Are Antenna Tuners a Lie?

[rwier]

No it isn't. An antenna tuner is a device that goes between the feedline and the antenna, not a part of the antenna itself.

lol

 

[jonwienke]

Calling a tail a leg doesn't make it one.

 

[forkeye]

Different ways to match.

There are different ways to match the coax to the antenna feedpoint, and make that work. There are a couple of work-arounds.

Some more savvy folks have what's called an LC type network they make for coupling at the antenna feedpoint to make load matching better. There's another factor most don't take into account. The distance between a groundplane and the ground will change the impedance value of the antenna load or feedpoint. A 1:1 electrical balun is maybe a better type of workaround, it uses and inductance/transformer type situation to take an unbalanced line like coax and make it match the antenna feedpoint.

Then there's coax. If you want to get technical, you may cut your coax so the total length from radio to feedpoint is in multiples of half-wave or quarter-wave lengths, etc. That's another argument. Others argue it should be cut in 3 foot multiple lengths or roughly 1/12th wave rather than half-wave or quarter-waves. The arguments are both valid by yet a third school of thought that says the type of coax will define how you cut it in length.

Certain coax is made for marine use, rather than land use. Other coax values may vary depending on the type of jacket and the braid inside and whether its nylon or foam or its an air or nitrogen rigid coax type..

But assume you have an antenna analyzer and you can show a 50 ohm antenna feedpoint, and the antenna is non-reactive and not affected by distance from the ground, and a 50 ohm coax that analyzes out to exactly 50 ohms. Then when connected to your radio all the energy going out to the antenna should transmit so all the power or virtually all of it is delivered to the main or driven element (the radiating part).

There's going to be line loss on two fronts: (1) length of coax will affect how much power is lost through heat dissipation because we're not in a vacuum; and (2) the higher the frequency the more the loss, and also the more your SWR's are affected.

Add to those difficulties the fact that you're limited to 4 watts (12 on Sideband) every bit of power counts.

An antenna coupler may work too and may be better. Depending on what kind you use, it will do the job for you. Most are automatic and tune the coax to the antenna and mounts at the antenna feedpoint. Its not a tuner because the tuner goes on the transmitter end. It basically matches the coax to the antenna. Probably results in less line loss and maximizes power to the antenna.

So okay there it is again. Best solution is to not have to use either a tuner or coupler but sometimes you have to pick one.

 

[jonwienke]

If you want to get technical, you may cut your coax so the total length from radio to feedpoint is in multiples of half-wave or quarter-wave lengths, etc. That's another argument. Others argue it should be cut in 3 foot multiple lengths or roughly 1/12th wave rather than half-wave or quarter-waves. The arguments are both valid by yet a third school of thought that says the type of coax will define how you cut it in length.

All of this is urban legend / hooey. If the feedline is an odd multiple of 1/4-wavelength electrically (taking into account the velocity factor of the feedline), then the phase relationship between the outgoing RF wave and the reflected wave bouncing back from the antenna will fool an SWR meter into giving an incorrectly low reading. People ignorant of how RF actually works jump to the confusion that the erroneously low SWR reading means that there is a benefit to certain coax lengths. But if you attach the SWR meter directly to the antenna (as opposed to the back of the radio), you will get the most accurate SWR reading, and coax length will have zero effect on SWR.

 

[KC4RAF]

As jonwienke just posted, that is the true way to measure SWR. At the antenna and not at the back of the transmitter. And length of coax will not correct the antenna.

 

[forkeye]

Urban Legend or Not None of Us Answered the Guy's Question.

First off, on a personal note, I tend to ignore the lengths of coax vs. random length because the difference will generally be negligible except for someone who's really trying to match the output of the radio to the antenna to deliver the most power to the radiating element of the antenna.

Second off, I guess we all went of on a tangent. His question was "Of the original 10 watts transmitted, (from the radio) are all 10 watts leaving my antenna OR is the originally reflected wattage being absorbed by the antenna tuner and therefore creating the illusion of a perfect SWR match?"

And the answer is the radio will see a perfect match, but whatever the SWR was on the feedline and the antenna will be the same. So if you start with a 2:1 SWR you'll still have a 2:1 SWR only the radio will see a closer match. The power is dissipated through heat, or is bounced back and forth between the tuner and the antenna and then dissipated as heat. In either event, it is not going out in transmit mode.

Third off, maybe the best way to deliver the most is the use of a matching network at the feedpoint.

But all of that can be fixed by using an antenna analyzer and other tips and tricks, or better still, tune the antenna so its impedance matches the feedline and the radio. All of that failing then maybe its a ground problem or some other electrical issue.

Some antennas can drive you nuts trying to figure out no matter what you do, you still can't get a good match and SWR. Then its something really ridiculous that answers the question.

Anyway, no matter what, we didn't answer the guy's question but its still a good discussion point about making it all match and getting good power delivery so you're power is out there not lost in the line or a loading coil, or inside a tuner.

 

[prcguy]

What you are describing is not a true picture of what really happens with an antenna tuner in line. Assuming the antenna tuner is feeding coax, a portion of the power will be radiated by the antenna and a portion will be reflected back to the tuner. If the feedline loss is very low, the power reflected from the antenna back to the tuner will be reflected back to the antenna and the same percentage of that power will be radiated by the antenna and reinforce the original signal.

Then a portion of that will reflect back to the tuner to be reflected back to the antenna to be radiated again and again in an endless cycle as long as you are transmitting. If there is any feedline loss between the tuner and antenna then power will be lost in heat but there will still be some power reflected back and forth and radiated by the antenna.

