Simple Part 15 Antennas

[RadioMan203]

Obstructions will always be a detriment to the signal, especially as the allowable power of Part 15.

While there will be some difference of opinion of height, especially regarding how a metal mounting structure may conceivably be considered ground or not by some, I still think height is the best advantage possible. I've seen Part 15 units mounted on top of institutional buildings and pass field inspections so I think it's more of how clean the operation is and how cooperative you are should you have an inspection from a field agent.

 

[rfry]

While there will be some difference of opinion of height, especially regarding how a metal mounting structure may conceivably be considered ground or not by some,

Radiation from the conductor(s) attached to a Part 15 AM transmitter is not a matter of opinion, but a fact of physics -- even if some users and manufacturers of Part 15 AM transmitters define such conductors as being "ground." But if such conductors were ground, they would not radiate (that is one definition of an r-f ground).

The link below leads to a study about this using NEC (Numerical Electromagnetics Code). NEC can accurately calculate the radiation properties of structures, and is used in the broadcast and other industries to design antenna systems.

It is clear to see that from this study that the length of the conductor(s) leading from an elevated Part 15 AM transmitter to a true r-f ground (buried in the earth) produces significantly higher fields than one mounted at earth level, other things equal.

These higher fields may be desirable for some Part 15 AM users, but they also may attract FCC attention and action.

http://rfry.org/Software Download/Elevated Part 15 AM Antennas.pdf

//

 

[RadioMan203]

Radiation from the conductor(s) attached to a Part 15 AM transmitter is not a matter of opinion, but a fact of physics -- even if some users and manufacturers of Part 15 AM transmitters define such conductors as being "ground." But if such conductors were ground, they would not radiate (that is one definition of an r-f ground).

I wasn't disputing the fact of whether or not a true ground would radiate or not, only the fact that an elevated Part 15 transmitter can indeed achieve better range when elevated. I would think a proper RF choke placed between the transmitter's ground connection and tower/monopole mounting structure (if that's the chosen ground point) would satisfy the most strict interpretation of a "rule abiding" ground made in a field inspection.

 

[rfry]

I wasn't disputing the fact of whether or not a true ground would radiate or not, only the fact that an elevated Part 15 transmitter can indeed achieve better range when elevated.

A Part 15 AM setup elevated above the earth, and having no conductors attached to it except a 3-meter whip would not have better range for the fact that it was elevated. This can be shown with NEC.

It is not the elevation of the 3-meter section above the earth that gives the signal improvement. It is the added radiation from the conductors leading away from transmitter -- the "safety ground," power and program lines. If all of these conductors are de-coupled for r-f so that they did not radiate, then there would be no benefit to installing a Part 15 AM system at any elevation above the earth.

MW AM monopole antennas are unbalanced, need an r-f ground and use the surface, or ground wave.

FM/TV antennas are balanced, do not need an r-f ground, and use the space wave. So these VHF/UHF systems will benefit when the antennas at both ends of the path are elevated above the earth.

There may be some confusion between the characteristics of the radiators and the propagation modes for these two applications.

//

 

[Rattan]

"A Part 15 AM setup elevated above the earth, and having no conductors attached to it except a 3-meter whip would not have better range for the fact that it was elevated. This can be shown with NEC."

NEC may be an excellent emulation, but I would have to wonder if it takes certain variables into account such as residential terrain, flora, buildings made of diverse materials, and etc. I do not argue NEC shows no advantage to elevation for a 3 meter whip, but what sort of environment is actually being emulated? If it isn't programmed with the hundreds of variables such as trees and buildings in the near field of the antenna, then I would question if it can be accurate for predicting a precise picture of the radiation from the antenna in a given spot in the real world.

While NEC or any emulator might be very good at predicting behavior on a flat antenna field, few part15 stations are built on perfectly level and featureless areas. In residential areas, even soil conductivity could vary, considering one direction may be mostly pavement/blacktop while another could be largely trees and yet another direction might be mostly buildings.

I am not saying that those features would be likely to *help* a part15 AM signal. I would hypothesize that in all but a few random instances they would be detrimental to one degree or another. Elevating an antenna could minimize those losses somewhat, even though it would not be as much of a benefit as would be seen in getting an FM transmitter a few wavelengths above the surface of the earth.

