Capacitance and inductance of coax

[ghz24]

fyi the coax with a lower vf has MORE loss which means less transmitted signal reaches your antenna.

And the lower VF, higher loss cable will further attenuate the signal that's reflected before the SWR meter can measure it. So it's double loss.

I'm still puzzling the add a no loss feed line and get a higher swr than at the feed point with no feed line.
Lower yes, higher didn't think it was possible
`

 

[Beetle]

The velocity factor of a transmission line is directly related to the composition of the dielectric. Most common coaces (my plural for "coax") use either solid polyethylene or foam polyethylene.

http://paragonmfgcorp.com/37/COMMON_COAX_COMPARISON_CHART.htm

The perfect dielectric would be a vacuum, which would give a Vf of 1.0, but that's a practical impossibility. Regular old atmospheric air is a good substitute and it's readily available. The foam in "foamed poly" dielectric is, of course, tiny air bubbles in a poly matrix. This is why, in the chart at the very bottom of the coax comparison chart (link is above), we see that solid PE (polyethylene) has a lower Vf than the foam PE varieties. The more air there is in the dielectric material, the higher the Vf.

In order to keep the characteristic impedance of coax right at or near 50 ohms, the manufacturer has to figure the diameter of the center conductor and the inner diameter of the shield. He also has to keep the finished outside diameter of the cable to a standard size: 0.405 inches for "RG-8", which also includes RG-213 and other 50-52 ohm coaces. The diameter of the center conductor will vary, but not enough to make any differences in the real world. A change of one or two millimeters in diameter will not cause any detectable loss at HF.

So RG-213 has solid polyethylene dielectric, and a relatively low Vf. Does this mean it's a "bad" choice? Not on your life.

 

[ghz24]

I didn't mean to insinuate that high VF and low loss are directly correlated.
There may be no engineering reason for the two to be related but there sure seams to be a practical correlation between the two in the cables I've compared.
And I wouldn't suggest that RG-213 is a bad choice.

Regarding VF's effect on SWR : Why do we want to use a 1/2 wavelength jumper to test SWR ? Why do some people insist that SWR can be "tuned " by cutting the coax feed to a different length?

Those are rhetorical questions please don't feel obliged to answer them !
My modeling program doesn't do feed lines.
I am curious what the model shows with feed lines the same electrical length.

 

[Marconi]

Thanks for the explanation Marconi. I'm not that familiar with modeling software.


The DB

I wasn't sure.

I would disregard what I've posted regarding the feed lines. Something is still not right with my models with feed line added, because when I change the velocity factor the model always seems to go south.

Maybe one day I'll figure it out. I think the manual says this function is at best virtual, so that may mean something.

 

[KM3F]

Antenna impedance, VSWR and VF are inter related depending on the values being measured.
Example 1. Antenna is a perfect 50 ohm resistive termination with no reactive component.
A 1/2 wave electrical line length 'reflects the antenna inmpedance at the end of this line. AS WELL, the line length makes no difference under these conditions because there is no VSWR to measure under these perfect conditions since there is no reflected power.
Only item that is in effect is the line loss in one direction going out.
.
Example 2. If the antenna is not a match, more parameters come into play.
A. The electrical line length is governed by VF and dictates the 1/2 wave physical length.
B. To measure only the actual antenna mismatch, the line length has to be corrected for VF.
C. If this is not done, then the feedline length appears to have an effect on the VSWR reading.
D. With a VSWR caused by antenna mismatch, the length of the line becomes important, the VF becomes important, the feedline loss become involved.
E. The following integrate into the final result. 1. the feedlline losses 'both directions', the VF and most of all the point along the feedline the VSWR is measured.
F. In a mismatched condition all points along the feedline is composed of 'different values' of voltage and currents due to phase differences.
Your VSWR meter can only detect what the values are at the measurement point.
Move the metering point one or two feet along the feedline and the meter sees a different set of values. This is the reason the VSWR changes with feedline length while nothing else has changed in the total system.
Lastly to clear up another misunderstanding, changing the feedline length has little effect on the antenna match but you can 'arrive' at a line length the radio likes as near 50 ohms at that 'specific point'.
In effect this is where the term 'tuned feedline comes from because the feedline becomes frequency and load sensitive (when there is an antenna mismatch)..
Bottom line is it pays to have the antenna match the feedline so you stay out of these complications that can also cause common mode currents on the outside of the feedline and can cause other issues with the radio feedback etc.
The subject is complex and not easily understood unless you make an effort to understand all the parts of it (and) how they interact under 'different' conditions.
Hope this is a help in understanding the relationships.
Good luck.

