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Old 19th Nov 2005, 10:57 pm   #21
Oskar.B
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Values for the output filter matches approximatetly what I calculated ... with this:

http://my.athenet.net/~multiplx/cgi-bin/pinet.main.cgi



I figured, why do it by hand, when someone has done a nice web-calculator.

I have read the theory though.

http://my.integritynet.com.au/purdic/lowpass.html

And in school we calculated values in smith-charts, beginning by plotting Q-curves in the smith chart that you must follow. Interesting .
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Old 19th Nov 2005, 11:33 pm   #22
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Oskar.B
I figured, why do it by hand, when someone has done a nice web-calculator.
Because then you will be missing out on the joys of having your sign errors exposed by someone with access to a nice web calculator...

I now get 1066pF, close enough for comfort.

Frank N.
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Old 21st Nov 2005, 9:13 pm   #23
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi everyone,

Progress report:

I have been CAD:ing the PSU box so that I can print a A3 sheet, apply it to the box and use directly as a drill chart. Basically finished. Just waiting fot the mod. transformer.

A quickie question:

Switching in the ATU (connecting different taps of the matching coil, moving cap to left, right side of coil). At my power level (25W approximately), can this be done with "regular" spec switches? Just wanted to know if I absolutely, definetly need RF-high-voltage-switches. It's hard to tell exactly what voltages will be present in the ATU. But say 200 volts maybe?

You can never have enough of them junk-boxes. Allways something missing . I'm looking around for some koax connectors too, they're not cheap! The electronics industry should give away hardware for free to experimenters!

/Oskar
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Old 21st Nov 2005, 10:00 pm   #24
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

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Originally Posted by Oskar.B
Switching in the ATU (connecting different taps of the matching coil, moving cap to left, right side of coil). At my power level (25W approximately), can this be done with "regular" spec switches?
Allow me to attempt to answer this one with a firm and clear 'Hmmm...?".

This one is impossible to answer in the general case I'm afraid. The classical solution is to use something like high quality banana plugs and sockets, moving the wire by hand (with no RF power applied, of course.)

Quote:
Originally Posted by Oskar.B
Just wanted to know if I absolutely, definetly need RF-high-voltage-switches. It's hard to tell exactly what voltages will be present in the ATU. But say 200 volts maybe?
By definition the impedances an ATU/transmatch will encounter are unknown. Yet my experiences leads me to conclude that at 25W you will have to be fairly unlucky for almost anything you can make not to work. Save the high quality switches for the KW amp, which will be down the road some day.

Where you may run into problems is if you accidentally encounter a very high impedance at a point, where a less than perfect insulator touches a conductor. That is what happened to the nylon coil form, which I mentioned catching fire at 100W.

Truth be told, then I'm more worried about you switching the B+ to the modulation transformer, than what might happen to the switches in your ATU. Do I smell burned out modulation transformer insulation?

Quote:
Originally Posted by Oskar.B
You can never have enough of them junk-boxes.
Absolutely. Also makes for fun digging experiences, as you come across something you had forgotten was in your possession.

Quote:
Originally Posted by Oskar.B
Allways something missing . I'm looking around for some koax connectors too, they're not cheap!
You don't need BNC or N connectors for this project. SO/PL-239 will do quite nicely. Also remember to add an independent safety ground bus wire between the PSU and the TX. I usually add a M8 or M6 bolt, nut and wingnut to the back of all my homemade projects for this very purpose.

No progress on my project today, as I got sidetracked by finding a box of valves in my junkbox, which I just had to test then and there. The bounty of the valves that work includes 7x EL84 and 9x EF80.

Frank N.

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Old 21st Nov 2005, 10:37 pm   #25
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Aaah, I've searched for them PL-239s, but never found them at any supplier. Now I see that suppliers tend to call them UHF connectors. Stupid, I didn't even bother to look at these, I thought they were TV connectors .

I like the prices of these much better .

About the switching in the final. Yes I've been worried about this. Would it not be enough with the P.T.T switch? It would remove the carrier. I guess what one wants is the back of the TX to be safe for disconnecting and reconnecting of antenna cables?

I would still need to connect an extra resistor into the plate circuit of the oscillator to compensate for the higher voltage when the P.T.T is open though.
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Old 21st Nov 2005, 10:46 pm   #26
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

PL259 (not PL239) connectors are also known as UHF or F&E. Despite being called UHF they are most unsuitable for UHF since their impedance is not guaranteed. I would be slightly worried about using them for high power RF though they would probably be OK. BNC is fine but will only take the smaller co-ax cables and may not take very high power. N connectors are ideal but more expensive unless you can find some surplus ones.
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Old 21st Nov 2005, 11:04 pm   #27
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hmm, PL259s..... Work of the devil!

