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- Thread starter JDWilbourn
- Start date
It really boils down to the fact that an antenna is never purely 50 ohms resistive across its entire BW unless there are wideband impedance matching devices at the antena input and then the antenna is still not 50 ohms across the BW even though it appears so...
If any of the impedances measured at the transmitters output, coax, or antennas feedpoint is different from one of the others measured values, the coax will exhibit changing impedance properties at various sub-multiples of 1/2 its wavelength. Since VSWR is a constant no matter where it is measured, the coax will always be part of any tuning done at the antenna unless the coax is exactly a multiple of 1/2 wavelength long of the tuning frequency.
If the VSWR changes when the coax lengths are changed, then the antenna feedpoint impedance is not exactly 50 ohms resistive. So, the coax is now part of the tuned system which allowed the VSWR to be adjusted to 1:1 - the coax created a conjugate of the antennas reactive impedance. This is called a resonant line.
The only way to make the coax nearly invisible when tuning the antenna is to use a 1/2 wavelength multiple of coax at the frequency being tuned, and the result of this tuning will give a good indication of the antennas feedpoint impedance, and it may or may not be 1:1 even after much work, and it is only valid at this one specific frequency.
If the VSWR is 1:1 after tuning with a 1/2 wavelength multiple, then coax length should not matter now since the antenna should be 50 nearly ohms resistive. But again, this is only true at the tuned frequency - not across the BW.
If the VSWR is not 1:1, then changing the length of the coax away from a 1/2 wavelength multiple may allow the antenna to be tuned to 1:1 by using the properties of a resonant transmission line, which is really no different than using an antenna tuner.
Just my $0.02
EDIT: VSWR does not give a good indication of radiated power, it only gives an indication of maximum power transfer. So, if an antenna has 48 ohms of resistive losses and 2 ohms of radiation resistance, the VSWR would be 1:1 but very little energy would be transmitted through the air - just like a dummy load. So don't get too hung up on VSWR...
If any of the impedances measured at the transmitters output, coax, or antennas feedpoint is different from one of the others measured values, the coax will exhibit changing impedance properties at various sub-multiples of 1/2 its wavelength. Since VSWR is a constant no matter where it is measured, the coax will always be part of any tuning done at the antenna unless the coax is exactly a multiple of 1/2 wavelength long of the tuning frequency.
If the VSWR changes when the coax lengths are changed, then the antenna feedpoint impedance is not exactly 50 ohms resistive. So, the coax is now part of the tuned system which allowed the VSWR to be adjusted to 1:1 - the coax created a conjugate of the antennas reactive impedance. This is called a resonant line.
The only way to make the coax nearly invisible when tuning the antenna is to use a 1/2 wavelength multiple of coax at the frequency being tuned, and the result of this tuning will give a good indication of the antennas feedpoint impedance, and it may or may not be 1:1 even after much work, and it is only valid at this one specific frequency.
If the VSWR is 1:1 after tuning with a 1/2 wavelength multiple, then coax length should not matter now since the antenna should be 50 nearly ohms resistive. But again, this is only true at the tuned frequency - not across the BW.
If the VSWR is not 1:1, then changing the length of the coax away from a 1/2 wavelength multiple may allow the antenna to be tuned to 1:1 by using the properties of a resonant transmission line, which is really no different than using an antenna tuner.
Just my $0.02
EDIT: VSWR does not give a good indication of radiated power, it only gives an indication of maximum power transfer. So, if an antenna has 48 ohms of resistive losses and 2 ohms of radiation resistance, the VSWR would be 1:1 but very little energy would be transmitted through the air - just like a dummy load. So don't get too hung up on VSWR...
rich :shock: :shock: we have talked about this in another thread,
i cant believe you thought i meant the feedline was changing the actual feedpoint of the antenna,
that would be like saying that when we look at a flea through a magnifying glass the flea grows 10 times larger :twisted:
take care.
i cant believe you thought i meant the feedline was changing the actual feedpoint of the antenna,
that would be like saying that when we look at a flea through a magnifying glass the flea grows 10 times larger :twisted:
take care.
"i cant believe you thought i meant the feedline was changing the actual feedpoint of the antenna,........"
changing what exactly at the feedpoint?
changing what exactly at the feedpoint?
The problem with the 1/2 wave coax is: What is a 1/2 wave? The OP has admitted that he is not up to snuff on radio theory, so is unlikely to be able to determine the proper length of coax. There are too many variables here.
