The brain adapts to low-power life. We have actually found an advantage to maintaining a less efficient transmitter site, in addition to the regular “more efficient” site.
The brain adapts to low-power life. We have actually found an advantage to maintaining a less efficient transmitter site, in addition to the regular “more efficient” site.
The “efficient” site is 1670 AM, which is “more” efficient precisely because it is at a higher frequency. But the DISADVANTAGE is when I try to make improvements in antenna performance. It’s necessary to go two blocks away to check for the result.
Aha! A second transmitter operates at 1550kHz and the efficiency is PERFECTLY LESS, so that the normal reach of the signal fades out within the confines of the yard’s borders. Therefore when we tinker with antenna ideas the result can be tested within convenient bounds.
IF antenna improvements are discovered using the weaker system, those improvements can be added to the “better” system.
Smart? Nobel Prize?
Weak Signal
Carl,
Interesting proposal. Weak signals are commonly used to optimize receiving systems since overload and saturation need to be avoided while doing adjustments and I can see an advantage to what you describe for the transmitting system for this reason.
If you are near the transmitter you are receiving the near field. Does optimization of the near field result in optimization of the far field?
Neil
Near and far fields
The border between the near and far fields is at wavelength/Pi. The antenna circuit (in which the displacement current from the antenna returns to the antenna) extends to wavelength/2. Beyond a half wavelength, the electromagnetic waves propagate, and there is not much return current to the antenna.
If you are closer than a half wavelength from the antenna, you are part of the antenna circuit. You should stay more than a half wavelength away when making measurements. This is why it is so difficult to adjust a transmitter that is attached to the antenna. You interfere with the antenna circuit more the closer you get to the antenna.
The wavelength is longer for lower frequencies, which means that you need to be farther away to make measurements at lower frequencies. A half wavelength at 1670 kHz is 285 feet and at 1550 kHz it is 318 feet.
The antenna circuit is linear, which means that the performance does not depend upon the power level. A weaker signal gets buried in the noise at a shorter distance from the antenna. This actually makes the measurements worse, and not better.
RE: Near and far fields
At these extremely low power levels with a short antenna, the overall application of propagation performance theory seems to fail. IOW, real-world relative measurable differences between tuning methods are minimal; there are so few cuttings in the harvest, perfect separation of wheat from chaff isn’t very practical.
I’ve found that adjustments for resonance can be made if your body proximity is only more than about 3′ from the antenna, nothing like a half wavelength, which is impractical for antenna adjustments of 1-man homemade gear in almost any case. As it was, it took 11 back-breaking hours over three days running up and down the ladder to find peak resonance. But it has held for more than 6 months.
With my Talking House and home brew ATU, at less than 1/4″ difference in antenna length, the signal jumped from something less than 300 ft. to over 1/2 mile, depending on direction, which has to do with valley vs. town obstructions … some things in town can obstruct or interfere, while heading out the valley side allows it to go a long ways … some days it can be heard (though noisy) as far as 3 mi. Though as you know, it’s grounded to the sea.
What I’m saying is just the ‘ground truth’ of it. With these severely low levels and restrictions, I’d think it would take state-of-the-art sensitivity to detect differences between a reasonably practical tuning method and one where the person making adjustments takes themselves completely out of the near field to avoid proximity effect. Plus, any remote gear used to make adjustments from outside the near field will also affect the antenna circuit, and at these weak levels may even worsen the problem … in addition to increased noise interfering with measurements as already mentioned, from outside the near field.
Having said that, if there was a more practical way to guarantee better performance, I’d do it right now. Everything we do involves compromise, so we can only try to guide the process in a way that benefits what we believe our goals to be from wherever we stand in overall ability, i.e., knowledge, skill, available tools, etc. IOW, growth notwithstanding, we all fall somewhere between limitations and possibilities.
Anyways … that’s my story and I’m stickin’ to it! 😉
Just Throw Money
I can certainly vouch for how changing the radiator 1/4″ will drastically affect your signal.