Ok, if you must use them, but I grumble at the sight of them, should be ok at 25 w, but try to find PTFE dielectric, if you get a high feedline VSWR the cheap ones can arc and melt......

I have seen them used up to a kilowatt at 144 Mhz, I have smelt the aroma too

Use N types, they arent that expensive, and they are a far superior device!

Cheers
Sean
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Old 21st Nov 2005, 11:23 pm   #28
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

I agree with Sean about PL259. I was just being a bit too polite in my previous post. PL259 used to be widely used for video and I hated them in that application too. I used to make UHF to BNC adaptors by soldering a BNC socket into the back fo a UHF plug. Only took a few minutes and cost nothing since the bits were in the junk box.

An alternative to N is the C connector. Not so common but if you've got them in the junk box they're good. Imagine a BNC on steroids
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Old 21st Nov 2005, 11:27 pm   #29
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

To bad I had allready ordered a couple then .


The difference in price as compared to the "N" connectors, at least to me, was enormous!

Edit:

Unfortunatly, my junk-boxes contains nil communications and TX bits. At this stage I have to buy everything, and my wallet is not enjoying it
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Old 21st Nov 2005, 11:37 pm   #30
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,

Well, depends what bits you are missing (Im sure I read that you were going to get a Ham ticket )

If you get stuck for odds and ends I could rake through the junk here (nothing heavy mind you, the postage would be more than the parts are worth )

So, if you get stuck let me know.

Im sure Frank has access to a huge junk box

Cheers
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Old 21st Nov 2005, 11:50 pm   #31
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Thanks alot

I'm not really stuck. I actually have all the bits now, except for the 807 plate RFC, but that I'll have a go at winding that myself. I will be winding all the other coils too. I acutally have a dos program I wrote to help me counting turns . You place the computer keyboard on the floor, and each turn you make with the wire you whack the keyboard with your foot. It doesn't matter wich key(s) you hit ... suprisingly useful!

I'm going to give my PL connectors a try, and if they fail me, the hole geometry will be the same (at least with the connectors I've been looking at), so it will be easy to replace them with "N" connectors.

/Oskar
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Old 22nd Nov 2005, 9:50 am   #32
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Oskar.B
I'm going to give my PL connectors a try, and if they fail me, the hole geometry will be the same (at least with the connectors I've been looking at), so it will be easy to replace them with "N" connectors.
No need to worry, they won't fail you.

The 'UHF' connectors, imperfect as they are, are quite good enough for anything amateur power level below 30MHz. They have been fitted to tens of thousands of commercially made 100W amateur transmitters for literally decades, and they are regularly used at higher power levels than that. My 500W Yeasu FC-901 ATU are fitted with these, and even some (most) 1.5KW linear amps have them as well. If it is good enough for Alpha Power, then it will be good enough for you too.

An N-connector is a rare sight indeed on a piece of amateur equipment below 30MHz. At least I never recall seeing one.

My main complaint about them is actually that the male cable connectors can be a bit difficult to connect to the cable.

I would definitely forget about switching the B+ to the PA. If the PTT switch in the cathode of the 807 isn't sufficient to cut off the valve, then you can easily generate an additional -140V of fixed grid bias from the HT transformer bias tap. I even believe I made a circuit diagram showing how easy this is, though I never photographed and posted it.

Stay tuned, I'll find it.

Frank N.
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Old 22nd Nov 2005, 11:46 am   #33
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

OK, here is the circuit I mentioned. It is simply a slightly unconventional half-wave voltage doubler attached to the 50V bias tap on the HT transformer. A simple switch arrangement couples the extra -140V bias voltage to the 807 grid during standby by simply cutting the ground return for the grid current. I forgot to draw the grid and cathode current meters in this circuit. The grid current meter should preferably be permanently connected between the standby switch and ground. Same for the cathode current.

This means your standby switch needs to have three poles, and one set of them is needed on the PSU chassis...

The component values in the bias rectifier are totally uncritical, as long as they can cope with the voltages involved. The pair of 1N4007s can be replaced by 1N5408s, BY255s or similar, and the 10uF/250V capacitors can be substituted by anything of larger capacitance/working voltage (within reason, of course).

Something else: The reason why I like to place the meters at ground potential is that often you cannot be sure at what DC insulation voltage your meters are specified for. Obviously the face of the meters can be touched easily enough.

So if possible I like to wire my circuits such that one connection of all meters is always at ground potential. This is always a good policy, especially at the seriously high HT voltage encountered in even larger amplifiers.

Frank N.
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Old 22nd Nov 2005, 12:36 pm   #34
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

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The grid current meter should preferably be permanently connected between the standby switch and ground.
Bummer, that is not right, of course.

