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Neil E. said:
But that would only happen if a Part 15 AM station owner blew off the FCC after a visit or a letter, which would be a stupid thing to do. The field agents are actually quite helpful and will suggest rule-complying changes if asked.
The practice of using an MOV (Metal Oxide Varistor) lightning arrestor between the transmitter's or ATU's ground terminal and the ground lead (because it is an open DC circuit but will pass RF like a somewhat lossy capacitor) was suggested by an FCC field agent to a Part 15 AM station operator (and no, it wasn't me! <GRIN>) who was using a "Whip 'n Mast" antenna installation.
-- Black Shire
Black_Shire said:
Wouldn't you want to use an inductor that blocked r-f and had low DC loss? That should be better, if the goal is to prevent the ground lead from radiating, but also keep the tx chassis close to DC earth potential. These are commonly used by AM broadcast stations, and are called "static drain chokes."
Whether or not they would provide a low-resistance earth ground at DC for the 3-m antenna to drain off static buildup will depend on the design of the tx output circuits.
//
When an FCC inspector says a stations construction is just fine with him, then, that should be just
fine with the rest of us as well.
Every station I have ever been involved with had the goal of getting as much range as the
FCC would allow us.
Local community broadcasting is a good thing. Maybe you guys need to build your own stations.
You can put it in the spare bedroom.
fine with the rest of us as well.
Every station I have ever been involved with had the goal of getting as much range as the
FCC would allow us.
Local community broadcasting is a good thing. Maybe you guys need to build your own stations.
You can put it in the spare bedroom.
R. Fry said:
I always prefer a DC path to ground to drain off static charges, and a large high-impedance RF choke like that would be my preference. A 1/2 watt, 5K ohm or 10K ohm resistor between the antenna base and the ground lead also provides a DC static drain path (although with a bit of signal loss), as does a variometer with multiple autotransformer-type taps near the bottom of the fixed outer coil (used to obtain a good impedance match to the transmitter).
The Part 15 AM station operator who posted about using the MOV (Metal Oxide Varistor) arrestor was told by the FCC field agent that he didn't want to see a DC connection between the transmitter's ground terminal and the top of the metal mast. When the station operator asked the field agent to suggest a solution that would allow him to keep using the mast-mounted transmitter, the field agent suggested using an MOV arrestor.
-- Black Shire
Hello again.. Just need to make one thing clear...WRPO is NOT the station in Marysville..it is the "log in" name for me. WRPO is an LPFM station that I work for at Russells Point, Ohio ( Indian Lake) Check out our web site at www.wrporadio.com and you'll see a "pretty" picture of me on the first page. The Part 15 in Marysville is WMHO-1620 AM...NOT WRPO, which is a licensed low power ( 100 watts) at 93.5.. on the air for 5 years. I have been reading all the comments on the board for some time...got a good one going, huh? 
Have fun..the FCC is NOT a buch of bad guys, if you treat them right! Have fun.. Best, Gene
Have fun..the FCC is NOT a buch of bad guys, if you treat them right! Have fun.. Best, Gene
K
kc8gpd
Guest
Well this guy seems to be doing good for himself.
see...
http://www.ohiounioncountyfair.org/
bottom left hand corner of the page
see...
http://www.ohiounioncountyfair.org/
bottom left hand corner of the page
Hamilton said:
However the ground/power/program conductor(s) leading to earth ground can be decoupled for r-f current, which will inhibit radiation from them, but at the same time prevent the buildup of static charges that can attract lightning. I believe the Rangemaster website even includes a statement that such devices are available for the AM1000 (correction invited).
An interesting point in this situation is that if r-f chokes or other means were used to prevent the ground/program/power conductors from radiating, then there would be no useful purpose in installing Part 15 AM systems on rooftops, flagpoles, towers, billboards or whatever, because the total radiation then would be the same as if the system was installed with the base of the ~3-meter whip at the surface of the earth -- where its chances of getting hit by lightning are much less.
But no matter what the elevation of the tx and whip, and the length of the ground conductor, whether or not the whip has a DC path to the ground conductor/chassis depends on the tx system design. Even if the tx chassis has a good connection to DC earth ground, that may not prevent static charges from building up on the whip, itself -- which of course is the highest point in the system, and the most susceptible to lightning effects.
