Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
Current across the antenna loading coil - from scratch
Richard Clark wrote:
"This must be a convention that is particular to only a very few hams. The FCC database describes AM antennas in both electrical degrees and physical height as follows." It is the convention to describe AM broadcast towers in electrical degrees. Harold Ennes reprints an RCA resistance chart for heights between 50 and 200 degrees in "AM-FM Broadcast Maintenance". Formula given is: Height in electrical degrees = Height in feet X frequency in kc X 1.016 X 10 to the minus 6 power. Example Towers: 50-degrees self-supporting: R=7. jx=-j100 50-degrees guyed mast: R=8, jx=-j222 90-degrees self-supporting: R=40, jx=+j35 90-degrees guyed mast: R=36, jx=j0 200-degrees self-supporting: R=23, jx=-j50 200-degrees guyed mast: R=80, jx=-400 There are values of R and X for 16 different heights. If you are interested, look at the book. Best regards, Richard Harrison, KB5WZI |
#2
|
|||
|
|||
Current across the antenna loading coil - from scratch
"Richard Harrison" wrote:
It is the convention to describe AM broadcast towers in electrical degrees. Harold Ennes reprints an RCA resistance chart for heights between 50 and 200 degrees in "AM-FM Broadcast Maintenance". Formula given is: Height in electrical degrees = Height in feet X frequency in kc X 1.016 X 10 to the minus 6 power. _______________ If electrical length is defined as the physical condition where feedpoint reactance is zero (e.g., resonance), then the true electrical length of an AM broadcast radiator on a given frequency is a function of the physical length AND physical width of that radiator. This was proven experimentally, and documented by George Brown of RCA Labs in his paper "Experimentally Determined Impedance Characteristics of Cylindrical Antennas" published in the Proceedings of the I.R.E. in April, 1945. It also has been proven in thousands of independent measurements of AM broadcast radiators ever since. The curves in Figure 3 of Brown's paper show the feedpoint reactance terms of the base impedance of an unloaded monopole of various lengths and widths, working against a nearly perfect ground plane. Those values cross the zero reactance axis at physical heights ranging from about 80 degrees (for the widest radiator) to about 86 degrees for the most narrow. Brown calculated height in degrees as (Physical Height in feet x Frequency in kHz ) / 2725 . Brown's equation, the one in the Harold Ennes quote above, and the one that the FCC uses in their published data all define only the relationship of the physical length of the radiator to its free-space wavelength in degrees at that frequency. But clearly these lengths in degrees do not define the self-resonant length of that radiator. The self-resonant length, invariably, will be shorter by several percent. This fact is easily confirmed by simple NEC models, for those who want to probe into George Brown's data. Tables relating a single value of base impedance as typical for towers of various electrical heights (only) must be read with an understanding of the above realities. For example, Ennes' list shows a tower of 90 electrical degrees to have zero reactance. But Brown's 1945 paper and a great amount of later field experience shows that this is incorrect, for the conventional use of this term. RF |
#3
|
|||
|
|||
Current across the antenna loading coil - from scratch
Richard Fry wrote:
But clearly these lengths in degrees do not define the self-resonant length of that radiator. Could it be that the resonant 80 degrees of physical length is 90 degrees of electrical length? -- 73, Cecil http://www.qsl.net/w5dxp |
#4
|
|||
|
|||
Current across the antenna loading coil - from scratch
"Cecil Moore" wrote ...