Foam, or solid?
RG-8X, RG-58, brand differences, etc.
Did you know that the characteristic impedance of your coax will vary, depending upon which part of the roll it comes from. The center conductor tends to migrate towards the shield with sharper bends and it gets worse the longer it is on the spool. Foam dielectric is the worst for this.
Small things, but they add up to big trouble when you try to cut a 1/2 wave coax without the proper equipment.
My problem with the 18 feet of coax theory is, that it does not take into account the differences in coaxes, so that just throws it out the window as so much hooey. Forget about it.
Rich
Foam, or solid?
RG-8X, RG-58, brand differences, etc.
Did you know that the characteristic impedance of your coax will vary, depending upon which part of the roll it comes from. The center conductor tends to migrate towards the shield with sharper bends and it gets worse the longer it is on the spool. Foam dielectric is the worst for this.
Small things, but they add up to big trouble when you try to cut a 1/2 wave coax without the proper equipment.
My problem with the 18 feet of coax theory is, that it does not take into account the differences in coaxes, so that just throws it out the window as so much hooey. Forget about it.
Rich
Hamin' X said:
What does the OP not being up to snuff have to do with theory and practices if they are correct?
If the hobbiest cannot accurately calculate a 1/2 wavelength piece of coax based on frequency and velocity factor, than VSWR probably isn't much of a concern.
If the coax is damaged, and I agree it is easy to damage, then the VSWR will not be able to be adjusted correctly even if the antenna and CB are matched, so it will need to be replaced anyway if it is a serious problem.
The 18' coax length comes from 3/4 (odd 1/4 wave multiple) wavelength theory, calculated with a 0.66 velocity factor at the center of the CB band, which will exhibit a low impedance point at the far end when open. Now if a 1/4 wave monopole has a nice flat reflective surface, then it will operate similar to a 1/2 wave dipole, which also has a low impedance feedpoint. This means tuning should be easier since the antenna and coax are both at their low points. But as you have stated, and I agree, there are way to many variables to know for sure.
My question is, why not start tuning where the probability for success is best even if there are too many variables?
exactly. the majority of vehicles don't have a nice flat reflective surface and moreover they don't have enough running surface in any directions away from the antenna to mirror the antenna properly as in the case of the halfwave dipole in the first place. the ground plane surface is lacking and causes antenna input impedances to shrink below 50 ohms, so far in fact that tuning the antenna alone doesn't get the job done.
attaching antennas such as these to a feedline with a characteristic impedance of 50 ohms leaves the feedline repeating a range of impedances varying anywhere from 33 - 78 ohms all along the feedline at regular intervals. knowing where those values are along the line can be useful in introducing values that can be used to a large degree in correcting the match between the feedline and antenna and more importantly the match between the transmitter and the feedline as the real problem here isn't the high swr at the antenna but the reduction in power output from the transmitter as the conditions at the antenna feedpoint are translated back to the transmitter.
if the mismatch is corrected at the antenna then the transmitter delivers all available power to the load or the condition can be mitigated at the transmitter and that change is reflected back down to the antenna input.
if the load does not equal 50 ohms resistive then the 50 ohm characteristic impedance of the feedline no longer exists. now that can be perceived as an insurmountable problem or it can offer further solutions if the dynamic characteristics of the feedline in use are properly understood.
"The 18' coax length comes from 3/4 (odd 1/4 wave multiple) wavelength theory, calculated with a 0.66 velocity factor at the center of the CB band, which will exhibit a low impedance point at the far end when open."
and to demonstrate the fact that velocity factor is not as critical as some seem to think it is the 18' lines that we use here are of the foam variety generally assumed to possess a vf of approximately .78. the discrepancy there is many times larger than the manufacturers stated vf variance figure for mass manufactured feedline, aside of spot irregularities easily seen when the jacket is inspected as the line comes off of the roll. all the op has to do is use the calculator included in every windows platform. 492/fmhz. X vf = a tuned 1/2 wave line. if the length is short or long it's a simple matter to scan the frequencies above and below the design frequency to find the frequency where the line is a 1/2 wave with an antenna analyzer or an swr meter with a slightly higher degree of difficulty. if two swr meters are available the job can be made somewhat easier.
