Does different coax affect swr?

[Tallman]

The only caution I would give you is to be sure that the coax is good enough if you are running a linear. I saw one guy keying an 800 watts rms amplifier through some RG-58 coaxial cable. It was really warm to the touch and I advised him to change it to RG-213.
He didn't listen and the cable shorted out and he let the "Magic Smoke" of the amplifier.
The first thing he said, "DON'T YOU DARE SAY I TOLD YOU SO!"

 

[Robb]

Not all coax is made well. I consider Radio Shack coax is better used as jump rope - rather than used as coax. Cheap coax isn't a bargain. Poor shielding/braid and the outer jacket isn't well UV stabilized - will crack when exposed to weather after a short period. Keep in mind that coax losses continue to rise above 24mhz as you go up in freq with even the best coax; so quality is important.

I still can remember the day some thirty years ago when I changed from Radio Shack/Tandy RG-8 to some real Belden RG-8. My receive meter seemed to go up 2-3 S-units. Eye opening.

 

[hotrod]

use coax thats 50 ohms and make sure it will handle the power you plan on putting thru it

 

[JoeDirt]

Short answer is NO, coax doesn't change the SWR only the antenna can.

If your antenna is a 50 Ohm load no special coax lengths needed.

If your antenna is anything other than 50 ohms (ie swr greater than 1.5:1) Or you have some common mode issues, then coax length make a huge difference. You can mask a poorly tuned antenna with carefully cut coax or visa versa make a poorly tuned antenna worse.

This subject has been beaten to death, here is the best explanation that I can find.



In mobile applications where a perfectly tuned antenna isn't always ideal.(102" whip should be 1.5:1 SWR or 75 ohms) I will use a custom length coax so the mobile amp sees 50 ohms.

 

[Captain Kilowatt]

In mobile applications where a perfectly tuned antenna isn't always ideal.(102" whip should be 1.5:1 SWR or 75 ohms) I will use a custom length coax so the mobile amp sees 50 ohms.

That 102 whip should be about 35 ohms not 75 ohms. If you see 75 ohms then your ground losses are ridiculously high.

 

[JoeDirt]

That 102 whip should be about 35 ohms not 75 ohms. If you see 75 ohms then your ground losses are ridiculously high.

I was always told 72 ohms never measured..... Or am I confusing that with a standard dipole?

 

[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.

Model 1. with a 106.8" inch whip over a horizontal ground plane with 4 x 108" radials at 12" inches above Earth. I think this is how Theory predicts the 1/4 wave over an infinite ground.

A mobile object used as a ground plane is very different and is probably why we hear so many stories of difficult installs with mobiles. My mobile object is not perfect, but this one with a conservative approach in wire density for the object shows to be closer to being predictable as a mobile than my more complicated mobile object with 3 times the wire density in the model.

This little truck design was taken from a Blazer or Bronco model several years ago that was sent to me by Henry HPSD.

Model 2. Here is my mobile object of that small pickup truck with a 102" whip on top and it more or less agrees with your claim of 75 ohms at 1.75:1 SWR at the feed point.

You will find the matching details in the Source Data report attached to each model.

Good work and I appreciate your comments.

 

[Marconi]

Joe I found some images of a 259B reading a mobile and it does show some very good readings in the CB band, but I did not read the context of the discussion, but here is the link.

Best Moblie Antenna for OTR Truck

I will make this comment however. These analyzer readings were likely taken with a feed line involved and therefore might have some effect. Also note that almost all his reading show little to no reactance across the entire bandwidth he scanned...I do not understand how that happens on the 259b, but I have seen it before and thus far no one has been able to explain why he sees resonance just about everywhere in the CB band.

My model record the match at the feed point, just to let you know.

 

[JoeDirt]

Joe I found some images of a 259B reading a mobile and it does show some very good readings in the CB band, but I did not read the context of the discussion, but here is the link.

Best Moblie Antenna for OTR Truck

I will make this comment however. These analyzer readings were likely taken with a feed line involved and therefore might have some effect. Also note that almost all his reading show little to no reactance across the entire bandwidth he scanned...I do not understand how that happens on the 259b, but I have seen it before and thus far no one has been able to explain why he sees resonance just about everywhere in the CB band.

My model record the match at the feed point, just to let you know.

Thanks Marconi however the link didn't work. I saw that discussion a while back, I haven't taken the time to read it maybe its time I should!

