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Overheard yesterday.....
- Thread starter PA629
- Start date
W5LZ said:Ken,
Ah, but that's the point! If the matching is done at the antenna input, then the antenna is the same impedance as the coax, as the radio's output. Yeah, I know that's at only one frequency, but considering the normal variations in normal coax, radio outputs, and antenna inputs, it's about as 'practical' as you can get.
C2,
Color me as in Ross Pirot. NOT that inventor of the internet!
CWM,
I'll take the 'something else', as in ladder-line, then it just doesn't make a lot of difference.
- 'Doc
I know, Doc, I was just goin' on..........I'm using ladderline myself!
73
CWM
Ya-but... if the antenna and coax are compatible, and they are both near 50 ohms, why shouldn't the radio be happy too?
Don't feel sorry for the SWR's. They are used to confusion, think it's normal. So they get even more confused when things are close to right and are even more happy with the confusion!
- 'Doc
If you figure out what I just said, I wish you'd tell me.
Don't feel sorry for the SWR's. They are used to confusion, think it's normal. So they get even more confused when things are close to right and are even more happy with the confusion!
- 'Doc
If you figure out what I just said, I wish you'd tell me.
The SWR that is measured with a power meter (or SWR meter) only exists inside the coax, and only when the coax is connected between a source and load (which can be an open, short, resistor, antenna, etc...).
If the source, load, and coax are all the same impedance (matched), then the coax acts as an ideal transmission medium, which means it appears as an impedance equal to its characteristic value, and it allows maximum power transfer between the source and load. This condition is ideal and somewhat unrealistic for most installations.
If the source or load is a different value from the other, or from the characteristic imepedance of the coax, then the coax behaves as a varying impedance that is a function of its wavelength (resistive or reactive, or as a tuned filter) and the impedances connected at each end. This condition is non-ideal, but fairly common for most installations.
The VSWR is generated at the source, but is controlled by the load since the source has a fixed impedance and is very short in wavelength when compared to the signal, coax's wavelength, and antenna's wavelength.
When the antenna is tuned, the coax and antenna both appear as a load to the transmitter, and therefore when combined together they create/control the VSWR that is measured inside the coax.
So even though the antenna's actual feedpoint impedance may not be anywhere near 50 ohms resistive, it can be made to appear near this value at the transmitters output due to the coax's varying impedance properties that exist when impedances are not matched. The coax is acting as part of the load with the antenna...
The standing waves that are measured only exist inside the coax, and are constant in value along the coax's entire length, as long as the coax overall length, and transmitter output, and antenna's feedpoint do not change. Remember the definition of SWR is the maximum value of standing wave voltage or current divided by the minimum value of standing wave voltage or current that is measured within a given length of coax.
The only time that the coax length will not effect VSWR is when all impedances are matched.
The only time that the VSWR of the antenna feedpoint can be close to being measured, is when the coax is a multiple of 1/2 wavelength long, which means the same impedance is seen on both ends of the coax.
The only way to be sure though is to try a few tests...
ken
ken,
if after tuning the antenna you cannot get a good match to the feedline does cutting the coax to get a decent match at the transmitter also give a decent match at the antenna and cancell the standing waves or is the mismatch at the antenna still there and are the standing waves still present or does neither of the above happen when cutting coaxial for a good match :?:
if after tuning the antenna you cannot get a good match to the feedline does cutting the coax to get a decent match at the transmitter also give a decent match at the antenna and cancell the standing waves or is the mismatch at the antenna still there and are the standing waves still present or does neither of the above happen when cutting coaxial for a good match :?:
i will experiment with 2 meters 2t's 3 loads and some coax,
see what happens at both ends of the feedline when it is as close as i can get it to an electrical halfwave including line sections and after i cut it for best match at the transmitter :?
see what happens at both ends of the feedline when it is as close as i can get it to an electrical halfwave including line sections and after i cut it for best match at the transmitter :?
bob85,
"if after tuning the antenna you cannot get a good match to the feedline does cutting the coax to get a decent match at the transmitter also give a decent match at the antenna"
The antenna feedpoint will stay the same which means the antenna VSWR has not changed, but the VSWR in the coax has been reduced. Since the feedpoint didn't change, the antenna is still just as inefficient as it was before. However, more power is sent to it due to the reduced VSWR, so the ERP will increase some.
