Swr vs ohms

[phantom309]

Now that I have an antenna analyzer I get a much bigger picture of what's going on then I ever did with just a swr meter.


So I pose this question.


Which actually makes the bigger difference?


Let say pickup truck roof mounted Wilson 5000, mini 8, cobra 29 is what we are working with, which is a fairly common setup.

Now you tune the antenna to 1.1:1 which has widely been accepted as the perfect tune, but the analyzer shows it at 40 ohms, not 50. So, now you start to play with the antenna to get 50 ohms, but now your swr is 1.3:1.

Which is really the better tuning, and why?

 

[vkrules]

OK..That's because that's not the whole story. The 40 ohms is impedance not resistance .We need the other figure your not using.The X or J depending on the brand and type of meter shows us how reactive the antenna is .
Or how far off resonance it is. Higher the figure the further out of tune it is. 0 is perfect. So you may have for instance 40 ohms +4 J. and the 50 ohm might be + 18 j so the 40 is better.

Scan up and down a little and see where the reading is 0 or close to it. thats the resonate frequency.

Secondly your antenna wont be exactly 50 ohm it could be anywhere between 35 and 60 ohms at resonance. Don't confuse resonance with SWR the swr only shows that the radio output and feed line is matched to the antenna feed point.

Some thing to think about ,,,without some sort of matching circuit a halfwave vertical has a impedance well over 1000 ohms at resonance .
a dipole 70 ohms
folded dipole 300 ohms
5/8th 200 ohms
And a yagi can be 15 ohms

Don't get bent out of shape chasing the almost mythical perfect SWR. A resonate ant with a 1.5 swr is better to have than one with a perfect swr thats not resonate.

 

[Beetle]

That "perfect tune" should be 1:1, not "1.1:1".

And don't forget: as you change channels or frequency, that 1:1 WILL change unless you have a lot of loss in your feedline.

A typical vertical on a typical mount on a typical vehicle, if properly installed, should have a feedpoint impedance of around 30-35 ohms, not 50 like your feedline impedance probably is.

 

[The DB]

That so called "perfect" SWR really doesn't tell you much. The only reason people ever used it to tune antennas is the price that said meters were available for. The SWR reading you get has little to nothing to do with performance, and it is possible to have a low SWR for the wrong reasons. A dummy load is an example of this, near if not "perfect" SWR, but almost nothing gets radiated.

What you want to look for, as mentioned above, is X. You want it to be as low as possible. There is a problem with this, however. X, along with R, will typically change as you move up the coax, so unless you are plugged directly into the antenna, which is impossible in your setup assuming you are using a Wilson 5000 magnet mount (if I am mistaken about this, please say so). What you need is to tune is X (and possibly R) at the antenna, not the X that you get at the end of some length of coax. The next best option is to use an electrical half wavelength of coax, and that will be good for exactly one frequency. Other nearby frequencies will at least be close, but the further you get away from the one frequency the more off the device will be. Unfortunately, I'm not sure as to the electrical length of the feed line that comes with that antenna, and that is assuming they all have a consistent electrical length...

My recommendation would be to, if you haven't done it already, bolt the antenna to the vehicle, and use a mount that has a PL-259 built in. When tuning the antenna analyzer use a male to male coupler if possible, and if not use as short a length of coax as possible, definitely try and keep it under a foot, if not 6 inches. Distance from the feed point matters, and the further away you are form the feed point you are the more off the readings will be. With a magnet mount you have no control over the length of coax unless you have taken the magnet mount apart and installed a new coax. You might get lucky, but I wouldn't count on it.

And if you cannot get direct access to plug the antenna analyzer directly into the antenna's feed point, and you don't have a coax of known (and desirable) electrical length, that brings us back to SWR. Unlike X and R, SWR doesn't change over the length of coax, and if it does it is a sign that you have a problem. Unfortunately that makes it the only stable means of tuning, which for many antennas makes these devices little more than expensive SWR meters.

I know, that isn't what people want to hear/read, especially after dropping a lot of money on such a device, but it is part of the learning curve to learn how to use them properly. If you just use them and hope for the best, you might well get lucky, but you might also make your antenna perform worse than just using an SWR meter in the first place.

