An Idea I Want To Try
According to Tim in Bovey's analysis of the EDM FM Transmitter, the lowest possible RF power setting is likely over the Part 15.239 level, unless perhaps a very small and inefficient antenna is used.
That's why our EDM Transmitter, set to lowest internal RF power, is probably over the 15.239 limit because we are driving a Decade GPL-32 FM Dipole using the 1/4 Wave Length setting according to a supplied chart.
Here at KDX we strongly believe that a matched antenna will give the best field reliability with a minimum of bad signal areas and multi-path.
Therefore we plan to re-set the GPL-32 Antenna to 1/8 Wave Length and make fresh measurements.
That should match up, don't you think?
Multiple propagation paths result from the propagation environment, and as such, are independent of radiated power.
However the greater the field intensity arriving at the receive antenna, the greater the "capture effect" of that signal in that FM receiver, to a certain limit.
When the arriving desired signal drives a good FM receiver into full limiting/quieting, it can reject undesired co-channel signals only ~1 dB below the field intensity of that desired signal.
This improvement in signal capture performance can result in less audible multipath distortion in the audio output(s) of the receiver, because the multipath effects tend to be better rejected by the receiver, itself.
So even though a certain reflection may be X dB below the direct wave for both high and low field intensities arriving at the receive antenna, the receiver will perform better (less distortion) when receiving signals of high(er) field intensities.
The detailed essay on "Multipath" by member Rich is comprehensive in considering most of numerous reasons for multipath experienced indoors between hobby transmitters and their receivers all of which is unquestionable.
In my opening post the matter of the many secondary causes and sources of multipath and broken signals was intentially omitted because the scenario described was presented in isolation so as to consider only those elements raised.
I'll pursue the matter further since Rich's essay did not touch upon the particulars of my inquiry.
Consider the transmission antenna by itself, deducting the physical environment for the sake of discussion.
In my unsderstanding the matched tuning of a transmission antenna determines whether the antenna itself will produce nulls and other impurities in the radiated RF field. It is therefore my belief that even with low power a weak field can be made incrementally better the moreso the antenna is matched.
Toward this end I have proposed shortening a horizontal FM dipole antenna from 1/4 Wavelength to 1/8 Wavelength by literally reducing the antenna length by half. It is my expectation this will maintain a decent match (compared to a stubby rubber bunny antenna) while also reducing the RF output power.
The secondary imperfections caused by reflections in the surrounding environment are, for the purposes of this discussion, not consequential.
Sorry, but antenna engineering textbooks, antenna modeling software such as NEC, and decades of industry experience all show that the radiation pattern and nulls produced by a transmit antenna are not related to the impedance match that antenna has to the transmission line/transmittter connected to its feedpoint.
It was in the early 1970s when a radio transmitter engineer explained to me that the bays of his station's antenna were mounted an incorrect distance from each other relative to the wavelength of the FM frequency of the staion, thereby causing nulls and multipath effects throughout the field of the signal at periodic intervals.
According to this pertinent information my viewpoint is substantiated.
A similar story was related to me regarding some type of mis-match with a TV station's antenna, resulting in much ghosting throughout the city, again, at periodic intervals. I recall having difficulty receiving a solid signal from the station until the problem was repaired, at which time the station's picture quality was corrected at my receiving location.
Perhaps I'm reading too much into what's being claimed, but the statement "not related to the impedance match that antenna has to the transmission line/transmittter connected to its feedpoint" seems to limit the discussion to the feedpoint between transmitter/antenna whereas I specified no such limitation.
EDITOR: To Clarify - I am referring to "periodic intervals" in topographic space, not time.
RE: ... Perhaps I'm reading too much into what's being claimed, but the statement "not related to the impedance match that antenna has to the transmission line/transmittter connected to its feedpoint" seems to limit the discussion to the feedpoint between transmitter/antenna whereas I specified no such limitation.
But you did specify that you are dealing with a single, (shortened) dipole antenna, which is not a multibay antenna such as commonly used by FM and TV broadcast stations.
It is true that a vertically-stacked, multibay antenna array can experience unexpected nulls in its elevation pattern if the bays are not properly spaced from each other, and/or are improperly phased.
However such array problems are not possible for the radiation pattern of a single dipole antenna of any length, and also are not produced by a poor impedance match between the input connector of the antenna and the r-f source connected there.
Mr. Rich writes: "such array problems are not possible for the radiation pattern of a single dipole antenna of any length, and also are not produced by a poor impedance match between the input connector of the antenna and the r-f source connected there."
That is written so as to sound like a final conclusion, however I do not accept it as such and will keep my thread open in search of other opinions.
Thank you Rich.
Carl queried "
Therefore we plan to re-set the GPL-32 Antenna to 1/8 Wave Length and make fresh measurements.
That should match up, don't you think?"
Not sure what you mean by match up but at these low power levels antenna match is not important for the well being of the transmitter.
Shortening the radiators will decrease the field strength. This is the method I use here to housecast and limit the range to a hundred feet or so outdoors. The transmitter is in my basement and is connected via RG 6 coax to the antenna on the second floor inside the house. The antenna is a straight length of wire soldered to the inner conductor of the coax and is about six inches long and there is a toroid placed over the outer conductor about six inches below the radiator in an attempt to form a short dipole. The length was determined by experiement to give the desired range limit..
I have not noted any multipath effects either indoors or outdoors.
Neil
We have one opinion stating in absolute terms that matching of an FM antenna plays no role in the integrity of the signal's field.
A second opinion states in terms of experimental observation that matching an FM antenna produces no noticable degradation.
Both of the opinions refer to transmitters operating at very low power.