All antenna tuners have some inherent loss and that can vary depending on if the tuner is matching to a very high or very low impedance. I wont speculate on that loss but some tuner mfrs publish those specs and its usually not much.
prcguy

First off, on a personal note, I tend to ignore the lengths of coax vs. random length because the difference will generally be negligible except for someone who's really trying to match the output of the radio to the antenna to deliver the most power to the radiating element of the antenna.

Second off, I guess we all went of on a tangent. His question was "Of the original 10 watts transmitted, (from the radio) are all 10 watts leaving my antenna OR is the originally reflected wattage being absorbed by the antenna tuner and therefore creating the illusion of a perfect SWR match?"

And the answer is the radio will see a perfect match, but whatever the SWR was on the feedline and the antenna will be the same. So if you start with a 2:1 SWR you'll still have a 2:1 SWR only the radio will see a closer match. The power is dissipated through heat, or is bounced back and forth between the tuner and the antenna and then dissipated as heat. In either event, it is not going out in transmit mode.

Third off, maybe the best way to deliver the most is the use of a matching network at the feedpoint.

But all of that can be fixed by using an antenna analyzer and other tips and tricks, or better still, tune the antenna so its impedance matches the feedline and the radio. All of that failing then maybe its a ground problem or some other electrical issue.

Some antennas can drive you nuts trying to figure out no matter what you do, you still can't get a good match and SWR. Then its something really ridiculous that answers the question.

Anyway, no matter what, we didn't answer the guy's question but its still a good discussion point about making it all match and getting good power delivery so you're power is out there not lost in the line or a loading coil, or inside a tuner.

 

[jonwienke]

First off, on a personal note, I tend to ignore the lengths of coax vs. random length because the difference will generally be negligible except for someone who's really trying to match the output of the radio to the antenna to deliver the most power to the radiating element of the antenna.

WRONG. Feedline length has ZERO effect on "matching the output of the radio to the antenna". It's an old wives tale that has no factual basis whatsoever, for the reasons I outlined in my previous post.

There are only 4 ways to maximize antenna efficiency to a given TX frequency and length of feedline:

1. Ensure that your connections are good--no corroded contacts or frayed/broken wires, all crimped connections crimped firmly, all soldered connections soldered properly, no shorts, no water intrusion in coax, etc.

2. Ensure that the antenna resonates at the TX frequency. This is typically done by physically altering the length of the antenna.

3. Transform the antenna impedance to the feedline impedance. This can be done by adjusting the angle of ground plane radials (which will affect the antenna's resonant frequency), a transformer, or a tuner. A tuner can partially compensate for an impedance mismatch caused by an antenna resonating at the wrong frequency, but antenna performance will not be as good as having the antenna properly tuned to resonate at the TX frequency. All you accomplish by tuning a non-resonant antenna is preventing transmitter damage from excess power being reflected back to the transmitter.

4. If necessary, ensure that there are no common-mode RF currents on the feedline. This is usually done with a choke balun, but common-mode RF can be minimized by proprly following the previous steps.

As a general rule, the steps should be done in order, with the caveat that adjusting ground radials may require adjustments to antenna length to get the antenna to resonate at the proper frequency. Anything else, including playing with feedline length, is a complete waste of time.

 

[forkeye]

The impedance of the antenna is going to increase as the feedpoint is moved closer to the end of the driven element.

A couple of things happen with groundplanes and horizontals. Impedance increases the closer to the ground you get. So if you bend the radials of a groundplane downward and slope them, you will increase impedance of the antenna. With the horizontal the closer to ground you get, the more likely your reflecting element will increase the impedance of the driven element too at the feedpoint.

So say you tune your antenna and it calculates out to a 9:1 SWR, then you want to use 50 ohm coax. But the braid in the coax can also act as a ground and if any part of the coax sits on the ground, that creates an issue too. Say you mount it on the ground, and bury radials down into the earth, so now you have a vertical. You can tune it and the radials, but now the coax is on or near ground. And part of the coax is going to run underneath your radials or parallel to them anyway.

Say you go with a Zeppelin design, and stub the antenna. Depending on how high up your driven element is, your feedline which is part of the antenna will affect it too. Each kind of antenna is going to present all sorts of issues, even the premanufactured stuff. Heard some guy talk about his Imax 2000 and how he used guy wires instead of radials and somehow that magically fixed his issues. But did it really? If you're in an arid region where you have no water in the ground, that might or might not work. If you're in a humid region where the ground is saturated with water, it might make a perfect ground or a lousy one. All sorts of unknown or better explained undiscovered phenomena work on antennas.

The tuner or matcher, or coupler, or other devices are all tricks used to compensate for something or another.

As far as feedline, line length may have an effect at some point. Also the kind of coax may make a difference too. Some feedlines are way more efficient than others with the same designation RG58 or RG8 or mini-RG8, etc. Some feedlines are air cored or nitrogen cored for UHF. Its not like a telephone line where you have step ups and step downs, switching networks and power added along the line to compensate for losses not to mention the callout station having its own matching network to make additional compensation.

Basically the idea is you're making a circuit in free space. So current flow is important to make a radiation pattern. In a closed circuit system or resonant system the antenna is a tuned circuit not dependent on ground reflection or a ground plane.

So the age-old question and answer is a tuner a myth or a lie(?) really gets answered with a "depends" as in depends on what you're trying to do, what frequency you're on, what material you're working with, etc.

Note also: Its possible to feed a 50 ohm transciever's power through a 75 ohm feedline to a 73 ohm feedpoint and not have any loss despite what the SWR reads. In that case the tuner should have negligble loss, if any. Again it all depends on what you're trying to do.