Reading too much lately and doing too little experimenting. So I defer to the radio gods here.
About to get back to the tiny FM transmitter I've been toying with.
The importer commonly recommended a formula for cutting a length of wire and attaching it for increased range (legalities aside here).
So we end up with around a 3 meter length of antenna wire based on frequency.
Assuming with FM, we'd want to mount the wire vertically?
What sort of signal pattern will such emit? Will it be essentially an omni?
How much more efficient will something like a dipole be beyond the simple wire antenna at these micro power rates?
Three meters seems long for the FM band. A quarter wavelength is closer to 1 meter.
Short antennas produce a pattern shaped like a do-nut centered on the conductor.
It is difficult to compare a 1/2 wave dipole to a short wire since the relative radiated power of a short wire is determined by the impedance match to the transmitter. For example, I adjust the length of the whip on my transmitter to get the range I want which is more a function of the mismatch to the transmitter with length than the length itself.
That being said, generally a longer wire will radiate more signal.
A basic 1/2 wave wire antenna can be figured by dividing 486 by the frequency in mHz. (in feet/inches) The center of the FM broadcast band is ~98 mHz. A 1/2 wave length of wire is 4.96 ft.
Put 4 horizontal 1/2 wave radials at the base of a 1/2 wave vertical wire and the impedance is ~75 ohms. Put a 1/2 wave counterpoise vertically under the active element and the impedance will be ~300 ohms. A 1/2 wave folded dipole will be ~300 ohms..
Most of those ElCheepo xmtrs. have an unbalanced antenna connection. 300 ohms seems to work the best..
A folded dipole will be somewhat directional "through the loop". A "groundplane" configuration will be omnidirectional using a horizontal or conterpoise ground return..
"The importer commonly recommended a formula for cutting a length of wire and attaching it for increased range (legalities aside here)."
Setting aside the legalities, a 1/2 wavelength dipole will produce far better results than even a 1/4 wave dipole or 1/4 wave single wire.
"So we end up with around a 3 meter length of antenna wire based on frequency."
3 meters is long for the FM frequencies..particularly a single element (wire) length. However divide that single wire in half, you got about 1.5 meters per wire for a close full wavelength dipole. (about 4.5 feet each). Obviously this will produce a better radiated field as now your bringing the antenna closer to a proper match.
Even at the tiny power levels I never am comfortable with obtaining proper field strength by mis-matching the load..ie extending a whip or retracting it. A mis-match, also known as VSWR, will kick even that tiny flea nano-power right back into the TX, which if running stereo, wreaks havoc with stereo separation and can cause unwanted side effects like force signal down power connections and/or audio regardless of chokes and filters. My advice is to build a proper match load for that TX, and to meet the field strength legalities, use an in-line attenuator rather than creating a mis-match.
An inline attenuator can be a simple resistor in series with the feed line center conductor. Resistance of the attenuator will be based on how much attenuation is needed to obtain the 250uV field strength at the 3 meter distance point from that antenna.
Before the preaching of proper compliance blah blah get's thrown in here, let us all recognize that it is highly probable that OP does not have one of those fancy 9 grand FIM 41's or anything near it that would be considered an adequate measuring device.
What could be used is a known 15.239 certified transmitter and test it's range with a common hand held radio. Mark the point where the radio's reception of the certified transmitter starts to dump into the static of the noise floor. Then take the small FM TX board and begin inserting a resistor in series with the antenna until you reach that same "fade out" point at the same distance. Your close enough for muster.
"Assuming with FM, we'd want to mount the wire vertically?"
Yes. A "doughnut" pattern is produced. Mounted horizontally you turn the doughnut pattern sideways, and the radiation is 90* and 270* (emitting from the flat side and not from the ends) of the wire/dipole.
"Assuming with FM, we'd want to mount the wire vertically?
What sort of signal pattern will such emit? Will it be essentially an omni?"
Yes.