//
R. Fry said:
Two factors make elevated Part 15 AM installations preferable even when the long ground lead is RF-decoupled and radiates little or no signal.
The 3 meter antenna radiates the upper AM frequencies (1600 kHz - 1700 kHz) about 10 times more efficiently than the frequencies at the bottom of the AM broadcast band, so given the minuscule radiated power at the bottom frequencies it doesn't pay to try to take advantage of groundwave propagation. Groundwave propagation is considerably less effective at the highest AM frequencies, but elevating the Part 15 AM antenna increases the range (up to a point) by line-of-sight propagation. The range increases as the antenna height increases, up to about 30 feet. Above that, the signal begins to overshoot the local area because the short antenna has a rather high angle of radiation.
Also, the highest AM channels are less noisy than the lowest ones. This allows Part 15 AM station operators to take advantage of the Automatic Gain Control (AGC) characteristics of AM receivers, which can "hear" low-power transmitters from farther away if the channel is quiet.
-- Black Shire
William C. Walker said:
Mr. Walker, where is exactly is this particular P15 station you speak of near Lake Ontario? What is the frequency? I'd love to tune it in. Thanks.
W
WCWalker
Guest
Watertown, NY. 1630 AM.
Black_Shire said:
Some considerations here... the reactance of an elevated, 3-m, Part 15 AM whip with no reference to r-f ground will be extremely high, so the coil it takes to resonate it will be very lossy, and the system will have very narrow r-f and audio bandwidth (very high Q).
But that aside, the radiation pattern of a vertical radiator for all elevations of the radiator up to about 5/8 wavelength always has maximum relative field in the horizontal plane. So installing the 3-m vertical even 20 meters above the earth, and using no r-f ground reference does not result in more relative field radiated toward nearby receivers. Its relative field is essentially the same as if the 3-m whip was was mounted with its base at earth level. However due to increased losses in the elevated system with no r-f ground, that elevated system produces lower fields in the horizontal plane than if it was on the earth, and used with an r-f ground.
It is true that groundwave losses are higher for higher medium-wave frequencies than for lower ones. But it is also true that even for the high frequencies, those losses are fairly small for the short path lengths that can be served by a Part 15 AM system. That's why the better antenna system efficiencies possible from a 3-m radiator result in better performance for the higher frequencies, when used for Part 15 AM -- not because of an elevated "space wave" propagated over a line of sight path.
Below is a link to a NEC calculation comparing a Part 15 system elevated 20 meters above the earth and with no r-f earth ground references, to one installed with its base at earth level, with a 30 ohm connection to r-f earth ground. The peak field of the elevated system is not as high as the one at earth level, although both systems have maximum gain in the horizontal plane. So the relative fields at all radiation angles are the same -- neither system has its radiation peak at some "high angle" above the horizontal plane.
http://i62.photobucket.com/albums/h85/rfry-100/Part1520mvsEarthElev.gif
R. Fry said:
I wish they really did work that way, as it's a pain to set up even a 20' mast-mounted Part 15 AM system. Using a Talking House transmitter with the supplied outdoor ATU/8.5' whip antenna set up on the ground in the yard (with no ground rod), I got a QRM/QRN-free signal on a Walkman radio only up to ~120 feet away at 530 kHz, but up to ~1/8 mile away at 1700 kHz. The 530 kHz signal faded away to background noise quickly as I walked away from the antenna, while the 1700 kHz signal faded out much more gradually.
Propping the ATU atop my 6' high wooden fence increased the 530 kHz and 1700 kHz ranges to ~300' and ~1200', respectively. That has to be due to line-of-sight propagation.
I didn't notice any difference in audio quality between the two frequencies within their strong-signal radii from the antenna, but the Talking House doesn't have a 10 kHz audio bandwidth (it's around 8 kHz or so).
-- Black Shire
Black_Shire said:
The line-of-sight distance is about 3.3 miles from the center of a 3-m Part 15 antenna with its base only 6 inches above the earth to the radio horizon, over a smooth earth. This 3.3 miles is a longer path than you were hearing for that system when elevated to 6 feet. Something other than the existence of a line-of-sight path must account for what you report in your post.
