Here's another link to a transmitter - hopefully it doesn't duplicate the others here (I really haven't gone over any of them) http://www.indianapolis.net/QRPp-I/talking_pixie2.html
Even though this is for the 20 meter band, it should be relatively easy to modify for the 22m Part 15 frequencies. Other interesting designs can be found by just searching for 'qrpp' - you'll get a lot of hits for amateur radio micro power transmitters and transceivers.
Thanks for that link, ArtisanRadio, that's a significant update from the old MWA item - the proto board pics are great!
In the document linked in the Talking Pixie post above the output filter is shown in the XMTR schematic and the component values are supposed to be shown using the fliter page link. That link is dead but even if it wasn't it's no help - I checked it on the Internet Archive and none of the values specified were for 13.5x anyway. John Smick suggests a better design and calculated the proper values, shown below.
I've found almost all the parts to build the "Talking Pixie 2" 13.56 MHz, 22-meter transmitter at jameco.com
The closest crystal they have is 13.5168 MHz, but proper AM requires a definite 13.560 MHz.
Can anyone suggest a source for this exact crystal?
13.560 MHz crystal from PCS Electronics
UPDATE - here is another
13.5168 MHZ XTAL
from Alltronics.
IIRC, we can operate from 13.553 MHz to 13.567 MHz, so 13.560 and 13.5168 are right in there
Carl,
SCWIS's posted source is worth pursuing but keep in mind the tolerance needed. Sometimes crystals used for computer applications have a wider tolerance than would work for communications and I don't know which they are offering.
To stay in band you need a 13.560 MHz crystal with at least at tolerance of +/- 0.05 percent or better. Considering that this would put you at the band edge I would go for +/- 0.005 percent. Inquire before you buy. Also, the frequency of the crystal depends on whether it is operated in series or parallel mode. Mfgs. of communication crystals usually need to know the load capacitance and operating mode to get you the correct crystal. If it were I, I would buy one from the source posted and try it. The price is not hateful and it could very well work just fine. Just be advised that it may not.
Another thought is that crystals can be "netted" with a padding capacitor. From my experience, a 13.560 MHz crystal should be able to "net" up or down about 500 Hz or so with a variable capacitor.
If you run into problems getting some other hard to find RF type parts let me know. I have a pretty well stocked junk box and am willing to share with you.
Neil
Dear scwis
THANK YOU for the tip on finding a crystal. Now every part is located.
Onward!
Thank you Neil for your comments of 10/6.
I've ordered the 13.56 MHz crystal from PCS without knowing its characteristics, because it's the only crystal I've found for this frequency and at $7 it's worth taking a chance. If you can recommend other crystals suppliers I'd be grateful.
Once the transmitter either works or doesn't work I'll begin looking for ways to refine it.
I'll remember that you have a collection of parts, as do I from the years I operated a recording service. The irony of life has it that whatever part one needs is always the part one doesn't have.
Since my two shortwave radios have tuning dials, I've ordered a radio with digital tuning so I can actually see what frequency I'm on. It's the Sangean ATS505P available from C.Crane and Universal. It's got LW 153 - 279 KHz, SW 1711 - 29.999KHz, and AM / FM.
All the parts and the crystal for the 13.56 MHz shortwave transmitter project are in transit, hopefully to arrive in time for weekend building.
Looking ahead, there will need to be an antenna of some kind to get that signal up into the air. What is known about that? At the low Part 15 power level is it realistic to beam toward the ionosphere for a mini-skywave? Should it be angled or just plain vertical or horizontal? Is grounding important at 22-meters?
This website, part15.us, has been so informative there ought to be a scholarly degree offered, with a fancy certificate for framing.
I would start my experiments with a horizontal half-wave dipole for an antenna, simply because it is relatively easy to construct. A vertical antenna, like a quarter-wave monopole with a radial ground system, would be a lot more difficult to set up. An electrically-short monopole with a poor ground system is OK, because the power can be increased until the field strength limit is met; but it is best to use a standard antenna for which the approximate relationship between power input and the field strength is known (unless one has a legal field strength meter, which isn't likely).
At a half mile, the ground wave at 13.56 MHz is attenuated by 20 to 30 dB compared to the upper end of the AM BCB. This diminishes some of the advantage that a vertical antenna has over a horizontal antenna at 13.56 MHz. A horizontal antenna produces hardly any ground wave at all, and communication would be mostly by the direct ray. Because of that, it would be best to get the horizontal antenna as much above ground level as possible.
The horizontal dipole is not omnidirectional, so the broadside should be facing the desired direction of the transmission.
