Antenna matching circuit versus an Antenna Tuner?

[guitar_199]

I haven't been over here much but a subject just popped into my head and I wanted to "kick this can" around a little bit to see where it goes.......

For years, I have heard people question the use of an outboard antenna tuner. I hear that they are lossy (reduce radiated power) and a few other things...but mostly lossy. One of the chief "refrains" that I tend to hear is.... "all they do is trick your radio into seeing a safe load" so that it (the radio) does not see the problems of a larger mismatch.

Fine!

But.... isn't that what a gamma match is doing? Or any other kind of matching network that is attached to the antenna?

THAT is the basic idea I would like some opinions on. Why is a matching network on a 5/8 wave "OK"..... but if you take 5/8 wave worth of element and hook an antenna tuner to it and tune it in.... that is "NOT OK"?

What difference is there?

Thanks in advance!

Bob R

 

[Shadetree Mechanic]

I think the main argument is that if the radiating element length is not correct for your frequency, the antenna tuner won't fix it as far as being able to make the antenna resonant on your frequency. The tuner makes the radio happy, and radios are expensive and deserve to be happy. Hahaha.

 

[guitar_199]

But.....in my 5/8 wave example....... 5/8 of a wave is not resonant.....

but the matching network makes it resonant.

How is it any different from what the tuner is doing?

That is that part that has me scratching my head......

 

[The DB]

The antenna tuner and a matching circuit at the antenna feed point does the exact same thing in the exact same way, often with the exact same type of circuit. They take the reactance at the antennas feed point and feed it with and equal amount of the opposite reactance. This is, by definition, resonance.

A matching circuit at the antenna has the benefit of being right at the feed point. The thing is, the antenna tuner works with a high SWR between the antenna tuner and the antenna, which also has its draw backs, namely when it comes to losses. Because of these added losses, people who use an antenna tuner across an HF spectrum tend to use ladder line and such, which comparatively speaking makes LMR-400 look like a bad piece of RG-58. Because of its very low losses, ladder line makes antenna tuners viable for a much wider range of antennas. If you are using coax, I wouldn't use an antenna tuner with an SWR of higher than 5, and that is pushing it to me.

The strange thing to me is many people point to this high SWR as the antenna tuner not tuning the antenna, but it is actually required for said antenna tuner to tune said antenna.

Also, using an antenna tuner does increase the output of the antenna, even with a high SWR on the feed line between the tuner and antenna, and there are at least two videos out there that show this...

41:15 into this video is one example.


Here is another.


Without the antenna tuner actually tuning doing something more than "hiding a high SWR", neither of these videos would have been possible...

The people who believe that antenna tuners don't tune antennas refuse to question it. Anyone who disagrees with them is often simply wrong to them. Further, aside from mentioning the high SWR between the antenna tuner and antenna, most refuse to talk about it and continue to believe. Its almost like a cult, there are things that you must not question...

If you want a deeper look at what is actually going on, take a look at a freely available .pdf file called "Another Look at Reflections" by M. Walter Maxwell, or perhaps one of his books, Reflections, Reflections II or Reflections III. I think Reflections III is still in print. It doesn't matter which book you get, the newer version add to the old, and update some small part of the text but that is all, most of the content related to this issue is still the same.


The DB

 

[W4KVW]

Having a Remote Antenna Tuner at the antenna is far better than having it at the radio or operating position.At the operating position requires that the entire feedline & antenna are being matched not just the antenna.Having an antenna that is built for the correct band & using a gamma match,T-match or other matching system on the antenna is far more efficient than any antenna tuner ever will be & having a multi band antenna makes it even less efficient because it's not the proper length for any band so it's a compromise antenna on every band.I have great success with my Mosley TA-53M (5) band HF yagi with the coils for each band so I can't imagine it being better without the coils but I have heard that a mono band antenna is even better.

SIX-SHOOTER
W4KVW
WRMX520

 

[freecell]

any antenna is resonant (purely resistive) when feedpoint reactance is equal to 0.

 

[The DB]

any antenna is resonant (purely resistive) when feedpoint reactance is equal to 0.

This is true, but also incomplete. This is actually more of a special case than the whole story on resonance, but on the CB band so many antennas use this special case that many people think that this is all there is. For example, take any (well most) 5/8 wavelength antennas. A lot of people will try and say they are shortened 3/4 wavelength antennas but this really isn't true. Most of them are in fact impedance corrected 5/8 wavelength antennas. One way to tell the difference is looking at SWR bandwidth. A shortened 3/4 wavelength antenna will have far more bandwidth than, say, an impedance corrected 5/8 antenna like the Maco 5/8 antennas.

If an antennas feed point has 10 ohms of capacitive reactance, for example, and you feed it with 10 ohms of inductive reactance, by definition this is also resonance, and is how all real world matching circuits (aside from loads up on the antenna itself) actually work. While not self resonance, a more accurate name for what you are putting forth, it is still resonance, and has all the benefits thereof.


The DB