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 Post subject: Part 15 medium wave transmitter tank design
PostPosted: Aug Thu 15, 2019 3:58 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
...
Proposed circuit:
Attachment:
Tank_Antenna_Coupling_100mW.jpg
Tank_Antenna_Coupling_100mW.jpg [ 302.3 KiB | Viewed 2054 times ]


Example:
Given:
Class C operation
1400 kHz
100 mW dc power input to tube plate
70% efficiency dc plate input to RF output
Tank (C2, L2) loaded Q = 12
3 meter vertical antenna, radiation resistance (Rr) 0.08 ohm,
capacitive reactance 4600 ohms.
Antenna coupling transformer T1: 50 ohm Z primary, 0.08 ohm Z secondary,
25 to 1 primary to secondary turns ratio, impedance ratio 0.0016.

Calculations:
100 mW x 70% = 70 mW into tank, with zero loss = 70 mW into antenna.
Sqrt(70 mW / 0.08 Rr) = 0.935 amperes antenna base current.
0.935 x 0.08 = 75 mV RMS antenna base voltage.
70 mW x 12 Q = 0.84 VA tank reactive power
Sqrt(70 mW antenna coupling transformer primary power/50 ohms) = 37 mA tank circulating current.
0.84 VA / 37 mA circulating current = 22.7 V RMS across tank.
22.7 V RMS / 37 mA = 613 ohms X.
613 ohms @ 1400 kHz = L2 70 uH, C2 185 pF.
70 mW / 22.7 V RMS = 3.08 mA RMS current into tank.
22.7 V RMS / 3.08 mA = 5973 ohms tank input Z
(22.7 V RMS x 1.414) + tube saturation drop, around 10 V = 42 V dc plate supply (adjust B+ for 100 mW dc plate input)
100 mW / 42 V = 2.4 mA dc plate current
4600 ohms L3 @ 1400 kHz = 523 uH

Ref. B. W. Griffith, Radio-Electronic Transmission Fundamentals, 1962.

With an ideal ground the Q of the antenna may be excessive, but ground loss should reduce the Q.

Completion of the design of T1 is left for other forum participants.
Operating frequency 1000 kHz to 1600 kHz:
Toroidal core of appropriate material and physical size.
20 gauge wire should be okay for secondary.
Each winding to occupy full circumference.
Primary reactance 200 ohms @ 1000 kHz

Hopefully nothing is too goofed up in assumptions, method or math.
I would like to breadboard this but not able to at this time.
Eric LaGess
WB5HDF


Last edited by infzqi on Aug Fri 16, 2019 3:32 pm, edited 2 times in total.

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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Fri 16, 2019 3:26 am 
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Posts: 3997
Location: Tucson, Arizona U.S.A.
It appears that you are assuming that the plate of the tube can be pulled completely to ground. This would be a good approximation if you used a transistor. But a tube will have substantial voltage drop, perhaps 10 V if you are lucky (it will depend on tube type and screen voltage if you use a pentode). The 70% efficiency estimate doesn't seem realistic.

I would suggest a higher B+ voltage so that the "saturation voltage" of the tube would be less important.

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Jim Mueller


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Fri 16, 2019 3:33 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
Thank you Mr. Mueller for mentioning the tube voltage drop at saturation. I had not questioned the validity of applying the 90% of B+ that is appropriate for much higher dc plate supply voltage power tubes to operation of a tube at much lower B+ voltage. I've improved the B+ voltage specification.
Yes, 70% efficiency is optimistic.

Eric LaGess
WB5HDF


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Fri 16, 2019 3:43 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 2152
Location: Roanoke, VA
The rest of the transmitter design is not shown, but note that if the oscillator stage is directly modulated the modulation will be limited to about 30% because of increasing envelope distortion and incidental frequency modulation above the 30% level.

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


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Fri 16, 2019 11:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 8788
Location: Beaver Falls, PA. USA
Were you thinking of using the 6888?

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Tim KA3JRT


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Sat 17, 2019 2:20 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
....
No particular tube in mind for studying tank circuit designs.

Right now (if I'm not making some sort of error), assuming double sideband A.M., it looks like the challenge is to get more than a fraction of a percent of the roughly 50 to 70 mW power out of the plate circuit into such a short antenna at low enough Q to get the desired 10 to 20 kHz RF bandwidth.

