I have a question that is somewhat about technical feasability, but probably more about perception.
I have a Radio Systems TR-20 (well, I'll have it once it gets here; just won it on eBay) and I am planning to use it as a "dual mode" portable station. The station will be on board a motorhome, and when commercial electric grid is available I will be operating carrier current, subject to applicable regs. When off-grid, I would operate 100mw with a 3-meter antenna, following those regs.
The TR-20 actually contains two parts - an exciter plus a linear amp. I won't know for sure until I test it (perhaps someone here knows?) but I am guessing the exciter probably puts out about a watt or less. What I hope to do is, when in antenna mode, is to disconnect the linear amp and use just the exciter. The reason is three-fold. First, it will be much easier to achieve and maintain the legal 100mw input power using only the exciter. Second, it should be more energy efficient (important while I am off-grid and relying on solar/battery power). Thirdly, it would hopefully appear to be more appropriate in the eyes of an FCC inspector, in the event I was to be inspected. Much more credible that I am using just a small exciter (even if it is potentially capable of exceeding legal limit) than hoping they will believe I am running a 20-watt rig at 100mw.
My actual questions are this: Has anyone here ever electrically separated the exciter from the linear amp in a TR-20 to use just the exciter? Any tips or suggestions on the technical feasability? It certainly looks very do-able.
Also, In the (hopefully unlikely) event of an FCC inspection, will the inspector be adept enough (and sufficiently motivated) to determine that I am, in fact, operating only the exciter, and throttled down to 100mw at that, or will they just look, see a 20-watt box and declare me to be non-compliant on appearances alone?
According to my schematic for a TR-20 Transmitter the modification you want to make should be simple. Probably some other members will have suggestions to improve doing it.
1. The RF Final Stage receives its power through Fuse F101. Wire a switch in line with the fuse to turn off the final.
2. The Exciter RF Output is found coming from C24, add a two-way switch here to switch the RF away from the Final Amp and into your own antenna matching stage.
3. Controlling the RF Output Power coming from the Exciter can be done with R25, which is the power control for either full power or low power use of the transmitter.
4. Your biggest challenge will be designing a low power output filter to prevent harmonic output from exceeding the rules. There are a lot of postings about building such filters.
5. Transistor Q2 is the RF Ouput of the Exciter and you will want to measure it to be sure that the input to Q2 is 100mW, as controlled by R25.
DONE!
Thanks, Carl. Your comments basically agree with my thoughts.
As for de-powering the finals, I wouldn't open the line at the fuse. Looking at my schematic, that would leave the bias circuit for the finals powered, which may or may not harm anything, but at the least would waste some power. So I'd put teh switch in the +30v line between the main board and the amp.
As for harmonic filtering, I hadn't really thought of that (thanks for reminding me!). It looks like maybe the filter on the amp board could be used if one were to disconnect the secondary of T102 (and maybe C106) and then connect the output of the exciter to the input of the filter at L101/C107.
Power input should be easy enough to determine by measuring the voltage drop across R40 and then doing a bit of math.
I am not familiar with the TR20 but it sounds as if what you propose is feasible. A problem with using the existing filter is that it may not be designed to properly load the exciter but it is worth a try.
If you use a base coil loaded antenna properly tuned to resonance this will provide pretty good filtering by itself but, again, the impedance match to the exciter could be a problem. At 100 mW power levels it is not probable that you will damage anything by experimenting so it is worth a try. Lacking a spectrum analyzer it is difficult to make sure the harmonics are suppressed enough but a check is to listen on the second, third, and fifth multiples of the carrier frequency and you should either barely hear your signal or ideally not at all.
Neil
Yes, Mr. NaturalAZ, switching the +30 where it will also cut the final bias is better than what I suggested.
In the schematic the exciter output is transformed up and loaded by 30-ohms consisting of 2x15-ohm resistors in series with one-another.
