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Post by Druid Hills Radio on Oct 30, 2017 12:58:44 GMT
Have a few questions...(1) Not sure exactly what a "top hat" is but if it's non radiating, what's it for? and (2) same question about the ground lead that's not connected to anything? But the wire could be considered as part of the antenna and over the 10ft limit. Mark From a Rank Amateur: The Top Hat adds capacitance to the antenna circuit and as such reduces the amount of inductance required for a physically short antenna. Answer 2: Yes you are correct. Effectively one would have an off-centered fed dipole.
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Post by thelegacy on Oct 31, 2017 2:49:03 GMT
The ground wire that's not actually connected to the ground or a ground rod is like a counterpoise to a dipole antenna.
A top hat although it's not physically connected is sort of like a capacitance radiator. Some people call it a synthetic radiator or sympathetic radiator either way it does sort of increase range. The top hats real job though is to try to make up for the shortness of the antenna and supposedly make it tune better.
There is lots of controversy about the effectiveness of a top hat but one thing's for sure about the ground lead that's not connected to anything it does make a counterpoise to a dipole and will increase range.
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Post by Boomer on Oct 31, 2017 11:54:36 GMT
Really, it's fun to think about, but for best output you would want to have your ground and capacity hat actually connected to the system. I don't think the capacitance hat would do one bit of good just hovering over the antenna, and you'd need the ground to go to a stake in the earth to drain off static charges, even from the wind!
Boom
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Post by Druid Hills Radio on Oct 31, 2017 13:13:27 GMT
Top hats are physically connected.
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Post by Deleted on Nov 1, 2017 19:21:21 GMT
Have a few questions...(1) Not sure exactly what a "top hat" is but if it's non radiating, what's it for? and (2) same question about the ground lead that's not connected to anything? But the wire could be considered as part of the antenna and over the 10ft limit. Mark Hi Mark. A top hat on a vertical antenna is usually a horizontal disk, or a number of spokes mounted at the top, and it offers several benefits. 1. Increases the antenna "current-area," Current-area is the antenna current integrated along the antenna. For example, a simple base-loaded monopole has maximum current at the base tapering linearly to near-zero at the tip; the current shape is triangular. For a 3 meter single wire monopole the current area for 1 amp at the base, is 1.5 A-m (1.5 amp-meters). A top hat tends to make the current along the antenna uniform. E.g. 1 amp at the base and 1 amp at the top. The current-area of a 3 meter top hat monopole can be 3 A-m. All else equal, the E-field in the far field is doubled and the AM transmitter range is doubled. The station coverage area is quadrupled. Let's take the case of a 2 meter tall antenna with a 1 meter radius top hat made with 16 spokes. Some think that this meets the FCC 3 meter rule when measured from the end of spoke to the antenna and down to the ground. The current-area of this antenna is 2 A-m, providing 33% more range and 78% more coverage area compared to a 3 meter single wire monpole. 2. Reduces the antenna loading reactance and can lead to increased signal strength. This means the loading inductance is reduced and the loading coil loss is lower. And, the antenna bandwidth is increased. Given low enough ground loss and loading coil loss antenna bandwidth can become so narrow that the AM sidebands are attenuated; the high audio frequency content is attenuated. Taking the case of a 3 meter CB whip antenna at 1500 kHz the reactance is 4000 ohms. The radiation resistance is 0.09 ohms. If the combined ground and loading coil loss are 40 ohms the antenna Q is 4000/40 = 100. The antenna bandwidth is 1500kHz/100 = 15 kHz, barely enough for decent AM. So, for this antenna greatly improving the ground system might result in a stronger carrier but attenuated sidebands. The antenna radiation efficiency is (0.09ohms/40ohms) x 100%= 0.2%. Now let's make the antenna 2 meters long and top it with 12 metal spokes 1 meter in length. Some think that this meets the FCC 3 meter length rule. The reactance is 1700 ohms and the radiation resistance is 0.11 ohms. We can reduce the ground and loading coil loss to 17 ohms and have the same bandwidth. The antenna radiation efficiency is (0.11/17) x 100% = 0.65%. The signal improvement is 0.65/0.2 = 3.25; the antenna radiates 3.25X more power and the E-field in the far field has increased by a factor of 1.8, providing 80% greater range and 3.25X more coverage area. Making the the antenna 3 meters long topped with 12 metal spokes 1 meter in length is a further improvement. Does this meet the FCC 3 meter length rule, being that the top hat does not radiate? The reactance is 1600 ohms and the radiation resistance is 0.25 ohms. We can reduce the ground and loading coil loss to 16 ohms and have the same bandwidth. The antenna radiation efficiency is (0.25/16) x 100% = 1.6%. The signal improvement is 1.6/0.2 = 8; the antenna radiates 8X more power and the E-field in the far field has increased by a factor of 2.8, providing 180% greater range and 8X more coverage area. In the original post the ungrounded 25' (7.6 meters) can add 1/2 x 7.6 = 3.8 A-m of current-area. Compared to a 3 meter monopole having a current-area of 1.5 A-m the E-field is increased by 3.5X.
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Post by mark on Nov 2, 2017 23:23:09 GMT
@davec,
Thanks for the informative answer.
Mark
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Post by Boomer on Nov 4, 2017 1:25:00 GMT
You can add a resistor across the loading coil, or a low value capacitor between the top of the loading coil to ground to lower the Q, and it will give you more antenna bandwidth, and lower the efficiency too. It's what commercial Part-15 transmitters do, using the capacitor as a fine tuner to peak the antenna output. With higher Q antennas, you might have to tune them every day because of changing conditions.
Nice study Dave on using a 2 meter antenna and 1 meter hat, and that it actually increases the efficiency, very cool! I hadn't thought of that one, shorter radiator still, which will support a top hat's bulk better, but wider, but maybe an option for more restricted space areas where the lower profile is needed.
Boomer
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