"How much more efficient will something like a dipole be beyond the simple wire antenna at these micro power rates?"
Far better than a single wire.
RFB
The length reported by censoredship sounds like a full-wave length.
At full-wave 88.1mHz is 11.17-ft
At full wave 107.9mHz is 9.12-ft.
Since, as Radio8Z reports, a longer wire produces a stronger signal, why not use full-wave for FM?
A vertically polarized transmitting antenna will put the donut in an omni-directional pattern and is best received on a vertical antenna.
I have not seen a circularly polarizing antenna that can be built for part 15 use, but it would be interesting to talk about the possibility.
Of course this posting will attract the usual rule-thumper, so I yield the floor.
"I have not seen a circularly polarizing antenna that can be built for part 15 use, but it would be interesting to talk about the possibility."
I've built several all based on commercially available C-Pol antennas for licensed use.
Most easiest to construct is the turnstile or 45* X type. Again it's a matter of using in-line attenuation to obtain the proper field strength permitted while maintaining the proper match.
Though a lot of "talk" suggests a C-Pol is not very useful at 15.239 because of the ridiculous limited field strength, I beg to differ because it DOES help tremendously with the multi-path plague always encountered with the tiny tot power level..such as that mutli-path problem you ran into with your cage monopole sending FM STL to a receiver directly below in the basement.
RFB
Probably there isn't much point in optimizing the gain and radiated power of a transmit antenna operating under FCC 15.239, because the radiated power needed to meet the FCC limit on radiated fields is extremely small.
Using a very efficient antenna, and then attenuating the r-f power applied to it is a bit at cross purposes.
High SWR at the antenna input has few effects on the received signal for a configuration of a transmit antenna connected directly at/to the output connector of the transmitter. Those effects require a rather long transmission line between the transmitter and the antenna input, so that the reflected energy has undergone a significant r-f phase shift when arriving back at the transmitter.
Member rich has moved the subject into foreign territory by replying to a point that was never raised.
Let me put the thread back in focus before it gets locked for drifting.
We were talking about transmitting on both the vertical and horizontal polar planes from a part 15 compatible antenna.
We were NOT talking about increasing the antenna gain.
Member blare wrote: Let me put the thread back in focus before it gets locked for drifting. We were talking about transmitting on both the vertical and horizontal polar planes from a part 15 compatible antenna.
Transmitting in horizontal and vertical planes together was introduced in the above clip from Post 5 of this thread (as a thread drift).
Prior to that it was addressing linear radiators, including my Post 7, and a bit about antenna mis-matching -- which was introduced earlier by others.
We were NOT talking about increasing the antenna gain.
That concept was part of the original post, "How much more efficient will something like a dipole be beyond the simple wire antenna at these micro power rates?"
It is important in technical discussion to keep from blending two separate contexts.
The main theme of this thread pertained to questions of vertical/horizontal antenna patterns.
My introduction of the circular antenna went straight to this consideration.
Reference in passing to "antenna efficiency" brought a secondary theme, still separate from the fist theme of polarity, but was not more than an aside.
By combining the two themes we have the confusion now being sorted out.
"High SWR at the antenna input has few effects on the received signal for a configuration of a transmit antenna connected directly at/to the output connector of the transmitter. Those effects require a rather long transmission line between the transmitter and the antenna input, so that the reflected energy has undergone a significant r-f phase shift when arriving back at the transmitter."
15.239 does not specify that a certified transmitter with a permanently attached antenna be used to comply with 15.239. The reference to a permanently attached antenna, or antenna with a special connector not common place in electronic supply stores only refers to units being tested for FCC certification and sold within the US. It does not mean that one must use a certified transmitter with a special antenna connector or permanently attached antenna to be compliant with 15.239.
15.239 also does not say a coax cannot be used. 15.239 does not say that a transmitter with more than 0.000011 nanowatts cannot be used.