Richard Fry wrote: But clearly these lengths in degrees do not define the self-resonant length of that radiator. Could it be that the resonant 80 degrees of physical length is 90 degrees of electrical length? __________ That a self-resonant, unloaded broadcast radiator length is shorter than the 90 degree conventional "electrical length" defined by the FCC is a given. But this reality sometimes is not recognized. RF |
#5
|
|||
|
|||
Current across the antenna loading coil - from scratch
Richard Fry wrote:
"Cecil Moore" wrote ... Could it be that the resonant 80 degrees of physical length is 90 degrees of electrical length? That a self-resonant, unloaded broadcast radiator length is shorter than the 90 degree conventional "electrical length" defined by the FCC is a given. But this reality sometimes is not recognized. For instance, EZNEC says a 33 ft. vertical made of #30 wire is resonant on 7.265 MHz while a one foot diameter pipe is resonant on 6.9 MHz. Does the FCC define physical lengths or electrical lengths? -- 73, Cecil http://www.qsl.net/w5dxp |
#6
|
|||
|
|||
Current across the antenna loading coil - from scratch
"Cecil Moore"
Does the FCC define physical lengths or electrical lengths? ____________ They call it an electrical length, but calculate it as the number of free-space electrical degrees contained in the physical length of the radiating structure, at the carrier frequency. So really, FCC "electrical length" is a measure of a physical length, not of an effective electrical length. The effective electrical length of a MW monople radiator determines its resonant frequencies, and that must include the velocity of propagation along the structure -- which is a function of the height AND width of the radiator (mainly), and the operating frequency. RF |
#7
|
|||
|
|||
Current across the antenna loading coil - from scratch
On Tue, 4 Apr 2006 12:11:20 -0500, "Richard Fry"
wrote: The effective electrical length of a MW monople radiator determines its resonant frequencies, and that must include the velocity of propagation along the structure -- which is a function of the height AND width of the radiator (mainly), and the operating frequency. WGOP 80.00° tall 125.2 meters tall 540 kHz WWCS 63.50° tall 98.8 meters tall 540 kHz WFTD 79.00° tall 64.0 meters tall 1080 kHz KYMN 118.60° tall 92.3 meters tall 1080 kHz WWLV 90.00° tall 47.2 meters tall 1620 kHz WTAW 204.00° tall 106.7 meters tall 1620 kHz http://www.fcc.gov/mb/audio/amq.html The FCC provides BOTH measurements. The correlation is obvious. Any association between resonance, velocity of propagation, height, width, etc. and something like our 118.60° tall antenna needs a heap more explaining than resonance, velocity of propagation, height, width, etc. - but such explaining is a specialty occupation here in this group. 73's Richard Clark, KB7QHC |
#8
|
|||
|
|||
Current across the antenna loading coil - from scratch
I'm not at all sure what all the hoop-lah following Richard Fry's
posting reproduced below is all about. What Richard wrote is accurate, as he says confirmed by NEC simulation, and also from the King-Middleton second-order theory of linear antennas. From the date, it sounds like Brown's paper was a confirmation of the theory, actually. An antenna resonant at 95% of a freespace quarter wave, above perfect ground, would be about 150 times as long as its diameter--a 75 meter tower about half a meter effective diameter. NEC gives slightly different numbers, but perhaps more interesting is that even for VERY thin wires, the resonant length is noticably shorter than a freespace quarter wave. A wire a millionth as thick as it is long still shows resonance more than a percent shorter than the freespace wavelength. It's an interesting observation, but I thought everyone (with a serious interest in antennas) would know about it. The effect at full-wave dipole resonance/half-wave above a ground plane is considerably more pronounced, over ten percent for a moderately thick antenna. Cheers, Tom Richard Fry wrote in Message-ID: : "Richard Harrison" wrote: It is the convention to describe AM broadcast towers in electrical degrees. Harold Ennes reprints an RCA resistance chart for heights between 50 and 200 degrees in "AM-FM Broadcast Maintenance". Formula given is: Height in electrical degrees = Height in feet X frequency in kc X 1.016 X 10 to the minus 6 power. _______________ If electrical length is defined as the physical condition where feedpoint reactance is zero (e.g., resonance), then the true electrical length of an AM broadcast radiator on a given frequency is a function of the physical length AND physical width of that radiator. This was proven experimentally, and documented by George Brown of RCA Labs in his paper "Experimentally Determined Impedance Characteristics of Cylindrical Antennas" published in the Proceedings of the I.R.E. in April, 1945. It also has been proven in thousands of independent measurements of AM broadcast radiators ever since. The curves in Figure 3 of Brown's paper show the feedpoint reactance terms of the base impedance of an unloaded monopole of various lengths and widths, working against a nearly perfect ground plane. Those values cross the zero reactance axis at physical heights ranging from about 80 degrees (for the widest radiator) to about 86 degrees for the most narrow. Brown calculated height in degrees as (Physical Height in feet x Frequency in kHz ) / 2725 . Brown's equation, the one in the Harold Ennes quote above, and the one that the FCC uses in their published data all define only the relationship of the physical length of the radiator to its free-space wavelength in degrees at that frequency. But clearly these lengths in degrees do not define the self-resonant length of that radiator. The self-resonant length, invariably, will be shorter by several percent. This fact is easily confirmed by simple NEC models, for those who want to probe into George Brown's data. Tables relating a single value of base impedance as typical for towers of various electrical heights (only) must be read with an understanding of the above realities. For example, Ennes' list shows a tower of 90 electrical degrees to have zero reactance. But Brown's 1945 paper and a great amount of later field experience shows that this is incorrect, for the conventional use of this term. RF |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Imax ground plane question | CB | |||
Questions -?- Considering a 'small' Shortwave Listener's (SWLs) Antenna | Shortwave | |||
FS: sma-to-bnc custom fit rubber covered antenna adapter | Scanner | |||
FS: sma-to-bnc custom fit rubber covered antenna adapter | Swap | |||
Current in loading coil, EZNEC - helix | Antenna |