"why not start tuning where the probability for success is best even if there are too many variables?" the only thing better is knowing what some of those variables are beforehand. when you see a trend in a particular direction some of the variable values begin to reveal a consistent pattern.
unlike the questionable comparison between antennas and soda pop on page 1, most manufacturers build their antennas to exhibit input impedances in the neighborhood of 50 ohms and most do a good job of it. the problem is that when they are ultimately used in the environment they were built for they seldom exhibit the same input impedance seen in the manufacturers test bed because the average vehicle ground plane surface hardly resemebles the one used for antenna tuning by the manufacturer. the analogy would be something like tuning the antenna prior to distribution in the center of a circular metal surface with a radius of 9' in all directions away from the center and then expecting it to possess the same input impedance when installed on just about any personal or commercial vehicle, it just isn't going to happen. in almost every case the input impedance will be lower than that exhibited during the manufacturers testing procedure, with very few exceptions.
attaching antennas such as these to a feedline with a characteristic impedance of 50 ohms leaves the feedline repeating a range of impedances varying anywhere from 33 - 78 ohms all along the feedline at regular intervals. knowing where those values are along the line can be useful in introducing values that can be used to a large degree in correcting the match between the feedline and antenna and more importantly the match between the transmitter and the feedline as the real problem here isn't the high swr at the antenna but the reduction in power output from the transmitter as the conditions at the antenna feedpoint are translated back to the transmitter.
if the mismatch is corrected at the antenna then the transmitter delivers all available power to the load or the condition can be mitigated at the transmitter and that change is reflected back down to the antenna input.
if the load does not equal 50 ohms resistive then the 50 ohm characteristic impedance of the feedline no longer exists. now that can be perceived as an insurmountable problem or it can offer further solutions if the dynamic characteristics of the feedline in use are properly understood.
"The 18' coax length comes from 3/4 (odd 1/4 wave multiple) wavelength theory, calculated with a 0.66 velocity factor at the center of the CB band, which will exhibit a low impedance point at the far end when open."
and to demonstrate the fact that velocity factor is not as critical as some seem to think it is the 18' lines that we use here are of the foam variety generally assumed to possess a vf of approximately .78. the discrepancy there is many times larger than the manufacturers stated vf variance figure for mass manufactured feedline, aside of spot irregularities easily seen when the jacket is inspected as the line comes off of the roll. all the op has to do is use the calculator included in every windows platform. 492/fmhz. X vf = a tuned 1/2 wave line. if the length is short or long it's a simple matter to scan the frequencies above and below the design frequency to find the frequency where the line is a 1/2 wave with an antenna analyzer or an swr meter with a slightly higher degree of difficulty. if two swr meters are available the job can be made somewhat easier.
"why not start tuning where the probability for success is best even if there are too many variables?" the only thing better is knowing what some of those variables are beforehand. when you see a trend in a particular direction some of the variable values begin to reveal a consistent pattern.
unlike the questionable comparison between antennas and soda pop on page 1, most manufacturers build their antennas to exhibit input impedances in the neighborhood of 50 ohms and most do a good job of it. the problem is that when they are ultimately used in the environment they were built for they seldom exhibit the same input impedance seen in the manufacturers test bed because the average vehicle ground plane surface hardly resemebles the one used for antenna tuning by the manufacturer. the analogy would be something like tuning the antenna prior to distribution in the center of a circular metal surface with a radius of 9' in all directions away from the center and then expecting it to possess the same input impedance when installed on just about any personal or commercial vehicle, it just isn't going to happen. in almost every case the input impedance will be lower than that exhibited during the manufacturers testing procedure, with very few exceptions.
can the low feedpoint impedance of a 1/4 wave for 27mhz when mounted on an average car be corrected with a dollar match at the feedpoint ( loop or coil of wire to ground from the feedpoint ) in the same way a very low impedance shortened resonant 1/4 for say 80mtrs can be matched at the feedpoint rather than tune the antenna to get a decent match and lose resonance??,
would that be the best way to go for best performance?,
the arrl handbook says its better to resonate the antenna then match the low feedpoint impedance to the feedline so as not to waste energy at high angles due to altering the radiator just to get a decent match to the line,
what do you guys think??
i want to try this on my 40mtr mobile antennas and see if the men in the blue book speak the truth
would that be the best way to go for best performance?,
the arrl handbook says its better to resonate the antenna then match the low feedpoint impedance to the feedline so as not to waste energy at high angles due to altering the radiator just to get a decent match to the line,
what do you guys think??
