I am not sure what "linear probe" is being talked about here. There are multiple ways to monitor AM and I have talked about a few here in times past. Easiest way is to obviously connect a diode right to the output (if low power) and run a cap across the output to ground to block RF then monitor the modulation with either headphones or a scope. Another way is to make a simple ferrite coil, connect a tuning cap and diode to make a simple crystal radio and monitor with same method. You can X/Y the scope with both the RF right from the ferrite coil and the audio output from diode/blocking-cap to see any linearity problems w/o even having to touch the RF output of a transmitter. So many ways to do simple testing like this.
If you are talking about linearity of a linear amplifier that can be done multiple ways too. With matched transistors it is quite easy and requires little adjustment but of course that is in a perfect world. With my MW AM linear I designed it so that not only could I adjust quiescent current on the gates of the FETs but the balance between the two in case they were slightly mismatched, which is fairly obvious on a scope using the first methods I mentioned above.
Just set the scope for a flat line, apply RF through a "probe" and a proper modulated sine wave, monitor the divisions and make sure they are equal (both positive and negative) upon increasing vertical on the scope. If one end of the sine is larger, you have a balance problem with the push pull circuit. Also make sure to monitor X:Y mode for trapezoid to make sure the sides/envelopes are flat and not bulging or shrinking for true linearity with modulation to carrier.
Amplitude Modulation 101
Edit: BTW using a single transistor as the linear output power amplifier has the same principals here. This does not just apply to dual finals.
The pallet amplifier I spoke about, was out of the question, being that this device www.rfsource.gr/files/PM62FDC.pdf Is unavailable, I have not been able to contact the seller since I first mentioned that device here.
I seen a 5 WATT wide banded linear amplifier on Ebay and figured I'd grab one while they were available.
The listed specs are as follows: 100 KHZ - 30MHZ rf amplifier, indicators are as follows:
The working frequency of 100 KHZ - 40 MHZ
The input power 0 DBM
37 DBM output power, that is, 5 watts
Harmonic suppression (typical) - 20 db
Working voltage 12 -- 15 v
The working current 0.8 A
Input and output impedance 50 euro
This baby is a linear amplifier is suitable for all kinds of AM and FM signal amplification, medium-wave radio amplifier, can also be used as a power driving circuit of ultrasonic probe similar device.
I have not hooked this device up as of yet, I still want to get a rack mount metal cabinet and a 15 volt 50 amp internal power supply to supply power to both the Chris Cuff AM transmitter and the linear amplifier, then a few short pieces of 50 Ohm shielded patch wire to interconnect the transmitter to the amplifier and then to an external RF output connector, which will be either an "N" connector or an SO-239 female connector for coaxial hook up to an AM broadcast antenna.
I know I will have my work cut out for me, when it comes to drilling or cutting holes in the metal case for switches, meters and power LEDs. I have an LED stereo meter I pulled from a stereo audio VCR unit and the LED has the LED drivers on the unit, it would just be a matter of adjusting things so that (LEFT/RIGHT CHANNEL) LED unit is adjusted for proper audio level reading.
I have a hell of a project ahead of me, since I do not have access to a machine shop and a good drill press for drilling professional looking holes in the rack mount cabinet I end up purchasing.
Once this thing is put together, then I have my next nightmare to work on and that is an AM broadcast antenna that will work well at 1250KHz.
A store near me, sells 100 foot spools of 16 Gauge primary wire for $9.99, I was thinking geeze, 10 rolls of this wire comes out to 1000 feet, which is slightly more than the required length for a full wave antenna at 1250KHz.
Perhaps if I bought 2 10 foot lengths of heavy PVC pipe, then wrap 800 feet of that 16 gauge wire around the pipe, then at the top, have a 10 foot length of 1/2 inch copper tubing, capped with an end cap and then solder two pieces of 1/2 inch copper tubing 2 feet long, criss-crossed at the top as a top-hat and then string a 16 gauge copper wire from the end of each top-hat to create a diamond shape.
Then I could try using the same 16 gauge wire as a set of ground radials buried slightly under the ground.
Over towering the trees here, will be impossible however, most are over 150 feet tall, I can't in any way possible over tower those monsters unfortunately.
So basically a short helical over a ground plane is what you are thinking of trying. Personally I would just use a shortened tuned random length of vertical wire. It would save a lot on money if you already have a roller inductor and variable capacitor laying around to tune up the vertical length of wire instead of wrapping hundreds of feet of wire around PVC to self resonate. Plus you can slingshot the insulated holding wire up into a tree to get it high up. Use two trees to get a horizontal insulated section up to hold your vertical wire. Also consider how long this will take to wrap all that around a pipe! To make matters worse you have to make sure it will all fit. Another idea is to do either of these ideas but extend the top out horizontally to a insulator and tree to make a type of inverted L antenna to give you both some horizontal and vertical polarization which could help with nighttime skip. I've said it before to people and I will say it again, your best bet is to look at 160m ham radio antenna design ideas. The guys on that band have already figured it all out for you, just recalculate lengths for the lower frequency. Luckily with medium wave frequencies trees will have little effect on signal strength, but of course the more wire you can get up in the sky the farther out you will reach. Any reason you are sticking with 1250kHz? 1610-1710kHz would be far more preferable if there are open channels in your area.
