Does different coax affect swr?

[fourstringburn]

No fourstringburn, not any more. I'm 78 years old and in bad health, and can't even walk far or drive anymore. For years however, I did all kinds of things with the several CB vertical and beam antennas I acquired over the years. And I videoed a small part of it in the last few years before diabetes disabled me.

If you check out the link in my signature below and click on the YouTube link...you'll see a lot of videos I once did trying to demonstrate a little of the things I use to do with my antennas, trying to understand better what I was hearing on this forum and others.

Modeling is about all I can do anymore...and I do make mistakes, it can be tedious work.

Sorry to hear about your health. I"ll look at your videos when I have some time.

 

[Marconi]

Joe you are right according to my Eznec models. You know Joe, I never measured the match with an analyzer at the feed point on a mobile either. I know there must be some images of guys using an analyzer on the forum somewhere and it should show the real truth of the matter. I depend on my models for information, but I'm not sure about these mobile objects I use. This is why I created the 1/4 wave theory model I posted below, I was curious how they came to those numbers.

Actually no he was not right. A dipole is 72 ohms and a 1/4 wave vertical over a perfect ground is about 35-36 ohms like I said. You REALLY need to stop putting so much faith in YOUR modeling.

Captain Kilowatt, like I said above...I do depend on my models, but I also said (in bold above) I wasn't sure about the mobile objects I have been using. I was in error telling Joe he was right about the 1/4 wave installed on a mobile showing 72 ohms at the feed point.

One thing about the mobile 102 whip installations...........people are "tuned" (pun intended) to see 50 ohms and consider that to be the be-all and end-all of a perfect tuning job and stop there. The issue is that since a 1/4 wave antenna is really 35-36 ohms if they were to continue improving their ground the SWR will actually go up slightly which runs contrary to what they think or understand. If they were to continue improving their ground not only would the SWR go up slightly but the efficiency would also go up considerable.

My old antenna mentor always told me to look for about 1.40-1.50 SWR for a 102" whip installed on a mobile. He also told me if I got below this SWR then I would just be adding losses...just like you say above. I knew better, but I just messed up.

I have been working on my mobile objects to try and get one of them to respond by showing <>35 ohms at resonance, but nothing I've done so far has changed the high readings I've been getting. My models also do not produce a good Average Gain report results either, and when I first saw that is when these mobile objects started giving me concerns the models were not working right.

I have made a new model of a 14' foot Bronco mobile object so I could fill the model with more wires hoping to mimic a mobile vehicle body better, but that does not appear to help either. I filled out my model with 12" inch sections and made my wires 5.75" inches in diameter. This basically made the model a continuous exterior tin surface from top to bottom and from front to back and all the wires are connected together. I still don't see the match, using a 102" - 108" inch whip should show us.

I know The DB has made some of these mobile objects but I'm not sure if he has noticed this high impedance issue in his mobile models or not.

I recently asked him, in his thread on his new Explorer model, if his mobile object showed a good AG result or not, but I haven't heard from him yet. I don't think he would have miss such a glaring mistake in his mobile objects however.

I'm still working for a solution and thanks for helping me see the light.

 

[Captain Kilowatt]

I'll be honest with you, it seems that most modelling is more accurate with fixed installations that they are with mobile installations. Whether it is the lack of functionality in the software or simply a result of too many unknown variables I am not sure but trying to use wires to represent a solid groundplane seems like a downfall of the software.

 

[Marconi]

I'll be honest with you, it seems that most modelling is more accurate with fixed installations that they are with mobile installations. Whether it is the lack of functionality in the software or simply a result of too many unknown variables I am not sure but trying to use wires to represent a solid groundplane seems like a downfall of the software.

I read some articles and one is in ARRL Antenna Compendium Vol. 7, p. 25-30, Computer Modeling the HF Mobile Antenna. According to this article short wires with only one segment per wire is problematic and this is the problem I have with my Eznec segment limitation of 500 segments per model.

The article does indicate that the impedance at 30MHz was 82 ohms on the small mobile object they used and a HP 4085A Vector Impedance meter was used in these determinations.

 

[The DB]

I am not sure but trying to use wires to represent a solid groundplane seems like a downfall of the software.

Yet we have an analogue of this in real life, at least to some extent. If you take a 1/4 wavelength ground mounted vertical and put it over 120 ground plane radials you get very close to the same feed point impedance that you get as if the earth was a perfect ground, and modeling shows this as well.

The solution above is the basis of how to deal with such a problem, the main part of the solution is more wires, and a grid pattern works very well. However, they are not needed everywhere, you put them where the currents are going to be naturally the strongest, and that is at the feed point of the antenna. When you get further away from the feed point the currents are spread out over a plane, such as the metal roof of a vehicle, you can use fewer wires, as with a sheet of metal, their will be far fewer currents in this area.

