Remember that milliwatts used to light the bulb are milliwatts not producing a signal. They also change the load on the transmitter so if only used in setup and removed things probably have changed. Probably good for experimenting but I would hesitate to use them for constant monitoring.
Neil
Indeed Neil. The setup would constantly be absorbing some of the energy and robbing it from the radiator. It would take quite a bit of experimentation to find a coupling point where a meaningful indication is obtained while trying to prevent too much of the signal being absorbed and turned into heat/light.
A much more fancier and elaborate monitoring system would need to be constructed, one with voltage amplifiers to boost the very loosely coupled signal of forward and reverse so as to provide isolation and enough coupling to obtain a sample signal in both directions, then feeding either a dual meter setup or single meter or set of bulbs. Problem with that approach is the calibration and matching. However with a general range field of the VSWR meter, perhaps a "ball park" range could be built into the meter system with a range adjustment control to compensate for the various setups out there.
Pretty much all VSWR meters have a calibrate control to set the reflected meter. Maybe this concept can work as well with the proposed flea meter here.
RFB
Holy thread drift, Batman! But that is what makes things interesting.
RFB is on the right track. We have two very common types of SWR meter pickups which essentially differentiate between forward power and reverse power. A simple type uses pickup loops in a coax line or stripline. The second uses toroid transformer coupling. Both rely on diodes to produce DC indications to drive a meter.
The two limitations I see for very low power use is the forward voltage drop across the diode and the very small energy available to move the meter indicator. Expanding RFB's comments, amplification most likely is the key here.
There is a rather simple op-amp circuit which simulates a nearly perfect diode producing near zero forward drop with no appreciable reverse leakage current (Many op-amp references have this circuit schematic). I have used this circuit for some instrumentation applications, the most recent being in a high speed (tens of nanoseconds rise time) peak detector. To get this to work at this speed and presumably at RF requires very high bandwidth and slew from the op amp. The amps I used have (Unity Gain) BWs upward of 50 MHz and slews of hundred of volts per microsecond. These are readiably available and not too expensive but I don't have the numbers at hand as I post. If anyone is interested in experimenting with these let me know and I will dig out and post the part numbers.
Edit to Add: The op amps are the AD841, AD843, and AD847 types. The precision diode circuit is shown in the left half of the schematic in Fig. 29 of this data sheet: http://www.datasheetcatalog.org/datasheet/analogdevices/565502ad843.pdf
Edited again to add more: Another approach which might be better (from the circuit stability perspective) and cheaper is to use lower performing op-amps to boost the signals from the pickups and then use conventional diodes and meters to process the amplified signals.
Neil
The loop pickup coax or stripline SWR pickups are easy to build but probably will not give a satisfactory signal at BCB frequencies. This is because the coupling is a combination of magnetic and capacitive effects with the capacitive effects being small at these frequencies. The lowest frequency pickup of this design I have seen operates down to 3.5 MHz but possibly the design would work at BCB frequencies. It could be tried to find out.
The other more complex design with the toroid transformers uses only magnetic and direct voltage taps and would work well at BCB frequencies with the proper toroid material such as type 2.
Just thinking out loud.
Neil
If such a two-tiered device were built, showing forward RF and reflecting RF, and added, say, to an AMT3000, what is it that would get adjusted to optimize the SWR? Selecting taps above or below peak on a loading coil?
Adjust for minimum reflected power.
Neil
If the new part15.us owner is willing to buy an AMT-5000 kit, I suggest that Neil would be about the best man for the job of doing a review. He is a moderator of this site, he has already made an AMT-3000, and he is an EE and a professor. What beter qualifications would you want?
if a rice bulb is used, it will not register actual numbers of any kind, plus, it doesn't show, as mentioned, reflected energy ratios.
With that in mind, I'm still working on what might be called a near-field coupler, with which I could certainly use some expert advice. It will consist of a loop antenna with appropriate diode, cap, and resister (which I will likely eliminate) device which is measured with a digital multimeter.
The digital multimeter I have can measure down to two decimal places of microvolts. I used in in conjunction with a similar device when I tuned my antenna on the boat. The thing is, although they are meaningless units (because the loop antenna, or short whip with a few turns on a wood dowel, won't produce actual broadcast voltages, and are affected by distance and interference ... of course), it can still give you readable numbers, and more useable than an analog meter ... because analog meters are slow to react and readings don't show small numbers, making it difficult to detect peaks.
In the end, the one I used still took several afternoons of work climbing up and down (plus backing away from it to help eliminate body proximity effects) to adjust length, and peaked in the line width of one relatively new Sharpie pen (either side of it, the numbers started going down). It worked quite well at the time, but lately I have perceived some signal degradation ... which could be due to weathering of the antenna and loading system, seaweed buildup on the ground plate. etc.
In fact, if such a device could be reasonably calibrated using a borrowed one-time professional measuring device, such as the FCC uses in the field, and keeping the test parameters equal, it could be used over and over for the same test (except for changes in parts/component wear). xx uV measured @ 1M.
Not a bad idea Ken! But I think that with such elaborate build of a meter to measure these low power levels would take the meter well beyond the scope of hobby and Part 15 budgets. Having such a meter matched up to a professional device no doubt would raise the cost of such a meter, probably beyond most people's budget.
