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Antenna tuner question.

I dis-agree

A antenna is either properly resident on a frequency or it isn't

A tuner may get a radio to be happy
but not persay enable any more power to radiate on a antenna that suffers from poor swr (residence on a given frequency).

is still best to be sure the antenna is fully resident on the frequency(s) you want to use it for

multiband antennas still are supposed to have reaonable swr over several bands
 
The problem with resonance is that an antenna is only resonant on one frequency. It can maybe be sort of resonant on a range of frequencies, but very seldom over a wide range. By definition, resonance is the absence of reactances. Even so, when at resonance, an antenna's input impedance is almost never very close to what the average transmitter wants to see, about 50 ohms, without some sort of impedance matching being done. That impedance matching for a single band antenna is usually some fixed device. For most multi-band antennas, there's an impedance matching device and then the lengths, or positioning of 'traps', is selected so that that matching device can be used for all of the covered bands. The measured lengths are not what would normally be called resonant, but combined with the matching device and trap, the resulting length would exhibit no reactances making the thing resonant. One example of such a thingy is the tuner and whip combination produced by 'SGC', where the tuner is at the base of the antenna. What 'SGC' doesn't advertise is that the further away from the frequency that whip would normally be resonant on, the worse the efficiency of the thingy is. Where the whip is something close to one of the 'typical' lengths thought resonant (1/4, 1/2, 5/8 wave), it does work fairly well. On bands where the whip is extremely electrically short, you can shout further than the resulting signal will go (an exaggeration, but not much of one).
The biggest draw back for tuners is the feed line between tuner and antenna. If that feed line has much loss, or is subject to deterioration because of the resulting voltages from the SWR, it ain't the best situation in the world. If you don't have to worry about the feed line being destroyed by high SWR, then it can work very well indeed, in terms of radiated signal getting to where you want it because of non-resonant antenna lengths. That doesn't mean that always using a tuner is the best possible means of doing things, but it does mean that in certain circumstances it will mean making an inappropriate antenna usable. Which is basically the point of this thread. It doesn't say anything about how appropriate any particular length of antenna is, some are just not going to be very good (usually too short).
- 'Doc
 
Doc has some good info there. First do not get hung up on the word "resonant". Probably 99% of the people that use that word use it in the wrong context. Like Doc said it simply means having no reactance. Most people think it means having an impedance of 50 ohms. I have seen that over and over again in antenna posts. I have an antenna that is resonant on one band but presents an impedance of about 5 ohms,that means an SWR of 10:1. The tuner allows me to use it and allows my TX to deliver full power into the feedline. The absolute best place for any antenna tuner is directly at the antenna feedpoint. I use a Yaesu FC-40 auto-tuner in the mobile and the tuner is mounted on the rear hatch door of my Ford Escape with about 1 foot of heavy braid connecting it to the antenna base. This braid is very well insulated as the RF voltage can on some freqs. be several hundred volts or even more. The use of coax in this case presents two problems. First is break down voltage rating and the other is capacity. With a very low impedance like one would see with an 8 foot whip on 80m, the coax capacitance appears in parallel with the low impedance and effectivly shunts the RF to ground thereby making a bad performing antenna even worse. Commercial AM broadcast stations have an antenna tuning unit (ATU) at the base of each tower transforming whatever the tower impedance is to a normal 50 ohms and then use 50 ohm cable back to the TX building where it is fed from a phaser or power divider. The ATU is NEVER at the TX end of the cable for the above mentioned reasons.
 
"I'm sorry, but you're mistaken. The only thing a tuner does is match impedances between whatever is on either side of it. That has nothing to do with resonance."

no he isn't mistaken, you are. Impedance = Resistance + Reactance. the "tuner", whether used at the antenna or the transmitter exhibits the ability to cancel reactance. a pre-existing mismatch coupled with another calculated mismatch provides the "conjugate" match and since impedance consists of both resistance and reactance and the absence of reactance determines resonance it is you who is in error.

