Impedance Matching

[Greatvalue1]

Forgive my repetition in this question:

I know just enough about electronics to get myself into trouble, and especially the stuff concerning capacitance and impedance.

And I want to understand, in a very rudimentary sense, how impedance effects the signal traveling through the coax and antenna.

More so as to how a mismatched impedance effects, or possibly distorts the wave of the signal...? Maybe? I'm not sure.

All I know is that the antenna and coax need to be 50 Ohm's to properly propagate the signal, but what happens to the signal when the Ohm's are different?

I'm not looking for a complete physics outline, but just an understanding that I can picture in my head.

 

[The DB]

Throwing out a simplified description, perhaps oversimplified.

If your antenna and coax have the same impedance (or ohms as you mentioned in your post) all of the signal coming down the coax will transfer to the antenna.

If they do not match perfectly some of the energy will not properly transfer into the antenna, and thus take the remaining path back up the coax. The more of a difference between the two the more of this energy does not transfer into the antenna and thus more energy is reflected back up the coax.

This energy going back up the coax is what is measured when using the REF setting on most SWR meters.

Hope this helps.


The DB

 

[W5LZ]

What 'DB' said and then this...
"More so as to how a mismatched impedance effects, or possibly distorts the wave of the signal...?"
There's no distortion of the signal, only less of it getting to the antenna to be radiated. It's something like you wanting a particular amount of water (volume) at a particular amount of pressure at a nozzle on a hose. If you change the size of the hose at some point, the amount of water and it's pressure at the nozzle will change. A big change is that hose's size means a big change in the amount of water and it's pressure coming out of that nozzle. Small changes mean a smaller change in the water's volume and pressure. If you could change the pump's pressure then you could compensate for that change in hose size, but with the typical transmitter that isn't that easy to do. It's much easier to stick with the same/right size hose. Or, if you could vary the size of the opening in that nozzle along with the pump's pressure, you could keep the same water stream coming out of the nozzle. That's not the best comparison in the world, but it's close. (It's also done all the time, ask a firefighter's pump operator. The 'tuner' on a firetruck, sort of. )
- 'Doc

 

[roadrage]

What 'DB' said and then this...
"More so as to how a mismatched impedance effects, or possibly distorts the wave of the signal...?"
There's no distortion of the signal, only less of it getting to the antenna to be radiated. It's something like you wanting a particular amount of water (volume) at a particular amount of pressure at a nozzle on a hose. If you change the size of the hose at some point, the amount of water and it's pressure at the nozzle will change. A big change is that hose's size means a big change in the amount of water and it's pressure coming out of that nozzle. Small changes mean a smaller change in the water's volume and pressure. If you could change the pump's pressure then you could compensate for that change in hose size, but with the typical transmitter that isn't that easy to do. It's much easier to stick with the same/right size hose. Or, if you could vary the size of the opening in that nozzle along with the pump's pressure, you could keep the same water stream coming out of the nozzle. That's not the best comparison in the world, but it's close. (It's also done all the time, ask a firefighter's pump operator. The 'tuner' on a firetruck, sort of. )
- 'Doc

I actually thought that was a pretty good analogy. When I took basic hydrolics and electronics, while at tech school to be a heavy equipment mechanic, I thought there were many similarities in the theory of operation between hydrolic systems and electrical circuits.

 

[Greatvalue1]

Thanks for the description, hydraulics is more familiar to me sometimes too.

My confusion stems from a logic breakdown when Chokes, Coils, or Baluns are applied to the antenna chain.

When I have built my Dipoles, and 1/4 wave ground-plane, the research I have done has told me that efficiency of the antenna must be corrected in the antenna design itself.

And that these corrections can be made by adjusting the Angle of the Ground-Plane(s) (for impedance), and the length of the elements (for wavelength resonance). (forgive me if these are not the technical terms).

I don't see how the antenna could be any more efficient than its inherent design with or without these Coils, but changes only how the Radio sees it.

 

[Mudfoot]

Hydraulic math is fun.

 

[Greatvalue1]

To clarify my logic breakdown: Why not just use a really short antenna with a coil that makes it resonate like a 5/8 wave (Firestick):tongue:.

I have to conclude that there must be some sort of sacrifice made in power loss using one of these coils to help improve the characteristics of an antenna. Maybe...?

And does this improvement in efficiency by the coil, surpass any losses that it will establish?

 

[The DB]

To clarify my logic breakdown: Why not just use a really short antenna with a coil that makes it resonate like a 5/8 wave (Firestick):tongue:.

I have to conclude that there must be some sort of sacrifice made in power loss using one of these coils to help improve the characteristics of an antenna. Maybe...?

And does this improvement in efficiency by the coil, surpass any losses that it will establish?

A really short 5/8 wave antenna? Doesn't exist. A 5/8 wave antenna will be longer then 20 feet long. Anything shorter will not be or act like a 5/8 wave antenna. The fiberglass mobile cb antennas you see advertised as 5/8 wave are not 5/8 wave antennas. What they did is use a 5/8 wave long wire to make the antenna, and then advertise it as a 5/8 wave antenna. The problem is the actual antenna length determines how the antenna radiates, not the length of the wire used to make it.

