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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Wed 03, 2018 9:44 am 
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Location: Saskatoon
Hi Mark, Thanks for the update. Glad to hear that it's working.
What do you mean when you say that it's not hifi? I don't think we would expect high audio bandwidth on the old receivers discussed on this forum. Is that what you meant, or were there other problems too?


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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Wed 03, 2018 12:04 pm 
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Location: Baton Rouge, Louisiana
BobWeaver wrote:
Hi Mark, Thanks for the update. Glad to hear that it's working.
What do you mean when you say that it's not hifi? I don't think we would expect high audio bandwidth on the old receivers discussed on this forum. Is that what you meant, or were there other problems too?

Hi Bob, I am pleased that you are interested.
Yes, there are odd harmonics present in music. Some, but not all musical passages sound pretty bad. When certain chords or harmonies are played one can hear the "discord".
I am inclined to believe that this can be tuned out, as it was present when I connected the MPX unit directly to amplifiers, but to a much lesser extent. From here, some "scoping around" will be needed.

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sat 06, 2018 10:03 pm 
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Location: Baton Rouge, Louisiana
I have antenna and ground questions. Here is the plate tank as per the 6888 transmitter project.
The only difference is that I used 100 uh inductors.
Attachment:
plate_tank.png
plate_tank.png [ 32.54 KiB | Viewed 298 times ]

The questions are:
(1) What should I shoot for as far as loading coils? 1400khz & 1600kz (b) does it matter?
(2) What is the impedance of the transmitter and how do I know? (b) does it matter?
(3) Are rf ground radials really important for just whole house coverage? (b) would a counterpoise be sufficient?
(4) I have access to 1" copper, actual diameter 1.125". is there a reason not to use this for radiators?

Here are a couple of shots of the top and bottom of the chassis as finished to date.
Attachment:
finished_top.jpg
finished_top.jpg [ 68.08 KiB | Viewed 298 times ]
Attachment:
finished_bottom.jpg
finished_bottom.jpg [ 84.13 KiB | Viewed 298 times ]

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 12:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2108
Location: Saskatoon
The largest value coil that will still resonate with the variable cap, when it's near minimum capacitance, will give you the most range. I'd be looking at replacing L1 with 470µH or 680µH.


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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 1:30 am 
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Location: Baton Rouge, Louisiana
Thanks Bob,
Does that mean that if I replace the tank coils that I may not need loading coils?

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 3:40 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Saskatoon
Correct.

The idea is that you should use the minimum possible amount of capacitance in the antenna tuning circuit, because the RF current is split between the variable capacitor and the antenna in proportion to their relative capacitance. So, the higher the capacitance of the variable capacitor, the more RF flows through it, and doens't flow into the antenna. And what doesn't go to the antenna doesn't get radiated.

The ideal solution for maximum radiated power would be to use a slug tuned inductor, and eliminate the variable capacitor altogether, but large value slug tuned inductors are pretty hard to find nowadays. So the next best thing is to use the largest possible fixed value inductor, and then just enough capacitance with the variable cap to get resonance.

Looking back at my notes, I may have been a bit high in my inductance recommendations. I think I got peak resonance at 1000kHz with a 470uH inductor. Since you're operating at higher frequencies, the inductance will be lower. But that also depends on the length of your antenna. Some experimentation will be in order.


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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 1:33 pm 
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Location: Baton Rouge, Louisiana
Thanks, Bob. I had been studying the short antenna problem and stumbled across your web page discussing such.
At first, I was very confused about seemingly conflicting information. Now I am beginning to see the light, but also realize
that there are some improvements that I need to make on the transmitters design, which is slightly disappointing.
Thanks again for your explanation of this subject.

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 5:11 pm 
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Location: Baton Rouge, Louisiana
kc5gym wrote:
BobWeaver wrote:
Hi Mark, Thanks for the update. Glad to hear that it's working.
What do you mean when you say that it's not hifi? I don't think we would expect high audio bandwidth on the old receivers discussed on this forum. Is that what you meant, or were there other problems too?

Hi Bob, I am pleased that you are interested.
Yes, there are odd harmonics present in music. Some, but not all musical passages sound pretty bad. When certain chords or harmonies are played one can hear the "discord".
I am inclined to believe that this can be tuned out, as it was present when I connected the MPX unit directly to amplifiers, but to a much lesser extent. From here, some "scoping around" will be needed.

I got the harmonic distortion cleared up, no scope required, just a little fiddling with the diddle stick took care of it. The 19 kc transformer was off a bit.
This project has doubled my antique radio listening pleasure! :D

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Sun 07, 2018 10:14 pm 
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Posts: 786
Location: Baton Rouge, Louisiana
Attachment:
front.jpg
front.jpg [ 83.57 KiB | Viewed 252 times ]
Attachment:
back.jpg
back.jpg [ 63.74 KiB | Viewed 252 times ]
Attachment:
end_view.jpg
end_view.jpg [ 47.94 KiB | Viewed 252 times ]

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Mon 08, 2018 3:30 am 
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Location: Beautiful Downtown Burbank CA
Wow! Really nice work Mark.


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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Mon 08, 2018 8:33 am 
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I'll say; a thing of beauty (and it woiks!).

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 Post subject: Re: Stereo Transmitters
PostPosted: Jan Wed 10, 2018 10:03 pm 
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Location: Baton Rouge, Louisiana
I would like to thank everyone who assisted me with this project.
At first, I was going to thank each person individually, but the post was getting lengthy.
Thanks again to those who provided technical assistance, encouragement, and compliments.
It has been fun. We will do something else one day.

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