what is your vswr at the antenna?

[Shockwave]

Well, I read it and still didn't believe it. Shame on me for not considering the sources and altering my views. I'm one of those people who sometimes need to see it to believe it and that is exactly what I just did. It was very easy to get a 2:1 VSWR while insuring no CMC just by connecting a 100 ohm carbon resistor directly to the connector on the MFJ analyzer.

Sure enough, a 2:1 VSWR was displayed across its range. Connecting 4 feet, 6 feet, and 20 feet of 50 ohm cable showed no change in VSWR other than a slight reduction due to cable loss. Changing the cable to 75 ohms immediately showed transformer action taking place in the line. With the 75 ohm wire, changing frequency or cable length made large changes in VSWR and at some points made the 100 ohm load a perfect 1:1.

Thanks for the guidance on figuring this out. I think the we may get misled here because of how often high VSWR is accompanied with significant common mode currents on the coax.

 

[bob85]

thanks for taking the time to do a test

i have not tested 75ohm coax with my analyzer but id say the 75ohm coax caused the odd readings because the analyzer is calibrated for 50ohm vswr,

i wish i knew what i know now years ago when i was in love with 1/2wave multiples and knew nothing about common mode currents.

 

[Marconi]

thanks for taking the time to do a test

i have not tested 75ohm coax with my analyzer but id say the 75ohm coax caused the odd readings because the analyzer is calibrated for 50ohm vswr,

i wish i knew what i know now years ago when i was in love with 1/2wave multiples and knew nothing about common mode currents.

Bob, I use to wonder my head silly trying to think about this stuff too. I also remember you alerting me to the idea of the transforming effects of different lengths in a feed line length...when I was trying to add a <>9' foot long piece of coax, as a choke to my S4, and I saw strange changes in results.

At the time I also did not realize much if anything specific about common mode currents either, so I was left wondering still...even though you provided me with new information to consider.

I have made up my mind not to ever again talk about stuff I'm not fully informed about...so these are my last words on this subject as I stated earlier. However I will try and read whatever is to follow on the subject as long as I can. At this point I'm not even sure what my idea is or was, and for sure I'm not sure how any comments so far relate to my long standing idea, but at this point I really don't care what the truth is on this particular matter.

I thank you, for all the help you have provided me over time...even if at times I felt I could not agree with everything you told me. At times I probably misunderstood what you said, and other times I didn't do my follow up testing correctly as you suggested, but I always appreciated your ideas and words. I can't say that about to many folks in my life long experiences in radio.

 

[bob85]

thanks eddie,
its like your sig says
"We're just lucky that Mother Nature doesn't require our antenna systems to be perfect in order to work our radio"

 

[Shockwave]

i have not tested 75ohm coax with my analyzer but id say the 75ohm coax caused the odd readings because the analyzer is calibrated for 50ohm vswr,

From the tests I did today I have to say beyond a doubt that the 75 ohm cable produced the classic impedance transformation we are familiar with. The 100 ohm resistor on the end of an electrical 1/2 wave showed just under a 2:1 VSWR. When the MFJ frequency was cut in half (effectively making the cable an electrical 1/4 wave), the 100 ohm resistor was transformed into a 50 ohm load at 1:1.

 

[Marconi]

thanks eddie,
its like your sig says
"We're just lucky that Mother Nature doesn't require our antenna systems to be perfect in order to work our radio"

BTW Bob, during that time testing my S4, when you informed me about the likely hood of transformation, I think I was pretty sure the antenna was not well balanced at the feed point, and thus it had CMC flowing pretty good. Like you've said already...

"...no argument that measured resonant frequency can change with line length,
its the idea that line length effects vswr i don't agree with unless you have common mode current on the braid, then relatively small changes in line length can effect vswr significantly,"

I think the reason for my adding the choke was probably due to some TVI type problems I was experiencing at the time.

 

[wavrider]

Ok guys.

First let me say there are many respected members posting in this thread.

And I have learned much from many on this forum.