Also, remember what the purpose of advertising is, truth is often only a metaphor where advertising is concerned.


The DB

 

[Greatvalue1]

I was being facetious about the Firestick of course.

Too many contradictory interpretations on the internet as to why and how these coils improve an antenna. I wish I could know more for sure, but I'll have to go back to school for that one.

 

[W5LZ]

It's certainly possible to make a really short antenna resonant at a particular frequency. The problem with that is that it's radiation pattern, where it puts a signal, goes to pot very quickly. That radiation pattern of a 5/8 wave antenna is determined by it's physical length, not it's resonance. To put a signal where a full sized 5/8 wave antenna would put a signal, an antenna would have to be physically a 5/8 wave length long. Like a 1/4 wave's pattern? Then a 1/4 wave length long. Same for any 'size' antenna. Changing the length of an antenna also changes it's radiation pattern's shape. That can range from 'not very much', to very drastically. That length change works in both directions, shorter or longer.
A rubberduck antenna can be made resonant on 11 meters, but it would be silly to think that it's going to put a signal where a 'full sized' antenna would. Antennas are NOT one of the things you can miniaturize with no change in performance. It'd be nice, wouldn't it? Don't hold your breath though, you'll turn a pretty shaped of blue waiting on it to happen...
- 'Doc

 

[The DB]

I was being facetious about the Firestick of course.

Too many contradictory interpretations on the internet as to why and how these coils improve an antenna. I wish I could know more for sure, but I'll have to go back to school for that one.

A coil built into an antenna improves that antenna by shortening the resonant length of the antenna. In the case of a mobile cb antenna it allows the resonant antenna be be shorter than 9 feet long. For some strange reason some CB operators don't seem to like such an antenna on their car thinking it is to long... They also typically raise the q of the antenna, which narrows the bandwidth, and in essence makes the antenna reject more out of band radiation. This effect can lower noise (static) the radio tends to picks up.

On the negative side the shorter antenna does affect the radiation pattern, allowing more of the energy to go up than out to the horizon. If DX is your goal this isn't necessarily a bad thing, depending on conditions.

Any coil adds resistance to an AC signal, like the signal the antenna is there to transmit. Resistance directly equates to loss. The energy lost in this way is not transmitted, and instead is turned into heat. Any antenna with a coil built into it will there for be less efficient than an antenna without a coil. When comparing coils in antennas the bigger coils tend to be more efficient than the smaller ones, hence the popularity of the Predator type of antennas. Also the further up the antenna the coil is the more efficient it is.

Hope that gives you a starting point.


The DB

 

[wavrider]

Understanding Antennas For the Non-technical Ham by N4JA - An online Book!

Good reading to get a basic understanding of antennas.

 

[KM3F]

A loading coil "never" improves an antenna over a full length at the frequency of operation.
A full quarter wave has an input impedence of close to 70 ohms not 50.
As you go longer to 5/8 wave, the input impdeance goes lower such that you must do an impedenace transformation up to the coax impedence of 50 ohms or the result is an SWR missmatch.
A 5/8 wave also pulls upward the max pattern point a bit higher above the ground.
This compresses the pattern in order to get more gain, but at a higher angle.
By using a loading coil to resonate a shorter element, you lose efficiency with a shorter radiator and the losses in the coil from the I squared R losses result in RF Power loss to heat in the RF and DC resistance of the wire used in the loading coil..
Loading coils, depending on where they are placed above the feed point, also change the feed impedence such that the feed method will also have an effect on total antenna efficiency from added losses.
The more inductance that is added to an antenna design to get to resonance the higher the Q and the narrower the bandwidth between chosen SWR measuring points.
Antennas are very inefficient without doing things that make it worse unless there is a physical reason to do so and you accept the result.
This antenna theroy is played out and proven on the Ham bands of 6 meters, 2 meters and 440 mhz, in the mobile repeater arenas and all these variations and effects are well known.
11 meters is no different, just larger in size and even worse in some cases.
.
Just imagine the mobile that operates on 80 meters where a 1/4 wave is 63 feet +/- and 'must' have a loading coil to have any chance of a transmitter loading into a short radiator.
The best that can be done is a total length of close to 13 feet to be legal in max height and a loading coil some 5 inches in diameter and close to 12 inches long.
The total efficiency is very low but it has to be lived with and still works well enough to make it worthwhile.
Get the theroy right and not go on hear-sey please
Good luck..

 

[W5LZ]

Pssst - change that '70 ohms' to '25 - 35 ohms' and I'll agree with you...

 

[Shockwave]

Change the 5/8 wave as having a higher impedance than the 1/4 wave too. The 5/8 wave antenna has a lower impedance then the 1/2 wave end fed but is still much higher then the 1/4 wave.