Both the coax and antenna are physically long when compared to the wavelength. An antenna is just an open piece of wire that appears as a low resistance to the transmitter based on its wavelength. So, the coax and antenna when combined can do the same thing even though the antenna may not be truely resonant.
"and cancell the standing waves"
The standing waves in the coax can be reduced even though the antenna feedpoint has not, or cannot be changed. This is because the coax and antenna now make up the load that the transmitter sees.
"or is the mismatch at the antenna still there and are the standing waves still present"
The 50 ohm mismatch is still at the antenna, but the SWR can be reduced within the coax by using the coax's varying impedance properties that exist when there are impedance mismatches. The correct length of coax can closely match the impedance at the antenna (even though it is not near 50 ohms) and at the trasnmitter, near 50 ohms.
"or does neither of the above happen when cutting coaxial for a good match"
Coax length will effect SWR when loads are mismatched. How much it is effected depends on how bad the starting points are.
"if after tuning the antenna you cannot get a good match to the feedline does cutting the coax to get a decent match at the transmitter also give a decent match at the antenna"
The antenna feedpoint will stay the same which means the antenna VSWR has not changed, but the VSWR in the coax has been reduced. Since the feedpoint didn't change, the antenna is still just as inefficient as it was before. However, more power is sent to it due to the reduced VSWR, so the ERP will increase some.
Both the coax and antenna are physically long when compared to the wavelength. An antenna is just an open piece of wire that appears as a low resistance to the transmitter based on its wavelength. So, the coax and antenna when combined can do the same thing even though the antenna may not be truely resonant.
"and cancell the standing waves"
The standing waves in the coax can be reduced even though the antenna feedpoint has not, or cannot be changed. This is because the coax and antenna now make up the load that the transmitter sees.
"or is the mismatch at the antenna still there and are the standing waves still present"
The 50 ohm mismatch is still at the antenna, but the SWR can be reduced within the coax by using the coax's varying impedance properties that exist when there are impedance mismatches. The correct length of coax can closely match the impedance at the antenna (even though it is not near 50 ohms) and at the trasnmitter, near 50 ohms.
"or does neither of the above happen when cutting coaxial for a good match"
Coax length will effect SWR when loads are mismatched. How much it is effected depends on how bad the starting points are.
C W Morse said:
Sometimes..... you can't get a good tune with an antennas current location and it can't be moved for various reasons. So what do you do now?
Trimming coax, using an inexpensive antenna tuner, adding reactive components to the antenna base, all do the same thing don't they?
........
the relative unimportance of low SWR when feed-line attenuation is low is demonstrated rather vividly in the following two examples of spacecraft antenna applications. first, in the Tiros-ESSA-Itos-APT weather satellites, of which the entire multifrequency antenna-systems design was the work of the author, the dipole terminal impedance at the beacon-telemetry frequency (108 MHz in early models) was 150 - j100 ohms, for a VSWR of 4.4, reflected power 40 percent. matching was performed at the line input, where it was fed by a 30 milliwatt telemetry transmitter. (we can't afford much power loss here!) the feedline and matching-network attenuation was 0.2 dB, and the additional loss from SWR on the feed line was 0.24 dB (5.4 percent), for a total loss of 0.44 dB (9.6 percent). on the prevalent but erroneous assumption that all reflected power (40 percent) is lost, only 18.1 milliwatts would reach the antenna, and efficiency, determined on the same erroneous basis, would be only 60 percent. but 27.1 milliwatts were measured; of the 2.9 milliwatts lost in total attenuation, only 1.6 milliwatts of it was from the 4.4:1 VSWR. so the real efficiency would have been 95.5 percent if perfectly matched at the load, but reduces to 90.4 percent by allowing the 4.4 VSWR to remain on the feed line. second, in the Navy Navigational Satellite (NAVSAT), used for precise position indications for ships at sea, the antenna terminal impedance at 150 MHz is 10.5 - j48 ohms, for a VSWR of 9.8, reflected power 66 percent. also matched at the line input, flat-line attenuation is 0.25 dB, and the additional loss from SWR is 0.9 dB, for a total system loss of 1.15 dB, approximately 1/6 of an S unit. this is an insignificant amount of loss for this situation, even in a space environment where power is at a premium. why did we match at the line input? because critical interrelated electrical, mechanical and thermal design problems made it impractical to match at the load. line-input matching provided a simple solution by permitting the matching elements to be moved to a noncritical location. this design freedom afforded tremendous saving in engineering effort with negligible compromise in rf efficiency, in spite of SWR levels many amateurs would consider unthinkable.
