Ermi:
Your comments relating to the VSWR bandwidth of my NEC antenna model are exactly right -- it is about 1/2 of what my plot showed. But it turns out that was not the fault of the NEC program, but a case of GIGO. I had entered the coil values in a form where NEC used the values for 1610 kHz when calculating the net impedance at other frequencies. Fixing that produced plots that agree with your bandwidth analysis. I'll disable those two links with bad data that I posted so as not to mislead anyone clicking on them later. Thanks for pointing this out.
//
The most popular method of maximizing the efficiency of Part 15 antennas is by stretching the meaning of "ground" and "ground lead" in Section 15.219(b) of the FCC rules. There is said to be an ambiguity in the rules. This "ambiguity" is a recent invention. The many years ago that this rule was introduced, there was no ambiguity at all. It was well-understood that amateur radio operators usually did not have the means of making an adequate ground radial system for the 160 meter and 80 meter bands. A reasonably good ground was obtained by attaching a ground lead to the water pipe where it emerges from the earth to supply water to the house. This ground lead counted toward the total antenna length. The antenna length mattered, not because of any legal restrictions, as in Part 15 AM, but the desire to have a resonant antenna. Connecting to just any cold water pipe in the house was discouraged, because it made it difficult to estimate the true length of the ground lead. You see, it was recognized that the pipe above the earth was part of the antenna.
Section 15.219 was written for hobbyists and experimenters who were technically knowledgeable, but didn't own expensive professional test equipment, like field strength meters. This is why only a VOM was originally necessary to test A Part 15 AM transmitter.
Antennas connected to "ground" on top of buildings or other tall structures can have efficiencies of more than 10%, because most of the radiation comes from the conductive path to earth ground, not the 3 meter whip. The efficiency comes from considerably higher radiation resistance.
The ground needs to be an existing ground near the earth. You are not allowed to create a ground or improve it. Any conductors you add will be counted toward the 3 meter total length allowed.
Just for reference, a 1 inch OD vertical pipe 10 feet long, in the earth, gives about 40 ohms of ground loss resistance in moist earh, and about 400 ohms in dry earth.
It may simply not be possible to find a suitable ground that already exists to use with a Part 15 AM installation. For that reason, I will describe an antenna that does not require a ground connection at all--a vertical dipole. It is best to use a large diameter antenna, to increase antenna capacitance, and therefore, antenna efficiency. However, the diameter must not be so large that the FCC would consider it excessive. I think that about 3 inches is the most the diameter can be without attracting unwanted attention. While it is not desirable to operate a Part 15 AM station at 1700 kHz, since it is difficult to suppress out-of-band spurious emissions, I have selected this frequency for my example, because it gives the highest antenna efficiency.
The radiation resistance of the 3 meter long dipole is .057 ohms. The capacitive reactance is 7563 ohms. The Q required of the antenna for 10kHz audio bandwidth is 1700/20 = 85. The loss resistance required for this bandwidth is 7563/85 = 89 ohms. The efficiency of the dipole is .057/89 = .064%.
To improve the efficiency, the antenna can be mounted over moist earth, with no electrical connection made to the earth. Without any connection to earth, the ground losses are low. Reflection from the earth will cause the radiation resistance to double, causing the efficiency to double (to .128%.)
If a good ground connection is available, the lower arm of the dipole may be connected to earth ground. This greatly changes the characteristics of the dipole. The RF current distribution of the lower arm, instead of tapering off to zero at the end, becomes more uniform along the lower arm, causing the radiation resistance to approximately double. Since now there is an RF displacement current path to the earth, instead of just between the arms of the dipole, the capacitive reactance is decreased. I don't have a formula for the capacitive reactance for this grounded vertical dipole. I just know that it is lower than the reactance of the ungrounded dipole, and higher than the reactance of the unipole of equal length above ground. The capacitive reactance for the unipole above ground is 2280 ohms. I will just guess a capacitive reactance of 4000 ohms for the grounded dipole. This gives a required loss resistance of 4000/85 = 47 ohms. The current density at the earth is about a third of that at the base of the unipole above ground. So, the grounded dipole is more tolerant of ground loss resistance than the unipole. The efficiency of the grounded dipole is .228/47 = .485%.
The unipole above ground has the same radiation resistance as the ungrounded dipole above reflecting ground. The loss resistance is 2280/85 = 26.8 ohms. The efficiency is .114/ 26.8 = .425%. This is comparable to the efficiency of the grounded dipole, but the loss resistance required is less.
