I'm sure Rich could simulate "under ideal condition" type stuff.
My air inductor and 9.8' antenna with the tx grounded to a mast which is grounded to "infinate" ground might be a good place to start - or the am1000 with it's whip antenna and the same ground situation could give "best case scenario" for a perfectly tuned antenna setup.
I'd really be curious to see...
Rich?
Gosh, if we all believed the simulations, there wouldn't be any Part 15 broadcasting, and then how much fun would we have?
Computer simulations that are valid for predicting the performance of MW AM broadcast antenna systems (and they are) do not suddenly fail because the radiator is electrically short, the "ground" is poor, and the available power is low. The equations allow for that.
A computer simulation does not imply that Part 15 AM "can't work." It simply calculates the field strength vs distance from the Part 15 radiator for a given set of conditions, according to equations that originated from carefully measured data taken under controlled, real-world conditions.
People may get enough fun and "coverage" from their Part 15 AM station for it to be completely satisfactory to them. I am not discounting that at all. But that may require listening to a noisy signal on the fringes of the coverage radius -- which is something that a commercial radio station could not tolerate.
I'm sure Rich could simulate "under ideal condition" type stuff. My air inductor and 9.8' antenna with the tx grounded to a mast which is grounded to "infinate" ground might be a good place to start - or the am1000 with it's whip antenna and the same ground situation could give "best case scenario" for a perfectly tuned antenna setup....Rich?
For that I'd need to know the inductive reactance and the DC resistance of the coil used in series with the antenna. I think the Rangemaster AM1000 has a loading coil built in, but I don't know what those values are for it. Maybe some reader can tell me?
The DC resistance of the conductor(s) from the transmitter chassis to real Earth ground potential is also highly important, but also very likely to be unknown. I'd have to estimate that, I think.
My earlier posts in this thread give some insight into the answers to your question, based on a set of rather optimistic conditions that were given there.
For the technically curious, and with credit to Reg Edwards, below the line are his calculated parameters for a 3-meter Part 15 AM vertical and its loading coil, for the conditions shown.
The inductance of the loading coil offsets the capacitance of this electrically short antenna, resulting in the values in bold face black (below). This antenna system impedance would be an acceptable SWR match to most transmitters designed for a 50 ohm load.
Note that even though the antenna is fairly well matched to 50 ohms non-reactive by the loading coil, the DC resistance of that coil has >400 times the radiation resistance of the 3-meter whip. Therefore the coil absorbs the majority of the transmitter power, and much of what's left is lost in the resistance of the ground connection. This results in poor radiation efficiency for the configuration (see the line in red text below).
This should give Part 15 AM-ers some insight into the reasons why the coverage of these systems is limited -- although still capable of giving useful service for the purpose it is authorized.
_______________________
H. Height overall, metres ... 3.20
P. Coil height as percent of Ht ... 5.0
U. Upper antenna dia, mm. ... 6.0
L. Lower antenna dia, mm. ... 6.0
C. Coil length, mm ...... 100
F. Frequency, megahertz ....... 1.00
J. Coil diameter, mm .......... 150.0
G. Ground loss resistance, ohms ... 6.0
Overall antenna height 0.011 wavelengths.
Height of coil midpoint 0.20 metres above antenna base.
Coil inductance 769.03 micro-Henrys
Number of turns on coil 76
Coil wire diameter 0.87 milli-metres = 20 AWG
Self-resonant frequency of coil 2.02 MHz, with self-capacitance.
Coil Q 311 incl effect of self-capacitance.
Radiation resistance 0.05 ohms transformed to feedpoint.
Coil wire loss 20.57 ohms .. .. ..
Antenna conductor loss 0.02 ohms .. .. ..
[b:1c39f5027e]Feedpoint resistance 26.64 ohms
Feedpoint reactance 0.1 ohms [/b:1c39f5027e]
[b:1c39f5027e][color=red:1c39f5027e]Radiating efficiency 0.18 percent.[/color:1c39f5027e][/b:1c39f5027e]
Loss relative to 1/4-wave vertical 26.9 dB when both above same ground.
Efficiency of 1/4-wave vertical 85.9 percent when both above same ground.
//
This should give Part 15 AM-ers some insight
Excellent, Rich!
This is probably one of the most important posts we've had on this site. This is VERY cool. I have given Rich a bit of a rough time, taking him to task on simulations, but this is critcally important, hard data we can really use!!!