"That said if you were to take a reading at the antenna you would find that the swr was through the roof and your radiation efficency was a fraction of what a properly tuned antenna for a particular frequecy would be."

the radiation efficiency of the antenna has nothing to do with swr.

swr on the line between the antenna and transmatch* is determined ONLY by the matching
conditions at the load, and is not changed or "brought down" by the matching device. "low
swr" obtained by using the device indicates only the mismatch remaining between the input
impedance of the transmatch and impedance of the line from the transmitter.

*designed for use between the transmitter output and line input.
 
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You are right, as far as you went. But that's only if the tuner is at the antenna's input. If it isn't, such as at the transmitter end, then the feed line also becomes part of that impedance mismatch, not just the antenna. That's where the 'problem' occurs, if the feed line isn't 'SWR-resistant'. Coax as a feed line is not 'SWR-resistant' by any means. It can be damaged quite easily by a combination of high SWR and too much power for that particular circumstance (that power doesn't have to be high, even a few watts can do the damage).

You're also right about the efficiency of an antenna not being dependent on SWR. But, if presented with a high SWR a transmitter tends not to produce it's full output. The antenna can only radiate what power gets to it, so if there is reduced power getting there, then the antenna's radiation efficiency will -appear- less than a properly matched exact same antenna. My mistake, should have said antenna system (antenna and feed line), and where the tuner was located. Oops, sorry 'bout that.

SWR on the feed line being determined by the load's input impedance. True, if the feed line and transmitter's output impedance are reasonably close. You can also 'transform' that mismatch by using a suitable length of coaxial feed line as the 'tuner'. The draw back to that is that coax was never designed, or intended, to do that transformation. Coax IS affected by SWR, and not to the betterment of it's self. Meaning that it will degrade, change it's characteristics, basically 'eats' it's self up. I wouldn't say that was a very good thing to do.

You're also right about what a transmatch/tuner is doing, changing the apparent SWR that the transmitter see. It doesn't change the actual SWR of the antenna system. And like I said, there are other feed lines besides coax, that reduce that 'loss' because they are just not affected by SWR as much. They can't be used in all circumstances without a -lot- of trouble because of their own particular characteristics.

Taking another 'step', the losses associated with tuners is almost always less than that when using a coaxial feed line as the 'tuner'. The exception being when a tuner/transmatch is used in a circumstance where it's ratings are exceeded (holds true for anything, not just tuners).

Knowing why coaxial cable was invented to start with is helpful. It wasn't because it did the job better than anything else. It was because it made a radio installation in vehicles much easier. Same for 'standardizing' transmitter impedance outputs at 50 ohms. Not cuz it was 'best', it just made other things easier.
- 'Doc
 
"You are right, as far as you went. But that's only if the tuner is at the antenna's input. If it isn't, such as at the transmitter end, then the feed line also becomes part of that impedance mismatch, not just the antenna. That's where the 'problem' occurs, if the feed line isn't 'SWR-resistant'. Coax as a feed line is not 'SWR-resistant' by any means. It can be damaged quite easily by a combination of high SWR and too much power for that particular circumstance (that power doesn't have to be high, even a few watts can do the damage)."

what i stated applies no matter where the matching takes place. impedance still consists of both resistance AND reactance regardless of which end of the line the conjugate match is being applied. in the majority of instances where feedline is destroyed using nominal amounts of power referenced to the feedlines power handling capability the situation is generally brought about by improper tuning of the matching network where excessive inductance and inadequate amounts of capacitance are employed instead of Min. L & Max. C.. i've used them right up to their maximum ratings into a diverse range of load impedances with no ill effects as long as the feedline ratings and the proper network tuning were given proper consideration and i've rebuilt many of them where these parameters were ignored. they either fly or fry based on these guidelines.

"You're also right about the efficiency of an antenna not being dependent on SWR. But, if presented with a high SWR a transmitter tends not to produce it's full output."

and the reason for that is a direct result of the load mismatch presenting itself at the transmitter output.