Thanks for this detailed and specific info about antenna experimentation. Some of these data are especially useful in providing inputs for the antenna modeling programs at
http://webpages.charter.net/crstrode/calcs/RFcalcs.htm
and
http://www.btinternet.com/~g4fgq.regp/page3.html#S301
Experimental broadcasting for a better tomorrow!
Just for reference, a 1 inch OD vertical pipe 10 feet long, in the earth, gives about 40 ohms of ground loss resistance in moist earh, and about 400 ohms in dry earth.
Just to point out that even if such a pipe had zero ohms of loss to the earth in contact with it, that doesn't mean that it would present a zero ohm loss to the r-f earth currents returning to the antenna system from as far as 1/2-wavelength from the antenna. Those r-f currents would be greatly attenuated by earth losses before arriving at the pipe, and the net r-f ground resistance presented to the antenna system would not be zero ohms. Such r-f loss is in series with the antenna current, and therefore will reduce system radiation efficiency.
An effective, buried r-f ground needs to gather the induced r-f earth currents physically as close as possible to where they enter the earth, which is the reason that AM broadcast stations install 120 or more radials of 1/4-wave or longer each. This was all studied and documented in 1937 by Brown, Lewis and Epstein of RCA Labs, and remains the present practice of AM broadcast stations.
.. I will describe an antenna that does not require a ground connection at all--a vertical dipole.
Regarding the comments about this configuration and the two that followed in the quoted post, a NEC-2 study was done for 3" OD structures on 1700 kHz to show how they compared to each other and to a "standard" 3-m, base fed Part 15 AM system mounted at ground level (see following link). The NEC study answered the question about the input Z of a 3-m vertical dipole with the bottom end of the lower arm connected to an earth ground (Configuration 4 in the link).
The NEC study shows for these conditions that, while vertical dipoles either in free space or near the earth do not need an r-f ground, neither of them is as efficient a radiation system as the "standard" 3-m base-fed, ground-mounted monopole shown in Configuration 1. The dipole with the lower arm connected to earth ground (Configuration 4) does show a 1.2 dB improvement over Configuration 1. This is going in the right direction -- but would not produce much of an increase in the distance to a given value of field strength.
Hopefully this will give some useful additional information on these subjects. Discussion/corrections invited.
Rich,
Thank you for checking my calculations with your computer program. I will list your and my results below so that they can be easily compared. I give your results first, and then, after the slant bar, mine. All of the results listed below are in ohms.
MONOPOLE ABOVE GROUND
Input impedance: .13 - j2048 / .128 - j2280
Loss resistance: 24.1/26.8
DIPOLE REMOTE FROM GROUND
Input impedance: .057 - j6274 / .057 - j7563
Loss resistance: 73.8/89
DIPOLE NEAR GROUND
Input impedance: .119 - j6047 / .128 - j7563
Loss resistance: 71.1/89
DIPOLE CONNECTED TO GROUND
Input impedance: .238 - j3545 / .228 - j4000
Loss resistance: 41.7/47
I think that our results are close enough to be considered to be substantially in agreement. I believe that your results with your computer program are far more authoritatve than mine.
The point I was trying to make in comparing the grounded vertical dipole to the base-loaded monopole above ground is that the grounded dipole works better with a poor ground than the monopole. We can see that from the higher loss resistance the dipole can tolerate for a given audio bandwidth, compared to the monopole.
Poor grounds should be considered to be the norm for Part 15 AM, particularly because the FCC is hostile to improvements to the ground. I first learned about this hostility from Ed Devecka, former owner of LPB. Ed has sold LPB, but he assists the new owners as the chief technical expert of the company. Ed wrote to me in response to an inquiry: "The 100 mW transmitter can't be used with a new ground plane, such as copper sheet, or ground radials." My later inquiry to the FCC confirmed Ed's statement. So, whatever you use as a ground has to already exist, and you can't improvr it. Also, it has to be at the level of the earth, since the height above the earth counts agains the 3 meter limit in the FCC rules (since the conductive path to earth is considered to be part of the "ground lead").
My mention of a 10-foot, 1 inch OD, pipe in the earth was merely intended to to illustrate a typical object that one might realistically consider using as a ground. My own best candidate for a ground for my station is a well pipe in my yard.
The practice of licensed AM broadcast stations relating to low-loss ground systems is not relevant here. We Part 15 AM enthusiasts have to make do with far less.