In the past I have played with various models and irl counterparts, in various situations, at least as far as I am able to test, and the modeling software has accurately predicted to within a reasonable margin of error all of the parts I could confirm, which includes the feed point impedance. Things like the radiation pattern I could only confirm to the point that the pattern shown was in the general shape that the modeling software shows, but I only had a cheap field strength meter as a measuring instrument (I don't have a full line of lab grade test equipment unfortunately). I have also confirmed that if you use a grid of wires in place of a sheet of metal, assuming the grid is "fine enough", the difference between the grid and the sheet is very small.

I have also attempted to confirm my original Ford Explorer model's radiation pattern with the same method, and it didn't match up to what my field strength meter said it should. The model showed gain in some directions that the actual antenna didn't. The whole reason I put in the effort of making the second model of the same vehicle, which included things like curves in the design and such, is that the original model's pattern didn't match my actual test measurements. If it wasn't for that live testing, the second model would have never existed. As a bonus, that model also presents a much closer feed point impedance to what I actually get on said vehicle, and tuned resonance on the same frequency within 1/8 inch of length of the actual vehicle. Without more accurate equipment I'm not going to say it is perfect (I wouldn't anyway), but it is close enough to get an idea of what is going on.

I guess this ends up working as a response to an irrelevant comment fourstringburn made as well...

I wonder if Marconi ever goes outside and physically does antenna work and testing as opposed to just doing modeling. I said the same thing about the DB who does exactly that also.

I wasn't going to say anything about this initially as it is the comment of someone who doesn't know me nearly as well as he seems to think, and honestly, it wasn't worth a response. It was pointless, and didn't add to the conversation in any way, shape, or form. In my opinion it says more about the person who said it than anything, especially when he was clearly wrong. You don't get to where I am with antennas on theory alone, but you can't get here with experience alone either, it takes both. The truth is I likely have my hands on far more antennas every year than he does. Some of these are repair jobs after a storm, some of these are custom made for where they are going, but most are pre-made antennas that someone bought and needed help putting up. That is what I get for being on an active and growing ham radio club's antennas team.

I would love to be able to afford a full lab of lab grade equipment to experiment with antennas. Hell, I would love to have more accurate equipment than I do just for testing purposes. Unfortunately I am only one person and don't have unlimited resources at my disposal. Instead I do what I can do with what I have, and that will have to do, at least for the time being.


The DB

 

[Marconi]

The solution above is the basis of how to deal with such a problem, the main part of the solution is more wires, and a grid pattern works very well.

DB, attached below are two pdf files.

1. This is my old mobile objects with 102" - 108" inch whip on the center of the tailgate for the PU truck titled "Bronco Model 102" on TG" and is 180" long with a 102" inch whip. I just removed the cover over the back of my Bronco model and mounted the antenna on the center of the tailgate. This model uses 47 wires using 133 segments.

2. This is my New model object titled "Bronco 14' object" and is 168" long with a 108" whip on the top rear center at the back. I did this one today hoping the idea you you just posted above would work...your saying more wires is more likely to show better results and I hope it also shows a better Average Gain result in Free Space too. Captain Kilowatt, Joe, and I have been discussing the topic of the natural theoretical match for a 1/4 wave whip. This model uses 227 wires using 365 segments and is very busy so I turned the wire numbers off just to show the model.

I think you will be surprised at these results. These models are still showing bad results with the Average Gain test, and I'm not vouching for them, but nothing I've done in the last couple of days has lowered the end impedance for this 1/4 wave whip on a mobile body down anywhere near 35 ohms either. As you will note both of my models show a higher feed point impedance. The match is not bad and no matching was done on either model.

However, they are not needed everywhere, you put them where the currents are going to be naturally the strongest, and that is at the feed point of the antenna. When you get further away from the feed point the currents are spread out over a plane, such as the metal roof of a vehicle, you can use fewer wires, as with a sheet of metal, their will be far fewer currents in this area.

I realized this from making these mobile models too. The articles I've read from guys modeling and also doing real life testing with an HP vector analyzer...caution about the same thing concerning using grid patterns larger than 0.1% wavelength among other things. My grid patter here is 12" inches but my wire size is 5.75" inches...so there is very little empty space in the model.

I did my model subject similar to yours without the curves however, and made the grid everywhere mostly so I could move the antenna around on the vehicle hoping for some consistency in effects.

That still doesn't help us explain how my object with few wires did nearly as good as my more complicated object with far more wires.

I have also confirmed that if you use a grid of wires in place of a sheet of metal, assuming the grid is "fine enough", the difference between the grid and the sheet is very small.

I can't make a grid per se with my version of Eznec, but I agree with your comments here. The article in ARRL Compendium #7, also claims the same as you note above in addition to other factors.