Exploring further, picture a high power station (10 kW) switching from a quarter-wave antenna to a 1/8th-wave antenna.
Once tuned to the shorter length all will be well, as I guess. But am I right? Will the signal field be pure and and produce the best result in the station's range?
On the other hand using a duck-ears 11" antenna at 10 kW, besides possibly damaging the output stage of the transmitter, would have what affect on the resulting signal in the field? It is implausible that the signal would be as solid throughout its range as a properly matched antenna.
Seems to me that Rich was saying, and I stand corrected if wrong, that the way the signal radiates from the antenna doesn't matter if there's a mis-match in impedence.
Makes sense. by shortening the leads you will lessen the field strength but the radiation pattern would be the same.
If you have the EDM at the 1-10mW setting and you then have the RF trim as low as it can go you don't have to worry about an impedence mismatch as it doesn't matter. There's not enough power that could make enough heat to hurt the output transister.
Mark
Admittedly, an understanding of the radiation pattern of a 1/2-wave, center-fed dipole compared to a shorter such dipole is not very intuitive.
The following NEC4.2 analysis compares Neil's center-fed dipole operating at 98.1 MHz and having a total length of 12" (0.1 wavelength) to a 98.1 MHz, center-fed dipole with an overall length of 60" (0.5 wavelength). There is no practical difference between these two patterns.
This is to be expected, as the impedance match at the antenna system feedpoint has no bearing on the radiation pattern shape of an antenna. The difference in these two patterns is due to the differing lengths of the radiating apertures of these antennas, alone.
An 0.1-wave dipole has a much higher input impedance than a 0.5-wave dipole. So more of the available power is reflected back through an unmatched transmission line to the transmitter -- which together dissipate most of the available output power as heat, rather than by e-m radiation from the antenna.
That is the reason for the reduced system performance for this scenario, which Neil has utilized to control the coverage area of his FM transmit system toward meeting FCC §15.239.
"A second opinion states in terms of experimental observation that matching an FM antenna produces no noticable degradation."
In terms of radiated power and field strength shortening the antenna indeed does produce degradation. I have no way to observe the pattern so I didn't comment about that. As far as multipath is concerned, I did not observe any effects before or after changing the antenna length.
Rich had commented about the antenna pattern and I did not so comparing my experience to Rich's pattern effect is not valid since we are posting about two different effects. My post related to decreased range due to less power being radiated as I shortened the antenna. Rich's post immediately above confirms his statement that the pattern does not change and his comment about mismatch reflecting power back to the transmitter is confirmed by my observation of what happened when I shortened the antenna.
Neil
For the Record
When I stated: "A second opinion states in terms of experimental observation that matching an FM antenna produces no noticable degradation,"
I meant to say: A second opinion states in terms of experimental observation that mis-matching an FM antenna produces no noticable degradation.
Even with that correction, it appears the follow-up remarks continue to reflect the position of Niel's experience.
Because we have universal agreement that "mismatch reflecting power back to the transmitter is confirmed by (my) observation of what happened when I shortened the antenna," I will comment...
It seems to me that reflection back into the final RF output stage of the FM transmitter would cause a distortion of the RF signal in some form, perhaps a second image in reverse phase or out-of-sync with the output signal.
Mismatch at any point along the signal path is suspect (to me) of causing some kind of contortion/warping/distortion of the final RF output. It seems (again, to me) that no technical imperfection along the RF signal path would be without unwanted result; not benign.
Carl has postulated that reflections on a transmission line due to antenna mismatch could cause distortion in the modulated signal radiated and he is probably right.
But the answer to any engineering question is always "it depends". When studying transmission line theory I drew space/amplitude diagrams of a transmission line connected between a source and a load with various permutations of source and load impedance. The worst case was when neither the source nor load Z matched that of the line. The hypothetical pulse would travel to the load and be partially reflected and upon reaching the source would be partially reflected again, and back and forth it would go along the line until the amplitude died out due to attenuation.
With a perfect Z match at the load there is no reflection so the pulse exits the system at the load. If there is a perfect match at the source then the reflected pulse stops there.
It is usually easy to provide the proper load Z but not so at the transmitter. Taking the case where the load is not properly terminated and the source Z does not match the line Z then the signal can bounce back and forth many times superimposing upon the desired signal.
It would thereby seem possible that distortion could be present on the radiated signal in such a system, but what kind and to what effect? In a situation of a Part15 transmitter, short coax, and mismatched antenna then the unwanted signal component due to reflections would be the wanted signal with the unwanted signal being the wanted signal delayed by tens of nanoseconds. Someone very clever probably has figured this out but my intuition is that such delays would not be a problem for a stereo FM signal. Just a guess and it could be wrong.
Neil
Neil's time forecast for reflected RF signals: "the unwanted signal being the wanted signal delayed by tens of nanoseconds."
It's a likely bet that such small time increments would result due to the speed of light in an extremely short length of wire, regardless of the fact that light is not carried by wire, but its speed is, with small resistance (slowing).
If this reflection effect is the only artifact extant in my intuitive imaginings it would, for practical purposes, render my reservations little more than minutia.
Unless there's more going on that hasn't been uncovered as yet.
Be all that as it may, this week we plan to reduce our Decade GPL-32 FM dipole-antenna to 1/8 wavelength with the objective of attenuating the RF field from an EDM FM Transmitter for compliance with FCC Part 15.239.
Signal readings on both a spectrum analyzer and dBu read-out from a TECSUN PL-310 FM Radio will be compared before and after the change.
No such change is planned for the Ramsey FM 30b Transmitter into another GPL-32 (1/4 wave) because the lowest RF power setting of this device is already lower than that of the EDM.