Poor grounds should be considered to be the norm for Part 15 AM, particularly because the FCC is hostile to improvements to the ground. ... So, whatever you use as a ground has to already exist, and you can't improve it. Also, it has to be at the level of the earth, since the height above the earth counts against the 3 meter limit in the FCC rules (since the conductive path to earth is considered to be part of the "ground lead").
This is a puzzling position from the viewpoint of physics, and also does not appear to be defined by 47CFR 15.219 or any other published FCC Regulation or policy applying to Part 15 AM.
The overall length of a Part 15 AM radiating structure is limited by 15.219 to 3 meters, including the conducting path to whatever, in fact, constitutes an r-f ground reference. The nature of this r-f ground reference is not defined in the Rules, but physics shows us that it does not, in fact, include the top of a grounded flagpole/ tower/billboard steel/ "ground wire" etc, because all of those conductors will radiate -- and a true r-f ground reference does not.
It is possible to construct an r-f ground reference either buried in the earth or suspended above it. When properly designed and installed, neither one would add its own far-field radiation to that of a 3-m vertical monopole including its ground lead that was used with it.
Such an r-f ground can improve the radiation efficiency of the antenna system. But that is due to its reduced r-f losses, and not to radiation from the r-f ground reference, itself, or because the radiating structure was made functionally longer than 3 meters.
Other than informal and "unofficial" inputs, does any information exist showing that the FCC has issued an NOUO, NAL or whatever for any Part 15 AM system operating with an "improved" r-f ground such as described above?
//
Rich and Ermi,
It is interesting to read your posts since they bring some theoretical underpinning to the subject of AM part 15 antennas.
I think, though, it is not very useful to guess what the FCC will or will not accept regarding a ground without some real enforcement actions on the subject. I recall reading posts here and elsewhere where some contacted the FCC about this and I recall the answers were merely citations to without interpretation of the 3 meter rules.
Here is yet another opinion, not backed by fact, that buried ground radials could be defensible since they do not radiate yet an elevated ground lead is not since it does radiate. As far as "existing ground" is concerned, install the radials the day before the antenna. (This joke is about as silly as I think the exisiting ground is).
So where are we? My opinion is if one uses a buried radial system then this is defensible and I wouldn't hesitate to install one. This is not my advice to anyone, just own opinion. So far, I know of no documented (by NOV, NOUO) FCC objections to a buried radial system. Please help me understand what the phrase "the FCC frowns on this" means in terms which go beyond hearsay.
Neil
at what must be one of the WORST web pages ever created, there are some allegations to the opposite here:
http://www.netfeed.com/~jhill/1610.htm
If you can possibly make sense of the visual and written gibberish, the underlying position seems to be that you will receive very little scrutiny if you're not generating complaints from licensees with your content.
These are transmitters that use that specious existing ground concept.
More comments on Jon Smick's site, in the 1580, 1610 and 1700 KHz frequency sections, here:
http://home.att.net/~weatheradio/part15.htm
Most of us have seen a similar situation on FM, as well, during christmas light displays and similar events.
Experimental broadcasting for a better tomorrow!
It might be best to ask the FCC directly. Then you will get the information directly from the horse's mouth. They will usually reply within two business days. To submit a question electronically, first get on the FCC web site, www.fcc.gov. Then click on "E-Filing" at the top of the FCC home page. You will be presented with many options. Click on "OET Knowledge DataBase (KDB)." Then click on "Submit An Inquiry" at the left side of the OET Knowledge DataBase page. An Inquiry Form will appear, on which you can type your question and submit it.
It might be best to ask the FCC directly. Then you will get the information directly from the horse's mouth.
As I mentioned earlier, this has been done and the responses from the FCC as reported on the boards have been simply a citation of the rules with no interpretation, therefore I think this is meaningless to pursue with the expectation that a definitive answer will be provided. Perhaps those who expect more should ask the question, but I am content to leave it as it is.
Neil
Well, it might be good to remember that FCC actions are generally "complaint oriented". This is by necessity, since they have a limited amount of manpower/resources, and many small transmitters may be far outside the listening range of any official monitoring facility.
As far as I've been able to tell, if you cause interference which impedes the abilities of others to listen to other stations, or if you broadcast content others find offensive, and complaints get filed, you pretty much can count on trouble with the FCC. If your station is strictly legal by part15 standards, that *may* save your butt, but I wouldn't count too far on it.
I've noted in looking at some of the violation reports that the most common reason for someone to get nailed is that they are exceeding the power limits allowed for part15 and so they fall under "operating a broadcast station without a license", since they are above the limit set for unlicensed (part15) operation.
