Sifting PhilB's lessons through my "stuck in my ways" brain, I realized how I've been missing the point about RF, and returned to the task of matching the AMT5000 into my home-brew window frame antenna, intended to be a compromise between an indoor and outdoor antenna.
Using the guidelines carefully described in the Manual and on entries here at part.15.us, the multi-meter inched the way to 378mW into the window array!
Before I correct for proper output power I'm going to tidy up and take a little drive because now I can see how different things are at 1550kHz compared to the AMT3000, which, really, was doing alright... just maybe shy a few feet in all directions.
I've already gotten double my money's worth and am still thrilled over this new transmitter.
With the badly located 3-element cage monopole window-frame antenna the AMT3000 at full sun of day goes about 150-feet.
Now with the AMT5000 matched into the same antenna, but testing one hour after sundown with a Mexican station blasting in, the signal goes about 150-feet.
To make a fair comparison the test must be repeated in full sun tomorrow.
And remember, with this skimped and improvised atypical antenna nothing at all is really certain.
i sure hope i get more than that. i was expecting to at least cover the apartment building im in maybe the one across the lot too. i'm counting more on a carrier current effect then through the air
kc8gpd,
You'll get a lot better than my ridiculous test with my window frame antenna. Everything is wrong with my antenna. In addition to being metal mounted in a wood and stucco wall, the whole surrounding wall contains stucco metallic mesh. It is against all common sense, but I went for cheap and quick.
By using a standard 3-meter antenna and good ground you'll get better.
I posted my results hoping it would be obvious I was experimenting in silly land. No one should consider it a scientific test.
The transmitter is a real breakthrough for Part 15, better than what we had before.
"the signal goes about 150-feet."
Given the "window frame" antenna setup...150 feet is not bad.
I suggest taking the 10 foot long white wire and wrapping that around an extension cord of same length or a bit longer and then plug that extension cord into an outlet. Tune up and run that AMT5K as a CC system...see how well it performs that way.
Don't forget to run the black ground wire to a decent ground like a cold water pipe.
RFB
Here is a link to a graph that shows how the RF INPUT CURRENT varies as capacitor C1 is tuned through coil/antenna resonance.
This graph shows the peak RF INPUT CURRENT ar the point where C1=2.4pF. The best Class E efficiency point is near the center where C1=2.9 pF. Overall efficiency is down 15-20% at the peak current point. In this plot the efficiency will be within 1% of maximum over a range of C1 settings from 2.7 pF to 3.4 pF.
The actual position of the S plot will be shifted left or right depending on the actual antenna capacitance and the Toroid tap selected.
You will be able to tune successfully if the peak current point is anywhere in the C1 adjustment range AND you can rotate C1 clockwise 1 full turn without hitting the bottom stop.
If your antenna capacitance is lower than 25 pF (shorter than the recommended antenna lengths), you will need to set the Toroid tap to a higher inductance value. If your antenna capacitance is higher than 30 pF (longer or fatter antenna), then you will need to set the Toroid tap to a lower inductance value.
People who already have an AMT 3000 with the outdoor tapped loading coil and antenna installed, and have also purchased the AMT 5000, have the unique opporunity of comparing the AMT 5000 to the AMT 3000, as RFB already suggested. Whether the comparison can be done or not depends upon if the AMT 5000 is compatible with said AMT 3000 loading coil and antenna. Maybe SSTRAN can comment on the compatibility.
If the AMT 5000 is what it is advertised to be, it should have significantly greater range than the AMT 3000.
For me, this sort of comparison is crucial to my own decision to buy the AMT 5000, or not. If there is no significant range increase reported, I say "why bother?"
The AMT5000 Manual addresses some of your question, Ermi Roos. By jumpering S2 the onboard loading coil is bypassed and an external coil can be connected.
I do not yet have a proper antenna with loading coil and radials, so I can't contribute a decent comparison, but someone will, I expect.
I want to praise one design feature that is extra handy.... the test probe sockets in the lower right of the circuit board make it super handy to make measurements.
Mention has been made, in the AMT5000 Manual, and in notes in this string, that the simple 118" white antenna wire as supplied has a capacitance of 25 to 30pF, and that the coil windings are based on that general capacitance being present on the antenna.
I got to wondering about capacitance and hooked my capacitance meter probes between ground and each of several sample antennas. I started with the 118" white wire antenna, and the meter orbited between 26pF and 214pF. By selecting "Min." the meter locked on the minimum number (26pF), and selecting "Max" it locked on the higher number (214pF).
