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Post by Boomer on Apr 18, 2017 12:33:21 GMT
I've found that you really can't tune an AM hobby transmitter like these inside and take it outside and have it still tuned properly or for maximum output.
One difference is the ground, it's going to be all different outside than inside.
The antenna also interacts with everything around it, it's environmentally sensitive, very much so.
You really can only properly peak your power for maximum in the transmitter's final resting place, and with all radials and audio lines attached. I'd re-tune with every change, even adding one radial when you already have 11.
Class E tuning is better, not only more 'E'fficient, but it should have a broader tuning peak, which would make the system less sensitive to change in the environment. That would mean more consistent output power from your station.
Boomer
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Post by jimhenry2000 on Apr 18, 2017 14:59:53 GMT
JumpersThe Jumpers (S1 - S22) in the AMT5000 can be confusing and require careful attention to make sure each one is set right. When some of the jumpers are ON they DISable a function by shorting across it. When other jumpers are ON they ENable a function by making a connection to it. Some of the jumpers must be in place for proper operation. Other jumpers are optional and depend on your choice. S1 should be OFF for transmitters operating in the HIGH BAND. By being OFF it DIS-CONNECTS C2. Read that again Jim Henry. You were concerned about meshing the stator and rotor of C2, but with J1 OFF, C2 isn't even in the circuit! S2 should be OFF for antenna systems employing the built-in Toroid Loading Coil. By jumpering S2 the Internal Toroid Coil is bypassed in favor of an external loading coil; S3 - S11 are Taps to the Toroid Loading Coil. In our KDX transmitter S7 is the tap that places C1 in its proper tuning range. For your transmitter a different tap may be needed; S12 is ON for our transmitter bypassing L2. This is an optional jumper. See manual; S13 is ON for our transmitter bypassing L3. This is an optional jumper. See manual; S14 is OFF for our transmitter bypassing C8. This is an optional jumper. See manual; S15 is ON for our transmitter enabling C7. This is an optional jumper. See manual; S16 & S17 should be OFF except for transmitters on battery power. Important!S18 & 19 is ON for our transmitter disabling L5 & L6. These are optional jumpers. See manual; S20 does not exist. S21 Not Used; S22 is ON for our transmitter enabling NRSC PreEmphasis. This is optional. Setting certain jumpers incorrectly can prevent the transmitter from operating correctly. My next report titled Secrets of the AMT5000 Part 2 will provide a detailed description of parts and hookup for a professional audio line from indoors to outdoors.
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Post by jimhenry2000 on Apr 18, 2017 15:03:12 GMT
Carl, if I'm not mistaken both charts in the manual show reading in volts not amps, even though one of the charts does reference current.
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Post by Deleted on Apr 18, 2017 15:14:49 GMT
Point of Confusion
Jim Henry spots a confusing fact about the difference between "volts" and "volts": "Carl, if I'm not mistaken both charts in the manual show reading in volts not amps, even though one of the charts does reference current."
I know, that can get confusing.
The RF INPUT VOLTAGE Column on Page 39 Section 21 is read in DC Volts (1.50 to 5.45);
The RF INPUT CURRENT Column is also read in DC Volts (0.667 to 0.183.
But for the RF Tuning the meter probes are connected to T2 and T3 RF INPUT CURRENT.
The chart on Page 39 is for adjusting the Input to the Final RF stage to 100 mW, which is not the same as the RF Tuning procedure Page 20 Section 8.
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Post by Deleted on Apr 18, 2017 17:18:04 GMT
Learning Opportunity
The method used in the AMT5000 for calculating the 100 mW settings is based on a bona-fide method that measures DC Voltage at two key points in the circuit, with the 10 Ohm resistor being part of the formula under Ohm's Law. I am not conversant to the point where I know the "Name" or "Description" of this method of power calculation, therefore both Jim Henry and myself will benefit if one of our more learned engineers explains what this calculation is called.
READ - Adjust RF Input Power to 100 mW Page 21
The formula is well explained.
RF input power = RF INPUTVOLTAGE * RF INPUT CURRENT (in DC Volts)/10
The text explains why RF INPUT CURRENT is a Voltage reading and NOT a current reading in Amps or milliAmps.
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Post by Druid Hills Radio on Apr 18, 2017 17:31:24 GMT
Learning OpportunityThe method used in the AMT5000 for calculating the 100 mW settings is based on a bona-fide method that measures DC Voltage at two key points in the circuit, with the 10 Ohm resistor being part of the formula under Ohm's Law. I am not conversant to the point where I know the "Name" or "Description" of this method of power calculation, therefore both Jim Henry and myself will benefit if one of our more learned engineers explains what this calculation is called.READ - Adjust RF Input Power to 100 mW Page 21 The formula is well explained. RF input power = RF INPUTVOLTAGE * RF INPUT CURRENT (in DC Volts)/10 The text explains why RF INPUT CURRENT is a Voltage reading and NOT a current reading in Amps or milliAmps. DHR: "Indirect Method."
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Post by Deleted on Apr 18, 2017 18:22:43 GMT
Excellent!
Druid Hills Radio explained the RF output power formula used in AMT5000 manual:
DHR: "Indirect Method."
A man who paid attention in school.