 

[KM3F]

To address coax inherent makeup, the diameter of the center conductor, the diameter of the outside shield and what is in the space between them sets the impedance, the losses and the VF.
Capacitance is the surface area the center presents to the outside conductor along with the dielectric.
The inductance is the AC resistance the center conductor presents to the flow of RF current. With skin effect also part of the resistance.
DC resistance per unit length has little effect at RF frequencies.
The sum total of this would be represented by an electrical drawing of series inductances representing the line length with capacitor shunting the center to the outside shield in a continuous manner.
This you cannot get away from, only reduce the values by the physical design.
Good luck.

 

[The DB]

Antenna impedance, VSWR and VF are inter related depending on the values being measured.
Example 1. Antenna is a perfect 50 ohm resistive termination with no reactive component.
A 1/2 wave electrical line length 'reflects the antenna inmpedance at the end of this line. AS WELL, the line length makes no difference under these conditions because there is no VSWR to measure under these perfect conditions since there is no reflected power.
Only item that is in effect is the line loss in one direction going out.
.
Example 2. If the antenna is not a match, more parameters come into play.
A. The electrical line length is governed by VF and dictates the 1/2 wave physical length.
B. To measure only the actual antenna mismatch, the line length has to be corrected for VF.
C. If this is not done, then the feedline length appears to have an effect on the VSWR reading.
D. With a VSWR caused by antenna mismatch, the length of the line becomes important, the VF becomes important, the feedline loss become involved.
E. The following integrate into the final result. 1. the feedlline losses 'both directions', the VF and most of all the point along the feedline the VSWR is measured.
F. In a mismatched condition all points along the feedline is composed of 'different values' of voltage and currents due to phase differences.
Your VSWR meter can only detect what the values are at the measurement point.
Move the metering point one or two feet along the feedline and the meter sees a different set of values. This is the reason the VSWR changes with feedline length while nothing else has changed in the total system.
Lastly to clear up another misunderstanding, changing the feedline length has little effect on the antenna match but you can 'arrive' at a line length the radio likes as near 50 ohms at that 'specific point'.
In effect this is where the term 'tuned feedline comes from because the feedline becomes frequency and load sensitive (when there is an antenna mismatch)..
Bottom line is it pays to have the antenna match the feedline so you stay out of these complications that can also cause common mode currents on the outside of the feedline and can cause other issues with the radio feedback etc.
The subject is complex and not easily understood unless you make an effort to understand all the parts of it (and) how they interact under 'different' conditions.
Hope this is a help in understanding the relationships.
Good luck.

Not this again... Someone isn't up with the many discussions on the forum about SWR and feedline length. (Or if you prefer VSWR, and hell lets include ISWR as well, you know, for the hell of it... ) Perhaps we should make a sticky with all of the needed information including source information and quotes in it that we can simply link to...

You are correct about the rest (at least most of it), the R and X readings from an antenna analyzer will in fact change, as well as the phase angle. The thing is, while R and X change due to the transformer effect of the feedline, SWR (and all of its variant names) does not, except for the effects of attenuation.

If you could look at R and X at every point on the feedline as you travel down the feedline from the antenna you would come across every possible combination of R and X that makes up the existing SWR, if the feedline is long enough.

If your SWR changes noticeably simply because the feedline length changed a few feet then you have another issue in play, common mode currents.

We have had this discussion multiple times on this forum, and before you continue with that line of thought I highly recommend you find a freely available pdf file called "Another Look at Reflections" by M. Walter Maxwell and read it if you haven't already. It will explain the true nature of SWR and feedlines.


The DB

 

[Marconi]

DB, I might have missed something, but after reading KM3F's first post above, I sorta felt he was pretty much spot on. I would have made more of a distinction throughout his post regarding the load being perfect vs. a mismatched load, but that is just my way of considering all of this complicated stuff.

In your seeming criticism of him posting, I wonder if you can point to something specific?

DB to repeat, I'm just curious and maybe I don't fully understand all that is going on here, but you post the following:

Not this again... Someone isn't up with the many discussions on the forum about SWR and feed line length. (Or if you prefer VSWR, and hell lets include ISWR as well, you know, for the hell of it... ) Perhaps we should make a sticky with all of the needed information including source information and quotes in it that we can simply link to...

DB, I'm not trying to start something here. I consider you as being informative and important member here of WWDX, but can you be more specific with what your seeming complaint of KM3F's post is here? What exactly did he say or do?

