HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Intruder on January 23, 2019, 1236 UTC
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Hello Guys,
New to the forum! Interest in 48M been on and off over the years. Being so late in the game transmitters are a bit hard to get hold of and unfortunately I don't have a lot of cash, anyway I found the QRP Corsette transmitter design by Dave Martin. Seeing somewhere one person built it and was getting around 1.1watts.
Not really a constructor and have very basic building skills I decided to build the unit. I had most of components except for the transistors, which I sourced on eBay and the closes inductor was 15mH.
I first built the unit on breadboard to confirm the circuit worked before soldering on to copper clad board.
Doing it this way and plugging into VSWR meter and dummy load the max output was 200mW. I removed what I believe is the low pass filter a 470pf, T50-2 and 1000pf and the output increased a slightly.
Maybe components weren't right, I started with 15mH inductor. I wound wire 59turns onto a T50-2 and got close 17.38mH yes that made a difference.
I wondered because it's built on breadboard there wasn't enough grounding. I then rebuilt and soldered onto copper cladding.
The output increased to 300mW, the Motorola BFY51 gets quite warm so I stuck a fan on to help. (Still with no LPF connected)
The only time I get more output is when connected to antenna and the most I get 500-550mW but without the filter, I don't want to leave it on for long due to no filter.
Has anyone else made one of these ?
Can anyone advise if I'm doing something wrong not to get 1watt out.
Was after putting a QRP 5 watts or less station on air
Many thanks
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Tried to add images, dont know how?
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Anyone advise on attaching images of transmitter
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You'll need to 'Host' the images elsewhere as you cant directly upload images here.
Lots of sites but I've no idea which is best but am sure someone else will let you know, I'm sure.
Re the design your building...
What Frequency are you using?
I could save you a lot of trouble and send you a PCB for a better design, but wont force it upon you!
LuLu is the way to go....
Str.
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upload your image to imgur.com/upload. then add image tag in your message like this: [𝖴𝖱𝖫]𝗁𝗍𝗍𝗉𝗌://𝗂.𝗂𝗆𝗀𝗎𝗋.𝖼𝗈𝗆/𝖪𝟣𝖹𝖪𝖳𝟣𝖺.𝗃𝗉𝗀[/𝖴𝖱𝖫] your pic will show automagically in your post.
Good luck with your tx, sorry I am of no help, others will chime in hopefully
rgg
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Experiment with different values of chokes and caps, particularly the ones you removed and also the choke on the supply volts to the BFY51.
The chokes that you wound on the T50-2's can be varied slightly by squeezing together the turns or spacing the turns out a bit more. This will give you an indication if you need more or fewer turns.
I haven't built that particular rig but I have built a couple of the Grenades and small adjustments can have a significant influence on the output power.
Good luck.
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Thanks to all that have replied!
Mr Stretchyman, would be interested in the PCB, if ok will PM you ?
Brian, I did wonder is it because of using ceramic caps on the circuit. I recently what a video on YouTube about ham transceiver circuit and ceramics aren't great at RF output stages/filters. It was mentioned mica or poly caps? Is this right?
I'm also confused when into a 50ohm dummy load I don't see a 500mW output but into a antenna I do? Thought it should read the same or better? Suppose I really need test gear but out of my budget and knowledge on using one. Like I say have basic knowledge to get by.
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Your antenna is probably not 50 ohms at your test frequency.
What power meter are you using?. If it doesn't cover the band you're using, then it may be very inaccurate.
My meter goes down to 1.8 MHz but when I use it on MW, it reads about 20% lower than it should.
I "think" I'm using a ceramic in the output of one of my Grenades without a problem but I know I've had issues in higher power txs.
Another suggestion. My grenade uses a BFY51 to drive an IRF. The choke I use on the supply of the BFY is tunable. It's about 15 or 20 turns on a 10 mm former with a ferrite slug. Might be worth trying.
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Yes please PM me.
The info you have about 'Ceramics' is BS they are perfect as long as you use the correct type and rating 'COG' '200V' for a 10W TX (for instance).
Mica are fine but stoopidly expensive and poly ones def not, OK for audio ONLY.
C322C102J2G5TA is a KEMET 200V COG 1nF (for instance).
BTW when measuring power ALWAYS use a dummy load as it's Non 'Reactive'.
Str.
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Is it this one ..?
http://freeradiotx.blogspot.com/2009/06/corsette-low-power-sw-transmitter.html
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Your antenna is probably not 50 ohms at your test frequency.
