KE5FRF
02-20-2007, 05:02 PM
Hello QRPers.
I recently finished building the SW+40 QRP kit from Small Wonder Lab. The rig comes supplied by design with the neccessary components to align the rig within a 30-40 KHz range, and if you've built one of these, you know this entails winding the L1 toroid with 24 or 25 turns to get it either in the Novice range (7.100 MHz or above with 24 turns) or between 7.000-7.100 MHz with 25 turns. You also are aware that it is a matter of hit and miss where the rig will be when you start the alignment procedure. Per the instructions, there is a chart on page 14 that details how to install a specific capacitor value in the C7 spot to align the rig where you want it to operate. Most QRPers would align the rig between 7.030 and 7.060 MHz. As an example, if the rig without a capacitor installed is tuned to 7.090-7.120 or thereabouts, you would install either a 68,47, or 22 pF capacitor (supplied) to get the rig lower on the band.
I wasn't satisfied that the rig is only designed for 30-40 KHz coverage. I'm an Extra class and want to be able to maneuver throughout the band to find strong signals. And on field day or a contest weekend, only having 30 KHz to play with is pretty minimal.
OK, there is an opportunity for broadening the rig. Dave Benson K1SWL mentions in the "Theory of Operation" section that it is possible to expand the coverage of the TXCVR by replacing C8 with a higher value capacitor. However, he advises that at larger capacitance values, thermal stability might be an issue. It also dawned on me that if one chose to mod the rig this way, that the resolution of your external tuning pot would be minimalized, making it tougher to fine tune and zero beat on your intended signal. When I reached the alignment procedure, I KNEW there was a BETTER WAY to extand the coverage of this rig!
It dawned on me that since C7 is used to align the rig, and since the instructions detail how the list of capacitor values will move the rig into the desired range, I thought, "Why not get a rotary switch and give myself the ability to switch between values"? I knew it would be a simple matter of paralleling progressively larger values around several switch positions of a rotary switch.
http://img408.imageshack.us/img408/7716/sw40modfk2.jpg
So, here is a drawing of how I designed the simple modification. I used a SINGLE POLE switch, which only breaks one set of contacts to remove the unused capacitors from the circuit. A better method might be to use a double pole switch, and solder the caps between matching poles and then solder two leads to both of the common legs of the switch. If this isn't clear, you can PM me and I will draw another schematic to illustrate. Since I had only a single pole switch available, I chose to go that route. This requires, as illustrated, that one leg of each capacitor be common to each other and soldered to a lead wire.
Since the initial alignment of the rig is done WITHOUT ANY capacitor value installed, it would be advisable to get a rotary switch with several positions, and leave one or more switch positions open, this way when the switch is turned to that position, the rig is aligned without the aid of any capacitance at C7, and will be in the operating range that you started with. Each progressive value of capacitance will move you further down the band.
You MAY want to start out by winding L1 toroid with only 24 turns, to ensure that your rig will be aligned higher up on the band from the start, since adding capacitance by design MOVES IT DOWN THE BAND.
This mod provides added coverage without sacrificing the resolution or sensitivity of your tuning circuit. It was very easy and the only extra part I had to provide was a rotary switch.
Some interaction does oocur between the capacitors and the chassis of a metal enclosure, and the lead wires that you add to the circuit, so it is advised to secure these wires to keep them from moving around.
But I will tell you that the mod DOES WORK, and I personally set my rig up for coverage all the way from 7.000 to 7.100 MHz. On the bottom end, the rig does overshoot below the band, so I have to be aware of this when operating in the Extra portion.
Disclaimer: This mod works for me, but your mileage may vary. Also, I did an INTERNET search to see if this mod was thought of before, and didn't find anything publicized...It is a very simple mod, but it is possible that someone else has done this, so I don't want to take credit for something thought of before. But I assure you, I did this completely uninfluenced by any other publicized mods.
ENJOY!
I recently finished building the SW+40 QRP kit from Small Wonder Lab. The rig comes supplied by design with the neccessary components to align the rig within a 30-40 KHz range, and if you've built one of these, you know this entails winding the L1 toroid with 24 or 25 turns to get it either in the Novice range (7.100 MHz or above with 24 turns) or between 7.000-7.100 MHz with 25 turns. You also are aware that it is a matter of hit and miss where the rig will be when you start the alignment procedure. Per the instructions, there is a chart on page 14 that details how to install a specific capacitor value in the C7 spot to align the rig where you want it to operate. Most QRPers would align the rig between 7.030 and 7.060 MHz. As an example, if the rig without a capacitor installed is tuned to 7.090-7.120 or thereabouts, you would install either a 68,47, or 22 pF capacitor (supplied) to get the rig lower on the band.
I wasn't satisfied that the rig is only designed for 30-40 KHz coverage. I'm an Extra class and want to be able to maneuver throughout the band to find strong signals. And on field day or a contest weekend, only having 30 KHz to play with is pretty minimal.
OK, there is an opportunity for broadening the rig. Dave Benson K1SWL mentions in the "Theory of Operation" section that it is possible to expand the coverage of the TXCVR by replacing C8 with a higher value capacitor. However, he advises that at larger capacitance values, thermal stability might be an issue. It also dawned on me that if one chose to mod the rig this way, that the resolution of your external tuning pot would be minimalized, making it tougher to fine tune and zero beat on your intended signal. When I reached the alignment procedure, I KNEW there was a BETTER WAY to extand the coverage of this rig!
It dawned on me that since C7 is used to align the rig, and since the instructions detail how the list of capacitor values will move the rig into the desired range, I thought, "Why not get a rotary switch and give myself the ability to switch between values"? I knew it would be a simple matter of paralleling progressively larger values around several switch positions of a rotary switch.
http://img408.imageshack.us/img408/7716/sw40modfk2.jpg
So, here is a drawing of how I designed the simple modification. I used a SINGLE POLE switch, which only breaks one set of contacts to remove the unused capacitors from the circuit. A better method might be to use a double pole switch, and solder the caps between matching poles and then solder two leads to both of the common legs of the switch. If this isn't clear, you can PM me and I will draw another schematic to illustrate. Since I had only a single pole switch available, I chose to go that route. This requires, as illustrated, that one leg of each capacitor be common to each other and soldered to a lead wire.
Since the initial alignment of the rig is done WITHOUT ANY capacitor value installed, it would be advisable to get a rotary switch with several positions, and leave one or more switch positions open, this way when the switch is turned to that position, the rig is aligned without the aid of any capacitance at C7, and will be in the operating range that you started with. Each progressive value of capacitance will move you further down the band.
You MAY want to start out by winding L1 toroid with only 24 turns, to ensure that your rig will be aligned higher up on the band from the start, since adding capacitance by design MOVES IT DOWN THE BAND.
This mod provides added coverage without sacrificing the resolution or sensitivity of your tuning circuit. It was very easy and the only extra part I had to provide was a rotary switch.
Some interaction does oocur between the capacitors and the chassis of a metal enclosure, and the lead wires that you add to the circuit, so it is advised to secure these wires to keep them from moving around.
But I will tell you that the mod DOES WORK, and I personally set my rig up for coverage all the way from 7.000 to 7.100 MHz. On the bottom end, the rig does overshoot below the band, so I have to be aware of this when operating in the Extra portion.
Disclaimer: This mod works for me, but your mileage may vary. Also, I did an INTERNET search to see if this mod was thought of before, and didn't find anything publicized...It is a very simple mod, but it is possible that someone else has done this, so I don't want to take credit for something thought of before. But I assure you, I did this completely uninfluenced by any other publicized mods.
ENJOY!