Capacitance and inductance of coax

[roadrage]

I was reading some spec charts of coaxial cable and capacitance and the inductance values were given along with the velocity %, power handling, and attenuation. I was wondering what role the capacitance/ inductance values play, and if they should be considered when choosing coax for your system? If one cable has a capacitance of 16 and another has 26, will it make any difference?

 

[Beetle]

I was reading some spec charts of coaxial cable and capacitance and the inductance values were given along with the velocity %, power handling, and attenuation. I was wondering what role the capacitance/ inductance values play, and if they should be considered when choosing coax for your system? If one cable has a capacitance of 16 and another has 26, will it make any difference?

None that will make any difference unless you're making a coaxial capacitor as part of a feedpoint. The capacitance is generally stated as so many picofarads per foot or per meter. Inductance only comes into play at UHF and above, generally, unless you're making a coaxial choke. And that only involves the outside of the shield, not the inside.

 

[roadrage]

Thanks. I figured that would be the case. I was told the same of the velocity factor as well.

 

[Marconi]

Thanks. I figured that would be the case. I was told the same of the velocity factor as well.

roadrage, I believe that Beetle might be right on the issue you raised, but I'm not too sure that feed line VF doesn't matter like you were told.

It might be easy to make the claim without some experience or better some evidence, but my models of a horizontal M103 does indicate there is a notable difference using coax with a VF of .66 compared to .84 on the same antenna at the same height and length of line. All I changed was the VF.

I have the models, but I'll just post the result for gain/angle, R, X, SWR.

Here is the model for a M103 @ 36' feet.

Gain/Angle.....R...........X..........SWR.......VF

11.86@14*.....23.36.....-13.01...2.32.......None

13.07@14*.....25.17.....-13.14...2.17........66%

9.07@14*.......11.81.....+8.92...4.36.........84%

When I added the feed line data to the model without a feed line...the difference was small as noted above. You'll note a notable difference, however, when the VF was changed from .66% to .84%.

I can't tell you that Eznec does the feed line feature perfectly, as in perfect duplication for the feed line results, but modeling is based on theory IMO. I also did not include the feed line losses in the feed line feature.

This is not a claim that this is accurate beyond all doubt, but it is what my model shows.

Sorry the image is not so good.

 

[roadrage]

So the higher velocity factor raised the SWR? I didn't expect that. To be honest, I am surprised it would even affect it. I was told that for HF usage the velocity factor wouldn't be of much concern, but my original thought was that the higher VP would increase the performance of the system because of the speed of the transmitting signal through the feedline. When I was told it would be of no concern unless I was going to have some kind of phasing. I never really questioned it. But I didn't expect SWR readings to be affected. Can I ask why and how.

Does that mean I should try to use coax with a lower VP?

 

[Captain Kilowatt]

I can see no reason why velocity factor would alter the SWR. If the antenna impedance was not a pure 50 ohms then the IMPEDANCE seen at the transmitter may be altered due to the transformation property of a transmission line however the SWR should not change. Having a different velocity factor would mean that the electrical length of the transmission line would be different and therefore the impedance presented to the transmitter would be different.

In any event velocity factor is a non-issue when choosing a coaxial cable unless phasing is a concern and then it is only a matter of altering the length to compensate for a different velocity factor.

 

[Marconi]

So the higher velocity factor raised the SWR? I didn't expect that. To be honest, I am surprised it would even affect it. I was told that for HF usage the velocity factor wouldn't be of much concern, but my original thought was that the higher VP would increase the performance of the system because of the speed of the transmitting signal through the feedline. When I was told it would be of no concern unless I was going to have some kind of phasing. I never really questioned it. But I didn't expect SWR readings to be affected. Can I ask why and how.

Does that mean I should try to use coax with a lower VP?

Well I was surprised too when I ran the models. I've used different VF coax before and I noticed SWR changes, but I thought it might be the VF and length some how made some difference.

I wouldn't jump the gun and start spending money based on my post, but something made the difference, because all I changed in the models was the VF data.

I wouldn't worry about it however.

 

[Marconi]

I can see no reason why velocity factor would alter the SWR. If the antenna impedance was not a pure 50 ohms then the IMPEDANCE seen at the transmitter may be altered due to the transformation property of a transmission line however the SWR should not change. Having a different velocity factor would mean that the electrical length of the transmission line would be different and therefore the impedance presented to the transmitter would be different.

In any event velocity factor is a non-issue when choosing a coaxial cable unless phasing is a concern and then it is only a matter of altering the length to compensate for a different velocity factor.

I agree CK. The M103 was not matched well as noted in the 3rd model posted, and therefore we would expect some feedline transformation. The model indicates an abundance of currents flowing on the feed line, which you can easily see in the attached antenna view for the model.

I only got the idea from this thread, and this all surprised me too.

I have M104c beam model that is tuned near perfect. It is using a direct coax connection with no matching. I'll do the same on it, and we'll see if we see better results...which I might expect. I will also switch the VF to make sure.