"another look at reflections........excerpt"
the relative unimportance of low SWR when feed-line attenuation is low is demonstrated rather vividly in the following two examples of spacecraft antenna applications. first, in the Tiros-ESSA-Itos-APT weather satellites, of which the entire multifrequency antenna-systems design was the work of the author, the dipole terminal impedance at the beacon-telemetry frequency (108 MHz in early models) was 150 - j100 ohms, for a VSWR of 4.4, reflected power 40 percent. matching was performed at the line input, where it was fed by a 30 milliwatt telemetry transmitter. (we can't afford much power loss here!) the feedline and matching-network attenuation was 0.2 dB, and the additional loss from SWR on the feed line was 0.24 dB (5.4 percent), for a total loss of 0.44 dB (9.6 percent). on the prevalent but erroneous assumption that all reflected power (40 percent) is lost, only 18.1 milliwatts would reach the antenna, and efficiency, determined on the same erroneous basis, would be only 60 percent. but 27.1 milliwatts were measured; of the 2.9 milliwatts lost in total attenuation, only 1.6 milliwatts of it was from the 4.4:1 VSWR. so the real efficiency would have been 95.5 percent if perfectly matched at the load, but reduces to 90.4 percent by allowing the 4.4 VSWR to remain on the feed line. second, in the Navy Navigational Satellite (NAVSAT), used for precise position indications for ships at sea, the antenna terminal impedance at 150 MHz is 10.5 - j48 ohms, for a VSWR of 9.8, reflected power 66 percent. also matched at the line input, flat-line attenuation is 0.25 dB, and the additional loss from SWR is 0.9 dB, for a total system loss of 1.15 dB, approximately 1/6 of an S unit. this is an insignificant amount of loss for this situation, even in a space environment where power is at a premium. why did we match at the line input? because critical interrelated electrical, mechanical and thermal design problems made it impractical to match at the load. line-input matching provided a simple solution by permitting the matching elements to be moved to a noncritical location. this design freedom afforded tremendous saving in engineering effort with negligible compromise in rf efficiency, in spite of SWR levels many amateurs would consider unthinkable.
"another look at reflections........excerpt"
Nice quote, but doesn't have much to do with HF. Those "critical interrelated electrical, mechanical and thermal design problems made it impractical to match at the load" also don't apply at terrestrial HF. As in, match the load, not the feed line. Just as easy as trimming coax and can be designed to handle what it was meant to handle, unlike coax as a matching device. It can even be made adjustable, unlike trimming coax. Just seems like more the 'thing' to do.
Rather trim coax? Have at it. Just don't fool yourself into thinking it's the 'best' way of doing things. Never has been (except in very particular circumstances) and never will be.
- 'Doc
Rather trim coax? Have at it. Just don't fool yourself into thinking it's the 'best' way of doing things. Never has been (except in very particular circumstances) and never will be.
- 'Doc
YAM
it is entirely applicable to and has everything to do with hf, the frequency that the author chose to demonstrate his point in no way diminshes its importance. except for decreased line loss at the lower frequencies the principles are exactly the same.
for many applications, coax can be used as a tuned line in precisely the same manner as open wire, only the degree to which it can be done differs, limited by the lower characteristic impedance of coaxial line. the spacecraft systems mentioned earlier are merely examples and not meant to indicate that these principles are only attributed to lines used at these frequencies or any restricted range of operating frequencies as you have incorrectly assumed.
when you begin to realize that reflected power doesn't flow back into the transmitter and cause dissipation and other damage and that damage blamed on reflection is really caused by improper output-coupling adjustment and not by SWR then you begin to understand why those who want to avail themselves of the increased operating flexibility, bandwitdh and other benefits that come with the ability to manage the match conditions existing between the transmitter and the input to the line do what they do, whether it's done with the feedline itself or in conjunction with a matching network.