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Post by jimhenry2000 on Apr 18, 2017 19:37:01 GMT
Boomer, Thanks. I think I already get that. I am tuning the 5000 on my workbench but it is all connected to my antenna, ground rods, and (20) 30 foot radials. This is the only way I have used it so far, however I DO realize that once I put it in the outdoor enclosure, mount it on the antenna, and reduce the coax from 150ft to 6 inches, I know I will have to tune it again. Jim I've found that you really can't tune an AM hobby transmitter like these inside and take it outside and have it still tuned properly or for maximum output. One difference is the ground, it's going to be all different outside than inside. The antenna also interacts with everything around it, it's environmentally sensitive, very much so. You really can only properly peak your power for maximum in the transmitter's final resting place, and with all radials and audio lines attached. I'd re-tune with every change, even adding one radial when you already have 11. Class E tuning is better, not only more 'E'fficient, but it should have a broader tuning peak, which would make the system less sensitive to change in the environment. That would mean more consistent output power from your station. Boomer
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Post by Deleted on Apr 18, 2017 20:14:26 GMT
Oh, That's Right, the Coax
Over the course of the back and forths I lost sight of the coax and wasn't sure it was still in place or not.
Now that we know, I'm guessing that all that coax is swamping the signal with capacitance that is throwing it out of the designed range.
By adding a 20 pF or other small value of capacitor in line with the antenna you might be able to get the tuning back into range.
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Post by jimhenry2000 on Apr 18, 2017 20:19:04 GMT
Carl,et al, What do you folks use for a spectrum analyzer? I used them at Comcast occasionally but they were typically $100,000 boxes. I'm not about to buy one of those. Also same question about SS meters. Thanks. Jim Question To Be AnsweredDHR asks about the tuning results with an AMT5000 Transmitter: " Have you compared the tuning procedure to the actual output power? In other words, when you are at maximum efficiency do you see more power on a signal strength meter?" I have not done this in the past but I will do it today using a spectrum analyzer. At this moment, with the AMT5000 set on "old" settings, and with the spectrum analyzer being 35-feet away with its un-calibrated 13" vertical receiving antenna, the signal at 1680 kHz reads: 44 dBu, a relative reading, not true dBu.
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Post by jimhenry2000 on Apr 18, 2017 20:22:07 GMT
That's easy enough to do but what SHOULD the capacitance of the antenna be and how could I measure it? Oh, That's Right, the CoaxOver the course of the back and forths I lost sight of the coax and wasn't sure it was still in place or not. Now that we know, I'm guessing that all that coax is swamping the signal with capacitance that is throwing it out of the designed range. By adding a 20 pF or other small value of capacitor in line with the antenna you might be able to get the tuning back into range.
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Post by Deleted on Apr 18, 2017 21:36:05 GMT
Questions Answered
Choosing a small capacitor to try in line with the coax and antenna is a trial and error thing... I was not getting a tune and began playing around... I sort of found the 20 pF value by accident.
Previously, however, I measured the capacitance between my Metal Window Antenna and ground and the capacitance meter said 4,000 uF. I don't know of a relationship between the capacitance reading and the 20 pF capacitor.
The spectrum analyzer is an ATTEN AT5005 purchased from eBay for around $550, as recommended by a radio engineer. I have not regretted it and although it is not as full-featured as the super-priced units, it is very handy for Part 15 work.
By the way, there are several TECSUN portable radios that have digital dBu/SN readouts, which are also great for observing signal strength. Mine is the PL-310 ($50.00 eBay).
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Post by Deleted on Apr 19, 2017 18:02:44 GMT
Audio Transmission DataTo Jim Henry... preparing to design a professional audio transmission line to connect indoor computer audio to outdoor transmitter over a distance of 150-feet. I will recommend you purchase one of these: A 1-Channel Distribution Line AmplifierWhen the complete proposal is ready it will show where to order, accompanying power supply, type of cable, and wiring diagram.
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Post by jimhenry2000 on Apr 19, 2017 20:00:02 GMT
Thanks Carl. Audio Transmission DataTo Jim Henry... preparing to design a professional audio transmission line to connect indoor computer audio to outdoor transmitter over a distance of 150-feet. I will recommend you purchase one of these: A 1-Channel Distribution Line AmplifierWhen the complete proposal is ready it will show where to order, accompanying power supply, type of cable, and wiring diagram.
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Post by Deleted on Apr 19, 2017 20:18:47 GMT
Power Transmission TalkFeeding power to the AMT5000 transmitter is made simple by use of the included Wall Wart Power Supply which supplies 12 VAC which becomes rectified by on-board diodes inside the transmitter, based on appropriate jumper settings. A 12 VDC Wall Wart Supply will do the same job by appropriate changing of jumpers inside the transmitter per instructions. Either way, 12 Volts is a "low power" supply, and can be sent over long-wires out to a remotely located transmitter. I strongly suggest that, unless full 120 VAC house-power is needed for some unrelated purpose out at the transmitter site, low power is a safer and smarter option. Comments on this are welcome. The power can be sent on its own dedicated long-wire, or phantom-powered on top of a balanced audio line, so that both audio and power share a single 3-conductor cable. To my knowledge phantom powering requires DC Only and cannot be done with AC power. Phantom Power Explained
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