Either a horizontal or vertical antenna will send power to the ionosphere, but it is unlikely that there will be enough power to operate skip, unless you are using CW. Working skip would be an interesting experiment to perform, however.
According to Section 15.225 of the Rules, the allowed field strength is 15,848 (not 10,000) uV/m @ 30 m. I don't know how the FCC came up with this peculiar number for a limit.
For some time, I had wanted to determine if there is any possibility of using skip within the limits of the Section 15.225 rules. I thought that using CW was the only possibility, but I couldn't make a definite statement. A vital piece of information I was missing was the power loss due to an ionospheric reflection. I just learned from "V.T." that ionospheric reflection loss in the vicinity of the 20 m band varies considerably; but it is at least 6 dB, and could be 20 dB. He thought that 10 dB is typical. This bit of vital information is missing from all of the references I have about radio propagation. I also asked what the minimum skip distance is, and what the height of the ionosphere is corresponding to this skip distance. He told me that the minimum skip distance is about 500 km, and the corresponding height of the ionosphere is about 300 km. The total propagation distance, in this case, is 781 km. The field strength along this path varies as the inverse of distance.
According to Jasik (1961), the urban electrical noise level at 13.56 MHz over a 6 kHz bandwidth is slightly less tha 5 uV/m. The atmospheric noise in rural areas is quite low--about 0.1 uV/m, but hardly anybody is far enough away from urban life to enjoy such low noise. I am sure that the urban noise level is much more representative of what almost always happens. I am assuming a 10 dB signal-to-noise ratio, which is about the minimum needed for AM reception. I am assuming 6 dB ionospheric reflection loss, which is minimum.
With the urban noise level, the field strength needed for the transmitter to work skip is nearly 50 times what is allowed by Section 15.225. Depending on the type of antenna used, about a 5 watt or a 10 watt transmitter is needed. The use of CW, in which a 100 Hz RF bandwidth is used, and the signal-to-noise ratio is 0 db, instead of 10 dB, still does not allow working skip. Using a 6 dB Yagi antenna at the receiver input would just allow getting over the threshold where skip is possible.
In the unlikely event that the receiver environment is remote from urban electrical noise, skip is possible using AM.
In the calculations for this post, the field strength of the signal is assumed to be in the direction to the ionosphere. If a field strength meter is used, however, the field strength is measured in the horizontal direction. Because of the lack of a ground wave, the field strength in the horizontal direction may be much less than in the direction of the ionosphere. Antenna radiation patterns are not of much help in this case, because they show zero field strength in the horizontal direction. Of course, the true field stength in the horizontal direction is non-zero, but it is unknown just what it is. If there is a big difference between the field stength in the upward direction, compared to the field strength in the horizontal direction, an appreciable amount of power directed in the upward direction may give only a modest field strength reading in the horizontal direction.
A crystal for 13.56 MHz is on order from a part15.us sponsor, but the wait is now into a month with no word.
Meanwhile it would be great to learn another source for crystals. I've scanned shortwave related sights with no success.
I'm hoping here that someone can suggest a good source.
Based on specifications described in this thread on Oct 6 by Neil radio 8z I'm describing the desired crystal as +/- 0.005 % , parallel capacitance 320 pF
Realizing that the circuit is The Talking Pixie2, linked elsewhere.
Located a possible source 1cmfg.com
Hi Carl,
In the past I have purchased crystals made to order from ICM and JAN linked below:
In the good ole days one could order directly from them in unit quantities. I note that ICM says they supply Radio Shack so that could be interesting to you.
Also, 320 pF seems to be awfully high for a crystal load capacitance. You might want to check this. Maybe it is 32 pF instead?
Neil
Carl,
I just viewed the circuit for the pixie2 and I see where you got 320 pF (add C1 and C2) which is not the way to go. First, they're in series, not parallel. Second, the feedback from the emitter to the base will affect the capacitance seen by the crystal. Third, there is a thing called the Miller capacitance seen from the base to ground which depends on the transistor and the circuit gain.
It looks to me that this circuit is operating the crystal in parallel mode if that helps.
Too bad the author of the "pixie" didn't give crystal specs. If it wasn't for the very tight frequency requirements probably any junk box crystal would work.
I am not trying to dissuade you from proceeding by making things too complicated...just want to point out some the the potholes.
I usually had success designing and building my own crystal oscillators for ham work but frequency tolerance was very much less of an issue than here. When I wore my professional hat, I would not try to design my own but would buy oscillator modules ready to go. That way, I knew the crystal was matched to the circuit and my design efforts could be used for other things. At the time the cost of a module was about 150% the cost of the crystal (about $25 in 1980).
I think you should call either ICM or JAN and run this by them. Maybe they could look at the schematic and make a recommendation.
Neil