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Post by Deleted on Nov 1, 2017 20:35:57 GMT
It was told that psychic associational thinking had opened new vistas of awareness about the secret of Making Top Hats Work. Before launching into my own findings and conclusions, let's send you on a journey of discovery. Visit this link, study the novel AM antenna presented, think about it and top hats at the same time. Ask your family to hold it down so you can concentrate. THIS LINKThen watch back here for my prophesied return. This Major Forum Presentation is Being Released Simultaneously On the World's Two Major Low Power Radio Forums
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Post by Deleted on Nov 1, 2017 21:36:17 GMT
Yes, the Star-H. I did an analysis and magazine article on this antenna several years ago. In it I proposed a further reduction in footprint by bending the horizontal wires at 45 degrees, routing them toward an adjacent horizontal wire.
Modeling a 10' vertical with two opposing 137' top wires over perfect GND (with 1 ohms ground loss added to mimic a BC station GND) shows a bandwidth of 5 kHz. We would have to remove some radial wires or add a one ohm of resistance to get enough bandwidth. The radiation efficiency is 20% with the added resistance and that certainly beats our small top hat antenna. What might an FCC inspector make of the two long wires?
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Post by Deleted on Nov 1, 2017 21:57:24 GMT
Best Response
DaveC "New Member" knows where we are going with this conversation.
Tremendous work with your analysis of this type of antenna considered in terms of part 15 modification.
The phone-line remains open for further comments before I add my thoughts on the subject.
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Post by Deleted on Nov 1, 2017 23:00:53 GMT
SPECIAL ADVISORY TO COMPETING FORUM SITE
Over at your fortified highly exclusive bastion of jealousy please feel free to re-post our posts and threads UPON STATING PROPER ATTRIBUTION.
The ALPB is generous with public information.
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Post by Deleted on Nov 2, 2017 23:36:16 GMT
I was hoping to get affirmation that a top hat was ok. However, this post from radiobanter.com gives details on the FCC view of top hat loaded monopole antennas under 15.219. www.radiobanter.com/antenna/64165-part-15-am-antenna-help.htmlJohn Smith, posted Feb. 11, 2005 about his written inquiry to the FCC in which he asked about adding a top hat to a 3 meter monopole. The FCC response was: ""The intent of Section 15.219 is for a maximum 3 meter wire antenna. The total length of the antenna plus ground lead plus connecting lead must not exceed 3 meters. The rule doesn't permit a 3 meter diameter "top hat" but should be applied to the combined length of each radial. If the combined length of each radial added to the length of the antenna vertical structure added to the length of the ground wire exceeds 3 meters, it would not be permitted."" My comments: By "each radial" I understand the FCC to be referring to the top hat "spokes." I wonder how they would measure a solid disk top hat? So, it's back to the drawing board to see what can be done with a straight 3 meter wire. With this simple design the radiation efficiency bumps up against the VSWR bandwidth required for an AM signal before much efficiency has been rung out. Given fixed radiation resistance, the efficiency is a balancing act between reactance and system loss; we want the Q (reactance/loss) to be roughly 100 at the top of the AM BC band. My fix is to reduce the antenna reactance by using a very thick wire, as in 12" diameter. This offers the potential of having an efficiency of 1.0% at 1700 kHz.
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Post by Druid Hills Radio on Nov 3, 2017 13:09:36 GMT
David: One might be able to accomplish what you want by utilizing a 10 foot section of Rohn 25 insulated from ground. Comments?
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Post by Deleted on Nov 3, 2017 15:54:39 GMT
Electrically the Rohn 25 would work fine. Would an inspector call it a 10' "wire" or would she add up the lengths of the tubes it's composed of? I thought a tube might have a better chance of being called a single wire. The local Home Depot stocks 8" metal duct pipe and I'm working out the details of how to build with it.
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Post by Deleted on Nov 3, 2017 17:44:29 GMT
With reference with that response from the FCC, I think that it needs to be taken with a grain of salt. It's one particular response from one particular FCC agent.
If you ask multiple FCC inspectors the same question, you'll likely get multiple different answers.
The rules are not always exhaustive, and black and white. They can be interpreted in different ways. And it's ultimately up to a field FCC inspector to determine if a particular installation is compliant with the rules (or not).
The FCC used to pass Part 15 broadcasting installations that had elevated grounds. Now they don't much any more. The rules didn't change, just the way they were interpreted and then implemented in the field.
I've heard it said that the radiating element of a Part15.219 antenna cannot be longer than 3 meters. The rules don't state that - they just say that the antenna + ground lead + feed line can't be longer than 3 meters. The initial statement is just an interpretation.
