

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 10, 2020 10:17 pm 
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I've been experimenting with this transmitter (the original design posted early in the thread) and all works as expected except I don't seem to be getting the same range around the house as others have reported in the thread. My current draw seems correct (~11ma peaked antenna with no audio applied to the LM386), voltage is correct at the LM386 and the oscillator input voltage is 4.5 volts. I've tried straight wire antennas between 7-10 feet with and without a ground. The trimmer definitely works and the peak can be observed with the LED and/or current draw and/or listening to a receiver.

The best performance I've been able to achieve is a moderately noisy signal with the radio(s) about 15 feet from the wire antenna. I need to be within a few feet to get a quiet signal. I've tried vertical orientation, horizontal, etc. but without any real improvement.

So, can anyone help with ideas for something I might have missed? Perhaps a different antenna other than a straight wire? And what radiation pattern should I expect from the straight wire?

I've built the circuit several times and experimented with a 1 MHz oscillator and a 1.2288 MHz oscillator and I get consistent results/performance.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Thu 10, 2020 11:39 pm 
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You said trimmer. Are you using a trimmer with a screw or a var cap? I was wondering if your trimmer was maxed one way or the other. I don't use a ground, my wall wart is my ground.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 11, 2020 12:08 am 
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Mike Toon wrote:
Are you using a trimmer with a screw or a var cap? I was wondering if your trimmer was maxed one way or the other. I don't use a ground, my wall wart is my ground.

I used a 10-50pF through-hole trimmer. It goes through a peak, unless I have no antenna attached or something ridiculous. I also tried padding it with an 18pF since there was mention somewhere upthread that a 10-75 pF trimmer might be a better choice. On the bench, I had a wall wart (no ground) with a 7812 regulator and a small resistor to provide ~11volts to the LED, which ended up delivering just under 9V to the 386, which ended up as 4.5V on the oscillator. For my testing, and just to make it easy to move anywhere in the house, I use a 9V battery at the 386. Some of my test locations have been adjacent to metal electrical boxes or a cold water pipe, so I've tested and peaked with and without a ground. I remember some of your posts and testing out on the lawn with vertical and horizontal orientation, etc. I'm definitely not getting those kind of results.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 11, 2020 4:26 am 
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Going back to your toroid, what mix?

I test my proposed wound coil solenoid/toroid and var cap in parallel with a diode making a xtal radio. Small amp and this is/was my platform for finding a resonate circuit. If my calculations were correct, 1mHz is in the middle of my var cap or I wind/unwind...This will tell you if you are up or down.

Swap coils/toroids then compare.. I've tested 10 new oscillators at 4.5V, (386 9V) and comparing. Some are better than others. Toroids are all over the place no matter what they are marked. Have they been magnetized. :roll:

My best coverage is with a wall wort (as ground) and a small ant. I believe the wall wort acts/has capacitance ground.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Fri 11, 2020 1:12 pm 
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Mike Toon wrote:
/SNIP/Swap coils/toroids then compare.. My best coverage is with a wall wort (as ground) and a small ant. I believe the wall wort acts/has capacitance ground.


Good input; thanks. I have three oscillator modules on hand, so I'll compare to see if one has better performance. I have another axial 330uH choke for 1 MHz that I can try, but only one radial choke for 1.2288 MHz. I'll have to dig through my odd chokes/coils to see what else I can experiment with or dissect. RE: your wall wart cord, yes, I'm finding any lead length connected to my circuit ground interacts with antenna tuning more than I had expected. My 3' audio input cable hanging off the test board is an issue as I'm testing. When I get the best performance with my test setup, I'll try to duplicate the same jack/terminals/power setup I plan to use in my enclosure and check performance again.
Last night, I did find that maximum antenna length with the current coil/cap was ~14', then I could no longer tune the antenna. Today I'll test for the minimum reasonable length. I've been experimenting with antenna placement and orientation because ultimately, regardless of ideal length and orientation, the antenna has to go somewhere practical and out of sight like in a closet, behind a cabinet, etc.


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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Sat 12, 2020 3:46 am 
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jtiner wrote:
So, can anyone help with ideas for something I might have missed? Perhaps a different antenna other than a straight wire? And what radiation pattern should I expect from the straight wire?

If there were not any other metal objects in the vicinity and with uniform ground, a vertical monopole (Marconi) antenna would, ideally, have an omni-azimuthal pattern.

A roughly 3 meter vertical could be roughly 15 - 30 pF, depending on surrounding objects. An expedient to try for getting more power to the antenna from the TTL oscillator would be to put inductance alone between the oscillator output and the antenna, to resonate the antenna. This won't provide a very good impedance match, but should be an incremental improvement over the original L2 - C6 circuit, because all RF current must go through the antenna. If several coils are on hand they can be used in series as shown in the diagram. Ideally each would have self resonant frequency greater than the operating frequency and very low resistance; but combinations of whatever is on hand can be tried to maximize signal.
For example if the antenna is 20 pF and at 1.2288 MHz, 839 microhenries of inductance would be needed. http://www.1728.org/resfreq.htm
--------
WB5HDF


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Expedient_antenna_coupling.jpg [ 72.92 KiB | Viewed 324 times ]
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Short_Marconi_Ideal.jpg [ 192.58 KiB | Viewed 324 times ]

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 Post subject: Re: LM386N MK-XI BCB Transmitter
PostPosted: Dec Sat 12, 2020 6:02 pm 
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infzqi wrote:
jtiner wrote:
And what radiation pattern should I expect from the straight wire?


If there were not any other metal objects in the vicinity and with uniform ground, a vertical monopole (Marconi) antenna would, ideally, have an omni-azimuthal pattern./SNIP/


Great info; thank you. I spent some time yesterday getting things into a more compact form to facilitate moving everything to different locations and trying different antenna orientations and grounds around the house. I do have some odd coils kicking around, so I'll need to experiment with values as you (and another poster) suggest.


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