I have to check our City TIS station performance regularly. It operates at 1650 kHz at about 8 watts. With the start of wet weather the SWR on the antenna can go from 1:1 up to 3:1 or more. If the conditions are to remain wet for several days I retune the antenna to compensate. The antenna is tuned by adjusting the lenght of the radiator and 1/4″ usually does it. When things dry out I retune again.
There are automatic tuners available but they start at a couple hundred bucks. MFJ and others have units that work just above the BCB at 1.8 mHz for Amateur Radio installations. I would think a little tweaking would allow one of those to work in the upper AM band. As the antenna performance changes due to conditions, the auto-tuner will retune for maximum performance.
Still
Ermi Roos, I have been pacing back and then sometimes forth thinking about your tutorial on the near and far fields, and here is where I am right at the moment.
As I indicated, at 1670 kHz the signal goes about 2-blocks, without regard to which field is being received out at the boundary.
But at 1550kHz, granted from a different transmitter and located 75′ feet from the other transmitter, the signal only covers the immediate lot. The downhill slope gets a decent signal, but, as usual, the uphill slope loses reception entirely.
Also, at 1550kHz there is more noise because over four distant stations jumble together in an almost hypnotic crowd sound.
But the antennas are identical at both locations and when I find a way of improving the 1550 signal it is easy to detect out at the yard’s edge. When that happens, I can transplant the improvement to the 1670 transmitter and go out 2-blocks to see whether maybe I’m reaching 3-blocks.
Since this way of doing it seems to work, I’m not sure what it matters to consider the near/far field aspect.
Important to Remember
MRAM
you make an important point, one which of course I already knew, but which is so easily forgotten… that weather conditions affect the performance of outdoor facilities, and probably indoor facilities as well.
Right now we’re in a dry spell and performance is different than when the ground is wet or even walls of the house become moist. Maybe hot and cold cause differences.
Perfection comes before a change.
Tuning vs. Antenna/Ground Configuration
Tuning a transmitter and antenna to peak resonance is very important and should be the primary goal when setting up a transmitter. Resonance is very sharp so tuning must be done carefully.
Proper tuning will maximize signal strength regardless of the distance from the antenna, near field or far field. You can tune your antenna with a multimeter or an FS meter and be confident of best tuning, provided, as stated previously in this thread, you move your hands and body away at least 3 ft. before evaluating the meter reading. I’m sure everyone has seen the effect of body capacitance near the antenna. Just watch the meter, start with your hand touching the antenna and move your hand away slowly. You will see the reading change up or down until you are far enough away (3 ft. or so) where the meter reading stabilizes.
The same applies to meter leads. Place the meter as far below and away from the antenna as possible. Make sure the meter leads are dressed downward from the antenna. An FS meter should be located at the farthest practical distance from the antenna while still being able to read it during tuning. If it is up close to the antenna, the tuning will change when you take the FS meter away. A cheap, insensitive FS meter should be avoided. You probably won’t get a meaningful reading unless it is very close to the antenna, too close to avoid proximity affecting the antenna capacitance and therefore the tuning.
A workable passive FS meter should utilize a meter device rated at 100 uA or less full-scale. These meter devices are hard to find these days, but I did recently see that newark.com stocks some suitable analog meters for a decent price. An amplified FS is the way to go. Even a cheap one will be sensitive enough for locating at least 10 ft. away.
Whether you use a multimeter connected to the transmitter or an FS meter, the tuning peak will be indicated by maximum reading on the meter.
ALWAYS be certain you tune the transmitter to a PEAK reading. Select the best coil tap and adjust the capacitance for peak. Just because one coil tap is better than another does not always mean peak. You must adjust the capacitance through a range that rises to peak and then falls again, then go back to peak. If your best coil tap is at one extreme or the other and you can’t see a rise AND fall with capacitance adjustment, then you are not at the best possible tuning point.
As for near field vs. far field measurements, I don’t think that should be a concern. If you want to evaluate effectiveness of changing antenna structural elements like antenna diameters, top hats, and ground radials, just pick a fixed point well away from the antenna and ground radials and be sure to use a measurement method that is accurate enough to show small differences. A receiver with and S meter is a good choice. It is equivalent to a sensitive relative field strength meter.