Radio8Z's idea of using a loading coil in line with the antenna would be my choice, for probably better matching of so-short an antenna.
I have a Radio Systems Phase II 20 watt, very similar to your transmitter.
The power output adjustment on mine takes it to zero output. I have been able to run it between 0 and 100 milliwatts.
Unfortunately the Part 15.219 rule states "final input power" not output power.
You could get around that by operating Part 15.209 if you could verify the actual field strength.
How about setting the output power around 66 mW to allow for the average inefficiency?
That would put the input to the final somewhere near 100 mW.
Smart? It's my idea. Carl Blare.
It would be easy enough to measure input current to the finals - remove their fuse and insert a milliameter. I guess I can determine how efficient the amp is by comparing input to output power, but I fear it isn't very efficient, and even less so at low power. That's the reason I'd like to bypass the amp and just use the exciter - I'm guessing I'll get more of my 100 milliwatts back out of the exciter than I will out of the linear amp.
And on the suggestion to go with 15.209 emission limits, I really think I'd do better with 15.219 with an efficiently coupled antenna and good ground. Another option, if broadcasting at an event or campground, might be to consider the entire venue grounds as a "campus" and go with 15.221b.
Heck, I have declared my house as "Home School College" and that makes me a campus. The neighbors live in "dormatories" so my campus boundary is miles from here.
A local pirate (decades ago) refered
to the nonexistant campus and students
and vehicles and a student union, a dinning hall, parking lots,
snow problems (during the wintertime)
etc., etc., etc. Yup. Hidden Valley College
Bruce, DOGRADIO
P.S. I had to put this in for the fun of it.
Now back to the TR-20.
'I have declared my house as "Home School College" and that makes me a campus. The neighbors live in "dormatories" [sic] so my campus boundary is miles from here.'
Those who might believe that this approach is legitimate and certain to pass an FCC field inspection might want to review the definitions linked below.
I doubt that a list of such believers would include the author of that clip, but some of its readers may believe that his post contains useful/legal information.
http://www.merriam-webster.com/dictionary/flippant
http://www.merriam-webster.com/dictionary/facetious
Sort of heavy on the artillary coming at a guy with two words when either one of them would have sufficed, and there's an element of truth, but good member Rich has not entirely invalidated the matter for there is a serious grain in my POV about what constitutes a "campus."
The notion of a "campus" is abstract, and through customary use has come to describe real estate dedicated to an educational enterprise but no monopoly is held over intellectual pursuit, and very scholarly individuals maintain libraries and study reference right at home, and if cultural exchanges take place with neighbors and communities the final vocabulary definitions belong to the people who live the experience.
What an FCC agent would make of it depends on his or her depth of intelligence, and there is another guy making sure trash cans are put away by 8 PM on trash day. We also have mosquitos.
Being too constrained in one's world view can lead to problems of regularity.
"Being too constrained in one's world view can lead to problems of regularity."
My final comments on this topic...
Note that a "world view" such as suggested above for unlicensed operators subject to FCC Part 15 that is not properly researched, understood, and followed can lead to FCC problems for such operators.
Your concern for the welfare of Part 15 members in operating on the safe side of the regulations is commendable.
Thank you for your cautions and advisories.
My initial mention of the "campus rule" was actually semi-serious. Referring back to my original message, you'll see the station will be in a motorhome. Which might be parked in a campground. The manufacturer actually marketed this system as "Camp Radio" ( http://www.radiosystems.com/campradio.html ) though admittedly they were thinking of summer camps for kids, and carrier current operation.
Actually I plan to do carrier current when appropriate and I expect it to work quite well in a typical RV park or campground. But when off-grid, I wonder if I have to stick with 15.219 (or radiated field strength) if I could legally get away with calling the campground a "campus" and go with perimeter field strength instead. Or is that not likely to be any real advantage anyway, from a coverage standpoint? I suppose it would depend on the size of the "CAMPus".