15.239 does not specify anything other than 250uV at 3 meters of field strength from the transmit antenna. If someone can measure for that 250uV at 3 meters field strength from a system using an outdoor antenna system fed by a coax, there is no violation whatsoever, or worry about being in compliance.
Thus there are in fact some setups..like mine for example, that does use an outdoor antenna system being fed by 25 feet of RG-214 coax which is being fed drive from a Ramsey FM 100 (25mW TPO) through a 7 pole low pass filter, then into an attenuator box, adjusted so that the field strength from the antenna mounted 20 feet up measures about 270uV at 3 meters. And to put the icing on the cake, the inspecting agent was perfectly fine with it. The antenna is a turnstile C-Pol antenna built out of scrap TV rabbit ear whips, which isn't very stable in the wind. I plan to rebuild it using 1/4 inch copper tubing and PVC pipe fittings..probably this summer sometime.
I do transmit in stereo as I don't believe in transmitting mono audio in the 21st century..even on my AM 15.221 CC system, which transmits in C-QUAM AM stereo. Therefore I have the FM system very well built and tuned to prevent any return VSWR from mucking up that nice stereo sound. I don't care that it fades out 300 feet down the street or that the stereo fades to mono a little before 300 feet, nor do I care that the stereo sub carrier occupies a little bandwidth.
If done properly, a C-Pol antenna, outdoor setup and use of a coax with appropriate power and if necessary, attenuation, is not "a bit cross purpose". It is all in the spirit of building a system to be proud of and especially proud of when an inspecting field agent smiles and impressed by it's compliance and use of recognized measuring equipment (Motorola R2012D) to ensure compliance.
RFB
"That concept was part of the original post, "How much more efficient will something like a dipole be beyond the simple wire antenna at these micro power rates?"
Obtaining antenna efficiency is not necessarily obtaining gain of an antenna. The efficiency in the antenna can be to obtain a better signal pattern than what would come from a single wire. Again with proper construction and attention to field strength, there is no reason why anyone should not consider an antenna other than a simple wire or whip. As long as that antenna system meets 15.239's 250uV at 3 meters, what is the problem? I see none at all.
In fact there is more to be learned from building an antenna system beyond stringing a simple wire, and a lot to be discovered how a more elaborate antenna solves multi-path issues at these low power levels compared to a simple wire or whip.
It's all about experimenting and finding out what can work, and there are more options available than the simple wire.
Isn't that the whole point of the OP's question..and this thread?
RFB
As long as that antenna system meets 15.239's 250uV at 3 meters, what is the problem?
None at all.
Indeed Carl, a full wave wire antenna.
The dipoles seem interesting and a possible DIY project. Have to find a good, cheap and DIY style project plan for one.
1 - 1/2" PVC T fitting.
2 - 1/2" Copper tubing and caps (each at least 5 feet in length)
2 - Lock screws (for securing copper tubing and feed connection)
1 - 1/2" PVC pipe (mounting arm)
1 - 1/2" L PVC elbow (mounting arm to support structure)
Pretty easy to construct. Insert 2 of the copper tubes into T fitting. Drill hole at the insert point to put in lock screws. Insert 1/2" PVC pipe to single end of T fitting. Drill hole at end of PVC pipe at the base of the T fitting for the feed line to pass through. Run coax (RG-59 or other) through the PVC pipe and coil it about half way (at least 6 turns close wound) to provide a proper balun match. Insert PVC elbow L and lock it all down with PVC pipe glue. Attach coax ends to lock screws securing the copper tubes.
Cut copper tubes to wavelength given also the coax velocity factor and length. Drill small hole in end caps and solder nut to outside of end caps. Use long screws and 2nd nut for "locknut". Solder caps to ends of copper tubes. Adjust fine trimming of both upper and lower dipole elements with the screws for lowest VSWR and lock down with locking nut. Mount antenna using 2 hose clamps to support structure.
RFB