The added range may have resulted from a difference in the power radiated from components of the system, such as from cables leading down from the tx, when elevated to six feet -- which then would radiate additional field with vertical polarization that would add to that from the whip. Or it could be a localized effect such as different coupling into overhead wires. More investigation would be needed to know exactly what is causing it.
//
R. Fry said:
The 25' coax does radiate, but its radiation is even more feeble than what the 6' AC power cord radiates (and the power cord's emissions barely budge the Micronta Field Strength Meter's needle at 30 cm [12"] distance).
The neighbor's power line drop lead runs to her house fairly close (but well out of falling distance) to where I set the ATU/8.5' whip atop the wooden fence, so it could have been excited to re-radiate the signal. A Rangemaster user once posted about a significant range increase he noted when he set up the transmitter/antenna near a wire clothesline.
There is an abandoned 320' tall Zenith Wincharger radio tower near here that still has an intact radial system. It is 1/4 wavelength resonant at ~730 kHz and 1/2 wavelength resonant at ~1460 kHz. Setting up a Part 15 AM transmitter with the 3 meter antenna next to the tower (or perhaps wrapped around the tower base without touching it, sort of like a link coupling coil) would excite the tower to re-radiate. Whether this would be efficient enough to be worth the trouble (or what an FCC field agent might think about its legality) are interesting questions, though.
-- Black Shire
K
kc8gpd
Guest
Well,
Stop messing around and Bang it out man!!! Get you a car battery a tape or MP3 player get up there and do some experiments. then get back to us about how well (or Bad) it works
I would not let something as ripe for experimenting as that pass by w/o doing some testing.
Stop messing around and Bang it out man!!! Get you a car battery a tape or MP3 player get up there and do some experiments. then get back to us about how well (or Bad) it works
I would not let something as ripe for experimenting as that pass by w/o doing some testing.
-----Black_Shire said:
Suggest that more investigation is in order. A metal clothesline (or railroad tracks, power lines, etc) usually are oriented essentially in one compass direction from the v-pol radiator of a Part 15 AM tx system.
That could, to some extent, generate a weak, localized, H-pol EM field produced by re-radiation from a nearby Part 15 AM v-pol antenna in this scenario (although H-pol and V-pol fields typically are decoupled by 20 dB or more).
But that does not necessarily mean that such added h-pol radiation will be useful to a typical AM receiver, even in the geographic sectors where it may exist.
Ground-wave losses in the MW spectrum are much higher for h-pol than for v-pol.
//
R. Fry said:
Indeed. That localized H-pol phenomenon has also fooled new LowFER Part 15 Long Wave beacon operators who were using "Inverted L" antennas instead of "Marconi Tee" antennas. The horizontal leg of a Long Wave or Medium Wave "Inverted L" antenna creates a nice strong local H-pol EM field (often helped by re-radiating power lines in the vicinity), but as the beacon operators drove away from their LowFER beacons they wondered, "Where did my strong signal go, and so suddenly?"
-- Black Shire
Hi Guys,
I have had the stomach flu and have been out of it (along with being busy with trying to make a living), I see this thread is still going! I have some comments that may be useful.
First about the inductor comment on our website (nice to know people are reading the site) we do provide a bulk inductor as a resource to the site engineer, we are not saying it should be used necessarily, it just depends on the situation. For the common scenario the manual should be followed.
Second about John Reed’s comment, I have talked to him personally about this issue. Who knows what he was talking about or what the context was, When I was speaking to him he seemed supportive of the issues on the other side of the equation so ? no telling.
Third there was something about the customer not complying with the FCC? I forget exactly what the comment was, we encourage people to comply with the FCC in all situations. We make our recommendations on the best information we have.
Funny thing about the ground, I have customers that call, worried about the ground thing, and they have disconnected their ground. Of course I tell them they have no lightning protection! The problem (and funny thing) is that they often are reluctant to hook the ground back up because they are getting better range without the ground! What is probably happening is that instead of the transmitter getting it’s ground through the main ground terminal, when there is no ground there, it finds a ground through the power supply or audio system, even though these grounds are isolated in the system. The transmitter isn’t designed to be ungrounded.