But I've seen no sign that the FCC is hiding in every bush and hedge monitoring all frequencies trying to catch part15 operators.
What it looks like happens is someone files a complaint, the FCC looks into it, and if they find you're over the allowed power limits, you are in trouble. I read quite a few of the FCC action reports available on the net before I ever put my little transmitter on the air, and the first things I did were to make sure as well as I could that I wasn't getting out more than I should and also that nearby neighbors knew they could come to me if it caused any possible interference.
That *reduces* the chances of a complaint being filed in the first place.
I also added a passive low-pass filter, since I'm not too many miles from a small airport that uses the band right over the FM band, and I wanted to reduce the chances I might inadvertently have a harmonic or spur that could cause them any trouble.
None of that guarantees that I couldn't get into trouble if there were complaints, but it's part of what I did to try and make sure I wasn't causing anyone any problems to complain about.
But logically, there has to be a complaint made to bring you to the FCC's attention in the first place. If you're causing interference or offending people in your community, even if the transmitter is in strict compliance with any and all part15 standards, the FCC still would have the legal right to tell you to take it off the air and/or change frequency, since we are guaranteed no "rights" due to part15. Whether it's technically compliant or not, if you're told to cease operation on a given frequency and you keep on transmitting it'd be a violation.
I'd like to think that what measures a part15 operator takes to try and be compliant and to avoid causing interference would be taken into account, but there's no guarantee of that anywhere in Part 15 so far as I've ever noted when reading it. But still, doing what we can to avoid complaints happening only makes sense.
Daniel
Daniel,
I appreciate your comments on this topic and your common sense approach.
I also have read several of the NOUOs and they are almost always for operation in the FM band. The reports mostly begin "... Office received information that an unlicensed broadcast radio station on XXX MHz was allegedly operating..." which affirms your comment about complaints initiating an investigation. But, as you said, there are no guarantees concerning this either.
Here, I have no way of knowing whether someone with a good antenna is listening to a distant station in their home on what might appear to be a clear frequency. Since, in my opinion, the greatest exposure in terms of causing interference is with part 15 operation in the FM band I limit my FM range to the confines of my home by using a very short antenna.
For "yardcasting" I use a SSTRAN transmitter on the AM band where I follow your advice regarding content and interference. I have a very good outdoor AM antenna and a communications grade receiver so I am confident the frequency I chose is not being used for reception by my neighbors.
I doubt that anyone using a transmitter such as the SSTRAN unit with a 3 meter base loaded antenna is going to have a problem due to a technical violation of the rules, but they should recognize that content and interference need to be considered.
Neil
Actually instead of limiting licensed LPAM to one watt, I'd like to see the current proposal that is before the FCC for LPAM to take form. Two power classes of LPAM, one being 30 watts and the other class being 100 watts. 100 watt station would typically be in the more rural areas while the 30 watt stations would typically be found in more populated areas. Antennas would be limited to 50 feet in lenght, vertical only. non directional only. Power would be constant day and night. You'd be surprised what 30 watts or even 100 watts into a 50 foot antenna with a decent ground system would do on the upper end of the MW band. Plus LPAM would be of a commerical nature meaning that the average everyday Joe and Sally could obtain a license. IF it were to be non-commerical, that would limit LPAM stations being owned by not for profit groups ....you know those "groups" that were created in many cases for todays LPFM stations just to obtain a license....no individuals could get a license and or own the station. Spacing requirements for these 30 and 100 watt stations would be based on using the idea that the stations are actually running 1kw. This thought in mind would give plently of room between all LPAMs and current high powered AMs as to keep interference to a min. LPAM would be restricted to the expanded band from 1620 to 1700.
I know many fear that LPAM could really put many part 15 stations off the air, but how many part 15 AM stations here would jump at the chance to "upgrade" your facilities so you'd have the ability to have a stronger signal in your communities and have a wider reach to cover out lying communities that would benefit for having a community radio voice? I know that every proposal out there isn't perfect and nearly each one of them have the potential to displace some part 15 AM operations, but on the flip side of the coin, there isn't anything keeping some new high powered station from popping up almost anywhere on the dial these days with the relaxed rules and interference tolerances. At least with LPAM, many people would have the chance to try and "stay in the game" so to speak.
Bob Carter - KC4QLP
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Mid-Atlantic-Engineering-Services of Utica NY / Elizabeth City NC
http://www.geocities.com/midatlanticengineeringservice/index.html
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WMHV 1700 AM - Part 15 radio
http://geocities.com/wmhv1700am