In my own experiments, I've noted that the receiver being uphill vs downhill in relation to the transmitter consistently makes a difference, and the "downhill" range is nearly always better. Interestingly, one of the *worst* results I've gotten with the experiments was with the transmitter and antenna at ground level. Even in the basement, where the antenna and transmitter were both entirely below ground level gave a better range. In all my experiments at this stage, there is no ground wire or etc in use. Battery power for the transmitter and the small FM receiver used as an STL, and the antenna rig has a nonconductive base and the whole rig can be picked up and moved around and set on the ground, in the basement, in different rooms, on accessible parts of the roof, etc. The best results so far with the experimental rig have been with the antenna in the attic, which would be 3rd floor elevation.

But even moving the test apparatus as little as 5 ft to one side or another sometimes has resulted in the signal being clear for an additional 10-20 meters in a some directions.

I am not saying that NEC isn't a fine emulator. I've never used such software, but I've gained enough respect for Rich's knowlege over the past couple years to be confident that he wouldn't be using it if it wasn't one of the best emulators. But *any* emulation has it's limits and in residential installations of very low powered antenna/transmitter systems, I would question whether it takes enough variables into account for it's conclusion that there can be *no* advantage in different elevations or lateral positions for the placement of a transmitter/antenna system.

Daniel

 

[rfry]

NEC may be an excellent emulation, but I would have to wonder if it takes certain variables into account such as residential terrain, flora, buildings made of diverse materials, and etc.

NEC models the radiation launched by a structure when it is driven by an r-f source. No version of NEC accounts for buildings and other structures/objects in the propagation environment unless they are included in the model that is constructed in NEC.

Ground conductivity over the useful coverage areas of most Part 15 AM setups hardly varies at all, so that has little affect on groundwave field variations in any direction from the antenna.

In my own experiments, I've noted that the receiver being uphill vs downhill in relation to the transmitter consistently makes a difference, and the "downhill" range is nearly always better. Interestingly, one of the *worst* results I've gotten with the experiments was with the transmitter and antenna at ground level. Even in the basement, where the antenna and transmitter were both entirely below ground level gave a better range. In all my experiments at this stage, there is no ground wire or etc in use.

The elevation pattern of a vertical monopole has maximum response in the horizontal plane, and reduces to zero directly above the antenna. This changing relative field could account for some or all of your experience.

A 3-meter, base-fed, MW whip has very high reactance even when driven against a good r-f ground. With no ground it will be much higher still, and more susceptible to change as the antenna is moved to different locations.

It would be difficult to provide a good and stable impedance match between the transmitter and the antenna system under these conditions, so the power actually radiated by the antenna could change significantly as it moved around (vertically and horizontally). This will affect the conclusions made about the performance of the system at those locations.

I would question whether it (NEC) takes enough variables into account for it's conclusion that there can be *no* advantage in different elevations or lateral positions for the placement of a transmitter/antenna system.

The horizontal location of the transmitter/antenna system will have some affect on the net fields received in various compass directions, based on the propagation environment.

The prime advantage of increasing the height of the transmitter and 3-m antenna system above the earth is the added radiation produced by the r-f ground, power and audio wires connected to it. That can increase the radiated field by 15 dB or more (other things equal). That 15 dB improvement is greater than the obstruction loss produced in a typical area of "urban clutter." So it is not the height of the 3-m section that produces the benefit -- it is related to the greater fields that the elevated system (including its attached wires) can produce.

//

 

[Tom Wells]

Another consideration for low power is that many of us who would be pt 15 AM operators have other radio interests and other antennas
around which will grab some of radiated power and do different things depending on spacing and relationship to the pt 15 radiator.

I have typically grounded unused antennas, and have several folded longwire and sloper dipoles, etc.
Grounding/ungrounding them makes a good deal of difference in directionality, as currents aid/oppose radiation and
nearby "wiring" tries to become a director/reflector to the radiating element even at such a small percentage of wavelength.
This of couse is why guy wiring for AMs must be made of shorter lengths broken up with insulators.
I know such current if fed back to the common point from a ground radial system aids radiation.
I'm seeing that elevated wires being grounded seem to influence the pattern.
Also seems like whether or not the earth ground is directly bonded to house electrical ground/metal water piping makes a difference.


I ungrounded one of my AM vertical indoor wire antennas and found coverage to the west greatly improved.

There was a decided minima thataway before.

I haven't checked to the east yet, maybe I lost coverage that way.

I made a few experiments many years ago with the transmitter/antenna in the attic, and found I did not do as well
as with the transmitter in the basement, and the antenna at ground level.
I too had hoped to "get above" the clutter.

 

[Hamilton]

Hello all!
Rich e-mailed me directly, I think he wanted my input into the discussion. I have been out of the country.

Rich,
It seems to me that you are just speculating, and presenting the speculation as dogmatic fact here. You haven’t ever done any actual Part 15 field testing relating to this speculation the last time we talked, I don’t think you even had ever owned a Part 15 except for a basic house only coverage system.

