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Author Topic: Reaffirm my understanding about antenna tuners  (Read 1815 times)
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WA2SQQ
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« on: January 01, 2024, 06:20:50 PM »

Got into a discussion about antenna tuners. My belief has always been that a tuner will provide a good match to the transmitter, but after the tuner the SWR is anything but optimal. So the tuner allows your transmitter to operate, without fold back, but the antenna performance of a non resonant antenna would will be a compromise.

The rebuttal was that as long as the transmitter is happy, any antenna will radiate.

I’ve always believed that there is no substitute for a resonant antenna. I can use a 40 ft pice of wire with a tuner on 160, but it’s no match for a full sized resonant antenna
What say?
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W1ITT
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« Reply #1 on: January 01, 2024, 06:58:06 PM »

Every decade or so this discussion comes up and creates all sorts of semantic and technical argument.  But you are correct.  I could be pedantic and state that there is no such thing as an "antenna tuner", unless you are talking about a pair of wire cutters or a hacksaw..  The "tuner" is actually a matching network that makes things happy on the inboard side.
But, and it's a big "but", anything will radiate as long as you can feed power into it efficiently.  As a broad brush statement, the components in matching networks have losses, some more, some less.  And your 40 foot wire generally operates against a ground which is awfully lossy.  Add up all the losses with a compromise antenna and a tuner, especially when it's trying to match very low impedances (high currents) and the high resonant antenna that's inherently matched to its feed system comes out the winner.
I made most of my living working with antenna systems that were paid for by well financed companies or taxpayer funded government money. They were all placed optimumly and well executed.  At home, at my own station, there are compromises to deal with, so I do the best I can within the limits of money and circumstance  I have 6 homebrew matching networks and one little MFJ autotuner that matches a 66 foot open wire fed dipole as the do everything backup antenna.  For better or worse, all those "tuners" just exist to keep my transmitters happy.  They don't alter the rules of physics.
73 de Norm W1ITT
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WA2SQQ
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« Reply #2 on: January 01, 2024, 07:45:05 PM »

Thanks Norm, I plan on sharing this discussion with the other person. I’m a few days from another birthday so I starting questioning my own beliefs!
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W7TFO
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« Reply #3 on: January 02, 2024, 02:03:59 AM »

Wrap your head around this:

Any piece of metal can be an antenna, regardless of frequency.

All one has to do is come to terms with the resistance & reactance it represents.

An R & X matching unit will try to do this with little power thru loss when properly designed. 

This quest makes us the search for the ideal "Q" of that unit, being a balance of efficiency vs. bandwidth.

Getting back to the opening statement, loading high power/low freq into a tiny wire  will just develop such high RF Voltage it isn't practical, with corona and dielectric losses eating up your power.  Arc-overs and exploding caps come to mind.

This is a form of distortion.

Doing the opposite (a loooong antenna) requires very low feed resistance and high RF current that can burn things up.  Again, distortion.

A resonant antenna is nice, but few actually are as their natural impedance isn't what your transmitter wants.  The real-world dictates antenna styles and sizes that we deal with all the time.

It really isn't any different than matching impedances with a hi-fi amp and speaker.

Another 'no free lunch' deal.

73DG
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W1ITT
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« Reply #4 on: January 02, 2024, 07:49:08 AM »

Dennis..  You are spot on when you state that anything metal can be an antenna.  A few years ago there was a group that sponsored an on-air event where participants loaded up anything conductive and put a signal out.  One particularly innovative bunch supposedly loaded up a railroad trestle bridge and did well.  I'm guessing they shunt fed it.  But it's all about matching networks and managing losses.  RF gotta go somewhere.
73 de Norm W1ITT
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K8DI
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« Reply #5 on: January 02, 2024, 10:23:27 AM »

It helps to keep in mind that there is nothing magical about 50 ohms. It's just as easy to design a transmitter to drive 10,000 ohms, or 10 ohms. The output network changes. Now, given that we all use non-ideal, real-world components, there's generally more loss in those components at very low or very high impedances.

It helps to realize that antenna efficiency is not magically better at 50 ohms. Same as with output networks, antenna materials themselves are generally lossier at very low and very high impedances.

A one ohm output transmitter will have high internal losses. A one ohm antenna will have high material losses. A one ohm feedline will have high losses as well. But the SWR will be perfect at all points and a 'tuner' or trans-match will not be needed.  The same is true of the SWR if everything is 50 ohms, or 5,000, but the parts will each have less loss so the system will have higher efficiency.

None of this says anything about radiation. What our real goal is  is to have all of the "loss" be radiation resistance, so that all of our power is dissipated in that radiation resistance -- and out into the air.  Increased radiation resistance is generally good, as it is then a larger proportion of the total resistance/impedance, meaning higher efficiency. Again, the actual number of ohms doesn't matter. Maybe the best antenna SYSTEM has 180 ohms of radiation resistance and 20 ohms of other loss... it will be very efficient. Now we feed it with 200 ohm line and a transmitter with a designed 200 ohm output. Perfect SWR, works great. If your transmitter has a 50 ohm output, though, the SWR will be a bit high and you'll want a 'tuner'/trans-match.  The real issue as I see it is the use of a tuner at the radio, with 50 ohm feedline/coax, and an antenna far from 50 ohms. This is the wrong place.. the tuner needs to be where the mismatch is, not between the 50 ohm radio and 50 ohm coax.