Maybe Ermi can comment on this: I don’t know of any way that a part 15 short vertical antenna structure can be changed such that the near field increases while the far field decreases, or vice versa. This has a direct bearing on the original post here. I’m limiting this to a part 15 transmitter/antenna in which the transmitter is approximately ground level with a 3-meter antenna directly above the transmitter. If undesirable nearby conductive surfaces are present (overhead wires, proximity to a building with wiring, etc.) they become part of the antenna in the near field, but keep these constant. Only changes to the antenna structure are allowed. We need to change only one variable: the antenna structure, which includes antenna diameter, top hat, and ground radials. I think a field strength measurement at a fixed, fairly nearby point will indicate a positive improvement for both near and far fields with a higher reading. So, a drive out to the fringe isn’t necessary for evaluating an antenna structure change.
Tuning vs. Near/Far Field
Tuning a transmitter and antenna to peak resonance is very important and should be the primary goal when setting up a transmitter. Resonance is very sharp so tuning must be done carefully.
Proper tuning will maximize signal strength regardless of the distance from the antenna, near field or far field. You can tune your antenna with a multimeter or an FS meter and be confident of best tuning, provided, as stated previously in this thread, you move your hands and body away at least 3 ft. before evaluating the meter reading. I’m sure everyone has seen the effect of body capacitance near the antenna. Just watch the meter, start with your hand touching the antenna and move your hand away slowly. You will see the reading change up or down until you are far enough away (3 ft. or so) where the meter reading stabilizes.
The same applies to meter leads. Place the meter as far below and away from the antenna as possible. Make sure the meter leads are dressed downward from the antenna. An FS meter should be located at the farthest practical distance from the antenna while still being able to read it during tuning. If it is up close to the antenna, the tuning will change when you take the FS meter away. A cheap, insensitive FS meter should be avoided. You probably won’t get a meaningful reading unless it is very close to the antenna, too close to avoid proximity affecting the antenna capacitance and therefore the tuning.
A workable passive FS meter should utilize a meter movement rated at 100 uA or less full-scale. These are hard to find these days, but I did recently see that newark.com stocks some suitable analog meters for a decent price. An amplified FS is the way to go. Even a cheap one will be sensitive enough for locating at least 10 ft. away.
Whether you use a multimeter connected to the transmitter or an FS meter, the tuning peak will be indicated by maximum reading on the meter.
ALWAYS be certain you tune the transmitter to a PEAK reading. Select the best coil tap and adjust the capacitance for peak. Just because one coil tap is better than another does not always mean peak. You must adjust the capacitance through a range that rises to peak and then falls again, then go back to peak. If your best coil tap is at one extreme or the other and you can’t see a rise AND fall with capacitance adjustment, then you are not at the best possible tuning point.
As for near field vs. far field measurements, I don’t think that should be a concern. If you want to evaluate effectiveness of changing antenna structural elements like antenna diameters, top hats, and ground radials, just pick a fixed point well away from the antenna and ground radials and be sure to use a measurement method that is accurate enough to show small differences. A receiver with and S meter is a good choice. It is equivalent to a sensitive relative field strength meter.
Maybe Ermi can comment on this: I don’t know of any way that a part 15 short vertical antenna structure can be changed such that the near field increases while the far field decreases, or vice versa. This has a direct bearing on the original post here. I’m limiting this to a part 15 transmitter/antenna in which the transmitter is approximately ground level with a 3-meter antenna directly above the transmitter. If undesirable nearby conductive surfaces are present (overhead wires, proximity to a building with wiring, etc.) they become part of the antenna in the near field, but keep these constant. Only changes to the antenna structure are allowed. We need to change only one variable: the antenna structure, which includes antenna diameter, top hat, and ground radials. I think a field strength measurement at a fixed, fairly nearby point will indicate a positive improvement for both near and far fields with a higher reading. So, a drive out to the fringe isn’t necessary for evaluating an antenna structure change.