IMO however, I tend to believe that generally many CB type antennas operate with workable complex impedance matches...without many being any the wiser.

So, again you're right on the mark.

 

[ghz24]

length?

Lots of reports of replacing cable with better quality ,higher VF, cable causing SWR to go higher.
Usually the losses of the inferior cable is blamed for hiding the true value.
But Marconi wrote

I also did not include the feed line losses in the feed line feature.

Marconi did you adjust the length of the models cable to make them both the same electrical length?
There's a reason some people think you can tune/change SWR by changing the cables length.

 

[Marconi]

Lots of reports of replacing cable with better quality ,higher VF, cable causing SWR to go higher.
Usually the losses of the inferior cable is blamed for hiding the true value.
But Marconi wrote

Marconi did you adjust the length of the models cable to make them both the same electrical length?
There's a reason some people think you can tune/change SWR by changing the cables length.

24, I started with a model that had no feed line. I then add the feed line with a VF of .66, to the original model. Then I did the same with a .84, VF.

So, yes the feed lines were the same length, but I didn't try to figure electrical lengths into the models for the two VF's...if that is your question? I had the feed line feature set at "actual distance," for the length of the two models with feed lines, and I used the same frequency for all, 27.205 mhz.

Again, I think CK was right, these models at 36' were being effected by the mismatch at the feed point, so we are likely to see feed line transformation and the coax becomes more reactive to changes depending on the magnitude of the mismatch. This also possibly demonstrates why guys like W8JI, and Cebik make note of the fact that models without feed lines provide mis-information when using Eznec's Real Earth feature. I'm just not real sure how much difference it makes.

IMO, modeling is a mixed bag of ideas that needs to be pretty well understood regarding all its features and options. I'm not to that level of understanding, so I don't do what I do for the accuracy of a matter, but I sometimes post what I see with my work and I try to keep it simple, because I don't understand everything about Eznec. I figure my models are as good at taking the words in CB lore.

I think a model in free space may be more accurate for predicting the physical design of the antenna itself, because the antenna system is minimized and/or eliminated, the losses are basically removed, and that tends to isolates the design itself for close study. The idea is, the Earth's effects are considered absent from such modeling. To repeat, the antenna system's effects are also minimized or eliminated for study.

When we get over Real Earth, however, the antenna system, the losses, and stuff on the surface of the Earth...all start coming into play, so the real world reality produces quite different results. Modeling is tedious work, and little errors tend to mount up, and maybe that is what is going on here.

Due to the fact that I can't control the feed point match, I don't know how to use the matching features, I think if I used a model with a more perfect natural feed point match, these differences I noted earlier would likely not be so profound.

 

[The DB]

When modeling feedlines, what options besides the velocity factor can you adjust for feedlines?


The DB

 

[Marconi]

When modeling feed lines, what options besides the velocity factor can you adjust for feed lines?


The DB

DB, you can enter the following data elements:

end 1, wire #,
% from E1,
end 2, wire #
% from E2
Length,
ZO,
VF,
Rev/Norm,
Loss in db/100',
Loss Frequency

DB, I'll present the following as though you don't know how the transmission line feature works, just in case.

There are 10 data cells in this feature left to right. If you look at the earlier drawing of the Antenna View, maybe you can see the wires with their wire#'s I used. In the M103 model, I put wire 10 at 0%, in the first two data cells. I put wire 5 at 0%, in the second two data cells. You should be able to figure out the others...they are intuitive. I also put "Actual dist" in the cell for Length, (length of feed line, you can also enter a physical length in the cell like 50'. I typically leave the last two cells to default, they will pop up automatically on saving the data entered to the Transmission line feature when I hit my enter key.

I hope this is not too confusing...just in case you want to play with the idea, and see if you can get it to work. BTW, with the cells entitled %, you can enter whole numbers, and maybe you can even use decimal fractions like 10.5% too, not sure though. The most important thing is you have to have a wire assigned (like a mast in this case) for the Transmission Line feature to work, or else the feature will not work. You will see a little red square box appear along the wire somewhere, but the feed line added is virtually not a part of the mast, as in this case.

I haven't figured out how to post a picture that is nice and legible yet, but here is a fuzzy picture of what the screen view for the Transmission Lines feature looks like. It is the long rectangle box at the bottom of the two boxes in front of my 70 lb. Rojo's image on my Desktop.

 

[hotrod]

Lots of reports of replacing cable with better quality ,higher VF, cable causing SWR to go higher.
Usually the losses of the inferior cable is blamed for hiding the true value.

i agree with this, and been told this for many years. the high vf coax may show a higher swr , BUT its more accurate. fyi the coax with a lower vf has MORE loss which means less transmitted signal reaches your antenna.

 

[W5LZ]

Velocity factor has no affect on cable losses or SWR. The 'size' of the velocity factor only plays a part in 'timing'. That 'size' also makes no difference in quality, a high VF doesn't mean a 'good' cable and a low one a 'bad' cable. Velocity factors make a very good excuse for people who don't know what it is and understand it though...
- 'Doc

 

[The DB]

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


The DB