I'm glad you kept coming back for more, Rich, because this is gold. I had run some simulations on the WA7CS[/size:ec84f45da3] site, and I kept getting .27 pecent efficiency, which seemed [i:ec84f45da3]way[/i:ec84f45da3] too high, and my sims indcated a Q of over 400 - also unlikely.
Rich's work gives us a much more realistic target for our antenna tuning. I think many of us are probably getting around .09%, and it's very encouraging to see that we might get as much as double that if we really try.
These efficiency percentages seem awful because, of course, they are - that's why the FCC lets us do this 😀 but to have some accurately calculated goals is very helpful.
Also one of the best descriptions of the function of the loading coil I've seen on the 'net - I might steal that for our library 😳
Thanks again Rich - glad you're on board
🙂
I agree.. This info is great! Thanks Rich for your time and effort. Many of us are playing "Midnight Baseball" out here on this stuff and it's a pleasure to see input such as yours..
I have a question on this design. Would the FCC see the center load as part of the radiator and add the coil wire length to the antenna? I have read in other references that a center load would give the best efficiency but not too many folks apply this to a legal Part-15 antenna. I guess the quesion is, Does the loading coil "Radiate" being that it's at the center of the radiator or would it just be considered a "Load"?
Most folks here desire the best antenna but we (I) would like to stay within the legal bounderies. That's what makes it so much fun..
For what I understand, the Rangemaster is an awsome unit and is very efficient. If there was a design that could be applied to other units using the 102" whip theory that would just come close to the same efficiency, would be great! Most of the "kits" that I've been looking at are 50 ohm friendly at the output. This would make the mounting of most units much easier at a higher level in the air. I believe the "Line of Sight" issue helps on range also. Rangemaster uses a torrid(sp) core multi tap ring to match the output for what I see..
Thanks again for your time, Rich. I've approached others that I felt could give some pointers but.. I guess we're not that important in their eyes..
Would the FCC see the center load as part of the radiator and add the coil wire length to the antenna? I have read in other references that a center load would give the best efficiency but not too many folks apply this to a legal Part-15 antenna. I guess the quesion is, Does the loading coil "Radiate" being that it's at the center of the radiator or would it just be considered a "Load"?
Predicting what the FCC might think of this situation is risky. But as shown in my last post, a loading coil adds so much resistance in series with the antenna current that the coil will absorb much of the available power, no matter where it is placed. On the other hand, if a loading coil is [i:c05f5f4791]not[/i:c05f5f4791] used, the SWR of a 3-meter Part 15 AM antenna is very high to a transmitter expecting a 50 ohm non-reactive load, and it can't deliver much power into it. That is the dilemma.
For what I understand, the Rangemaster is an awsome unit and is very efficient. If there was a design that could be applied to other units using the 102" whip theory that would just come close to the same efficiency, would be great! Rangemaster uses a torrid(sp) core multi tap ring to match the output for what I see.
Any inductor that can produce the reactance needed for this short antenna and at the same time GREATLY reduce the DC resistance in the coil (with a good Q) will be an advantage. I don't know the characteristics of the Rangemaster coil, but even if its DC resistance was only one ohm, that is still 20 times higher than the radiation resistance of the antenna, and the coil would absorb >99% of the transmitter power even if ground system losses were zero.
I believe the "Line of Sight" issue helps on range also.
This is a characteristic of antenna designs like a 1/2-wave dipole used at VHF and above, where a surface wave along the Earth does not provide the signal to the receiving antenna. The useful path is by line of sight, only, and the higher the antennas, the better. But this isn't true for MW verticals, whose radiation pattern and gain can only be developed in reference to Earth, and where the usable signal propagates by the surface wave.
//
Just wanted to jump in here and thank RICH for his excellent technical posts. His info is accurate and he has described the Part 15 3-meter antenna very precisely.
All not-so-technical Part 15 enthusiasts should force themselves to pay attention here. There is no single thing you can do to get max range that is more important than installing a well designed antenna (tuned precisely, good high efficiency loading coil) and a really good ground system.
Over time, engineers and experimenters have perfected the nuances of the base-loaded 3-meter antenna system so much that I think they perform far better than the FCC originally expected when they wrote the rule. This is a testimony to good old inginuity.