once the transmitter is properly coupled to the line input the impedance present at the feedpoint of the load (and the line vswr) is virtually of no consequence and no reflected power whatsoever is able to travel past the match point towards the transmitter. that's the point of the excerpt and it doesn't just apply to the frequencies of the equipment used in the example and you either know it or you don't.
after learning of the benefits obtained with line-input, or conjugate matching in the two spacecraft examples described earlier, it is interesting to compare the results using this same input matching technique in typical 80- and 40- meter situations. eighty meters is the widest amateur band in terms of percent of center frequency and thus suffers the greatest SWR increase with frequency excursion to the band ends. a dipole cut for resonance at 3.75 MHz will yield an SWR in a 50-ohm feed line somewhat above 5:1 at both 3.5 and 4.0 MHz. in a 100-foot length of nonfoam RG-8/U, an SWR of 5:1 adds only 0.46 dB loss to the matched (i.e., flat line) loss of 0.32 dB at 4.0 MHz.. so out almost to the band ends, less than 1/12 of an S unit is lost because of the SWR, an imperceptible amount.
this further verifies the principle and proves that full-band, coax-fed dipole operation on 80 meters also is practical. even with the high SWR at the band ends, the loss cannot be distinguished from what it would have been had the SWR been a perfect one-to-one! At 40 meters, with the dipole resonated at 7.15 MHz, something is amiss if the SWR exceeds 2.5 at the band ends. this SWR adds only 0.18 dB to the matched loss, which at 7 MHz is 0.44 dB for 100 feet of RG-8/U coax.
yeah, it applies to hf.
it is entirely applicable to and has everything to do with hf, the frequency that the author chose to demonstrate his point in no way diminshes its importance. except for decreased line loss at the lower frequencies the principles are exactly the same.
for many applications, coax can be used as a tuned line in precisely the same manner as open wire, only the degree to which it can be done differs, limited by the lower characteristic impedance of coaxial line. the spacecraft systems mentioned earlier are merely examples and not meant to indicate that these principles are only attributed to lines used at these frequencies or any restricted range of operating frequencies as you have incorrectly assumed.
when you begin to realize that reflected power doesn't flow back into the transmitter and cause dissipation and other damage and that damage blamed on reflection is really caused by improper output-coupling adjustment and not by SWR then you begin to understand why those who want to avail themselves of the increased operating flexibility, bandwitdh and other benefits that come with the ability to manage the match conditions existing between the transmitter and the input to the line do what they do, whether it's done with the feedline itself or in conjunction with a matching network.
once the transmitter is properly coupled to the line input the impedance present at the feedpoint of the load (and the line vswr) is virtually of no consequence and no reflected power whatsoever is able to travel past the match point towards the transmitter. that's the point of the excerpt and it doesn't just apply to the frequencies of the equipment used in the example and you either know it or you don't.
after learning of the benefits obtained with line-input, or conjugate matching in the two spacecraft examples described earlier, it is interesting to compare the results using this same input matching technique in typical 80- and 40- meter situations. eighty meters is the widest amateur band in terms of percent of center frequency and thus suffers the greatest SWR increase with frequency excursion to the band ends. a dipole cut for resonance at 3.75 MHz will yield an SWR in a 50-ohm feed line somewhat above 5:1 at both 3.5 and 4.0 MHz. in a 100-foot length of nonfoam RG-8/U, an SWR of 5:1 adds only 0.46 dB loss to the matched (i.e., flat line) loss of 0.32 dB at 4.0 MHz.. so out almost to the band ends, less than 1/12 of an S unit is lost because of the SWR, an imperceptible amount.
this further verifies the principle and proves that full-band, coax-fed dipole operation on 80 meters also is practical. even with the high SWR at the band ends, the loss cannot be distinguished from what it would have been had the SWR been a perfect one-to-one! At 40 meters, with the dipole resonated at 7.15 MHz, something is amiss if the SWR exceeds 2.5 at the band ends. this SWR adds only 0.18 dB to the matched loss, which at 7 MHz is 0.44 dB for 100 feet of RG-8/U coax.
yeah, it applies to hf.