 

[JoeDirt]

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.

Model 1. with a 106.8" inch whip over a horizontal ground plane with 4 x 108" radials at 12" inches above Earth. I think this is how Theory predicts the 1/4 wave over an infinite ground.

A mobile object used as a ground plane is very different and is probably why we hear so many stories of difficult installs with mobiles. My mobile object is not perfect, but this one with a conservative approach in wire density for the object shows to be closer to being predictable as a mobile than my more complicated mobile object with 3 times the wire density in the model.

This little truck design was taken from a Blazer or Bronco model several years ago that was sent to me by Henry HPSD.

Model 2. Here is my mobile object of that small pickup truck with a 102" whip on top and it more or less agrees with your claim of 75 ohms at 1.75:1 SWR at the feed point.

You will find the matching details in the Source Data report attached to each model.

Good work and I appreciate your comments.

Thats really interesting! How was the ground modeled between the two antennas?

 

[Captain Kilowatt]

Joe you are right according to my Eznec models.

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. Either that or understand what it is the numbers are telling you.That model you did of a 1/4 wave on a PU truck may show about 75 ohms but that is because of losses that can easily be minimized thru bonding. Been there done that and ended up with 39 ohms when I got tired of scraping paint and adding ground straps across hinges etc. The fact that your other model over perfect ground showed 35 ohms proves my point about high losses in the model showing installation on the PU truck. the radiation resistance did not chance thus the ground losses HAD to have increased.

 

[fourstringburn]

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 once measured a 102" whip on a mobile with an analyzer and a 18 in. piece of coax and came up with a Z around 37 ohms. This makes sense because a 1/4 wave 102" whip is only one half of a half wave dipole at 27 MHz. A half wave dipole at 1 wavelength above ground should be around 75 ohms at 27 Mhz.

Tuning with coax lengths as Joe dirt basically suggests so an amp will see 50 ohms isn't the correct way to do it either. That's the Mark Sherman way to do it. The correct way if you really want to be precise is to add some inductance right at the antenna feed point. A simple home made shunt coil will do the trick.

 

[Marconi]

Thanks Marconi however the link didn't work. I saw that discussion a while back, I haven't taken the time to read it maybe its time I should!

Neither does my model apparently. I can't explain the link not working though...it works fine here. There were just 6 or 8 images of an MFG 259b readings. I see a mobile in the thread so the analyzer was likely attached to a mobile...maybe a big truck. Like I said I did not read the whole thing for context.

I just did a simple search, but I'll look for some other images. Those did tend to show closer to what Captain Kilowatts posted however.

BTW just a point, my 1/4 wave theory model is over a very good ground plane, but the model is set over Real Earth as the ground type...not Perfect just to be clear.

In light of Captain Kilowatts follow-up and sage comments, which makes total sense, I'll have to go back and check my mobile object and see why it responds like it does for me. I realize Eznec will not exactly duplicate the metal density of a real automobile frame and body, but based on what I've read from others and seen in demonstrations of modeling a mobile...my objects look to be close enough. I recently asked DB a similar question in the last post in his thread on his new Ford Explorer mobile model object.

Sorry folks, I can't explain nor support what the high impedance match I see on my mobile models.

DB and I both have discussed these objects, and I have some articles on modeling mobiles...and nary-a'-one seem to suggest the mobile will not work right. I don't have an answer, but I see the problem and I have been tussling with the Idea whether the mobile objects I use have too few or too many wires. Plus where the wires should be best located in a mobile object seems difficult to determine at this point.

Captain may have described my model well...it desperately needs a good bonding job. but I'm too old and CK is tired of the hard bonding work.

 

[Marconi]

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.

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.

 

[Captain Kilowatt]

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. This is especially the case with ground mounted verticals using a ground radial field.Without understanding what the impedance SHOULD be it is easy to fool yourself into thinking you have an efficient installation because you see 50 ohms +/- J0 on the analyzer. You can have two installations that both have the same 1.5:1 SWR and yet one will be 6 dB stronger than the other. One case is when you have an impedance of 75 ohms due to high ground losses and an efficiency of only about 50% and the other is when the impedance is near 36 ohms and the efficiency is near 99%. Better yet if the impedance is a perfect 50 ohms the efficiency is really only about 70% despite the apparent "perfect" SWR.