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I figure the transmitter has a 50-ohm pure resistive output and that the transmission line is 50-ohm as well. I think it's usually the antenna itself that actually needs to be tuned, yet it sounds like everyone places the tuner at the transmitter side. what are the effects and wouldn't it be better to have the tuner at the antenna? my guess is that the transmitter sees a "perfect" match and will have no complaints but that there are reflections from the antenna port going back and forth between the tuner and the antenna. i think this would create a power bottleneck where the transmitter could put out more power but the tuner/cable/antenna combo can't "absorb" that much power. i would think there are standing waves between tuner and antenna which will make it harder to send more power through as well as cause additional losses.

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There are a lot of topics in this question, so let's take them one at a time.

I figure the transmitter has a 50-ohm pure resistive output

Not necessarily. You're probably arriving at this conclusion based on the maximum power transfer theorem. Which of these circuits delivers more power to the load resistor?

schematic

simulate this circuit – Schematic created using CircuitLab

What we can say is the manufacturer has designed the transmitter expecting a 50 ohm load, and they've also designed it to minimize their cost, thus maximizing their profit. This means when the load is 50 ohms the transmitter will be able to make its rated power while staying within the ratings of the components of the transmitter, the finals especially. But there won't be much margin for deviation, because that would increase costs.

When the load isn't 50 ohms this might subject the finals to too much current, voltage, or power. If we're lucky this means the radio just reduces power. If we're unlucky the radio doesn't reduce power, and the finals are damaged.

So we want the transmitter to see 50 ohms so it can operate as designed.

I think it's usually the antenna itself that actually needs to be tuned

Usually, but not always. Most radios these days are designed for a 50 ohm load, and 50 ohm coax is very popular. But there is also 75 ohm coax, and there are balanced feedlines between 200 and 600 ohms which are readily available.

Also, not all radios are designed for a 50 ohm load. In particular, older tube radios typically have a variable output network, so they will work with a whole range of loads. And although coax has been around since the mid-19th century, it wasn't really until after WWII that it became available to regular folk. Prior to that the most common feedline was some kind of balanced feedline typically with a higher impedance.

my guess is that the transmitter sees a "perfect" match and will have no complaints but that there are reflections from the antenna port going back and forth between the tuner and the antenna.

This is exactly right. The reflected power from the antenna encounters the tuner, and the tuner (if you've managed to adjust it so the transmitter sees a 1:1 SWR) re-reflects that power back at the antenna. The consequence of these extra reflections is usually (but counter-intuitively, not always!) additional loss in the feedline.

i think this would create a power bottleneck where the transmitter could put out more power but the tuner/cable/antenna combo can't "absorb" that much power.

Not usually. Under normal circumstances, the feedline and antenna are linear systems, which means waves can be superimposed indefinitely. What happens is that each reflection is independent of what's happening with other reflections at the same time.

However, linearity does break down eventually for just about any real system. To give one example, that additional loss in the feedline causes the feedline to become warmer. At some point it will become warm enough that the dielectric may melt, and the center conductor will short to the shield. Or, the voltage can get high enough to arc through the coax dielectric. It's pretty much impossible to get to this point with 100W and LMR-400, but 2kW and some really terrible RG-58 might. For broadcast stations in the megawatts, it's definitely a concern.


In summary, on purely theoretical grounds it usually is better to put the tuner at the antenna end of the feedline. However this requires making it weatherproof, and having some mechanism to remotely operate it, which makes it more expensive and difficult to install. Those downsides may or may not outweigh the benefit, depending on circumstances and priorities.