The crystal for the transmission is cut for 1250KHz, that is what crystal the transmitter was sold with.
There are open frequencies above 1600KHz in my area.
This is the instructions on changing the frequency from 1250KHz to a higher frequency:
The Digi-Key part number is ECS-P85-A-ND and remember to multiply the frequency times 4 to get the value of the oscillator you want- for example, 1250Khz would be 1250 x 4= 5 Mhz, so you would want the oscillator programmed for 5 mhz. They do this at no additional charge.
So to change the crystal out, and move the transmitter up above 1605KHz that is what I'd have to do.
Let's say for 1650KHz we would take 1650 times 4 equals 6,600, so the oscillator would have to be programmed to 6,600MHz
I have a Realistic TM-152 AM Stereo Receiver, but since it is an older AM radio, the analog tuner dial does not appear to go much higher than 1605KHz, and I really do not like altering older radios with hacks and toggle switches sticking out all over to change the original factory alignment to get it to work in the extended band.
I would have to choose something like 1650KHz or in that general area, again, I do not think my Realistic TM-152 tuner can go that high without some type of mod, then again, perhaps loosing the lower AM broadcast band in the process.
The issue with trees in my area, is most of them sway in the wind, specially in the winter with nor-easters as they are called in the north-eastern USA.
Swaying trees can cause wires to rip loose and snap. Sure I've thought about climbing up trees to stretch a long wire up to the highest point, but when we get a winter storm or tropical storm here, it will whip that wire antenna all around. Right now Post-Tropical Storm Hermine is near the North-East Coastline and the trees are whipping about, even though that storm is never going to hit us directly.
OMG, FM 88.1MHz to 107.9Mhz is so much easier to set up than AM. I think we can all agree on that statement.
I have an AM C-Quam AM Stereo crystal controlled Transmitter which is hard wired for 1250 KHz to which I am considering an out door antenna for.
I looked at the antenna length for 1250 KHz and if I did it correctly it states 786.86 Feet is the total length for the main antenna radiator element.
I was curious what the results would be if I was to take several 10 Foot x 2" PVC pipes glued together and wrapped 786.86 feet of copper wire around the pipe from the ground up to the top of the antenna would this work? I have seen many antenna setups for AM broadcasting to which large coils are used and tuning capacitors to help tune the antenna, but most are tuned up to transmitters using the 1605 KHz to 1705 KHz part of the band which obviously require a MUCH shorter antenna.
So I was considering wrapping the copper wire around the pipe like a cork screw making the coils so they do not touch thus creating a solid rod but lets say a 30 foot pipe with 786.86 feet of copper wire wrapped around it.
For those who understand MW antennas better than I do, what do you think this antenna would do as far as performance goes compared to 10 feet antennas with large copper coils?
Bruce.
Hi Bruce ! You need 187.2 Feet per 'leg' to make a 1/2 wavelength for 1.250 MHz. The formula is: 468/1.250= 1/2 wavelength or 374.4 Feet for 1.250 Mhz, thus dividing that figure in half, you need to cut two identical wires 187.2 Feet for a 1/2 dipole at 1.250 MHz or 1250 KHz. Have fun ! Spooky
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
I have an AM C-Quam AM Stereo crystal controlled Transmitter which is hard wired for 1250 KHz to which I am considering an out door antenna for.
I looked at the antenna length for 1250 KHz and if I did it correctly it states 786.86 Feet is the total length for the main antenna radiator element.
I was curious what the results would be if I was to take several 10 Foot x 2" PVC pipes glued together and wrapped 786.86 feet of copper wire around the pipe from the ground up to the top of the antenna would this work? I have seen many antenna setups for AM broadcasting to which large coils are used and tuning capacitors to help tune the antenna, but most are tuned up to transmitters using the 1605 KHz to 1705 KHz part of the band which obviously require a MUCH shorter antenna.
So I was considering wrapping the copper wire around the pipe like a cork screw making the coils so they do not touch thus creating a solid rod but lets say a 30 foot pipe with 786.86 feet of copper wire wrapped around it.
For those who understand MW antennas better than I do, what do you think this antenna would do as far as performance goes compared to 10 feet antennas with large copper coils?
Bruce. I tend to believe you may fare better with the 1/2 wavelength dipole..
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
Oh I had forgotten about this thread. I changed the crystal to 1620 KHz, I did try a long wire, but like I posted previously, running a long wire in the trees in my area is prone to wind damage, well we had a large storm hit the northeast in October and that ripped the wire apart, stress on the wire as the trees swayed tightened it to the point it snapped.
The antenna was at a length of 187 feet if I'm not mistaken, but of course it was horizontally stretched along a tree to a mast on my house, back into another tree, it reached 1/4 mile, but I still believe vertical is better.
I will work on getting some 2 inch in diameter copper tubing and make a vertical radiator and some-how make a coil to tune it.
I'll try to update this thread, but with it being winter, I'll have to wait a bit before the weather favors such a project.
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