Hi I am the one who brought up the rf meter,and SWR.
Not sure where that post went to,but there is a really good meter
not for much money that can read down to 5 mw and show SWR also.
Please look at this site for the meter. It really does come in handy and it is
cheap.
http://www.ohr.com/wattmeter.htm
You guys will really love this meter.
Thanks
Sean
I would find comparisons based on PhilB's suggestion useful and I suspect that the results would show some differences between the units tested. Whether these differences would be significant would remain to be seen. What I mean by this is suppose Brand A produced 80 mW into the simulated 30 ohm load and Brand B produced 70 mW. Whether that would be a deciding factor in choosing one over the other for purchase would be up to the buyer.
This test method would work for transmitters with internal matching networks but would not work for those requiring an external loading coil unless the coil could be laboratory standardized across the testing labs. If the 30 pF capacitor is omitted from the load then these transmitters tested without the loading coil in place could be tested with just the resistor as the load. But then you get the situation where let's say one wanted to compare my AM High Efficiency design with the AMT-3000. Mine does not require the loading coil to drive a 30 ohm load directly but the AMT-3000 does since part of the coil inductance is used in the matching network. See the problem? Comparison between transmitters with internal loading coils and those with external coils would be confounded due to the coil also because of differing external coil losses and comparison may not be meaningful.
Additionally, the scope probe loading would need to be standardized.
Pursuing agreed to standardized testing may be helpful but interpretation of the results will need to be done carefully according to which transmitters are being compared.
Neil
The posting by PhilB suggesting a test method that is simple and expressly relevant to part 15 style transmitters seems to me like it would give equipment reviews from part15.us a special appeal, for being very realistic.
I wish to expand a little bit on the matter of 100mW to the final. PhilB said "The input power is always 100mW but that doesn't mean much except that it is legal." There are several things I have to say about this.
Perhaps the published specs from all manufacturers claim 100mW to the final, but in reality I don't think this holds true. One transmitter has been well reported as having far more than the legal power at the input to the final, and I have heard of transmitters registering less power.
The AMT5000 and the Rangemaster1000 both have adjustments to set the input to the final at an exact 100mW.
Therefore I think the measurement at the input to the final is "#1" on the list of important standard measurements.
I agree with Radio8Z that the several approaches to "loading" antennas at the output of the transmitters need to be reduced to standard and reproducible techniques.
The SWR meter referenced by Seankw40 is very attractive, but go back and read PhilB's explanation of why such meters will not work with part 15 transmitters.
After some minutes of walking in the garden and thinking about the output questions raised by Radio8Z, I have some thoughts on how to proceed.
I think all the part 15 transmitters come ready for an "out of the box" hook-up, typically consisting of adding a 3-meter vertical wire. Therefore, I believe the main body of any review should consist of testing these transmitters, using the PhilB method, in their "out-of-the-box" state.
Then, as an added part of the review, discussion could turn to loading options and some tests with well documented loading methods, i.e., diameters, wire-sizes, # of turns, etc., with results, still feeding the PhilB RC "dummy load".
I agree with PhilB's method of measuring the output power of a Part 15 transmitter on the test bench, using a 30 pF capacitor and a 30 ohm resistor in series to simulate the antenna load. Since he is the manufacturer of both the AMT-3000 and the AMT-5000, he is in the unique position of getting the ball rolling by publishing the results of his own output power tests for his transmitters on his website.
Other manufacturers should publish similar data, preferably using the "standard" dummy antenna suggested by PhilB. There is also nothing to stop individuals who own Part 15 transmitters from testing their transmitters the same way, and then publish the results here, or on any other Part 15 site.
I see an application for the "standard dummy load for part 15 transmitters", and that is, to first optimize a transmitter into the dummy load, then see how far different settings end up being when tuned for the antenna in use at the time.
Besides being simple curiosity, such a test will reveal how far an antenna is from being well matched.
I too agree with Phil's testing suggestions. However I don't agree about limiting it to a bench. The bench is not the real world nor is it the intent of the majority of Part 15 broadcasters to just broadcast to themselves in the house.
Here is where Neil's standardizing the testing comes into play. You cannot test a TX on a bench and call it a day and claim superiority over an outdoor setup. That kind of test is bogus and means absolutely nothing. The TX MUST be tested on an antenna system that is outdoors if we want to have any real world results when doing comparisons.
The lab is a great starting point, and that's it. Even one will find different results from their own testing on their own bench and another test done just outside their shack walls in the yard.
The meter linked. Well that's a fine meter indeed! Too bad it won't really work for this. Bulky and no doubt will cause all kinds of inductive influence on the tuning, it would have to be built without it's cabinet so as to fit inside the outdoor antenna loading coil enclosure. With the exception of TH/iAM/ATU units, no one is going to be running any coax to their outdoor antenna system while claiming 15.219 compliance.
Some good ideas being tossed nonetheless.
RFB
Just build the dummy loading system within the outdoor antenna box. Rest assured, your going to see a different set of results going from the dummy load on the bench to one inside your outdoor antenna system.
