If you can come up with a dipole that works within the 3 merter rule, you will not need a ground. That is supposedly what Station8 has done with his new antenna.
Responding to Reply 15 above:
For purposes of understanding the minutia of it, we would wonder by how many fractions of a foot such excessive antenna length would add to the transmitter's range, if any.
For these conditions in both configurations below ...
- 1650 kHz
- 50 Ω RF Loss in Connection to RF Ground
- 15 Ω RF Loss in Loading Coil, at System Resonance
- 85 mW RF Power at Loading Coil Input
- 8 mS/m, d.c. 13 Earth Conductivity
... the horizontal distance from the Part 15 AM antenna system to its 150 µV/m groundwave field is 0.5 miles for a 3-meter radiating length with its base at the surface of the earth, and 0.65 miles when the transmitter+3-m whip are elevated 3 feet above the earth, and connect to RF ground through a 3-foot conductor (as described by Mr Blare).
That elevated system gives a coverage radius increase of 30%, and a coverage area increase of about 70%.
(NEC4.2 data)
Station8 has amazing energy for Part 15. He has managed to track down and gain permission for the ALPB to post most of the Part 15 transmitter schematics and manuals in all of history, something he is still working on.
Station8 has collected and gained experience with almost every Part 15 transmitter out there, and that is a lot of transmitters.
Presently underway is a fascinating antenna experiment, which will be very interesting to follow as progress is made.
At the same time the FCC in its "AM Revitalization" has opened the door on using "shorter antennas" for AM stations, intended to ease the growing real estate problems for legacy full-size antennas. Station8's secret-formula antenna IS a shorter antenna, and might have applications in licensed broadcasting as well as Part 15.
We are all doing our part to introduce Part 15 low power as the medium of the future keeping radio alive.
The decimals south of the decimal point used to measure Part 15 power levels and range-distances can seem significant by the look of them, but tend to remain quite small when considered in there miniaturarity.
A million billionths might seem like a big gain, but it's a lot less than a penny.
Let's say a ten-footer goes 100-feet down to the side walk. If we increase it by another 30% we almost get across the street.
Minute by any measure.
Mr Blare posted: A million billionths might seem like a big gain, but it's a lot less than a penny.
1) In fact, a million billionths of any number represents a huge LOSS.
2) Please explain what a penny has to do with this.
You brought something I didn't think of when the statement was posted about Station8 making an antenna that could be used for licensed broadcasting. If that was to happen it could change the way some may feel about part 15 or Hobby Broadcasting. If us hobbyists experiment to as where our 6 foot antenna could help a commercial station I can't even begin to imagine how bright the future could be for Hobby Radio. It could educate the masses and thus open the eyes for the sheer need to keep us around for the long hall.
IIRC, The Isotron Antenna is not legal for broadcast use either in Full Power Stations.
It would be nice if Station8's got certified. That would help a sizeable number of Class D stations with low power output.
Part 15 power levels aren't worth a red cent.
Intended as a play on small units of measure.
Comments about Reply 21, including: ... If us hobbyists experiment to as where our 6 foot antenna could help a commercial station I can't even begin to imagine how bright the future could be for Hobby Radio. ...
Enthusiasm and self-confidence are valuable attributes, but just to note that many highly-regarded antenna engineers with PhDs (university professors) have done very extensive investigations of this topic over many decades -- yet no such short monopole antenna design/installation has been able to approach the system radiation efficiency of a linear, unloaded radiator 45 degrees or more in electrical length (height).
If otherwise, then commercial AM broadcast stations would not spend the money needed to install the antenna systems they do, and which the FCC requires.
For comparison, at 1650 kHz a 10-ft vertical monopole has an electrical height of about 6 degrees, and a 6-ft vertical monopole has an electrical height of about 3.6 degrees.
The electrical height of a linear monopole determines its radiation resistance at the operating frequency, and ultimately the radiation efficiency of that antenna system for a given set of r-f losses in the matching and r-f ground systems.
The shorter the monopole is below 45 degrees, the lower is its radiation resistance, and the lower the radiation efficiency of that antenna system will be.
I agree with you Rich, but all this new antenna has to do is to outperform the short monopole allowed by Part 15 rules, which we all agree, by both theory and practice, is highly inefficient. I have not heard anyone suggest that the Station8 antenna is more efficient than what commercial stations use.
Unfortunately, it is difficult to model some types of antennas at the AM frequencies that we use (at least, the ones that we are allowed). Here is a thread to refresh some memories on helical antennas: http://www.part15.us/forum/part15-forums/general-discussion/antennas?page=1. So we have to experiment.
I am both sceptical and enthusiastic about the work that Station8 is doing (talk about having a dual personality). It has prompted me to do some research on shortened dipoles. I have been looking into linear loaded dipoles as a result, and I am wondering if a helical antenna fashioned into a linear loaded dipole might also be an avenue to pursue. (if that is not what Station8 is doing). And to satisfy everyone - ignoring the legality of helical antennas in general with Part 15 (which, as the above thread suggests, is open to interpretation).
As mentioned earlier, the FCC's AM Revitalization will include allowing use of shorter antennas.
If shorter antennas for medium wave are a physical-electrical impossibility, why would the FCC be planning to allow them?
Comment to Reply 25, including: Here is a thread to refresh some memories on helical antennas: ...
That information is rather dated.
The NEC4.2 analysis below is more current/relevant.
Comment to Reply 26, which includes: If shorter antennas for medium wave are a physical-electrical impossibility, why would the FCC be planning to allow them?
I didn't state that shorter antennas for medium wave were an "impossibility," just that for the same system losses they aren't as efficient as longer antennas -- which is true.
To avoid a ground loop between an outside mounted 15.219 complainant AM transmitter and a computer/audio processing gear, either balanced audio with a transformer between the transmitter and the audio source or an optical link can be used. Then the only grounds the transmitter sees are the RF ground, such as from a ground rod/radial field, and any grounding provided through the power cable.
If balanced audio is run to the transmitter the common (ground) should be separated at the audio transformer; if there is a hum problem with the common disconnected it can be connected. In some circumstances a capacitor and resistor in parallel will need to be put between the two sides of the balanced audio feed. Balanced audio can be fed through a surge suppressor; ideally something designed for the control lines on a rotor, but for some protection even the phone protection section of a power strip will clamp down on spikes induced on the audio lines.
As with all grounding discussion, the above is only a suggestion and if you are unsure what you are doing get the services of a professional engineer.
Boy this thread got hijacked.