Antenna Tuner for Part 15 AM Transmitter?

Yes (qualified)

The schematic shown at http://www.mfjenterprises.com/man/pdf/MFJ-956.pdf shows the best circuit to load a part 15 antenna, namely a loading coil with a big series cap for fine tuning. The high end of the band will most likely tune at switch position 4 (330 uH)

The problem is that this is a small antenna tuner intended for receiving applications, so the inductors are physically small. Small, relatively high value inductors like the 330uH are usually wound on ferite cores with very fine wire. The DC resistance will be high (because of the fine wire) and inter-winding capacitance will be high. This adds up to big LOSSES for transmitting. A relatively huge coil (3.5" diameter, #16 wire) will perform dramatically better. I have experimented with both.

In the part 15 AM world nothing comes cheap or easy. We have to deal with crippled power and crippled antenna length. To get the best range, everything must be done to minimize losses.

Just as I suspected, except for the loss you mentioned. BTW what are we talking about loss wise 1,2, 3 db?

Here's the bottom line I have an sstran on the way and plan to go on 1610 in my area as soon as I can get the tx built and operating, however I am still undecided on an antenna so I thought i would start out with the old 70+ ft. sloper I have in the backyard strung to a tree that I was using for a shortwave receiver.

I have a small backyard with a retention pond full of water and whatever washes off the streets in it so a radial wire ground plane may work here?

Looking for sugestions on antennas, want something in kit form or plans for something I can go to home depot and build in an afternoon if possible.

And thanks for that info Phil!

If you'd like to experiment with full size antennas, one place to look might be the 160 meter band (~1800 KHz, right above the AM BCB) antenna experimenter pages. Some of these antennas could be easily adapted to 1600 - 1700 KHz.

Here are some starting points:

http://www.hamuniverse.com/cobraantenna.html

http://users.erols.com/k3mt/inv_u/u_160.htm

http://www.pi4cc.nl/link/bc.htm

http://www.cebik.com/wire/160new.html

http://www.hard-core-dx.com/nordicdx/antenna/wire/ant160m.html

http://www.qsl.net/wb1gfh/antenna.html

http://www.arising.com.au/people/Holland/Ralph/shortvert.htm

http://www.qsl.net/we6w/projects/160_loop.txt

http://www.hard-core-dx.com/nordicdx/antenna/wire/shortdipole.html

http://www.tuc.nrao.edu/~demerson/helixgain/helix.htm

http://www.yccc.org/Articles/double_l.htm

tregonsee is baaack...(numerous groans)...

About Part 15 AM antenna length (assuming you want to be legal):

1. Most Part 15 AM work is done under Section 15.219, which states:

- Maximum 100 mW power input to final RF amplifier

- No emissions below 510 kHz or above 1705 kHz that are less than 20 dB below the level of the unmodulated carrier

(NOTE) Total length of antenna, transmission line, and ground lead (optional) not over 3 meters (10 feet)

2. If you don't want to operate under 15.219, then you can run under the general provisions of Section 15.209. This means your Part 15 AM setup must have a field strength not over (24000/f) uV/m at 30 meters, where f is your operating frequency in kHz.

3. All this meaning what?

- If you want to run under 15.219, then there is no "random length" wire. Antenna plus feedline 10 feet max, period. 70' sloper is out.

- If you want to have a longer antenna, then go get your test-grade, tunable, calibrated field strength meter (capable of "CISPR quasi-peak" detection, by the way), or find someone who has one.

4. I echo PhilB's loss comments if you plan a 15.219 setup. Not as much a concern if you go under 15.209, as there are many ways to get to that (24000/f) field strength.

5. Looking for antennas? Go to the Part15.US Library and check out:

- Part 15 AM antenna construction (Base Loaded 3 Meter Vertical)

- A Tunable Shortened Vertical AM Antenna [a GOOD 3-meter design]

- The MWA Antenna Book [at least Chapter 2 - Shortened Verticals]

Rained on a parade again, didn't I? Sorry, it's what I do. Ask scwis, he knows...

- If you want to run under 15.219, then there is no "random length" wire. Antenna plus feedline 10 feet max, period. 70' sloper is out.

- If you want to have a longer antenna, then go get your test-grade, tunable, calibrated field strength meter (capable of "CISPR quasi-peak" detection, by the way), or find someone who has one.

These two alternate ways of complying are well known (at least to those who it is well known to) 🙂

Since a "calibrated field strength meter (capable of "CISPR quasi-peak" detection" is not readily available in real human budget terms, which of the two compliance specifications will get you the most range?

I suspect that the field strencth version is more restrictive, but I am only guessing. The reason I say this is that I don't think the FCC anticipated the use of a very elaborate 3-meter base-loaded antenna, elevated with an extensive ground. This type of antenna installation probably gives a higher field strength than the alternate limit (again, just guessing).

Anyone have one of those super-duper field strength meters? How does a really good 3-meter antenna installation read on the meter?