"SWR on the feed line being determined by the load's input impedance. True, if the feed line and transmitter's output impedance are reasonably close."

wrong. swr on the feedline (regardless of ANY pre-existing match between the transmitter and the line input prior to load termination) is solely dependant on the match/mismatch between the feedline and the load. conditions at the junction between the transmitter and the line input are a direct result of the degree of mismatch presented at the termination point of the feedline and the load.

"Knowing why coaxial cable was invented to start with is helpful. It wasn't because it did the job better than anything else. It was because it made a radio installation in vehicles much easier. Same for 'standardizing' transmitter impedance outputs at 50 ohms. Not cuz it was 'best', it just made other things easier."

By the early 1920s, AT&T engineers recognized that the open wire and cable in use at the time would be unable to carry the high frequencies needed for the broadband systems of the future. So Espenschied and Affel developed a new kind of wire system that could transmit a continuous range of high frequencies over long distances providing reduced signal attenuation and freedom from outside interference.

May 23, 1929 - Lloyd Espenschied and Herman Affel applied for a patent for broadband coaxial cable, the first broadband transmission medium.

the manufacturing of transmitter equipment was re-designed to conform to the newly available coaxial feedlines. convenience was an afterthought and not the primary motivation.
 
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...and if you had kept reading, you would have seen that coaxial cable became commercially available in the very early 1940's. '[SIZE=-1]Wikipedia' is a good place to find 'general' information about things. It's also NOT always correct in particular aspects of those 'things'. Nice try...
- 'Doc

[/SIZE]
 
See this is good
Explaining the true use of a item
Especially when most use the item the wrong way and or for the wrong reason(s)

Yes a tuner/transmatcher is best used at the actual antenna

No a tuner will not "enable" an antenna to work better and or great

but only allow the radio to be happy and unharmed by the true swr of the antenna
ofcourse if say a antenna is to be used for whatever frequency and at that frequency it has a 1.1 swr but since you are using 300 ohm laderline feedline and for this a tuner is used to enable radio to see the 50 ohms it is designed for
then this will help your antenna system work mush better and in so doing enable it to be fed with full power of your radio/transmitter without loss of effeciantcy
 
In a 'nut-shell', it just depends on what you have to work with, what you want to accomplish, and not doing any irreparable harm to things. I don't have any 'hang-ups' about using a tuner. They do not 'do' miracles, but they can certainly 'grease' the bearings in some/most cases. If a particular situation calls for one, great. If not, then that's great too, just don't use it. The 'trick' is to make it as reasonable as you can, then 'fudge' it from there. - 'Doc ...sort of like the 'trick' of picking up a dog t**d by the clean end, ain't it?
 
coaxial cable was not invented to make mobile radio installation easier and i didn't find the information on wikipedia.

a transmatch is named as such because it is designed to be used at the TRANSmitter to facilitate line input MATCHing.

when line input matching is used the swr at the line-load termination doesn't change. once the reduction in available power generated by the source created by the line-load mismatch is conjugately compensated for at the input it is of absolutely no consequence whatsoever. in either case, whether matching is effected at the line input or at the load the net result is the same. the load absorbs and radiates all power delivered to it regardless of swr.
 
"the load absorbs and radiates all power delivered to it regardless of swr." - That part's right, can't say the rest is. Where the 'matching' is done can certainly make a difference. Biggest part of that is the losses in the feed line used for that conjugate matching. Never ruined a section of coax by that conjugate matching? You've been very lucky. Coax was just not designed for doing the 'matching', doesn't have the ability to handle the currents/voltages of some of htat 'matching'. A 'transmatch'/tuner is designed to do just that. Sure, coax can be used for matching, but only within limits which aren't to broad. Is the radiation of the antenna affected by how that 'matching' is done? As far as it's radiation characteristics are concerned, no. How -much- is radiated is affected, but not how it's radiated. Wanna use coax as a matching transformer? Be my guest. I'll stick with a tuner and ladder line and not have to worry about SWR at all. Your choice. - 'Doc
 

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