Home |
Search |
Today's Posts |
#11
|
|||
|
|||
Where does it go? (mismatched power)
|
#12
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 6:16*am, Owen Duffy wrote:
... The transmitter output power is probably different ... Thank you, Owen. Do your comments apply to a transmitter designed/adjusted for, and expecting a 50 + j 0 ohm load? IOW, if the net output power of such a transmitter (which equals that dissipated in the load) probably is different with such a mismatch, do you expect the reason for that to be related to "reflected power?" RF |
#13
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 11:32*am, Richard Fry wrote:
On Jun 11, 6:16*am, Owen Duffy wrote: *... The transmitter output power is probably different ... Thank you, Owen. Do your comments apply to a transmitter designed/adjusted for, and expecting a 50 + j 0 ohm load? IOW, if the net output power of such a transmitter (which equals that dissipated in the load) probably is different with such a mismatch, do you expect the reason for that to be related to "reflected power?" RF NO! NO! NO! the difference is not due to 'reflected power'... any difference is due to the impedance change seen by the transmitter at its terminals. A VERY important method of analyzing sinusoidal steady state response of a system like a transmission line with a load on the end takes the load impedance and transforms it using the transmission line equations (with or without loss) back to the source terminals and then replacing it with an equivalent impedance. as long as the load is constant and linear this is a well known and easily proved substitution. you can then solve for conditions seen by the source ignoring the length of the line and load and using only a lumped impedance, therefore any reflected wave is irrelevant when analyzing the response of the source. |
#14
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 9:13*am, K1TTT wrote:
NO! NO! NO! *the difference is not due to 'reflected power'... any difference is due to the impedance change seen by the transmitter at its terminals. If the "forward" power that can be generated by a transmitter is dependent on the degree of match of the output load to the load specified for that transmitter, would that not mean that there would be little/no risk of damage to the transmitter if it was operated into a load with ~infinite SWR relative to the load it expects, such as a short or open directly at its r-f output connector? RF |
#15
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 10:45*am, K1TTT wrote:
On Jun 9, 10:38*pm, Jim Lux wrote: In fact, the ration between that stored energy and the amount flowing "through" (i.e. radiated away) is related to the directivity of the antenna: high directivity antennas have high stored energy (large magnetic and electric fields): *the ratio of stored to radiated energy is "antenna Q" (analogous to the stored energy in a LC circuit leading to resonant rise). So, high directivity = high stored energy = high circulating energy = high I2R losses. this is a relationship i haven't heard of before... and would be very wary of stating so simply. *it may be true for a specific type of antenna, MAYBE Yagi's, MAYBE rhombics or or close coupled wire arrays, but some of the most directive antennas are parabolic dishes which i would expect to have very low Q and extremely low losses. *you could also have an antenna with very high Q, very high i^2r losses, but very low directivity, so i would be careful about drawing a direct link between the two. and yes, this does work for complex loads and multiple stubs and connections to the line. this is a reasonable description of the derivation of these techniques, study especially the Thevenin equivalent impedance representation on page 2-13 and how it is applied: http://ee.sharif.edu/~comcir/readings/tran%20line.pdf |
#16
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 9:13*am, K1TTT wrote:
NO! NO! NO! *the difference is not due to 'reflected power'... any difference is due to the impedance change seen by the transmitter at its terminals. Please let's be careful to give all the details. The *virtual* impedance, the ratio of Vtotal/Itotal, seen by the transmitter at its terminals is: Z = Vtotal/Itotal = (Vfor + Vref)/(Ifor + Iref) where all math is phasor (vector) math. Vref and Iref are components of the reflected wave. So the mismatch is certainly related to the magnitude and phase of the reflected wave. If the Z0 of the transmission line is the impedance for which the transmitter was designed, we can go as far as to say that the reflected wave causes the mismatch, the virtual impedance that deviates from the Z0 of the line. Before the reflected wave arrives back at the transmitter, the transmitter sees the Z0 of the transmission line. The mismatch develops when the reflected wave arrives. -- 73, Cecil, w5dxp.com |
#17
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 3:03*pm, Cecil Moore wrote:
On Jun 11, 9:13*am, K1TTT wrote: NO! NO! NO! *the difference is not due to 'reflected power'... any difference is due to the impedance change seen by the transmitter at its terminals. Please let's be careful to give all the details. The *virtual* impedance, the ratio of Vtotal/Itotal, seen by the transmitter at its terminals is: Z = Vtotal/Itotal = (Vfor + Vref)/(Ifor + Iref) where all math is phasor (vector) math. Vref and Iref are components of the reflected wave. So the mismatch is certainly related to the magnitude and phase of the reflected wave. If the Z0 of the transmission line is the impedance for which the transmitter was designed, we can go as far as to say that the reflected wave causes the mismatch, the virtual impedance that deviates from the Z0 of the line. Before the reflected wave arrives back at the transmitter, the transmitter sees the Z0 of the transmission line. The mismatch develops when the reflected wave arrives. -- 73, Cecil, w5dxp.com i wouldn't call it a 'virtual' impedance, it is a very real impedance. it is the steady state impedance seen by the transmitter at its output terminals. once the transient response rings down the waves in the line beyond the terminals are irrelevant and the constant impedance can be used to calculate the voltage, current, and power in the source. in the steady state (which except in special cases is entirely adequate for amateur use) the mechanism of generating that impedance is irrelevant, waves or not it can be represented by a constant value and the source won't know the difference. |
#18
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 10:36*am, K1TTT wrote:
i wouldn't call it a 'virtual' impedance, it is a very real impedance. Yes, it meets the (B) non-dissipative definition of impedance from "The IEEE Dictionary". It doesn't meet the (A) dissipative definition and it is not an impedor as it only exists as a V/I ratio. That's why I call it a virtual impedance. There is no resistor, there is no inductor, and there is no capacitor. There is only a V/I ratio caused by something else. it is the steady state impedance seen by the transmitter at its output terminals. * And it is a V/I ratio caused by the superposition of the forward wave and reflected wave, i.e. it would NOT be the same impedance without the reflected wave. That fact of physics is undeniable. When we have an actual impedor, i.e. a resistor plus an inductor or a capacitor, the voltage/current ratio is caused by the impedor. When we have a virtual impedance, the cause/effect procedure is reversed and the impedance is caused by the voltage/current ratio which may (or may not) contain both forward and reflected values. For the two definitions of impedance, (A) and (B), given in "The IEEE Dictionary", cause and effect are reversed. Let's take an example. Assume a 50 ohm load resistor fed with 1/2WL of 300 ohm lossless line. If we assume a 100 watt (50 ohm) source, the forward power will be 204 watts and the reflected power will be 104 watts. 100w Source----1/2WL 300 ohm feedline----50 ohm load In the feedline, a forward power of 204 watts is a forward voltage of 247.3 volts and a forward current of 0.825 amps. In the feedline, a reflected power of 104 watts is a reflected voltage of 176.6 volts and a reflected current of 0.589 amps. The reflected voltage is 180 degrees out of phase with the forward voltage at the transmitter, so the superposed voltage at the transmitter is 70.7 volts. The reflected current is in phase with the forward current at the transmitter so the superposed current is 1.414 amps. Since everything is either in phase or 180 degrees out of phase, phasor addition is not needed. So I repeat, the impedance seen by the transmitter is: Z = (Vfor - Vref)/(Ifor + Iref) Z = (247.3 - 176.6)/(0.825 + 0.589) Z = 70.7/1.414 = 50 ohms, NON-DISSIPATIVE! It is clear that the superposition of the forward wave with the reflected wave is the CAUSE of the 50 ohm impedance. That particular impedance cannot exist without the effects of the reflected wave. Using a math model for answers to problems for so long that one comes to believe that the model dictates reality is a major contributor to the myths and old wives' tales that exist in amateur radio today. It's time to get back to the basics even if it causes a few old rusty brains to be put in gear for the first time in decades. -- 73, Cecil, w5dxp.com |
#19
|
|||
|
|||
Where does it go? (mismatched power)
Richard Fry wrote:
On Jun 11, 6:16 am, Owen Duffy wrote: ... The transmitter output power is probably different ... Thank you, Owen. Do your comments apply to a transmitter designed/adjusted for, and expecting a 50 + j 0 ohm load? IOW, if the net output power of such a transmitter (which equals that dissipated in the load) probably is different with such a mismatch, do you expect the reason for that to be related to "reflected power?" RF We've had this discussion many times before, but with no apparent change in your perceptions of the nature of "reflected power". But why would you expect the "reflected power" to have a different effect when the transmission line is a quarter wavelength than when it's a half wavelength? Can you write an equation giving the supposed dissipation caused by "reflected power" as a function of transmission line length? Roy Lewallen, W7EL |
#20
|
|||
|
|||
Where does it go? (mismatched power)
On Jun 11, 4:18*pm, Cecil Moore wrote:
On Jun 11, 10:36*am, K1TTT wrote: i wouldn't call it a 'virtual' impedance, it is a very real impedance. Yes, it meets the (B) non-dissipative definition of impedance from "The IEEE Dictionary". It doesn't meet the (A) dissipative definition and it is not an impedor as it only exists as a V/I ratio. That's why I call it a virtual impedance. There is no resistor, there is no and that is part of the problem in here... 'you call it'. what is the definition of 'virtual impedance' in the ieee dictionary? |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Mismatched Zo Connectors | Antenna | |||
Calculating loss on a mismatched line | Antenna | |||
Collins R390 power cord and power line filter | Boatanchors | |||
Collins R390 power cord and power line filter | Boatanchors | |||
Astron RS-20A Power Supply Great Condition - used to power a VHF radio | Swap |