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  • $\begingroup$ As usual you gave me a whole more to think about. I was wrong about the bottleneck situation because I was thinking in terms of a parallel resonant band-stop filter, when actually the antenna/tuner combo is more like a series resonant band-pass filter. The two circuits you posted really threw me for a loop. For some reason it doesn't seem fair to change the source impedance and compare them. Can I instead look at it as 50Ω source and 2500Ω load? I would see it as more efficient since most energy is dissipated in the load versus source but overall less power is transferred because of higher Ω. $\endgroup$
    – pgibbons
    Sep 13 at 17:08
  • $\begingroup$ and please help me out with this basic math, the way I calculated your circuits: A: 1V divided by 100Ω = 0.01A, then looking at the 50Ω load * 0.01A = 0.5W. B: 1V divided by 51Ω = 0.02A, looking at the load, 50Ω * 0.02A = 1W. So that tells me B transfers more power but because of different source impedance is that apples and oranges, am I getting this correctly or what am I missing? $\endgroup$
    – pgibbons
    Sep 13 at 17:13
  • $\begingroup$ @pgibbons There are many ways to analyze the circuit but I'd calculate the current by dividing 1V by the sum of the two resistances, and then you can calculate the power in each resistor with $P=I^2 R$. The efficiency is $R_{load} / (R_{src} + R_{load})$. So, efficiency is maximized by minimizing the source resistance. Of course maximum efficiency isn't the only design criteria, but the point is there's no reason the source impedance must be 50 ohms, and there are reasons you might not want it to be. $\endgroup$ Sep 13 at 17:59
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    $\begingroup$ @user10489 Sorry I didn't know that, it's always a drag when your stuff gets deleted. Thanks for pointing out the patent date, I'll edit the answer to reflect that. $\endgroup$ Sep 14 at 16:24
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    $\begingroup$ I don't know when coax became available to everyone, but your guess of "after WW2" is likely correct, and your new phrasing on that is pretty good. $\endgroup$
    – user10489
    Sep 14 at 22:45
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To make a long story short, if you are using 50 ohm coax with a 50 ohm radio, putting the tuner at the antenna is higher efficiency because it reduces losses in the coax caused by high SWR.

However, putting the tuner at the antenna has some huge disadvantages. It has to be weatherized. It is more prone to damage from lightning. It has to be remotely powered and possibly remotely controlled. If anything goes wrong, you have to pull down your antenna to service it, etc. It's much easier to just put the tuner next to the radio. Manual tuners (that need to be next to the radio) don't even need power.

Having said that, there are a number of antenna designs that integrate a simple cheap tuner into the antenna. These miss most of the disadvantages, and some don't even need to be powered.

However, the losses from SWR in the coax are usually not so bad that you can't operate anyway with a tuner at the radio, as long as you stick to under 100w and realize you might not be getting full power to the antenna. And as another answer pointed out, not all radios or feed lines are 50 ohms, so a remote tuner might not make sense in that case.

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  • $\begingroup$ A nitpick: "has to be weatherized" applies only to permanent installation; many hams use portable or temporary stations or antennas. $\endgroup$
    – Kevin Reid AG6YO
    Sep 13 at 2:40
  • $\begingroup$ @KevinReidAG6YO I'll agree with that. But I can't see putting a remote automatic tuner in that situation. That would be more like the case of a tuner integrated into the antenna, like the SuperAntenna. I don't think I've seen a remote tuner appropriate for portable/temp use that wasn't integrated into the antenna. (If you have, I want to know about it!) $\endgroup$
    – user10489
    Sep 13 at 2:59
  • $\begingroup$ Well, you can interrupt the feed line and put the tuner at the base of the antenna, say, rather than next to the radio, but that would have lesser benefits and isn't common. But your answer says “It has to be weatherized” as the very first thing, and I just think that the emphasis is a little off, especially since integrated tuners exist, and it'd be best to qualify it like "… huge disadvantages if your antenna is permanently installed." $\endgroup$
    – Kevin Reid AG6YO
    Sep 13 at 13:40
  • $\begingroup$ I get what you are saying. It would cost less to increase power that will make up for SWR losses than to implement a remote tuner and that is the main reason why the tuners are not at the antenna. $\endgroup$
    – pgibbons
    Sep 13 at 17:16
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    $\begingroup$ I agree with @KevinReidAG6YO's point about field operations. Often times, antennas used in the field or on field day events are much bigger compromises than anything else we normally do in a permanent installation. Additionally, we now have multiple people in one large shack. A remote tuner makes tremendous sense here to reduce radiated RF from feedlines in and around the shack, moving everything to the antenna feedpoints. $\endgroup$
    – David Hoelzer
    Sep 14 at 12:33
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I'll add that a tuner is used for tuning, and tuning, for a single or small set of frequencies is often done at the antenna, usually by adding or adjusting a loading coil or adding a capacitive top-load or hat (etc.), usually done during antenna design, construction, or deployment.