"Anyone have one of those super-duper field strength meters? How does a really good 3-meter antenna installation read on the meter?"
__________

A computer simulation based on Numerical Electromagnetics Code to model the 3-meter antenna on 1,000 kHz shows this:

Field Strength --> Distance
2mV/m --> 0.087 miles (very good signal)
0.5mV/m --> 0.333 miles (OK signal)
0.1mV/m --> 1.523 miles (marginal, depending on receiver and local noise level)

This assumes that the transmit antenna is vertical with its base mounted a few inches above Earth, the resistance in a typical connection from the transmitter to Earth ground, a ground conductivity of 8 mS/m (fairly good), and a path over open country (no overhead wires, buildings etc).

This also assumes that a full 100 milliwatts from the transmitter is radiated by the antenna. That is unlikely, though, because the radiation resistance of a 3-meter antenna in the AM broadcast band is very low, and the transmitter cannot drive it efficiently -- even with a loading coil. Also the transmitter output cannot be 100 milliwatts unless the final amplifier is 100% efficient (which is impossible).

So this should be considered as a "best case" scenario, not to say that the station couldn't be detected at greater distances.

Richard Fry, Broadcast Engineer

Hi Rich..

I understand the concept that you discuss above but would it be possible for a unit that is say.. 60' in the air get more coverage? At 10' off of the ground plane doesn't give the antenna much room to develop any pattern. I have a platform that is 20' high. I built a coil to load a section of mast pipe 9' long and I am getting at least 3/4mi. now with a poor grounding system. The signal is weak at the edges but still can be understood. I believe if I work on the grounding I'll get more clear range..

Is there any way to include some radiation elevation in that computer program?

Is there any way to include some radiation elevation in that computer program?

_____________

Yes. A model of a 3-meter vertical with a loading coil having 6 ohms of DC resistance inserted in series with its base input, mounted at the top of a 20 foot high aluminum cylinder with an OD of 100 millimeters has essentially the same radiation pattern and gain as a 3-meter vertical with its base near Earth. Therefore for equal applied power and other conditions the same, field strength from both configurations also will be essentially the same.

This assumes that a short "ground lead" from the transmitter is connected to the top of the aluminum cylinder supporting the 3-meter radiator, and a typical amount of DC resistance in the connection of the bottom of the cylinder to Earth ground.

The conductive path from the chassis connection of the transmitter to Earth ground at the bottom of the cylinder becomes one of the radiating elements of the antenna. However this doesn't much affect the intrinsic gain of the antenna, because that differs very little between one electrically short radiator and another.

The only practical way to increase radiation efficiency is to use a radiator of sufficient electrical length, say at least 60 degrees (164 feet at 1,000 kHz). This increases the radiation resistance of the antenna, and increases the amount of power that the antenna can radiate versus the amount that is lost in the ground system.

A loading coil increases the electrical length of a short radiator to some extent, but in doing that it adds DC resistance in series with the antenna current, which greatly reduces the efficiency of the antenna system in converting the applied power into useful radio signals.

Would the coil resistance value make much difference? I have a coil with 85 turns on a 4" form. I checked the resistance from end to end and the reading is less than 2 ohms.(1.7) I'm using 18ga. insulated solid copper wire. At the tap where I loaded an element, the reading was a little less.(1.2)
Thanks for your input..

Would the coil resistance value make much difference?

That's probably measuring DC resistance, which is not the critical factor in the influence of the coil on the efficiency of the radiator.

Two more important factors would be RF impedence, and Q factor. The RF impedence of a matching coil is difficult to measure with common workbench equipment but measuring it isn't that important, because the coil is necessary. The impedence is more of a given than a controllable factor.

The Q factor is determined, in part, by the diameter or the wire, the spacing of the windings and the length to diameter ratio of the coil form. There are some pretty good computer simulators to help predict that here, WA7CS RF Calculator Page, but the general approach outlined in the various articles on Part15.us come pretty close

Speaking of computer simulations,

A computer simulation based on Numerical Electromagnetics Code

The NEC application, and it's sister MININEC, have been used for years for HAM and other antenna application modeling. Visitors interested in learning more should visit the NEC home page at http://www.nec2.org/ .

Like all simulations, there are fairly straightforward limits on the nature of the designs it can test. I would be very interested in the NEC developer's opinion of using that application to simulate a 3 meter antenna for a 200 meter wavelength 😀 😀 😀

Like many computer simulations, NEC is also the source of endless theoretical discussions that have little applicability in highly specific, real-world applications. Remember the old story of the computer simulation that proved Bumble Bees can't fly? 🙄

It has also been observed that Part 15 AM broadcast reception is more influenced by groundwave and near field radiation than the far field radiation the NEC apps analyze, which would seem to limit the applicability of the simulation in real world scenarios.

Field Strength --> Distance
2mV/m --> 0.087 miles (very good signal)
0.5mV/m --> 0.333 miles (OK signal)
0.1mV/m --> 1.523 miles (marginal, depending on receiver and local noise level)

WOW! That's pretty encouraging if NEC shows far field radiation of that strength. When ground wave and near field are added, and a good Q factor coil is used for matching, a pretty satisfying range should be attainable!

Would the coil resistance value make much difference? That's probably measuring DC resistance, which is not the critical factor in the influence of the coil on the efficiency of the radiator.