Let me pop in here with a "What does the FCC say about the coil?" - or, more accurately, "What did the FCC tell Rick", and even more granular "What did one FCC field agent tell Rick?"
The coil can be looked at in 2 different ways: As part of the final (in the case of the AM1000 Rangemaster), or as part of the overall length of the antenna (the case of my Manteca Magnum and the SSTran, and the Antenna Guy antenna). However, the chances of an FCC agent coming for a visit and telling you to bring down your antenna for measurements, is, in a word, unlikely.
Now keep in mind, I was living in a neighborhood with two of these guys, and they could just knock on my door and ask me to make adjustments if there were ever any complaints. I do know that they do not like travelling out of state and having to leave thier families for a few days to go look at a radio. The moods probably vary. In all things, not just radio, use common sense and ask yourself "Is this hitting a grey area? Do I need to rethink this design?"
/end preaching
First, I would like to ask if anyone has ever heard of an actual case where the FCC faulted a part 15er for his antenna/feed/ground lead length being "slightly" over 3 meters? When I say "slightly" I mean inches or maybe a foot and certainly not blatantly longer. I haven't seen any evidence and I have been watching for several years. I may have missed a report somewhere.
I agree 100% with your "use common sense" statement. I'm sure anyone who has seen a few of the common part 15 am antenna arrangements could easily spot a significant violation of length. I assume FCC inspectors would be even more attuned to this.
In the case of the rangemaster, the coil is not visible inside the box, so it is not likely to even be considered as a contributor to the antenna length. The coil is a toroid type which is very good at confining its magnetic field, so, even with the plastic box, it contributes a negligible portion of the overall radiated signal. No-brainer here. Just measure the length of the whip. Interestingly, the whip could be close to 118 in. and still be legal, but a 118 in. whip is not available off-the-shelf.
Because the Manteca Magnum and the SSTran (and other similar types) have the big loading coil visible externally, the common sense approach is to include the coil in the 3 meter limit. This still allows the antenna pipe to be somewhat more than 102".
Even in the case of the external loading coil, very little of the overall signal is radiated by the coil (Rich may want to comment here). It would be pointless for the FCC to try to nitpick on dimensions of parts of the antenna that don't contribute significantly to the radiated signal. In fact, it would be counterproductive to FCC goals to do this. A long, skinny loading coil would require the antenna to be shortened to stay within the 3-meter limit and would not perform as well.
An even more foolproof approach would be to put the big loading coil inside a big grounded metal box along with the TX. Then the coil would radiate nothing and could absolutely not be considered in the 3-meter limit. Then the antenna pipe could be lengthened to nearly 118 in.
We already know the FCC doesn't bother about the "ground lead" length because they have never faulted a user for elevating their antenna above ground level, which implies that the ground path must be via the mast or a wire.
I hate to go non-technical because I feel more comfortable with facts than politics, but the actual bottom line on legality boils down to:
1. don't interfere with licensed stations.
2. don't broadcast offensive or controversial content that may invite complaints.
3. don't get into a p*ss*** match with a local neighbor who may retaliate with an FCC complaint.
If the FCC somehow does find fault with a serious attempt at compliance they will ask you to correct the problem or stop operation. If you do what they say you won't get fined or go to jail or need to hire a lawyer.
Even in the case of the external loading coil, very little of the overall signal is radiated by the coil (Rich may want to comment here).
I did a quick NEC-2 model of a 50-turn, air core, linear coil about 5" high and 4" in diameter to see what its 1.7 MHz free space radiation patterns would be, all by itself. Driving this calculated impedance of 4x10^-14 +j2x10^-6 is a practical impossibility, but this can be ignored because radiation patterns/gains don't depend on the impedance of a radiator. The peak gain of the coil pattern for the vertically-polarized field in the horizontal plane was about -65dBi -- which would add insignificantly to the field produced by a ~ 3-meter radiator, if it was attached. The coil also radiates some horizontally-polarized field, at a peak gain of
-125dBi at an elevation angle of about 40 degrees, and nulling in the horizontal plane (meaningless).
So the bottom line is that the loading coil for a 3-meter Part 15 AM vertical adds virtually nothing by itself to the radiation of the antenna system. Its only purpose is to reduce or cancel the capacitive reactance of the electrically short radiator, which when used with other matching techiques then can produce a reasonably good match to a transmitter expecting a 50 ohm, non-reactive load.