However a tuner is used for a wider range of tunings, thus has additional reactive components and switching components that (1) weigh something and (2) are only used in some tunings for some frequencies. Thus a tuner has addition weight, often unneeded, that might involve some difficulty adjusting, maintaining, and hoisting stuff perhaps a hundred feet in the air, far away from the operating point, to an antenna's feedpoint.

But for some field antenna's the answer is both, as the feedpoint for a random wire or an end-fed (e.g. highly off-center fed dipole) and counterpose might be right at the transmitter (in SOTA ops, et.al.). So tuning with a tuner can be done at both the transmitter and the feedpoint, since those two points are colocated.

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    $\begingroup$ Good point, the antenna is already (roughly) tuned for the most part and the tuner at the transmitter is used to take the edge off :) $\endgroup$
    – pgibbons
    Sep 13 at 17:18
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The conditions on a transmission line in terms of the line impedance depend on the characteristic impedance of the line and on the boundary conditions, which are in this case a transmitter at one end and an antenna at the other. In other words the source (radio + matcher) and load (antenna) both have an effect on what is happening everywhere on the line.

As mentioned in the question, if there is a mismatch at both antenna and radio ends of the line, then there will also be a two reflections bouncing backwards and forwards between both ends which interact with each other.

If the fed point impedance of the antenna doesn't match the characteristic impedance of the line, there will be a reflection at the antenna and the result is a standing wave on the line the voltage and current of which are out of phase with each other. This phase difference between voltage and current of the standing wave depends on the the value of reactance present in the impedance of the antenna.

If there is a reflection at the antenna feed point, then the impedance at any point along the line becomes a function of the distance from the end of the transmission line. If you position a matcher at the radio end, then when you adjust for lowest SWR, what you are really doing is adding a reactance which is opposite to that present in the antenna impedance such that phase difference between voltage and current on the line is returned to zero degrees. This removes the standing wave and the impedance everywhere on the line becomes the characteristic impedance of the line.

So to answer your question, using a matcher at the radio adjusted for a perfect SWR removes the standing wave from the line just as if the antenna were matched perfectly and there is no bottle neck and you get maximum power transferred from radio to antenna just as is the case for a perfectly matched system where a matcher is not required.

It seems many ham radio operators think that if you use a matching device at the radio end, then there is still a standing wave on the transmission line resulting in increased I²R losses, and it's better to place the matcher at the antenna, and this idea is false.

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    $\begingroup$ If when the tuner is on the radio end, and adjusted for a 1:1 SWR, this "is adding a reactance which is opposite to that present in the antenna impedance", then the tuner should not need to be readjusted if the feedline length changes. Perhaps you should give that a try. $\endgroup$ Sep 13 at 1:06
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    $\begingroup$ Also, if a radio-side tuner eliminates any standing wave on the feedline, how does the tuner "know" the antenna is mismatched without any reverse wave to inform it of such? $\endgroup$ Sep 13 at 1:17
  • $\begingroup$ A tuner at the radio side does not fix the impedance for the whole line. You still have SWR in the feed line, and for coax, this magnifies losses. For parallel line, it's not that big a deal. $\endgroup$
    – user10489
    Sep 13 at 2:06
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    $\begingroup$ @Andrew Yes, the reverse wave is what carries the information that the antenna is mismatched. electronics.stackexchange.com/q/19759/17608 $\endgroup$ Sep 13 at 13:19
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    $\begingroup$ @Andrew My point is if the tuner does need readjustment, then the tuner isn't compensating for the impedance of the antenna, because that doesn't change as the feedline changes length. Rather, it's compensating for the impedance of the antenna as transformed by the transmission line, which does change. Also consider that 25+0j ohms is nonreactive, but mismatched, and a tuner can adjust for that as well. So the tuner isn't merely inserting an opposite reactance. $\endgroup$ Sep 13 at 13:21

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