Does a top hat radiate? No, but it makes an antenna easier to match. Does a loading coil radiate? No, but it's essential to matching an antenna. I've seen plenty of base loaded antennas pass inspection. Why not a top hat? Again, it depends on the FCC inspector.
So hope for top hats is not lost. If an FCC inspector comes calling, and you treat him/her with respect, fully explaining what you are doing and why you are doing it (with preferably printed references), then you might pass, or you might not. Of course, that assumes that you aren't flagrantly flouting the rules in other areas, such as long ground leads, long feedlines, etc. If you don't, then they'll tell you why, and all you have to do is to follow the inspector's provided recommendations to become compliant and you'll be OK.
Unfortunately, this hobby has too many self professed 'experts', each with their own opinions. The only opinions that matter are those of your field FCC inspector.
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Post by Deleted on Nov 3, 2017 20:02:14 GMT
12" Diameter WireBiggest chunk of wire on earth, 12" diameter, proposed by DaveC (not to be confused with DavidC). Neighbor Rich at part15_us has some further thoughts on the subject. Link to Part15_us Forums
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Post by Deleted on Nov 3, 2017 20:40:47 GMT
12" Diameter WireBiggest chunk of wire on earth, 12" diameter, proposed by DaveC (not to be confused with DavidC). Neighbor Rich at part15_us has some further thoughts on the subject. Link to Part15_us Forums It looks like Rich did not read the other thread where the antenna reactance and system loss are to set for a Q of 100; enough for an AM BC signal at the top of the band. If we do that for the 12" x 3 meter wire the reactance is 1100 ohms and we can make the system loss 11 ohms. Practical numbers are 4 ohms for the loading coil and 7 ohms for GND. At 1.8 MHz I measured a radial system covering a 25' x 50' to be 10 ohms. 7 ohms should not be difficult to achieve. The 0.12 ohm radiation resistance divided by the system loss of 11 ohms, gives a radiation efficiency of 1.1%. Let's contrast this with a 102" CB whip having a reactance of 4300 ohms. To achieve a Q of 100 we make the system loss 43 ohms. The 0.12 ohm radiation resistance divided by the 43 ohm system loss gives us a radiation efficiency of 0.3%. Given some loading coil and ground system work the fat monopole achieves 4X the radiation efficiency, resulting in 2X the range and 4X the coverage area. The same coil Q of 275 applied to the CB whip antenna, along with a 7 ohm ground, results in system loss of 23 ohms and a system Q of 187. This antenna will rolloff the sidebands at 4.5 kHz and its radiation efficiency is 0.5%.
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Post by Deleted on Nov 3, 2017 21:04:19 GMT
David C. thank you for the top hat encouragement. There have been no NOUOs citing top hats and either no top hat monopoles have been involved in FCC inspections or they have been but were passed. I would certainly prefer to go with a top hat design. I've designed and built over a dozen short monopoles and all of the very short ones (0.02 wavelength) have had top hats. Pulling out the stops and building a 3 meter monopole with a 3 meter diameter top hat (8 spokes with a perimeter wire) the reactance is 900 ohms. For a Q of 100 we can reduce system loss to 9 ohms (4 ohms for the loading coil and 5 for GND) and achieve a radiation efficiency of 5%. For range this beats the 1.1% monopole by a factor of 2X. Is a Q of 100 what we want?. I based that on a -3dB bandwidth of 17 kHz, which allows for audio to 7.5 kHz and +/-2 kHz for resonance drift. The +/-7.5 kHz is based on the Radio World article Opinion: 'Let's keep AM Sounding Good.' Note that the NRSC standard calls out 10 kHz steep audio rolloff whereas the antenna has a 2-pole 7.5 kHz rolloff. www.radioworld.com/headlines/0045/opinion-39let39s-keep-am-sounding-good39/313449By the way, I tried to join the Part15 group but their CAPTCHA shows an error message for the site owner.
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Post by Deleted on Nov 3, 2017 21:55:55 GMT
Getting Things Done
DaveC ran into a roadblock when trying to join part15_us: "By the way, I tried to join the Part15 group but their CAPTCHA shows an error message for the site owner."
Tomorrow evening at the ALPB Meeting three of the moderators from part15_us may be present, so we'll bring the enrollment problem to their attention and things will get fixed.
The reverse is also happening in some cases, people have had trouble signing up here at ALPB.