 

[The DB]

DB, I'm not trying to start something here. I consider you as being informative and important member here of WWDX, but can you be more specific with what your seeming complaint of KM3F's post is here? What exactly did he say or do?

I was frustrated from something else when typing that post. That frustration colored my general relaxed tone. Apologies that it came out in that post, nothing was intended by it.

Several times over the last few months a debate crept in and out on weather a mismatched load caused the SWR in the feedline before it to change based on how far before said load the measurement was taken. I agree with several important parts of what KM3F said, however, I disagree with any reference that says or implies that SWR changes on the feedline depending on where the measurement was taken from with the exception of the effects of attenuation. Most lines in the post happen to refer to the one idea that I disagree with.

There have been several quoted sources in relation to that, some of them posted multiple times in a fair number of recent threads. There was also a recent test on an semi-related matter that also demonstrates this fact.

I'm not saying or intending to say that he is an idiot, he is right about this being a complicated matter that many people have trouble wrapping their head around. Added to that is the prevalence of incorrect information from what appears to be reputable sources definitely doesn't help either.


The DB

 

[Captain Kilowatt]

One of the big reasons people think SWR changes with line length in an unmatched system is that while they understand that the complex impedance changes along the line length however the actual SWR does not. Some folks do not realize that for any given value of SWR there is an almost infinite combination of R, Xc, and Xl that in the right combination will produce that value of SWR.This is where understanding a Smith Chart comes in handy.

 

[Marconi]

I think I understand DB.

I only have one question for anybody that will respond.

Does this issue, as briefly noted above by Captain Kilowatt, have anything to do with the idea of the conjugate match?

 

[W5LZ]

They are broadly related, but don't draw any hasty conclusions from that.
- 'Doc

 

[Captain Kilowatt]

I think I understand DB.

I only have one question for anybody that will respond.

Does this issue, as briefly noted above by Captain Kilowatt, have anything to do with the idea of the conjugate match?

Yes and no. Don't you love that kind of answer? A conjugate match is used when the load impedance does not equal the source impedance in order to maximize power transfer. It does not mean that there is no reflected power. It simply means that reflected power does not appear at the source (transmitter).

From the ever popular Wikipedia:
Theory

Impedance is the opposition by a system to the flow of energy from a source. For constant signals, this impedance can also be constant. For varying signals, it usually changes with frequency. The energy involved can be electrical, mechanical, magnetic or thermal. The concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms. In general, impedance has a complex value; this means that loads generally have a resistance component (symbol: R) which forms the real part of Z and a reactance component (symbol: X) which forms the imaginary part of Z.
In simple cases (such as low-frequency or direct-current power transmission) the reactance may be negligible or zero; the impedance can be considered a pure resistance, expressed as a real number. In the following summary we will consider the general case when resistance and reactance are both significant, and the special case in which the reactance is negligible.
Reflection-less matching

Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. Ideally, the source and load impedances should be purely resistive: in this special case reflection-less matching is the same as maximum power transfer matching. A transmission line connecting the source and load together must also be the same impedance: Zload = Zline = Zsource, where Zline is the characteristic impedance of the transmission line. The transmission line characteristic impedance should also ideally be purely resistive. Cable makers try to get as close to this ideal as possible and transmission lines are often assumed to have a purely real characteristic impedance in calculations, however, it is conventional to still use the term characteristic impedance rather than characteristic resistance.
Complex conjugate matching

Complex conjugate matching is used when maximum power transfer is required. This is different from reflection-less matching only when the source or load have a reactive component.
Zload = Zsource* (where * indicates the complex conjugate).
If the source has a reactive component, but the load is purely resistive then matching can be achieved by adding a reactance of the opposite sign to the load. This simple matching network consisting of a single element will usually only achieve a perfect match at a single frequency. This is because the added element will either be a capacitor or an inductor, both of which are frequency dependent and will not, in general, follow the frequency dependence of the source impedance. For wide bandwidth applications a more complex network needs to be designed.
Power transfer

Main article: Maximum power theorem
Whenever a source of power with a fixed output impedance such as an electric signal source, a radio transmitter or a mechanical sound (e.g., a loudspeaker) operates into a load, the maximum possible power is delivered to the load when the impedance of the load (load impedance or input impedance) is equal to the complex conjugate of the impedance of the source (that is, its internal impedance or output impedance). For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude but of opposite signs. In low-frequency or DC systems (or systems with purely resistive sources and loads) the reactances are zero, or small enough to be ignored. In this case, maximum power transfer occurs when the resistance of the load is equal to the resistance of the source (see maximum power theorem for a mathematical proof).