What power meter are you using?. If it doesn't cover the band you're using, then it may be very inaccurate.
My meter goes down to 1.8 MHz but when I use it on MW, it reads about 20% lower than it should.
I "think" I'm using a ceramic in the output of one of my Grenades without a problem but I know I've had issues in higher power txs.
Another suggestion. My grenade uses a BFY51 to drive an IRF. The choke I use on the supply of the BFY is tunable. It's about 15 or 20 turns on a 10 mm former with a ferrite slug. Might be worth trying.
The VSWR meter is a Diamond SX-600.
There are two inductors, one feeds the BF245 which is 470uH which value I had and checking on a Peak Systems LCR45 it reads 469uH. The second one is 17uH that's on the BFY51 the home made inductor is 17.38uH. It might be worth trying a variable type of inductor to get them closer.
Thanks for advice !
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Yes please PM me.
The info you have about 'Ceramics' is BS they are perfect as long as you use the correct type and rating 'COG' '200V' for a 10W TX (for instance).
Mica are fine but stoopidly expensive and poly ones def not, OK for audio ONLY.
C322C102J2G5TA is a KEMET 200V COG 1nF (for instance).
BTW when measuring power ALWAYS use a dummy load as it's Non 'Reactive'.
Str.
Hi Stretchyman,
Thanks for advice on caps, I did think this was bizarre! Plus like you say it saves on price. Yep will use dummy load for future reference.
Will PM you in a moment.
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Is it this one ..?
http://freeradiotx.blogspot.com/2009/06/corsette-low-power-sw-transmitter.html
Hi QRP,
Yes that the circuit!
I'm only going by this blog: http://www.radiohobbyist.org/blog/?p=271
It stated channel Z "First up, he used a Corsette transmitter, putting out 1.1 watts" - it doesn't say on Dave Martins website what output power it is just QRP!
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Is it this one ..?
http://freeradiotx.blogspot.com/2009/06/corsette-low-power-sw-transmitter.html
Hi QRP,
Yes that the circuit!
I'm only going by this blog: http://www.radiohobbyist.org/blog/?p=271
It stated channel Z "First up, he used a Corsette transmitter, putting out 1.1 watts" - it doesn't say on Dave Martins website what output power it is just QRP!
Dave Martin launched his Corsette transmitter already in June 2009 with the same design but Chris Smolinski wrote about it first in December 2011 in "RadioHobbyist.org".
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Your antenna is probably not 50 ohms at your test frequency.
What power meter are you using?. If it doesn't cover the band you're using, then it may be very inaccurate.
My meter goes down to 1.8 MHz but when I use it on MW, it reads about 20% lower than it should.
I "think" I'm using a ceramic in the output of one of my Grenades without a problem but I know I've had issues in higher power txs.
Another suggestion. My grenade uses a BFY51 to drive an IRF. The choke I use on the supply of the BFY is tunable. It's about 15 or 20 turns on a 10 mm former with a ferrite slug. Might be worth trying.
The VSWR meter is a Diamond SX-600.
There are two inductors, one feeds the BF245 which is 470uH which value I had and checking on a Peak Systems LCR45 it reads 469uH. The second one is 17uH that's on the BFY51 the home made inductor is 17.38uH. It might be worth trying a variable type of inductor to get them closer.
Thanks for advice !
That meter should be fine.
Don't get too hung up on the exact values in the circuit diagram! The values will change depending on frequency.
I would probably leave the 470 uH as it is. The 17 uH may have worked best on the frequency Dave was using but a lower or higher value may work better for you. Same with the output section. Leave the 1000pF cap as is but try to vary the 470pF cap. Add or take away 100pF and see what happens. The toroid can be varied as well.
Make one change at a time and you'll get a feel as to what is working best. There is an interaction between the various components. For example, if the output coil is slightly higher than required, then the 470pF may need to be reduced and vise versa.
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Z did quite well with his Corsette's in the winter months. I'm a long way from where he did a lot of those tx'es and normally could pick up some audio. Other times it was nothing, and sometimes sounded surprisingly well. It's all about your antenna and the propagation gods at low power.
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Intruder, check your message inbox, just sent you a PM.
Z
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Intruder, check your message inbox, just sent you a PM.
Z
Thanks :)
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To continue with progression of thread and the construction of the Corsette with help from Channel Z, I have been able to sort the issues and now got the transmitter working fine.
Produces a very close 1watt and works pretty well too!
Thanks to others that have given advice.
Not first build and won't be the last either! ;)