As a bonus, that model also presents a much closer feed point impedance to what I actually get on said vehicle, and tuned resonance on the same frequency within 1/8 inch of length of the actual vehicle.

Are you telling us here that your mobile object shows the theoretical match for a 1/4 wave at or near 35 ohms when mounted on you mobile object, and that adding the curves to the object did that?

Does you new object also show you a good AG test results?

If so, I sure would like to know how you did that.

 

[The DB]

I think you will be surprised at these results.

I'm not, mainly because of the results I got with my first attempt at modeling my vehicle.

These models are still showing bad results with the Average Gain test,

I'm getting an AGT of 0.96, I think the larger number of elements are actually helping here.

but nothing I've done in the last couple of days has lowered the end impedance for this 1/4 wave whip on a mobile body down anywhere near 35 ohms either.

Are you telling us here that your mobile object shows the theoretical match for a 1/4 wave at or near 35 ohms when mounted on you mobile object, and that adding the curves to the object did that?

That 36 ohm number, there are exactly three cases that that is relevant.

1. If you are mounted over a perfect ground, (that is a 1/4 wavelength antenna with the other half being the perfect ground, not a ground plane antenna mounted some distance above perfect ground) good luck with that irl.
2. If you are over an analog of a perfect ground, such as, say, 120 ground plane radials of 1/4 wavelength. That would be an 18 foot diameter ground plane, good luck fitting that on a car.
3. The things that affect the antennas feed point impedance just happen to balance each other out and give that as a result, but how do you know that happened?

Regarding number 3, impedance is a funny thing, there are things that will lower it, such as not enough metal underneath the antenna, you know, like on all vehicles that are road legal, and things that will raise it, like how close the antenna is to the imperfect earth below the antenna system. In a vehicle those both play their parts and modify the feed point impedance, as well as the antenna's radiation resistance. I agree that the number most people shoot for, namely 50 ohms, isn't really what they think it is, but at the same time, 36 ohms isn't necessarily the number to shoot for either.

Those are also not the only things that affect impedance, as well as the antenna's radiation resistance.

That still doesn't help us explain how my object with few wires did nearly as good as my more complicated object with far more wires.

Not having the models myself it is hard to say, but the model with more wires seems to be less efficient and more directional, at least as far as the models you posted are concerned. The maximum gain figure isn't all you need to be looking at, but the entire pattern, and in the case of these models the side gain as well, which is listed and noticeably different. The patterns don't look anything alike, so aside from the gain numbers appearing to be about the same, I would definitely not call them similar.

Does you new object also show you a good AG test results?

If so, I sure would like to know how you did that.

As I stated above, 0.96 AGT, and that was just how it ended up, I didn't have to modify the model in any way. Actually modifying various aspects of the model didn't actually change the AGT, so it seems pretty stable, more so than other models I have made in the past.


The DB

 

[fourstringburn]

I wasn't going to say anything about this initially as it is the comment of someone who doesn't know me nearly as well as he seems to think, and honestly, it wasn't worth a response. It was pointless, and didn't add to the conversation in any way, shape, or form. In my opinion it says more about the person who said it than anything, especially when he was clearly wrong

You can't resist because you are narcissistic and you can't accept the fact when you are wrong. You are not the end of all of everything antennas. At least Marconi said it but he's trying another attempt to prove himself otherwise like you would do to show instead of being wrong, he was mistaken.


You can't respond with a short and to the point reply, instead you run a 1000 word essay in an attempt to baffle people with B.S. This says everything about you!

You are completely off topic and haven't added anything to this thread. To prove my point you will reply back in your usually narcissistic and condescending way.

And if you don't respond, I know it will burn you up inside to where you won't sleep tonight!

Have a good day.

 

[Captain Kilowatt]

 

[The DB]

You can't resist because you are narcissistic and you can't accept the fact when you are wrong.

I did resist, it was only happenstance that I happened to talk about something that referred to irl tests I have done, you know, those things that go beyond the theory you said I was limited to?

You are not the end of all of everything antennas.

I am well aware of this, if I knew every detail about antennas I would not still be studying and learning about them, their would be no point. Actually, if I though I knew everything there was to know about antennas I would be bored after a month or two and would have moved on to another more interesting topic.

At least Marconi said it but he's trying another attempt to prove himself otherwise like you would do to show instead of being wrong, he was mistaken.

I have been wrong/mistaken several times, and when I am I have no problem owning up to it, all you have to do is demonstrate it, something that you have failed to do. Please don't say you did with the cap hat thing. I have done a irl test with the instructions right out of the 22'nd edition of the ARRL Antenna Book, written by broadcast engineers you said were flat wrong, the antenna mounted on my SUV. I was able to build a shortened antenna, with a cap hat and no loading coil (something you insisted would not and could not work), and it tuned and functioned beautifully. I invite you to try it yourself, not that I expect that you ever would.