Remember the big influence...INDUCTION! Even your test bench dummy load can cause changes to the resonance of the final coupling by nearby objects as it will outside. Your found "sweet spot" will change drastically.
RFB
Neil: I agree that the proposed antenna simulator circuit will not work for a transmitter that has no internal loading coil. In fact, even if there were a "lab standard" external loading coil, there still would not be a meaningful basis for comparing the results to any of the transmitters having internal coils. Each user of an externally tuned transmitter must construct his own loading coil, which may result in better or worse performance that a "lab standard" coil, so an apples-apples comparison to an internally tuned transmitter isn't really possible. For now, I am trying to focus on a way to compare the most popular transmitters, which all include an internal coil. We might be able to conjure up a standard way to test externally tuned transmitters as a secondary effort.
I mentioned using a 10x probe, that would part of the standard. 10x probes have a 10 meg resistance and a fairly low capacitance, typically somewhere around 10 - 20 pF. Since the probe is connected across the 30 ohm resistor, it will have no loading effect and the probe capacitance is swamped out by the low value resistor. I simulated the effect of probe capacitances in this range and found no meaningful affect on the results. A 1X probe is another story altogether. These probes can have 150 pF or higher capacitance. 1X probes would be explicitly excluded.
Carl: you have a good point concerning the one transmitter that has been reported to have an input power higher than 100 mW. That doesn't fit into the plan! I don't know how to handle that. Comparison to compliant transmitters would be invalid unless someone can discover a way to measure its input power and factor that into the measurement results. If a transmitter has less than 100mW input power and it is not adjustable, then the output power measurement is still valid for comparison to other transmitters. It will just measure less output and that is what we want to know in the comparison.
RFB: I agree that just measuring the power output on the bench does not reflect actual performance of a complete system with the antenna and ground. The goal is to provide meaningful information about the transmitter performance when decoupled from the wildly varying performance of individual antenna/ground systems. The idea is to provide meaningful comparison of transmitter performance. Overall performance of the complete transmitter-antenna-ground system will depend on two independent factors: (1) transmitter power out and (2) the effectiveness of the antenna-ground installation.
I think testing on the bench is valid. Part of the standard procedure would include placing the transmitter at least a foot away form any conductive surfaces. This can be done by placing it on a plastic or cardboard box at least a foot high. When the transmitter is mounted at its final location, there will likely be 1 or 2 more pF of stray capacitance due to the mounting hardware, but that shouldn't affect the antenna simulator measurements enough to invalidate the bench test measurements.
Phil
I agree 100% with what you posted, PhilB. So will you publish what the efficiency of the AMT-5000 is on the test bench using your own specified dummy antenna load connected to the output terminals, and the tuning procedure you instruct your customers to use for their assembled kits? I'm sure that this information is on file at SSTRAN.
In response to Phil's statement "Since the probe is connected across the 30 ohm resistor, it will have no loading effect and the probe capacitance is swamped out by the low value resistor." we need to be careful with this.
The best way to arrange the 30 pF and 30 ohm series load is, coming from the tx. output, connect to the capacitor to the resistor and then to the resistor to ground. This makes the "cold" end of the resistor ground so attaching the measuring instrument across the resistor will keep the instrument ground at circuit ground.
But with the probe attached if you take the Thevenin equivalent from the load resistor looking out toward the circuit you will see the 30 pF capacitor and the probe capacitance in series to ground with the common connection providing the source connection for the load resistor. The two capacitors form a voltage divider which reduces the voltage applied to the load R according to the relative values of the capacitors. A 30 pF series cap. with the 15 pF probe cap. will reduce the voltage by a factor of 0.667. So the probe capacitance, though small, is important.
The good news is that my Tektronix 10X probe is stamped that the probe capacitance is 1 pF which, if typical of 10X probes, would cause a much smaller loading effect.
Also, we might consider changing the agreed to load resistance from 30 ohms to 27 or 33 ohms which are standard JEDEC values.
Neil
What about building a sampling port fed by an isolating toroid coupler tapping the output? Let the toroid coupler become the loading while sampling. The toroid has a very low Q and would help provide a high degree of isolation from loading down the output when connecting the scope probe.
RFB
Professor Neil,
The 15 pF probe cap has a capacitive reactance of about 6000 ohms near the upper end of the AM BCB across the 30 ohm resistor, I think that if you redo your calculations, you will find that the probe capacitance has but a small effect on the voltage drop with 30 pF (about 3000 ohms capacitive reactance) in series.
Additional isolation is not necessary. The 15 pF probe capacitance has very little effect on the measrement of the RF voltage across the 30 ohm resistor in the dummy antenna load. A 10X probe will work fine. No FET probe is needed. Once again, I agree with PhilB.
Ermi and Phil are correct. The 15 pF probe cap. will have negligible effect on the circuit proposed. Should have carried my calculations through to the end! (or was the professor just testing the class?)
Hope all is forgiven and my HKN and TBP keys are safe.
Neil