With the ass kissing out of the way let's get to the point.

The transceiver, which is what we as operators, antenna system designers, installers, and users will all need to transmit the signal.

This transmitter, if modern model solid state likes a 50 ohm load or impedance.

AT THE TRANSMITTER.

As long as there is an acceptable load at the transmitter then a signal can be transmitted.

This has nothing to do with how efficient an antenna is. Just that the SYSTEM presents a 50 ohm load to the transmitter.

Example, build a dipole. Use the analyzer and make it as perfect a 50 ohm load X=0 by making it an inverted V configuration.

1:1 current balun at the feed point to help eliminate cmc and have equal current distribution.

Good antenna??

Build same dipole, use ladder line to feed it and an antenna coupler at the shack and use the analyzer to tune the antenna coupler "in the shack" to a 50 ohm X=0 impedance and you have the same antenna.

It does not matter what vswr you have at the antenna.

The WHOLE antenna system needs to present a 50 ohm impedance to the transmitter.

What matters is how efficiently you you can transmit your signal.

In order to transmit a signal in todays modern rigs you need to make a 50 ohm load impedance at the transmitter.

NOW change this around.

IF you build/tune the perfect antenna. 50 ohm load impedance X=0 perfect on the analyzer.

As soon as you connect a transmission line to it then it degrades the performance of the antenna SYSTEM.

Will you see a difference in vswr when you add coax?

Will the resonant frequency of the antenna change?

Will the resonant frequency of the antenna change when it is elevated above earth?

I see so many discussions about antennas. I enjoy all of them.

But in the real world of actually operating and not discussing antennas we need to look at the whole antenna system. Not just the antenna..

Example.

Friend of mine has a set of stacked 13 element 2 meter yagis, and he is using RG8X for feed line. 75'.

Great antennas, great gain, He is using 100 watts of TX power.

Looking at his ERP minus the cable loss he offsets the gain by using inferior feedline at the frequency of the system.

Once again the WHOLE system needs to be tuned not just the antenna.

Just my 2 cents guys, not singling out anyone, just throwing my thoughts out on the subject.

Comments, suggestions, bashing all is welcome.

 

[The DB]

I am curious as to the purpose of your post waverider. Bob85 posted a hypothetical question for reason's I don't know and there was a discussion about it, end of story.

What is wrong with a discussion about antennas and how they work? Sure, some people are interested in the easy route you suggest, but some of us want to know more.

Also, just because the antenna presents the settings of X=0 and R=50 does not necessarily mean the antenna (or antenna system if you would like) will work well, case in point, the dummy load. Oh wait you said something about this:

This has nothing to do with how efficient an antenna is. Just that the SYSTEM presents a 50 ohm load to the transmitter.

It takes more than a happy transmitter to get a signal out, and get it out well, or we would all be using dummy loads for antennas. Luckily antennas are forgiving, but that has a drawback as well, sometimes we don't know how much better our antenna can be than it is. That is where discussions like this come in.


The DB

 

[Marconi]

I am curious as to the purpose of your post waverider. Bob85 posted a hypothetical question for reason's I don't know and there was a discussion about it, end of story.

What is wrong with a discussion about antennas and how they work? Sure, some people are interested in the easy route you suggest, but some of us want to know more.

Also, just because the antenna presents the settings of X=0 and R=50 does not necessarily mean the antenna (or antenna system if you would like) will work well, case in point, the dummy load. Oh wait you said something about this:

It takes more than a happy transmitter to get a signal out, and get it out well, or we would all be using dummy loads for antennas. Luckily antennas are forgiving, but that has a drawback as well, sometimes we don't know how much better our antenna can be than it is. That is where discussions like this come in.

The DB

Hey DB what's going on? Do you really think any of us CBr's are really able to test antenna efficiency very well if at all? If you can, spell it all out...then let us hear all about efficiency testing. That might be interesting.

Is it still raining there? I'm a little scared to ask, but have you ever had a chance to follow up on your testing on the A99 ideas any more. I'm still patiently waiting on your promise.

