Was reading a very interesting pdf file entitled "1750mtxantennas" which describes a number of antenna designs. Nearly all of the designs call for a capacitance hat--usually 15 or so prongs extending downward from the top to about a third of the way down the antenna.
I was curious if such an addition would improve or hinder the efficiency of a Rangemaster using the recommended Radio Shack whip. For that matter would it improve a Procaster? Also (and I may be wrong) but I thought Keith Hamilton, at one time, mentioned placing a copper sheath (or pipe) over the Rangemaster's antenna to improve efficiency.
I think that many would agree, that adding such a capacitance had would jeopardize the FCC certification of the transmitter. Also, it is doubtful that Keith would recommend placing a copper sheath over the antenna as would would likely overstress the top of the transmitter case.
Now that being said, using a "fatter" conductor for a radiator increases transmit bandwidth, resulting in better audio on AM.
I think that many would agree, that adding such a capacitance had would jeopardize the FCC certification of the transmitter. Also, it is doubtful that Keith would recommend placing a copper sheath over the antenna as would would likely overstress the top of the transmitter case.
Now that being said, using a "fatter" conductor for a radiator increases transmit bandwidth, resulting in better audio on AM.
Just my opinion, but a capacitance hat
will change the way the antenna
resonates, so the transmitter's tuning
system might not work right.
I think most of the Part 15 longwave
experiments use capacitance hats.
But the longwave people are very low
key - they are not broadcasting - so
therefore they are under the radar. They
are mostly using morse code and other
digital languages. (And they are doing
amazing things.)
I think it is the feeling in the longwave Part 15
community that the top hat is not an antenna -
but a capacitor. Theoretically, for each spoke
in a top hat, there will be an opposite one that
will be out of phase. So any radiation from a
spoke will be canceled out by it's opposite.
However, I don't think it is a good idea for
Part 15 AM broadcasters to use top hats no
matter what kind of transmitter is in use.
I have to go, unfortunately.
Best Wishes,
Bruce, MICRO1690/1700
Just my opinion, but a capacitance hat
will change the way the antenna
resonates, so the transmitter's tuning
system might not work right.
I think most of the Part 15 longwave
experiments use capacitance hats.
But the longwave people are very low
key - they are not broadcasting - so
therefore they are under the radar. They
are mostly using morse code and other
digital languages. (And they are doing
amazing things.)
I think it is the feeling in the longwave Part 15
community that the top hat is not an antenna -
but a capacitor. Theoretically, for each spoke
in a top hat, there will be an opposite one that
will be out of phase. So any radiation from a
spoke will be canceled out by it's opposite.
However, I don't think it is a good idea for
Part 15 AM broadcasters to use top hats no
matter what kind of transmitter is in use.
I have to go, unfortunately.
Best Wishes,
Bruce, MICRO1690/1700
Bruce MICRO 1690/1700, you have for the first time given me a usable idea of what a so-called "capacitance hat" amounts to.
Your illustration of a spoke pointing, say, east, and an opposite spoke pointing west whose RF energy would exactly cancel out makes it clear that what we have is an upside-down "L" where the leg is VERTICAL and the little stub up-top is TRUNCATED-HORIZONTAL.
There is a design I've seen promoted (Star-H) for full-power situations in cases where great height is not an option. There are four poles set close together, with a total of 4-vertical wires up each pole, 45-feet, then each vertical goes horizontal at the top pointing east/west/north/south, out to the quarter-wavelength per each wire. It never occurred to me before, but now I am inclined to see the horizontal lengths on top as fully canceling each other out.
I'm now thinking the "capacitance" idea is mythical, like one hand clapping.
Bruce MICRO 1690/1700, you have for the first time given me a usable idea of what a so-called "capacitance hat" amounts to.
Your illustration of a spoke pointing, say, east, and an opposite spoke pointing west whose RF energy would exactly cancel out makes it clear that what we have is an upside-down "L" where the leg is VERTICAL and the little stub up-top is TRUNCATED-HORIZONTAL.
There is a design I've seen promoted (Star-H) for full-power situations in cases where great height is not an option. There are four poles set close together, with a total of 4-vertical wires up each pole, 45-feet, then each vertical goes horizontal at the top pointing east/west/north/south, out to the quarter-wavelength per each wire. It never occurred to me before, but now I am inclined to see the horizontal lengths on top as fully canceling each other out.