This is a complex situation due to the low frequency and the polarization, I don’t believe you can make such blanket statements accurately without have done some field testing.

I think in the field any radiation from the “safety ground” would be limited by the reactance in the long wire (if used), each situation being different. It has been my experience in the field that any radiation from the “safety ground” is not a large component of overall field strength, and that any radiation from the “safety ground can be controlled by how the ground system is done.

Generally I think one ground rod in the dirt is a poor RF ground, but is fine for a “safety ground” for lightning protection.

The use of a proper “safety ground” is critical for lightning protection, to prevent damage to life and property.

Rich, it seems you have been on this soapbox for years now.

I plan to do some additional testing this summer and will let all know the results I get.

I am guessing that while on a test range with no obstructions, that the elevation of the transmitter would have less of an effect on overall field strength then in the practical installations, due to obstructions, reflections, terrain, ect. SO, I do think that height for Part 15 systems is important.

 

[rfry]

I think in the field any radiation from the “safety ground” would be limited by the reactance in the long wire (if used), each situation being different. It has been my experience in the field that any radiation from the “safety ground” is not a large component of overall field strength, and that any radiation from the “safety ground" can be controlled by how the ground system is done.

Keith, the capacitive reactance seen by the transmitter between the input to a 3-m whip and the top of a long "safety ground" is offset by the inductive reactance of the loading coil used to resonate the antenna system. A resonant system has zero reactance, which certainly you as a circuit designer must already know. Therefore your claim that radiation from the safety ground would be limited by the reactance in the long wire is scientifically inaccurate.

Using a long conductor to reach an r-f ground from an elevated Part 15 AM system increases the radiation resistance of the antenna system, which means that the radiation efficiency* of the antenna system has increased. This is not conjecture -- it is proven in every antenna engineering textbook, and by decades of field experience in the broadcast industry.

This reality also is shown in the NEC analysis in the paper linked below. Notice in the figures on pages 2 and 3 of the paper how the added wire length has changed the r-f current distribution over the length of the radiating conductors on the elevated system, and the considerably higher field strength that the elevated system generates using the same applied power.

http://rfry.org/Software%20Download/Elevated%20Part%2015%20AM%20Antennas.pdf

* Radiation efficiency is the ratio of the radiation resistance to the sum of all the resistances in the antenna system.

I plan to do some additional testing this summer and will let all know the results I get. I am guessing that while on a test range with no obstructions, that the elevation of the transmitter would have less of an effect on overall field strength then in the practical installations, due to obstructions, reflections, terrain, ect. SO, I do think that height for Part 15 systems is important.

A well-designed and impartial test protocol will confirm the statements I have been posting on this topic.

//

 

[Hamilton]

Rich,
I am just guessing and conjecturing, (I don’t want to be dogmatic here). Like I said it is a complex situation, that can’t be explained without some field testing. I am interested enough to try a few things.

“Therefore your claim that radiation from the safety ground would be limited by the reactance in the long wire is scientifically inaccurate.”

That isn’t entirely what I meant, practical ground systems are not a connection to a perfect RF ground, any potential radiation just depends on the situation you are dealing with.

Rich you should do some field testing of your own, I think theory is just a starting point, as it cannot take into account unknowns that are present in the practical world.

 

[rfry]

Rich, I am just guessing and conjecturing, (I don’t want to be dogmatic here).

As a manufacturer, wouldn't it be preferable to be certain of the accuracy of what you write before advising Rangemaster and other Part 15 AM users about the way these systems operate?

(Fry)“Therefore your claim that radiation from the safety ground would be limited by the reactance in the long wire is scientifically inaccurate.”

(Hamilton) That isn’t entirely what I meant, practical ground systems are not a connection to a perfect RF ground, any potential radiation just depends on the situation you are dealing with.

If the long "ground" wire ended with no connection to an r-f ground whatsoever that wire would still radiate. An elevated Part 15 transmitter is connected to an off-center-fed dipole, in that case -- and that configuration will radiate more than one whose total radiating length is limited 3 meters, per 15.219(b).

Rich you should do some field testing of your own, I think theory is just a starting point, as it cannot take into account unknowns that are present in the practical world.

FYI my career field experience includes the installation, test, adjustment, pattern measurement and repair of commercial AM, FM and TV broadcast antenna systems over a 35+ year span, as well as writing and using commercial software to design and evaluate them.

Over that time I learned that the performance I measured in the field always was predictable when properly understanding and applying the underlying physics.

//