For anyone that cares or wants to care here is a Smith Chart tutorial. I will admit to only being a novice at it. I never used one much and it was a long time ago but keep coming back in attempt to learn more about using them.


http://www.google.ca/url?sa=t&rct=j...eQ_6UH1Caf-OyNw&bvm=bv.49967636,d.dmg&cad=rja

 

[Marconi]

Yes and no. Don't you love that kind of answer? A conjugate match is used when the load impedance does not equal the source impedance in order to maximize power transfer. It does not mean that there is no reflected power. It simply means that reflected power does not appear at the source (transmitter).

From the ever popular Wikipedia:
Theory

Impedance is the opposition by a system to the flow of energy from a source. For constant signals, this impedance can also be constant. For varying signals, it usually changes with frequency. The energy involved can be electrical, mechanical, magnetic or thermal. The concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms. In general, impedance has a complex value; this means that loads generally have a resistance component (symbol: R) which forms the real part of Z and a reactance component (symbol: X) which forms the imaginary part of Z.
In simple cases (such as low-frequency or direct-current power transmission) the reactance may be negligible or zero; the impedance can be considered a pure resistance, expressed as a real number. In the following summary we will consider the general case when resistance and reactance are both significant, and the special case in which the reactance is negligible.
Reflection-less matching

Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. Ideally, the source and load impedances should be purely resistive: in this special case reflection-less matching is the same as maximum power transfer matching. A transmission line connecting the source and load together must also be the same impedance: Zload = Zline = Zsource, where Zline is the characteristic impedance of the transmission line. The transmission line characteristic impedance should also ideally be purely resistive. Cable makers try to get as close to this ideal as possible and transmission lines are often assumed to have a purely real characteristic impedance in calculations, however, it is conventional to still use the term characteristic impedance rather than characteristic resistance.
Complex conjugate matching

Complex conjugate matching is used when maximum power transfer is required. This is different from reflection-less matching only when the source or load have a reactive component.
Zload = Zsource* (where * indicates the complex conjugate).
If the source has a reactive component, but the load is purely resistive then matching can be achieved by adding a reactance of the opposite sign to the load. This simple matching network consisting of a single element will usually only achieve a perfect match at a single frequency. This is because the added element will either be a capacitor or an inductor, both of which are frequency dependent and will not, in general, follow the frequency dependence of the source impedance. For wide bandwidth applications a more complex network needs to be designed.
Power transfer

Main article: Maximum power theorem
Whenever a source of power with a fixed output impedance such as an electric signal source, a radio transmitter or a mechanical sound (e.g., a loudspeaker) operates into a load, the maximum possible power is delivered to the load when the impedance of the load (load impedance or input impedance) is equal to the complex conjugate of the impedance of the source (that is, its internal impedance or output impedance). For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude but of opposite signs. In low-frequency or DC systems (or systems with purely resistive sources and loads) the reactances are zero, or small enough to be ignored. In this case, maximum power transfer occurs when the resistance of the load is equal to the resistance of the source (see maximum power theorem for a mathematical proof).

Thanks to both of you for responding. I have made a promise here on WWDX not to ever bring up my ideas about this subject again, because I could not get my ideas across, and, I've said it before, doing such surely does not suggest a good idea.

This is why I asked the question.

I will ask another question CK. What is the theory you posted above called, the Conjugate Match Theory?

 

[Captain Kilowatt]

Thanks to both of you for responding. I have made a promise here on WWDX not to ever bring up my ideas about this subject again, because I could not get my ideas across, and, I've said it before, doing such surely does not suggest a good idea.

This is why I asked the question.

I will ask another question CK. What is the theory you posted above called, the Conjugate Match Theory?

You mean this part?

"Theory

Impedance is the opposition by a system to the flow of energy from a source. For constant signals, this impedance can also be constant. For varying signals, it usually changes with frequency. The energy involved can be electrical, mechanical, magnetic or thermal. The concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms. In general, impedance has a complex value; this means that loads generally have a resistance component (symbol: R) which forms the real part of Z and a reactance component (symbol: X) which forms the imaginary part of Z.
In simple cases (such as low-frequency or direct-current power transmission) the reactance may be negligible or zero; the impedance can be considered a pure resistance, expressed as a real number. In the following summary we will consider the general case when resistance and reactance are both significant, and the special case in which the reactance is negligible."

That is just general impedance theory. It has nothing to do in particular to conjugate match theory.That text was not spaced well. The bold text is applicable ONLY to what immediately follows it. On subject is basic Theory the next is Reflection-less matching followed by Complex conjugate matching etc.