I'm now thinking the "capacitance" idea is mythical, like one hand clapping.
I have nothing to offer the original poster except a brief addendum on the theory of operation. As mentioned above, the spoke radials' radiation cancels if they are physically mated at 180 degrees. The principal of operation is that the hat changes the current distribution in the vertical radiator.
That helps because it is the current which produces radiation (accelerating electrons). Essentially, the more accelerating electrons in a conductor the more radiation. In a vertical monopole the current at the top of the antenna has to be zero since the electrons have nowhere to go beyond the tip. The current at the top of the radiator is zero and that near the top is small. The capacitive hat gives the electrons somewhere to go and thus raises the current at and near the top of the radiator in effect "pulling up" the current distribution along the whole radiator. This increase in current due to redistribution affects increased radiation.
I have not used hats on AM but for FM I have measured remarkable increases in radiation with a hat. One was simply a soda can placed over the tip of a short whip and another was a spoke type hat fashioned with a few inches of solid copper wire. I didn't pursue this beyond a simple experiment but the increase in radiation could have been due to tuning effects as well as current redistribution.
Neil
I have nothing to offer the original poster except a brief addendum on the theory of operation. As mentioned above, the spoke radials' radiation cancels if they are physically mated at 180 degrees. The principal of operation is that the hat changes the current distribution in the vertical radiator.
That helps because it is the current which produces radiation (accelerating electrons). Essentially, the more accelerating electrons in a conductor the more radiation. In a vertical monopole the current at the top of the antenna has to be zero since the electrons have nowhere to go beyond the tip. The current at the top of the radiator is zero and that near the top is small. The capacitive hat gives the electrons somewhere to go and thus raises the current at and near the top of the radiator in effect "pulling up" the current distribution along the whole radiator. This increase in current due to redistribution affects increased radiation.
I have not used hats on AM but for FM I have measured remarkable increases in radiation with a hat. One was simply a soda can placed over the tip of a short whip and another was a spoke type hat fashioned with a few inches of solid copper wire. I didn't pursue this beyond a simple experiment but the increase in radiation could have been due to tuning effects as well as current redistribution.
Neil
Capacitance hats on antenna systems are not magical. A radio antenna is a radio frequency energy load that possesses the properties of a radiator. In doing so, the antenna performance is, to some degree, governed by ohms law. Therefore, we can draw a schematic of the antenna as individual electronic components; resistor, capacitor and inductor. The inductor and capacitor produce REACTANCE (Xl/Xc), either inductive or capacitive. The resistor produces a pure RESISTANCE (R). The combination of net reactance and resistance equals the IMPEDANCE of the antenna. Each antenna has a specific impedance at a specific frequency. That means as the frequency changes something in the formula changes the impedance. That something is the reactance.
We can then draw the conclusion that a "capacitance hat" on an antenna adds capacitance to the antenna. This is done primarily to raise the impedance of the antenna load. The antenna need not be resonant to radiate. In fact, resonance only actually occurs in the antenna system at a single frequency when both flavors of reactance cancel out leaving the antenna nearly resistive only. You will notice most antennas are listed as having a specific working impedance, 50 Ohms and such. The working impedance changes as the frequency of the radio energy fed into the antenna changes. This helps determine the bandwidth of the antenna.
So, all that the capacitance hat would accomplish for the 102 inch whip on the Rangemaster is raise the impedance of the antenna. By doing so, the final tuning /tank circuit of the transmitter (which is designed for very low impedance, 2-3 Ohms) would very likely not tune the transmitter to the "cap-hat" antenna. In short, the transfer of energy from the transmitter to the antenna would be much less; certainly not enough to be overcome the small improvement in efficiency of the
cap-hat" antenna system.
This might be as clear as mud, but leave it said, others have expressed their skepticism here. I echo their thoughts.