As well, remember that an ideal dipole mounted at height is about 70 ohms, not 50. Similarly, an ideal vertical 1/4 wave is closer to 36 ohms, not 50. Both need feedline and transmitters at their impedance, or a matching network between the unmatched point(s)...

Back to the original post -- the difference between the 40' wire and the full-size, full-height dipole on 160 is the radiation resistance. The 40' wire will have very low radiation resistance, meaning most of the power is dissipated elsewhere, like in the feedline, the wire itself, and the matching network. Sure, it'll radiate, but not well.  as was said, you can load up anything with a suitable network. And RF will be radiated and received by some pretty unlikely conductors.. Remember the stories of fillings picking up AM radio? Certainly not resonant...And then there's my first HF contact...done on the service bench in my basement, before I was licensed, with an old Heathkit SB301/401, connected to an MFJ dummy load.  It radiated, just enough...

Ed

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wa2tak
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« Reply #6 on: January 02, 2024, 11:37:20 AM »

First...the so called antenna 'tuner' is completely mis-named.
They use to be called a transmatch...some what better.

Secondly, the equipment has NOTHING to do with 'tuning'.

Thirdly, it does NOT change / alter / effect......transmission line SWR TO THE ANTENNA or antenna PERFORMANCE...esp does NOT resonate anything.

It does one, single, solitary job = creates - via the circuitry...capacitance REACTANCE and inductive REACTANCE -
the conjugate IMPEDANCE of the INPUT LINE IMPEDANCE so that its OUTPUT IS A PURE 50 OHMS!!

Soooooooooooo it simply 'satisfies' the transmitter into 'thinking' it is outputting to a LOAD of 50 ohms VIA THE COAX BETWEEN THE UNIT AND THE TRANSMITTER.

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wa2tak
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« Reply #7 on: January 02, 2024, 12:20:38 PM »

PS
There are two FUNCTIONAL specifications that are most important:
1. power handling capability, naturally;
2  the unit impedance RANGE!

As we know, antenna impedance varies alllllll over the place as the frequency changes.
It's not unusual for it to be as high as 500 ohms...and higher for a single wire multiband operation.
That is 500 / 50 = SWR 10

These units have various ranges!
Generally...the more it cost...the bigger the box...the 'better' the unit.

Soooooooooooo it EXTREMELY important to SEE the RANGE of the unit for your operation.
Check out its spec!!!

Being an old timer, it was said that a PALSTAR tuner can load of a metal bathtub on 160!
In my view, they were...and STILL ARE...the best antenna tuner in the world.
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KD1SH
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« Reply #8 on: January 02, 2024, 01:05:08 PM »

  I remember, way back in the 70's on 11 meters, communicating with a friend who was using a length of coax with clip-leads, and a tuner, to connect to and "tune" pretty much any metal object within reach of the feed-line: an aluminum storm door, rain gutters—perhaps even a lawn-tractor outside the window—and I was hearing him on all those "antennas." Well, we were only three miles or so apart. We didn't realize until later that as much radiation was probably coming from the feed-line as from those metal objects, with that sort of mismatch, even though the rig didn't care, comfortably at the other end of the tuner.
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wa2tak
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« Reply #9 on: January 02, 2024, 01:35:37 PM »

Yup....TUBE rigs can take the 'mismatch'.
How many of us used light bulbs to 'tune' them...when connect whatever antenna we had.

The reason todays IC rigs have built-in tuners is to help prevent them from being fried.
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KD6VXI
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« Reply #10 on: January 02, 2024, 03:33:46 PM »

If the tuna is at the antenna end, it becomes part of the antenna system.  It will tune out the reactance and give a 50 ohm match (if that is what you're designing it for).  At this point, you are getting maximum power transfer from the transmitter into the antenna and coaxial losses are the lowest you'll have for this particular setup.

If the tuna is at the SHACK end, it has nothing to do with achieving resonance, etc.  It's only purpose is to show the shack end of the coax 50 ohms.  Period.
Coaxial swr losses will still be.  Losses at the feedpoint due to mismatch will still be a thing.

Your rig will be happy with either setup.  With the first, the most power gets coupled to the antenna
  With the second, the most power gets coupled to the coax.

Either scenario keeps the rigs happy.

--Shane
WP2ASS / ex KD6VXI
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« Reply #11 on: January 04, 2024, 01:28:20 AM »

Here is an example to hopefully explain the situation.

This is a simplified 6146 final circuit from say a DX-60.

The final is operating ClassC and has 650 volts on the plate with a plate current of 125 mA, so Rplate = 2400 ohms.

The input frequency is 7.2MHz and we assumed the final's Pi-Net was properly tuned into a 50 ohm dummy load, but is now switched to the tuner with an antenna connected which has a resistance of 300 ohms with an unknown reactance.

The impedance at the plate is 2400 ohms (with a capacitive reactive component), with the 'resistive' part of the plate impedance being 2400 ohms. The Pi-net  would like to see a 50 ohms termination resistance at the right side of the Pi-net since that is what we initially tuned it into, so the values seen in the pi-Net circuit are what would be expected to transform the 2400 ohms resistance to 50 ohms at 7.2 MHz with the reactive components cancelled out.

The tuner has inductive and capacitive reactive components as well so that the 50 ohms resistive transformation to 300 ohms takes place and any antenna reactance is cancelled out as well.

The 6146 final is a happy camper now because of 1) the Pi-net transformation from 2400 ohms to 50 ohms and because 2) the Tuner has transformed 50 ohms to 300 ohms.

* DX-60 6146 Final Example.pdf (28.24 KB - downloaded 57 times.)
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