Part 15 AM has special problems that more conventional transmitters do not have. Use of a 50 ohm load for bench testing is common with conventional transmitters, but Part 15 AM does not usually have a defined output inpedance, and the actual load impedance is generally unknown. There isn't much that can be done about that fact, and there is no alternative to trying to live with it.
It should be possible to tune up any Part 15 transmitter using PhilB's dummy load. Any number of of other dummy loads can also be speicified alternatively. The efficiency measurement will be different for different dummy loads because the tuning circuit loss resistance is part of the total transmitter load. A lower series resistor in the dummy load tends to give a lower efficiency measurement. This is why it is convenient to settle on some "standard" dummy load to have uniformity in testing.
By all means, other infornation should also be included in product reviews. FIM data would be great, but are rarely available if not provided for free (ha!) by FCC agents. Reports of range are good, but don't really tell a lot about the transmitter. They are something like reception reports in amateur radio, which are not very useful for evaluating the transmitter (or the antenna system).
It's highly possible I will be back after this statement with "Post Concluding Remarks", but this is my position at this stage of the discussion on doing Part 15 Equipment Reviews for the website.
Wholeheartedly YES, these reviews should be undertaken and Jon Paul Janze has invited members to decide how to accomplish this.
Without hesitation PhilB's contribution should form the basis for one review approach and these results can be permanently filed with our transmitter manual.
Also, RFB has a valid and scientific position that reviews of in-field performance are well worth some effort and YES, such reviews would make a very fascinating companion for comparison with the lab-bench reviews.
Thus far we have only discussed part 15 transmitters, but what about antennas? Not just home-brews, but the several commercially available antennas such as the oft mentioned Isotron and others.
Ermi Roos has spoken of FIM tests, and indeed Mr. Janze spoke of acquiring the necessary test gear to enable serious review testing. We have not yet informed him of the test gear needed for this purpose.
The final and probably most important step in moving forward has been avoided except for small mentions, and that is actual reviewers. Persons who are prepared to undertake this creative project. The entire idea could die into oblivion without real review people.
"It should be possible to tune up any Part 15 transmitter using PhilB's dummy load."
I think so as well. What I would be wondering is if the simulated antenna/load seen by various design configurations may be outside of the DUT's peak performance curve vs that performance curve being obtainable through an actual 3 meter setup.
The fact that we work with such low power doesn't give us much window range in tolerance.
And..what if this simulated antenna/load actually makes a DUT's peak performance go "over the line"..ie actually causes the TX under test to present a higher peak than it would feeding the antenna system it was designed to push?
FM is not the only band of transmitters where if you attach a different antenna/load, that could decrease or increase the TX's performance.
RFB
Hi Guys
I propose we keep it simple. Bench Test are good. Now what we want to know
is what transmitter and antenna perform the best.
Rangemaster and procaster both have the antenna'a on the transmitter.
Would it not be easy to set up both these transmitters in the same location
under the same conditions . Then take field strengths.
Really which ever one is putting out the strongest field strength is
the one performing the best. Also getting more signal into the air to
be picked up.
The SSTRAN 5000 could be tested in the same way.
This get rid of the bench test and get to real outdoor testing under real
conditions.
one just needs a very good field strength meter that give real readings
in microvolts.
This way each transmitter can be compared.
Please tell me if I am wrong about this, but when we were in the field
testing large transmitters we would peak the tunners up for the
strongest field strength.
Thanks
SKW40
"when we were in the field testing large transmitters we would peak the tunners up for the strongest field strength."
That is normally how it's done. One cannot get field strength readings via a simulated antenna/load system like that from an actual antenna system meant to radiate..be it a wire or loaded whip/rod.
IMO, the bench testing needs to test these transmitters with their intended radiator and not simulated loads.
RFB
IMO, the bench testing needs to test these transmitters with their intended radiator and not simulated loads.
RFB,
A desirable feature of a comparative test would be for it to be "lab independent", that is to say be reproducible anywhere. Do you have some suggestions on how this could be done using intended radiators? This is not a challenge of the idea, but rather a quest for how we can establish some test criteria.
One of the challenges to getting meaningful data on a budget is how to assess the field strength (or range) of the devices. I suggest one possibility is to tap into an inexpensive AM receiver to breakout a signal which would represent the received signal strength. Since accurate calibration would be unlikely, a way around this is to move the receiver until a given indication(s) is(are) read and measure the distance from the transmitter/radiator. This removes the requirement for instrument calibration. The same can be done for succeeding transmitters/radiators and the ranges for the given indication(s) can be compared.