A long, skinny loading coil would require the antenna to be shortened to stay within the 3-meter limit and would not perform as well.
True, but probably it is not all that important. A 1/2" OD, 98" copper pipe has a radiation resistance of about 0.08 ohms at 1.7 MHz. An 88" version has about 0.067 ohms. These two lengths would allow for a loading coil external to the transmitter, or internally -- plus 10 to 12 inches for the interconnect and ground lead.
If in both cases the loading coil resistance is 10 ohms, and the resistance of the path to Earth ground is also 10 ohms, then the radiation efficiency of the longer antenna is 0.398%, and for the shorter one 0.334%. This is a difference of 0.76 dB in radiated power, which converts to about a 6% difference in the radial distance to a given value of field strength. Not a huge benefit, there.
//
Hello,
First of all, i congratulate Rich for giving some advice about the 3-meter antenna.
I have three questions:
1) Since the peak of the wavelength is 5/8 of wavelength, what do you think if i make many turns on pvc form with 400 feet long wire (that is, my antenna would be my coil only on pvc form at the frequency of 1600 kHz)?
2) Does the AM signal propagate more at the end of the AM band than the beginning of the AM band (1600 kHZ vs. 550 kHz) ?
3) How using the NEC apps? For example, i want to simulate results the 3-meter antenna on 1630 kHz and 1460 kHz.
Regards!
Yves
1) Since the peak of the wavelength is 5/8 of wavelength, what do you think if i make many turns on pvc form with 400 feet long wire (that is, my antenna would be my coil only on pvc form at the frequency of 1600 kHz)?
It is true that a 5/8 electrical wavelength vertical radiator has the highest field strength in the horizontal plane, per watt of input power, of all non-sectionalized vertical radiators. But to do that, the entire radiating portion of the antenna must be an electrical 5/8-wave long -- in your case, about 376 feet of vertical, linear radiator. A loading coil used with a much shorter linear radiator is not the electrical equivalent of that, even if the total length of coil wire+radiator is 5/8 wavelength. Radiation from the coil itself is virtually nothing. And of course, Part 15 AM rules do not allow using a linear radiator longer than 3 meters.
2) Does the AM signal propagate more at the end of the AM band than the beginning of the AM band (1600 kHZ vs. 550 kHz) ?
Propagation loss along a given set of ground conditions is less for lower frequencies than for higher ones. But the radiation resistance of a Part 15 AM antenna rises as the frequency increases, which usually improves the radiation efficiency of the antenna system (other things equal). The net result tends to favor using the higher AM frequencies.
3) How using the NEC apps? For example, i want to simulate results the 3-meter antenna on 1630 kHz and 1460 kHz.
Modeling a complete Part 15 AM antenna including the coil and a buried ground system requires NEC-4, which is an expensive commercial program not widely available. A simpler way is to use some of the free programs downloadable from the website shown below, to calculate radiation efficiencies for various Part 15 parameters input by the user.
http://www.btinternet.com/~g4fgq.regp/
///
Hello,
I have here a 3-meter copper pipe with a diameter of 15 mm. I want to broadcast at 1500 kHz. To do so, I have to builld a loading coil of 332 uH (78.3 turns with wire of 29 AWG). And in order to maximize my ground system, i would make a ground system which consists a ring with 16 radials (16 wires of 20 AWG) attached to the ring. These radials will have 3-meter long as my vertical antenna.
Is that correct?
And my second question, will the AM signal be the same if my wire on my loading coil measure 20 AWG (134 turns) instead of 29 AWG ?
For your information, i have a Ramsey AM transmitter (AM25).
Regards!
Yves
I have to builld a loading coil of 332 uH (78.3 turns with wire of 29 AWG). And my second question, will the AM signal be the same if my wire on my loading coil measure 20 AWG (134 turns) instead of 29 AWG ?
Lowering the DC resistance in the coil increases the radiation efficiency of the antenna system, other things equal. The total length of wire in your coils will depend on the coil length and diameter. When you know the wire length you can calculate the total DC resistance of these coils at this website http://www.cirris.com/testing/resistance/wirecalc.html .
And in order to maximize my ground system, i would make a ground system which consists a ring with 16 radials (16 wires of 20 AWG) attached to the ring. These radials will have 3-meter long as my vertical antenna. Is that correct?
Should make a reasonable ground connection, but you may want to increase the wire diameter to make it more rugged when buried.