This takes a rather complex answer. The real purpose of the loading coil is to cancel all or most of the capacitive reactance of this very short radiator. The resistive term (eg, the radiation resistance) of the base input impedance of a 3-meter vertical radiator on 1,000 kHz is less than 0.1 ohms, and its (capacitive) reactance term is several thousand ohms. Even if the transmitter can drive very low values of load resistance, the high reactance in this short antenna will produce high load SWR to the transmitter, and the transmitter cannot deliver its rated power into such a load.

The reactance term needs to be zero before the transmitter can see a perfect match. That can be done by a suitable loading coil, but such a coil also can add several ohms of DC resistance in series with the antenna current. As in any series circuit, the larger resistance of the loading coil (several ohms) will dissipate much more of the available power than will the radiation resistance of the short antenna (<0.1 ohm).

It has also been observed that Part 15 AM broadcast reception is more influenced by groundwave and near field radiation than the far field radiation the NEC apps analyze, which would seem to limit the applicability of the simulation in real world scenarios.

The far-field boundary for a 3-meter vertical on 1,000 kHz is well within 50 feet, so this really is not a factor for distant signals in this application. In any case, my analysis first used NEC-2 to generate the radiation pattern shape and peak gain of this Part 15 radiator when the "ground terminal" of the transmitter is connected to a perfectly conducting Earth plane through a typical DC resistance. I used 5 ohms, which is no doubt optimistic for Part 15 stations. [AM broadcast station antennas with 120 radials each 1/4-wave long typically have around 1 or 2 ohms of DC resistance to real Earth.]

The NEC-2 program also outputs field strength at 1 mile for 1 kW of input power over this perfect Earth. This corresponds to the FCC's "efficiency" value for MW broadcast verticals vs height, which was based on measured data from 1937 (George Brown, et al).

Then I used the efficiency of the Part 15 radiator with the FCC's propagation curves for the frequency and a ground conductivity to determine the field strengths and distances you repeated below, for 100 milliwatts of power in the radiator.

So by this process the calculated values are as "real world" as possible without actually duplicating the assumed conditions in real hardware, and making the measurements with a real field strength meter.

Field Strength --> Distance
2mV/m --> 0.087 miles (very good signal)
0.5mV/m --> 0.333 miles (OK signal)
0.1mV/m --> 1.523 miles (marginal, depending on receiver and local noise level)

WOW! That's pretty encouraging if NEC shows far field radiation of that strength. When ground wave and near field are added, and a good Q factor coil is used for matching, a pretty satisfying range should be attainable!

My calculation already gives the end result, but it will be optimistic because the radiator will not have the full 100 milliwatts in it, and losses in the ground system of a Part 15 AM probably are not as low as my assumption for it. In fact I have a paper from Carl Smith showing about 95% system losses for a 3-meter vertical on 1,000 kHz using a ground consisting of 30 radials each of 0.15 wavelengths. This ground system is no doubt better than has been installed at any Part 15 AM station.
//

So by this process the calculated values are as "real world" as possible without actually duplicating the assumed conditions in real hardware, and making the measurements with a real field strength meter.

Yep, that's always the problem with simulations and papers - never can quite get to the real world

If I ever want to broadcast to a simulated audience, tho, this will be useful 😆 😆 😆

Rained on a parade again, didn't I? Sorry, it's what I do. Ask scwis, he knows...

Hey, tregonsee. sometimes rain is very refereshing
😉

So by this process the calculated values are as "real world" as possible without actually duplicating the assumed conditions in real hardware, and making the measurements with a real field strength meter.

Yep, that's always the problem with simulations and papers - never can quite get to the real world.

If I ever want to broadcast to a simulated audience, tho, this will be useful

BUT - the computer models for determining field strength vs distance that I used [b:b14d472587]are[/b:b14d472587] based on measured data. Their accuracy has been proven by thousands of field measurements made on directional and non-directional AM broadcast station antenna systems going back some 60 years, and should not be dismissed so easily. They are, in fact, real world.
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BUT - the computer models for determining field strength vs distance that I used are based on measured data. Their accuracy has been proven by thousands of field measurements made on directional and non-directional AM broadcast station antenna systems going back some 60 years, and should not be dismissed so easily. They are, in fact, real world.

It will certainly be interesting when some simulations that are relevant to Part 15 broadcasting are developed, although it's such an unusual combination of constraints that I doubt it really can be simulated.

[color=red:e347b7d2fc]OOPS! I was wrong here - Rich came up with a great simulation for part 15 - be sure to see it on the [/color:e347b7d2fc][/size:e347b7d2fc][u:e347b7d2fc]next page[/size:e347b7d2fc][/u:e347b7d2fc]

Too many unknowns and 'gotchas' to really simulate easily. Shoving all that wave length into a short antenna is sure a perversion of reality, simulated or otherwise. 😉

Thank goodness for good old empirical experience, and the contributions of those who do real experiments in the real world 😛

Gosh, if we all believed the simulations, there wouldn't be any Part 15 broadcasting, and then how much fun would we have?