You can't respond with a short and to the point reply, instead you run a 1000 word essay in an attempt to baffle people with B.S.

So my attempts to explain the why of things is just "baffling with BS"? That is funny as on multiple occasions several people that have thanked me for my descriptions of various aspects of antenna theory. Some people appreciate and learn from it, and those are the ones I write those long posts for. It takes me a long time to write many of those, and I often get a reference along the way to make sure what I put in is accurate, I wouldn't take that kind of time to spread BS. I don't need to baffle with BS, and doing so is not beneficial to my goals, or my time. Some things can't be explained properly with a short and to the point reply, and the why is never as short as the what, however it does include understanding.

This says everything about you!

Yep, helpful and able to explain not just the what but also the why to people, I'll agree with that without further comment.

You are completely off topic and haven't added anything to this thread. To prove my point you will reply back in your usually narcissistic and condescending way.

Your comment that mentioned me was exactly the same as you are accusing me of being now. I have no problem saying that I am off the original topic, however, several people went to a different sub-topic, which what I said is relevant to, so to say what I said is completely irrelevant to what was actually said in the thread is misleading at best. That happens quite often on this forum by the way, often in threads I never once posted in, or am I the only person not allowed to participate in a discussion that others have taken off topic?

And if you don't respond, I know it will burn you up inside to where you won't sleep tonight!

You think you keep me up at night? Now that is funny.

If you were really as big a concern to me as you think you are, I would have responded to your earlier remark much sooner than I did, and that response was just because of a convenience that a topic that had to do with some real world tests I did happened to come up, and that was relevant to when you mistakenly claimed that I was all theory, so you see, it was you that started this.

So I don't respond and let you spout crap about me that has no basis in reality, or I do respond to defend myself and I am a narcissist? I have news for you, defending myself from false accusations is not narcissistic.

Its like you are trying to manipulate the situation to work against me either way, you know damned if you do and damned if you don't type of deal? At least I don't drop your name in a negative way in some random thread like you did, I am able to continue to ignore you, and when you are right (as happened in another thread) I back you up. To me its not the person but the validity of what they say.

The reality is that once I am finished with this post, unless you make another based on BS like you now did twice in this thread, you the person are irrelevant to me (as are all of the people on this forum). State facts and I will back you up, spout BS and I will call you on it, it is as simple as that, and often I don't even bother with that much.

Seriously, do us both a favor and don't mention my forum name again, anywhere, ever. We will both be better off for it.

I should also point out that, at least as far as this thread is concerned, you are the only person talking bad about someone else. I didn't say anything negative about you.

Have a good day.

Have a good evening.


The DB

 

[fourstringburn]

Thanks for proving my point Mr. DB and in less than 2000 words, you are the greatest living legend in existence!

 

[Marconi]

I'll be honest with you, it seems that most modelling is more accurate with fixed installations that they are with mobile installations. Whether it is the lack of functionality in the software or simply a result of too many unknown variables I am not sure but trying to use wires to represent a solid groundplane seems like a downfall of the software.

DB, Captain Kilowatt made the comments above, and your response below.

Yet we have an analogue of this in real life, at least to some extent. If you take a 1/4 wavelength ground mounted vertical and put it over 120 ground plane radials you get very close to the same feed point impedance that you get as if the earth was a perfect ground, and modeling shows this as well.

I could be wrong, but I think QRN is suggesting problems with mobile modeling objects lacking an adequate ground plane in a mobile model. He also suggest there are too many variables to be sure and he questions if straight wires present enough mass (rather than empty space) to represent a solid ground plane like an automobile body does. And I further assume he is alluding to the issue why our mobile model objects are showing a high feed point impedance rather than being closer to 35-36 ohms at the feed point in this case, because he talked about improving the bonding in autos in real life (iRL) will possibly make the match closer to 35 ohms without added losses.

You suggest there is an analogue iRL. To me, this sounds like an important consideration on the subject. You also tell us modelling shows us the same as well. Can you post some details with your modelling that supports this idea so I can be sure I understand your point here?

I also think you are basically telling us that more wires are better in the case of mobile modelling, right?

You know the old saying, "...pictures are worth a thousand words."

 

[tba02]

I see a lot of resistance, now how about reactance?

 

[tba02]

To the OP - as mentioned, for that short a run there will be no significant difference between the two types of coax.

You may see a difference in SWR at the radio end if you were using a 100 foot run and comparing RG-8x and RG-213/LMR400. The additional loss in the RG-8x would show less reflect due to cable losses and therefore lowering the SWR reading at the radio. This doesn't change what the true SWR reading would be at the antenna.

 

[Captain Kilowatt]

I see a lot of resistance, now how about reactance?

There is no reactance at resonance. That is the definition of resonance.......the frequency at which Xc completely cancels Xl or vice versa.