Capacitance hats on antenna systems are not magical. A radio antenna is a radio frequency energy load that possesses the properties of a radiator. In doing so, the antenna performance is, to some degree, governed by ohms law. Therefore, we can draw a schematic of the antenna as individual electronic components; resistor, capacitor and inductor. The inductor and capacitor produce REACTANCE (Xl/Xc), either inductive or capacitive. The resistor produces a pure RESISTANCE (R). The combination of net reactance and resistance equals the IMPEDANCE of the antenna. Each antenna has a specific impedance at a specific frequency. That means as the frequency changes something in the formula changes the impedance. That something is the reactance.
We can then draw the conclusion that a "capacitance hat" on an antenna adds capacitance to the antenna. This is done primarily to raise the impedance of the antenna load. The antenna need not be resonant to radiate. In fact, resonance only actually occurs in the antenna system at a single frequency when both flavors of reactance cancel out leaving the antenna nearly resistive only. You will notice most antennas are listed as having a specific working impedance, 50 Ohms and such. The working impedance changes as the frequency of the radio energy fed into the antenna changes. This helps determine the bandwidth of the antenna.
So, all that the capacitance hat would accomplish for the 102 inch whip on the Rangemaster is raise the impedance of the antenna. By doing so, the final tuning /tank circuit of the transmitter (which is designed for very low impedance, 2-3 Ohms) would very likely not tune the transmitter to the "cap-hat" antenna. In short, the transfer of energy from the transmitter to the antenna would be much less; certainly not enough to be overcome the small improvement in efficiency of the
cap-hat" antenna system.
This might be as clear as mud, but leave it said, others have expressed their skepticism here. I echo their thoughts.
I've found the white-paper on the Star-H antenna which shows a photo of an array designed for 1680kHz under 250 Watts.
The description is as I stated earlier, 4-verticals clustered symmetrically near each other, up 45-foot poles and then making an L turn out in horizontal direction on all 4 poles of the compass, extending out to four distant poles. The base is fed by a single transmitter, therefore the parallel connection of all 4 elements lowers the impedance. Indeed, says the lit, the opposing horizontal wires cancel each other in phase, however they serve the purpose described in this thread also served by top hats, they alter the capacitance/reactive characteristics so as to help match the vertical section, all 4 parts of which merge in phase and strength.
So, putting everyone's view into the mix, it seems like the "hat" can help in some cases, hinder in others.
As a consequence of this discussion I am able to take the hat more seriously.
I've found the white-paper on the Star-H antenna which shows a photo of an array designed for 1680kHz under 250 Watts.
The description is as I stated earlier, 4-verticals clustered symmetrically near each other, up 45-foot poles and then making an L turn out in horizontal direction on all 4 poles of the compass, extending out to four distant poles. The base is fed by a single transmitter, therefore the parallel connection of all 4 elements lowers the impedance. Indeed, says the lit, the opposing horizontal wires cancel each other in phase, however they serve the purpose described in this thread also served by top hats, they alter the capacitance/reactive characteristics so as to help match the vertical section, all 4 parts of which merge in phase and strength.
So, putting everyone's view into the mix, it seems like the "hat" can help in some cases, hinder in others.
As a consequence of this discussion I am able to take the hat more seriously.
At first it would seem like a part 15 version of the Star-H antenna would be impermissible. Even smalling it down so that the vertical height would be under 3-meters and the upper horizontal off-shoots the remaining length, in final analysis the antenna would contain 4 upside down L's, pointing in 4 directions. No way?
Think Litz wire! On Wikipedia an illustration of a Litz wire shows 19 distinct insulated wires packed together inside an outer insulated binding. Litz WIRE: Singular!
Therefore we could view the Star-H as being based on a four-conductor Litz wire.
In practice there would not be the wavelength as if 4X 3-meter wires were placed in series. No. When put in parallel the only thing that changes is the impedance... and...
At first it would seem like a part 15 version of the Star-H antenna would be impermissible. Even smalling it down so that the vertical height would be under 3-meters and the upper horizontal off-shoots the remaining length, in final analysis the antenna would contain 4 upside down L's, pointing in 4 directions. No way?
Think Litz wire! On Wikipedia an illustration of a Litz wire shows 19 distinct insulated wires packed together inside an outer insulated binding. Litz WIRE: Singular!
Therefore we could view the Star-H as being based on a four-conductor Litz wire.
In practice there would not be the wavelength as if 4X 3-meter wires were placed in series. No. When put in parallel the only thing that changes is the impedance... and...