For example, a 3 meter vertical antenna at 1600 kHz is 5.76 degrees long.
Radiation resistance (Rr) of 5.76 degree vertical antenna roughly: 0.12 ohm (ref. H. Jasik, Antenna Engineering Handbook, 1961).
Referring to schematic diagram above,
Assuming 25 pF capacitance of antenna at 1600 kHz gives 3979 ohms capacitive reactance.
Required L3 reactance: 3979 ohms.
L3 series resistance, Rl:, 20 gauge wire on ferrite rod, assumed to be 1 ohm for example.
Ground resistance, Rg, assumed to be 20 ohms for example.
Rt = Rr + Rl + Rg = 0.12 + 1.0 + 20, Rt = 21.12 ohms
Q = XL / Rt, Q = 3979 / 21.12 = 188
Bandwidth = F / Q = 1600 kHz / 188 = 8.510 kHz RF bandwidth.
8.5 kHz is okay for communication but narrow for music.

Assuming 60 mW power from T1 secondary into antenna circuit,
Sqrt (60 mW / 21.12 ohms) = 53 mA antenna current.
Power = I squared x Rr = 0.053 squared x 0.12 ohms = 340 microwatts radiated.
340 uW / 60 mW = 0.57 % of power from T1 secondary is radiated.

Again hopefully no fundamental goof-ups. Probably I'm ignorant of some coupling technique.

Possibly a forum participant has an RF bridge and would like to make some measurements of (Rr + Rg) and X of a typical 3 meter vertical antenna setup at various frequencies.

Making the antenna an inverted L, such that the top 1.5 meters is horizontal, should raise the radiation resistance somewhat. That's what I conjectured, but running the inverted L in EZNEC shows that it has less radiation resistance than the straight vertical.
A capacitance hat 1.2 meters in diameter made of six wires at the top of a 2.4 meter vertical gives 0.15 ohm Rr at 1600 kHz.

...


Last edited by infzqi on Aug Mon 19, 2019 1:57 pm, edited 1 time in total.

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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Sun 18, 2019 1:31 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 3997
Location: Tucson, Arizona U.S.A.
The OP isn't proposing a transmitter, only a coupling network.

Since this seems to be a drive to get the maximum power possible radiated, I would propose using a tube with a low voltage drop to get the maximum efficiency from the final amplifier. Something like a 6CU5 or 6W6 would be a good starting point. After putting this much effort into it, it would seem natural to go for a first class transmitter. There would be a crystal oscillator or electron coupled VFO, possibly with a buffer stage, driving the final. The final would be plate (and screen of course) modulated. The oscillator (and buffer, if used) and their coupling networks should be shielded to prevent radiation. Radiation at the same frequency as the output could reduce the amplitude of the carrier resulting in distortion or increase the carrier reducing the percentage modulation or something in between depending on the phase relationship. Without using Compactrons, a transmitter like this would probably have four tubes not counting the power supply rectifier.

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Jim Mueller


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Sun 18, 2019 4:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
I am considering a rig like that as a future project; I already have a master oscillator chassis from an old RCA broadcast transmitter.

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Tim KA3JRT


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Sun 18, 2019 6:33 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 120
Location: Texas, U.S.A.
Yes it would be rewarding to construct a part 15 transmitter with best practice techniques and features. Thanks for mentioning possible effects of inadequate shielding. Will look at the data on the tubes.

An oscillator from a broadcast transmitter should be plenty stable, if one happens to have one of those.

Indeed maximizing radiated power with the prescribed power input and antenna limitations is what I'm studying.

Right now it looks like a double-tuned circuit would be applicable for increasing the bandwidth of the T1 secondary to antenna circuit, have to study more.


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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Mon 19, 2019 1:04 am 
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Joined: Sep Mon 16, 2013 2:42 am
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Location: Tucson, Arizona U.S.A.
It would be nice to have a doublet or loop antenna since the length of the ground wire is subtracted from the length of the antenna. With a doublet or loop, no ground would be required and the entire length could be radiating.

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 Post subject: Re: Part 15 medium wave transmitter tank design
PostPosted: Aug Sat 31, 2019 6:57 am 
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Joined: Mar Sun 14, 2010 2:51 am
Posts: 533
Location: Newport, Oregon, 97365 U.S.A.
If the antenna is ground mounted, you could increase ground effectiveness by ground plane wires or swamp or water location.
There is a company that sells very limited-space ham radio antennas and they have one for this Part 15 BC band also. It's just
basically a coil with metal capacitive plates at each end of coil, so not ground dependent. If you could get that way up high, it might
outperform a ground placed antenna, but otherwise I don't think so. The way some Part 15 people game the rule is by placing the transmitter way up high, then the ground and power lead radiates also til it reaches earth ground. This gets out well but only if the broadcasts don't assume too high a profile and come to the FCC's attention.


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