I believe also that such a meter should be kept simple for those who are not vested in translating numbers from a DVM to mW and uV. Using the bulb approach, if doable, provides the basic indicator in both forward and reverse, and very simple to tune by making the forward bulb bright, and the reverse bulb dim. No calculating, no conversions, no need for external metering with test leads dangling causing inductive changes to the antenna system.
The meter should be designed to fit within the enclosure of the antenna system..become a part of it full time. Everyone knows that even the slightest influence by a leaf off a tree or a bird perching on the top hat will shift that antenna's resonance big time. Not to mention the headache of tuning it while avoiding your own body's inductive influence on the thing.
Fun fun fun!! 🙂
RFB
"Let Neil do the review!"
Why not! I have a masters in EE and over 30 yrs in the radio and television broadcast engineering field. And I have the proper test gear to boot. But what I do not have is an open area suitable for an outdoor antenna system and part of the reasons why I run a CC AM station instead of a 219 setup. Too much clutter nearby along with neighboring homes and fences, which no doubt would hamper the testing results.
Neil has my nomination!
RFB
Perhaps using the same technique for measuring audio can be used here to measure VSWR..meaning the combination of Ken's op-amps and say a 5 step or more LED driver chip for the indicator. Have two segments, one LED segment showing forward, the other reverse.
Just tossing ideas.
RFB
The very interesting paper that was linked earlier (On the Topic of SWR - MRAM 1500) said something that impressed me. It said that (if) a reflection comes back to the transmitter and makes a quick u-turn and joins the transmitted RF IN PHASE, it will actually add to the resulting antenna power. I presume this is known as "antenna gain."
That might also be a condition worth achieving.
I appreciate the confidence but I can't commit to such a review project. Though I have access to 90 acres of farm land it is 100 miles away and it wouldn't be feasible to do a range there given the changing environment with crops growing 8 months of the year in the fields.
I tend to agree with RFB's point about getting meaningful numbers which would be useful in predictions about other installations. Maybe a field test comparing several different transmitter types would have some predictive value, but maybe not. Suppose brand A demonstrated 30% better range (however that is determined) than brand B. Would that be useful?
If someone wants to do the field work and take data that is fine and I am available to advise.
Neil
Usually my brain is too cluttered to reach the crisp clarity that I feel is just out there ready to be grabbed. But Radio8Z's note just now gave that slight nudge that might be a ..... well, there, I've already blabbed on until I'm confused again. Maybe I can still say it.
We have certain givens with AM part 15.
The input to the final may not exceed 100mW. If we can obtain that measurement FROM ANY TRANSMITTER, we start on an even playing field.
The next stage is the efficiency of the final stage. If we could calculate that factor for EACH transmitter you would know who was ahead in the race.
Now comes antenna matching, to a tried-and-true 3-meter vertical with ground radials.
The transmitter that did best in all three departments will probably provide the best result.
At a moment like this it seems simple.
Product reviews typically repeat features which are already available from manufacturers' web sites, followed by installation experiences and subjective evaluations of audio and range performance. I have never seen results of audio or range tests comparing two or more different AM transmitters performed with exactly identical installations, so meaningful comparisons of various brands is just not available.
Even exotic, expensive test equipment won't provide meaningful comparison information unless all tests of various transmitters are done under identical, controlled and repeatable conditions.
No transmitter manufacturer has ever published any information about RF output level, which is what any serious Part 15 broadcaster really wants to know when making a purchase decision. The input power is always 100mW, but that doesn't mean much except that it is legal.
I suggest that RF output testing of a transmitter under review be done in a way that is independent of any actual antenna/ground system installation. Instead of an antenna system, the transmitter should be tested with an antenna simulation circuit. In the old days the simulation circuit was called a "dummy load" and was just a 50 ohm resistor, but all the major, serious part 15 transmitters (Rangemaster, Procaster, Talking House, AMT5000) won't work when connected to a load resistor. They must be connected to a short vertical antenna which is effectively a capacitor in series with the ground resistance. The antenna capacitance is the C part of an LC series-tuned resonant circuit along with the L provided by an inductor located inside the transmitter. Likewise, conventional SWR and RF power meters won't work because they assume a 50 ohm near resistive impedance.
I suggest that RF output evaluations be done with the antenna disconnected and replaced by a capacitor and resistor in series between the TX antenna connector and the TX ground connector to simulate the antenna system under controlled conditions. TX output power can be calculated from the RF RMS voltage measured across the resistor. The RF voltage can be measured with a decent scope or RF voltmeter, either of which can be had for a few hundred dollars on Ebay (watch for the ones that say the unit has been tested and is operational).
A "part15.us standard" antenna simulation circuit can be defined by forum consensus (I suggest a 30pF 500V 5% mica capacitor and a 30 ohm 1/4 watt 5% carbon composition resistor).
Also, a "part15.us standard" measurement procedure and calculations could be published for all reviewers to follow. It will be easy to set up and measure two or more different transmitters under the same simple controlled conditions, and there is even the opportunity to get meaningful comparisons of different transmitters by different reviewers because the test conditions are acceptably simple and repeatable.
As an additional bonus, using a scope across the resistor also allows display of the modulated RF waveform to evaluate modulation level as part of the review of the transmitter audio performance..
Phil