Neil
Neil Radio8Z said
"I suggest one possibility is to tap into an inexpensive AM receiver to breakout a signal which would represent the received signal strength."
DO YOU MEAN "Add a signal level meter" to an AM radio?"
DO YOU MEAN "Add a signal level meter" to an AM radio?"
Yes. My rationale for connecting an external meter is based on my experience here with two AM radios which have S meters. The resolution of these meters is not high enough to accomplish what I have in mind.
Basically, there are three things to consider in a measuring instrument: Accuracy, Resolution, and Repeatability. A breakout signal strength voltage read on a DVM could provide resolution and repeatability though this would need to be determined.
There are radios which have a built in signal strength meter such as suggested by a friend "As a suggestion, the DSP-based Tecsun PL-310 and PL-380 include a display of
relative r-f signal level within the receiver." Not being personally familiar with these I am only passing along the suggestion.
Neil
My intentional repetition considers a laboratory whose mission is to study how to build a laboratory. The second laboratory would be for part 15 AM radio, the first one for experimentally building a better second one.
I have never been more sane in my life.
While looking forward to adding a meter to a portable radio, or more simply, finding a portable with a meter for "rough sketch" signal testing, here is a radio-oriented set of tests I did today.
The Alinco DJ-X3 handheld communications radio needed dusting and its battery's shelf-life was "off the shelf," so I started scanning.
On a long sheet of paper I wrote down every signal found from 1705kHz to up above 15mHz, and tried to account for each signal.
Most of them were from the nearby 13.560mHz dipole for KDX SW.
Then I compared the same signals on the Sangeon ATS 505, and the performance of the two radios is quite different.
Also operating was KDX CC at 670kHz and AM 1680.
Oh, and on FM the Alinco only hears KDX 107.1 FM out to 3-feet. But that's perfect, because it allows REALLY noticing any antenna improvements at the transmitter.
Other FM radios receive 107.1 out to 50-feet or better.
Make sure your stray radiation isn't leaving home.
But I am not an engineer.
The numbers that are displayed
on the screen are meaningless
to me. There are two parameters
expressed on the screen.
The first parameter is dbu. It's range
is 00 to 63. I don't know what it
means, but I think I heard that
250 uV per meter at 3 meters (the FM
field strength limit) is the same
as about 48 dBu. That is the extent
of my knowledge, because I am not as
educated as you guys are. So I could
be completely wrong about that.
Now, by itself, that one parameter might
be useful, but there is another parameter
on the screen right next to it. It is
simply marked dB S/N. This parameter
ranges from 00 to 25. So this apparently
is measuring the signal to noise ratio
above SOMETHING. But what? Is it
signal to noise above the dBu figure
right next to it? I don't know.
So, if you look at the two together,
the lowest reading is 00 00. And the
highest reading is 63 25. Although
the two values appear to be linked to
each other, they both change rapidly. So
it is hard to tell what is going on.
I'll go back to my Hammarlund HQ-140X,
with its wonderful analog meter. Although
it would be hard carrying the Hammarlund
around the yard trying to measure a Part 15
transmitter.
By the way, the PL-380 is a wonderful
portable, but it lacks an external antenna
jack, and is not shielded in any way.
I will try to do some research on why the
PL-380 is set up this way.
Bruce, DOGGRADIO STUDIO 2
In reading literature about the
Tecsun PL-380, I found that I
was wrong about the first
parameter. It is not dBu.
The letter "u" is really the
Greek letter "mu" which is a
whole different animal.
It's too early in the morning
for me to fully understand what
this means. I am not completely
awake yet. However, you guys will
understand, I'm sure.
I wish I had your knowledge, but right
now I'll just settle for more coffee.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
dBu is related to the voltage at the antenna terminals in microvolts (uV), but not the electric field intensity, uV/m. To convert uV/m to uV, it is necessary to multiply the field intensity by a factor related to the structure of the receiving antenna called the "effective height," which is usually not known.
dBu is a compressed representation of the voltage at the antenna terminals in uV, which is calculated by (Voltage in dBu) = 20 log(voltage in uV).
"u" does indeed represent the Greek letter "mu," which is an abbreviation for "micro," which means one millionth.
So, the big question is - would this
PL-380 radio be of any use to the
people who want to take measurements
of Part 15 signals?
I don't know. I can do some experiments
with mine and see if the readout is really
of any use.
Bruce, DOGGRADIO STUDIO 2
If you have a scientific calculator, and you want to convert 250 uV (not 250 uV/m) to dBu, first type "250," then press "log," and then multiply by 20.
To convert 48 dBu to uV, type "48," then divide by 20, and press "10 to the exponent x," which is the inverse of the "log" function.