The lower number is exactly what the directions specify, but why the fluctuations up and down? Is that caused by RF energy being sensed from the air? (the antenna is not connected to a transmitter)
"why the fluctuations up and down?"
The wire capacitance is being varied by inductive reaction to the surrounding environment. This could be nearby signals, close proximity objects, the orientation of the wire to ground.
Usually if your cap meter is set too high (sensitive), it will indicate these variables to the extreme. It is like turning up the gain of an amplifier, which a cap meter basically is an amplifier calibrated to indicate a reading corresponding to capacitance. Same is true with a meter measuring resistance or voltage. There are basic designs with less sensitivity and others that have a wide sensitivity. This is why some measurements call for a good DVM, those have an increased sensitivity compared to the basic types such as simple analog meters.
If your conducting measurements of this kind within the same room as the "window" antenna and its emitting RF energy, along with any other signals being generated, nearby lights and ballasts, that measurement will be influenced by these external sources acting upon that wire.
The effect can be observed when tuning a 3 meter antenna setup. As you move closer to the antenna, it is affected by inductive capacitance from your body, and the resonance will change a bit, and change again as you move away. That is why some of those all in one box transmitter units recommend a non-conductive, non-inductive tuning tool. Same is with aligning a sensitive receiver front end or transmitter intermediate stages with a non-conductive and non-inductive tuning tool.
Any wire, be it in a coil form or straight line, is subject to external influences and thus will react and measure accordingly.
Take a regular capacitor and connect it to your meter. Set to measure. Then take your fingers and "pinch", or grab the capacitor with two fingers and note how the meter changes its reading, perhaps even change as you get closer to the capacitor. The more sensitive the meter is, the greater the influence rage will be and in turn would be affected at a greater distance from an external influence.
RFB
"I do not yet have a proper antenna with loading coil and radials, so I can't contribute a decent comparison"
Perhaps not an actual outdoor 3 meter setup, but you can perform a comparison with your existing antenna arrangement and make notes on any differences between the 3k and 5k units into that same antenna system.
Think of it as a unique system designed and constructed to KDX specifications. You never know...there may be other Part 15'ers out there working with similar situations.
RFB
Looking back to your (RFB) much appreciated comments on the nature of the art of capacitive measurement, can I safely assume that the lowest Farad reading of the otherwise varying numbers is most likely the true capacitance?
I made the measurements I wrote about after all my transmitters were turned off, but there are ballasts and so on plus maybe roving spirits in the room.
Yesterday I got so involved that I re-built my whole back-room window antenna, so any comparisons will have to start from the beginning, with the AMT3000 first, etc. And today we have rain, so not much will happen.
This entire adventure is great for Part 15 radio, and products like PhilB's and of course this website and the wonderful members keep this hobby/profession alive and growing. As a promoter of personal radio stations I see one stubborn obstacle that discourages some people, namely the antenna.
People I've attempted to "convert" to Part 15 have balked at even having an inconspicuous white (or any color) wire discretely up the wall, and the other big attitude which keeps some people from adopting AM technology is a deep rooted preference for FM. I tend to align with promoting Part 15 AM, leaving the FMers on their own.
As I see it, there are three meaningful categories of Part 15 AM transmission antennas: the "Ultimate," as described by PhilB and the true objective of the most serious operators, namely outdoor vertical with radials, but this requires locating equipment outside; the indoor antenna for the most simple approach; and my vision of an "indoor/outdoor" antenna, with my window-frame example serving as a way of accomplishing the goal of reaching the yard and inner-sanctum while keeping the transmitter inside, albeit not perfect.
This track is about the AMT5000 and only incidentally the antenna question, so I have opened a separate track titled "Indoor Antennas" with my aim of better formalizing whatever methods we can invent of integrating an indoor antenna with home decor.
I wonder if the "one turn clockwise" business is still applicable when using an external loading coil and antenna.
I'm putting in my guess that the answer might be yes, that the same rule would apply to reach and then tune just past that peak-point.
"that the same rule would apply to reach and then tune just past that peak-point."
Is C1 still in the RF path to the antenna port when the internal toroid is bypassed?
I ask because it would seem that the circuit of C1 and the toroid is specific to their own purpose...using the internal matching network to a basic length of wire at the antenna port, and both C1 and the toroid are removed from the RF path when bypassing this internal matching network, passing on the matching task to the external loading coil/antenna system.
The TH transmitters have both internal matching circuits for the basic antenna wire setup, which has that fancy motorized ferrite bar coil system that automatically peaks the tuning for the wire, and in external "range extender" antenna mode, bypasses the entire internal matching system, passing on the task to the range extender outdoor antenna system.
RFB