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Post by Boomer on Nov 4, 2017 0:56:23 GMT
A super top hat on commercial stations, a good idea with stations that are in a flight path height restricted area, or a location with zoning that doesn't allow high erections. Some stations can use it, just don't have the angles go all the way to 90 degrees clockwise, or it will look like the symbol for the SS, and politicize your station in this day and age.. Too bad that a lot of the smaller broadcasters who might benefit are directional, not single towers.
I could see a zig-zag pattern being used on the top hat, more 45-90 degree angles, but then that might take more masts to do. Good engineering discussion, I like seeing the numbers and comparing notes on efficiencies, etc., and I'll copy this info to my files. All in all, it seems like it wouldn't be so hard to get ten times the efficiency of the average install with a top hat, thicker radiator and some careful design.
I'm all the way for keeping AM sounding good, I don't think the NRSC should have pushed for dropping response from 15 to 10 khz, (bandwidth 30 to 20 khz). It's a shame, because now we have digital receiving chips that can do so much more with wider bandwidths and keeping the sound clean at the same time, and having it in stereo. We all lost out on that one, except maybe Inovonics..
Boomer
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Post by Boomer on Nov 4, 2017 1:06:27 GMT
Behold the UmbrellaHere's something else I wanted to share, another version of the extreme top hat, the Umbrella Antenna. First, from the page on 'Medium wave' and a paragraph on umbrellas. "A popular choice for lower-powered stations is the umbrella antenna, which needs only one mast one-tenth wavelength or less in height. This antenna uses a single mast insulated from ground and fed at the lower end against ground. At the top of the mast, radial top-load wires are connected (usually about six) which slope downwards at an angle of 40–45 degrees as far as about one-third of the total height, where they are terminated in insulators and thence outwards to ground anchors. Thus the umbrella antenna uses the guy wires as the top-load part of the antenna. In all these antennas the smaller radiation resistance of the short radiator is increased by the capacitance added by the wires attached to the top of the antenna." en.wikipedia.org/wiki/Medium_waveThis might make Part 15 stations be able to think about using the middle or lower ends of the AM band. Now the page on the Umbrella antenna: "At the low frequencies used the height of the mast is typically 1/10 to 1/40 of a wavelength, so it makes a very electrically short antenna, and by itself would have very low radiation resistance and be a very inefficient radiator. The function of the umbrella-wires is to add capacitance to the top of the antenna, improving the current distribution on the vertical mast radiator to increase the radiation resistance and radiated power. The umbrella wires serve as the plate of a capacitor, with the ground serving as the other plate, which is charged and discharged by the radio frequency current from the transmitter. " en.wikipedia.org/wiki/Umbrella_antennaIf I make them, my copyright, trademark, patented version will be called the "Big Top Antenna" because of how it would resemble the framework for a big top circus tent. Boomer
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Post by Deleted on Nov 4, 2017 19:08:59 GMT
I'm not a big fan of the umbrella top hat because the downward sloping wires decrease the antenna current-area compared to a horizontal top hat. The top hat spokes for a 3 meter LPAM antenna are short enough to be horizontal tubing. On to my top hat antenna project and here's the recipe for a version with the loading coil near the top. I prefer the coil near the top because the coil form does not have to be as strong. Some of the loading inductance (20%) can be placed at the base for ease of tuning. 1. Chose the operating frequency. 2. Chose the antenna bandwidth. 3. Calculate the Q 4. Estimate system loss 5. Multiply Q by loss to find the top hat capacitance 6. Using NEC design a top hat with the required capacitive reactance 7. Design the loading coil(s) Example 1. 1700 kHz 2. 20 kHz to pass 7.5 kHz audio plus a 5 kHz guardband 3. Q = 1700/20 = 85 4. 15 ohms 5. 85 x 15 = 1275 ohms reactance 6. Eight 5' spokes (0.625" OD) with the loading coil at the 8' point, reactance is 1180 ohms and inductance is 110 uH. 7. 49 turns close-wound stranded #14 THHN house wire on a schedule-40 3" PVC pipe, or 83 turns on a 2" PVC pipe The pipe OD is 3/8" larger than the published pipe ID and we then add 0.1" to account for the coil diameter measured through the center of the wire. The wire O.D. is 0.1". 5' tubing is used because the construction uses 10' EMT from the hardware store and it's easy to simply cut them in half. NEC shows radiation resistance to be 0.48 ohms. Assuming the loss turns out to be 15 ohms the radiation efficiency is 0.48/15 x 100% = 3.2%. Solenoid inductance calculator electronbunker.ca/eb/InductanceCalc.html
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