The grid current meter must be placed in between the connection to the RF bypass capacitor at the cold end of the 22K grid bias resistor, and down to the 'T' connection with the brances off to the PTT switch and the second 22K resistor. If placed anywhere else, it will either be subjected to RF currents, or it will also measure the 'dummy' current supplied by the bias voltage source and through the secondary 22K resistor.

Frank N.
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Old 16th Dec 2005, 9:20 pm   #35
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi all,

Unfortunatly my job has taken to much time lately so no real progress in the building of the transmitter. The company I have ordered my mod transformer from had managed to forget about my order, so I wont get it before Christmas. That actually ****** me off! Sorry ..

Just to keep the discussion going I will request some opinions on my antenna. It is approximately 80 meters of 2,5mm² wire strung from my parents' house, over a field and up into a large tree. See the very nice picture taken today:

http://gargnas.net:3000/bilder/En%20...5/PC160005.JPG

I have about 20m of some very heavy gauge copper wire buried in the lawn under the antenna, plus 5 ground rods, maybe 1 meter each.

The grounding will be improved next summer. I just wanted some thoughts on the basic concept.

/Oskar
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Old 17th Dec 2005, 9:53 am   #36
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,
Quote:
Originally Posted by Oskar.B
Hi all,

Unfortunatly my job has taken to much time lately so no real progress in the building of the transmitter. The company I have ordered my mod transformer from had managed to forget about my order, so I wont get it before Christmas. That actually ****** me off! Sorry ..
This shouldn't discourage you. Take the drilling pattern for the mod transformer from Hammond's website and start working on the scrap chassis. Once you have the RF section and PSU sorted out, you just have to plug in the mod iron to finish the project.

I for my part haven't gotten any further either, but that is hardly unusual for my projects. Too many irons in the fire as we say here in Denmark.

Quote:
Originally Posted by Oskar.B
Just to keep the discussion going I will request some opinions on my antenna. It is approximately 80 meters of 2,5mm² wire strung from my parents' house, over a field and up into a large tree. See the very nice picture taken today:

http://gargnas.net:3000/bilder/En%20...5/PC160005.JPG

I have about 20m of some very heavy gauge copper wire buried in the lawn under the antenna, plus 5 ground rods, maybe 1 meter each.

The grounding will be improved next summer. I just wanted some thoughts on the basic concept.
Any long piece of wire up as high as possible will do nicely for reception on 3,5MHz and thereabouts.

How about posting some photos of your dummy load and/or your newly acquired amateur radio license, please? I for one am not going to help you put your 'science project' on the air.

Best regards

Frank N.
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Old 17th Dec 2005, 4:52 pm   #37
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Frank,

I can understand if you don't want to help an unlicensed operator out on the air. I don't think I will have time to take a license before the transmitter is finished though. My intention is just to test into the dummy load until I have a legal way of operating the transmitter on the air.

I don't even think that my current antenna system could be used to test the transmitter, thats because the grounding seem to be very poor now that the ground is frozen. When I tested the antenna with my Trio receiver this summer, the ground improved the signals really much, but now there is hardly any difference when I connect the ground.

I totally understand the problems people can have with an unlicensed guy like me building a transmitter. However, building and testing a transmitter into a dummy load should not cause any trouble I think. And as long as we keep the discussion technical and nothing but technical I don't think naughtiness is encouraged in any way.

Difficult dilemma though, as I might, for all you know, say one thing, and do another, and I understand if a moderator would like to close the thread(s).

/Oskar
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Old 22nd Dec 2005, 9:04 pm   #38
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Ok, I can at least prove that I have a dummy load:


http://gargnas.net:3000/bilder/dassr...s/PC220019.JPG

http://gargnas.net:3000/bilder/dassr...s/PC220017.JPG

A bit smallish, but it should handle shorter tests and tuneup of up to a couple of minutes at 25W.
/Oskar
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Old 22nd Jan 2006, 9:47 pm   #39
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Some progress at last:

http://gargnas.net:3000/bilder/dassrull/P1220001.JPG
http://gargnas.net:3000/bilder/dassrull/P1220003.JPG

*building exitement level = 7(10)*

/Oskar
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Old 24th Jan 2006, 1:36 am   #40
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

It's aliiiiive!

The OD3 is doing well, very well:
http://gargnas.net:3000/bilder/dassrull/P1240001.JPG

Oscillator B+ is ok too, a bit high, but its onloaded:
http://gargnas.net:3000/bilder/dassrull/P1240002.JPG

And finally .. the final B+:
http://gargnas.net:3000/bilder/dassrull/P1240003.JPG

If anyone want to laugh at me, maybe I'll get a pic of the chassis insides, I actually forgot to take one .. because I'm tired ...

Now I'm going to bed ... my head is aching from soldering fumes ..

Edit:
Build exitment level (right now = -1) )

/Oskar
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