Antenna analyzers aren't as simple of a device as SWR meters, and they can give you a lot of information. If you really want to take advantage of the tuning abilities these devices give you, you need to understand the various things they are telling you, as well as their limitations.

There are many people here that are happy to help, so don't be afraid to ask questions...


The DB

 

[vkrules]

Agree with you Db but it's all a bit much for a newbie. We most likely confused 309 more Tried to keep it simple .
One thing I will add though is the closer X is to 0 the less it will vary with coax length.The more reactive it is to start with the higher the variation.

 

[Captain Kilowatt]

Agree with you Db but it's all a bit much for a newbie. We most likely confused 309 more Tried to keep it simple .
One thing I will add though is the closer X is to 0 the less it will vary with coax length.The more reactive it is to start with the higher the variation.

So this wouldn't be a good time to talk about Smith Charts then I guess?

 

[vkrules]

I dont use smith charts, but only because I have to print them out extra large so I read them these days.
Just did a search looking for a youtube vid for 309 found several with Hams doing SWR sweeps and taking about Resonance at lowest swr points on Dipoles with no mention of "X". Unbelievable !!

 

[ExitThirteen]

I prefer Jones Charts over Smith Charts.


~Cheers~

 

[Tallman]

So this wouldn't be a good time to talk about Smith Charts then I guess?

NOT SMITH CHARTS! AAIIIIIEEE!
SWR is NOT an indicator of resonance.

http://www.hamradioschool.com/good-swr-and-antenna-resonance/

 

[Captain Kilowatt]

NOT SMITH CHARTS! AAIIIIIEEE!
SWR is NOT an indicator of resonance.

http://www.hamradioschool.com/good-swr-and-antenna-resonance/

I was thinking about the fact that there is almost an infinite number of resistance and reactance combinations that will result in the same SWR value and that a Smith Chart will show those values. That leads to the fact that while the displayed SWR may not change along the length of coax, the actual impedance will change. Some people have a hard time understanding that while the impedance changes the SWR does not. That brings us back full circle (no pun intended) to the Smith Chart again.

 

[Tallman]

Once I figured out how to plot the Smith charts accurately I had tons of friends in the Engineering Department all of a sudden. One of our customers wanted Smith charts for the entire design stage by stage.

 

[phantom309]

Just a quick reply.....


Been on the radio for 20+ years but over the last 2 I have taken the time and money to invest in equipment to actually learn what I am doing.

I drive a semi over the road currently, and actually do not have a cb in my personal vehicle at this time.

The example of the Wilson and cobra 29 I figured would be considered an average install in today's world.

I also ment this install to be soild, not mag mount.

I just drive 628 miles today, I am going to grab a shower and dinner. I will be more involved in the discussion tomorrow.

 

[gamegetter]

I prefer Jones Charts over Smith Charts.


~Cheers~

i was thinking mrs. jones

http://www.worldwidedx.com/threads/oldie-but-goodie.206263/

 

[M0GVZ]

Here's a bit more to think about. The feedpoint impedance at resonance (X=0) of a quarter wave over a perfect ground is 36.8 Ohms. We can use this knowledge to get an idea of how well our vehicle is performing as a ground plane by getting a whip with no loading coils etc at or close to 1/4 wave long for 11m and sticking it in the mount and seeing what the impedance is at resonance. The further away from 36.8 Ohms it is, the worse the RF ground is.

What we can further do with this whip and the analyser is use it to see if anything we try to do to improve the grounding is working. What would be an indication of this is both the feedpoint impedance getting close to 36.8 Ohms and the frequency where X=0 lowering. When the frequency stops lowering you're pretty much at the point where you've done as much improvement to your RF grounding on your vehicle as is reasonably possible.

 

[phantom309]

Ok, real life. This is the readings of my setup in my semi.

Sirio performer 5000 antenna, on a factory antenna stud, and factory coax, I am talking on a galaxy 99v2.

I am not allowed to change the coax cable or stud per company regulations.

The analyzer is extremely touchy to dial in a frequency but I got it as close to ch 1 ch 19 and ch 40 as it would allow me to go without going insane.

Looking at the pictures what should I do if anything?