i want to try this on my 40mtr mobile antennas and see if the men in the blue book speak the truth
bob85 said:can the low feedpoint impedance of a 1/4 wave for 27mhz when mounted on an average car be corrected with a dollar match at the feedpoint ( loop or coil of wire to ground from the feedpoint ) in the same way a very low impedance shortened resonant 1/4 for say 80mtrs can be matched at the feedpoint rather than tune the antenna to get a decent match and lose resonance??,
would that be the best way to go for best performance?,
the arrl handbook says its better to resonate the antenna then match the low feedpoint impedance to the feedline so as not to waste energy at high angles due to altering the radiator just to get a decent match to the line,
what do you guys think??
i want to try this on my 40mtr mobile antennas and see if the men in the blue book speak the truth
bob85, the best solution is for the antenna, coax and trasnmitter to all be matched for maximum energy transfer.
The next best solution is to use an impedance matching circuit right at the antennas feedpoint - aka a quality tuner. This will provide maximum power transfer from the transmitter to the tuner and then from the tuner to an antenna that non-resonant.
As for a dollar match, I have never used one and don't even know what it looks like, so I cannot tell you how well it will work.
Try it and let us know how it works. Oh, and take pictures...
a dollar special is multiple turns of copper wire attached to the feedpoint at one end and grounded at the other,
adjustments are made with a shorting tap from the grounded end moving the tapping point for a good match to shortened resonant 1/4wave hf mobile antennas,
anybody interested can look at page 16_15ff of the 17th edition ARRL antenna handbook,
an explanation and construction details are included
my ? is could this technique be used on 27mhz and achieve a better radiation pattern than detuning the radiator to get a match?
i was thinking that for 27mhz a few turns or a simple loop of hard wire would do the same thing.
adjustments are made with a shorting tap from the grounded end moving the tapping point for a good match to shortened resonant 1/4wave hf mobile antennas,
anybody interested can look at page 16_15ff of the 17th edition ARRL antenna handbook,
an explanation and construction details are included
my ? is could this technique be used on 27mhz and achieve a better radiation pattern than detuning the radiator to get a match?
i was thinking that for 27mhz a few turns or a simple loop of hard wire would do the same thing.
bob85, since the coil is added to the base of the antenna at the feedpoint, and since a 1/4 wave monopole is electrically short or capacitive, this will create a parallel tank circuit as a load to the transmitter. So, the inductor could be designed/sized to make the antenna appear resonant but there will be more ground loss due to the circulating current generated by the RF and ground plane. Plus the voltage will probably be higher at the feedpoint which may create higher common mode currents as well as interference. This is all hypothetical of course... I don't have an ARRL handbook.
:shock:
:shock:
Bob,
Can it work? Yes. Just make sure the ground tap is correct, the same way as with most tuning processes, watch the meter. Done so for years with a mobile antenna on all bands except 10/11 meters. Those two bands didn't match cuz the antenna was too long for them to start with, and inductive matching just wasn't the way to do that. A capacitive match done in the same way worked fine.
For antennas that are 'too short' to start with the inductive matching is certainly an option. For antennas 'too long', it isn't worth a 'hoot', but the capacitive match works. That 'too short/long' thingy is usually in reference to a 1/4 wave antenna (with or without a loading coil). You just don't see very many mobiles close to 1/4 wave length for bands lower than 15 meters, sort of (and 15 meters is really 'pushing' it - lol).
- 'Doc
Still looking for one of those 'radiation resistance' ohm meters... hard to find.
Can it work? Yes. Just make sure the ground tap is correct, the same way as with most tuning processes, watch the meter. Done so for years with a mobile antenna on all bands except 10/11 meters. Those two bands didn't match cuz the antenna was too long for them to start with, and inductive matching just wasn't the way to do that. A capacitive match done in the same way worked fine.
For antennas that are 'too short' to start with the inductive matching is certainly an option. For antennas 'too long', it isn't worth a 'hoot', but the capacitive match works. That 'too short/long' thingy is usually in reference to a 1/4 wave antenna (with or without a loading coil). You just don't see very many mobiles close to 1/4 wave length for bands lower than 15 meters, sort of (and 15 meters is really 'pushing' it - lol).
- 'Doc
Still looking for one of those 'radiation resistance' ohm meters... hard to find.