I have done some field testing concerning the ground radiation thing. Pat 15 engineering is a different world then full power broadcast engineering. I found the there was very little radiation from a longer ground, it was basically the height that contributed to increased signal. Such a short radiator is a point source at that low frequency.
Also a conduit around the ground can be effective, at frequencies of 1-2 Mhz a short length of conduit is a effective (though sure, not perfect) shield.
Some of the finer points of the engineering could be debated for decades. (I don’t enjoy that sort of thing)
I have had the stomach flu and have been out of it (along with being busy with trying to make a living), I see this thread is still going! I have some comments that may be useful.
First about the inductor comment on our website (nice to know people are reading the site) we do provide a bulk inductor as a resource to the site engineer, we are not saying it should be used necessarily, it just depends on the situation. For the common scenario the manual should be followed.
Second about John Reed’s comment, I have talked to him personally about this issue. Who knows what he was talking about or what the context was, When I was speaking to him he seemed supportive of the issues on the other side of the equation so ? no telling.
Third there was something about the customer not complying with the FCC? I forget exactly what the comment was, we encourage people to comply with the FCC in all situations. We make our recommendations on the best information we have.
Funny thing about the ground, I have customers that call, worried about the ground thing, and they have disconnected their ground. Of course I tell them they have no lightning protection! The problem (and funny thing) is that they often are reluctant to hook the ground back up because they are getting better range without the ground! What is probably happening is that instead of the transmitter getting it’s ground through the main ground terminal, when there is no ground there, it finds a ground through the power supply or audio system, even though these grounds are isolated in the system. The transmitter isn’t designed to be ungrounded.
I have done some field testing concerning the ground radiation thing. Pat 15 engineering is a different world then full power broadcast engineering. I found the there was very little radiation from a longer ground, it was basically the height that contributed to increased signal. Such a short radiator is a point source at that low frequency.
Also a conduit around the ground can be effective, at frequencies of 1-2 Mhz a short length of conduit is a effective (though sure, not perfect) shield.
Some of the finer points of the engineering could be debated for decades. (I don’t enjoy that sort of thing)
I hear you on the stomach flu (I had it recently myself)...
<I have done some field testing concerning the ground radiation thing. Pat 15 engineering is a different world then full power broadcast engineering. I found the there was very little radiation from a longer ground, it was basically the height that contributed to increased signal. Such a short radiator is a point source at that low frequency.>
I don't recall the reference, but someone (was it "Crash" Knorr?) did extensive testing of 3 meter Part 15 AM antennas and found that they are not far short of being isotropic, radiating almost equally well (actually, equally poorly) in all directions. "Point Source" is a good description, as the antenna would look much like a flare if it were emitting light instead of RF and was viewed from a distance, and it would appear almost uniformly bright from any direction.
I once read an account of a private pilot who was surprised that he could clearly hear Talking House Part 15 AM realty transmitters on his aircraft's ADF (Automatic Direction Finder) even at several thousand feet, so the 3 meter antennas do radiate a significant amount of signal skyward.
-- Black Shire
<I have done some field testing concerning the ground radiation thing. Pat 15 engineering is a different world then full power broadcast engineering. I found the there was very little radiation from a longer ground, it was basically the height that contributed to increased signal. Such a short radiator is a point source at that low frequency.>
I don't recall the reference, but someone (was it "Crash" Knorr?) did extensive testing of 3 meter Part 15 AM antennas and found that they are not far short of being isotropic, radiating almost equally well (actually, equally poorly) in all directions. "Point Source" is a good description, as the antenna would look much like a flare if it were emitting light instead of RF and was viewed from a distance, and it would appear almost uniformly bright from any direction.
I once read an account of a private pilot who was surprised that he could clearly hear Talking House Part 15 AM realty transmitters on his aircraft's ADF (Automatic Direction Finder) even at several thousand feet, so the 3 meter antennas do radiate a significant amount of signal skyward.
-- Black Shire
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