If you have forgotten what it was all about, let me know and I'll fill you in again on the ideas.<gotproof>

 

[bob85]

the 1/4wave of 75ohm coax should transform 100ohm x=0 to 50ohm x=0,
sqroot of load impedance x 50ohm,
i can't find my scientific calculator, no roots on my phone

the 1/2wave coax would mirror the load impedance looking into the coax so 100ohm x=0 at the analyser,

im not sure what the analyzer would report for vswr with the 1/2wave of mismatched 75ohm coax, i think 2:1 with a 50ohm analyzer and no reactance in the load,

i don't doubt your measurements

From the tests I did today I have to say beyond a doubt that the 75 ohm cable produced the classic impedance transformation we are familiar with. The 100 ohm resistor on the end of an electrical 1/2 wave showed just under a 2:1 VSWR. When the MFJ frequency was cut in half (effectively making the cable an electrical 1/4 wave), the 100 ohm resistor was transformed into a 50 ohm load at 1:1.

 

[bob85]

i will tell you why i made the post DB, i was trying to get some discussion about the VERY common misconception of vswr and line length,

i hold my hand up as been guilty in the past and i may offend in the future, but as i have said in the past i am ALWAYS willing to learn,
when im wrong i want to be steered in the right direction, walt maxwell set me straight on several topics, i still have much to learn,

if just one interested person learned something new its not posted in vain.

 

[wavrider]

I am curious as to the purpose of your post waverider. Bob85 posted a hypothetical question for reason's I don't know and there was a discussion about it, end of story.
What is wrong with a discussion about antennas and how they work? Sure, some people are interested in the easy route you suggest, but some of us want to know more.
Also, just because the antenna presents the settings of X=0 and R=50 does not necessarily mean the antenna (or antenna system if you would like) will work well, case in point, the dummy load. Oh wait you said something about this:
It takes more than a happy transmitter to get a signal out, and get it out well, or we would all be using dummy loads for antennas. Luckily antennas are forgiving, but that has a drawback as well, sometimes we don't know how much better our antenna can be than it is. That is where discussions like this come in.
The DB

D.B.

The post was to bring the discussion to the antenna system.

This forum has a wealth of information, and masses of members will read and follow what is posted on this forum.

With that said I posted to bring the discussion to represent the whole antenna system. Not just the antenna.

I see so many posts , "What is the best antenna.?"

The system is much more than just an antenna.

To get the most efficient system one needs to take into consideration the whole system not just the antenna.

I have yet to see a dummy load of X=0 or 50 ohms. they come close but not perfect.

I do not understand the easy way out? Easy way out of what?

As for efficiency of the antenna. It can be the most efficient antenna in the world. 99.9% efficient. But if you can not transmit into it due to impedance mismatch at the transceiver it is useless.

 

[The DB]

Hey DB what's going on? Do you really think any of us CBr's are really able to test antenna efficiency very well if at all? If you can, spell it all out...then let us hear all about efficiency testing. That might be interesting.

Take a look at M. Walter Maxwell's "Another Look at Reflections", Part 5, page 38. You will find a section called "Vertical Radiator over Earth".

Is it still raining there? I'm a little scared to ask, but have you ever had a chance to follow up on your testing on the A99 ideas any more. I'm still patiently waiting on your promise.

If you have forgotten what it was all about, let me know and I'll fill you in again on the ideas.<gotproof>

Unlike last year it has rained a lot here this year, but I think that is the story throughout most of the country, although I think the area you live has been an exception...

My testing of the A99??? Hmm? You sure you have the right person here? I haven't owned an A99 since long before joining this forum. I do recall you and someone else having such a discussion though.


The DB

 

[The DB]

D.B.

The post was to bring the discussion to the antenna system.

This forum has a wealth of information, and masses of members will read and follow what is posted on this forum.

With that said I posted to bring the discussion to represent the whole antenna system. Not just the antenna.

I see so many posts , "What is the best antenna.?"

The system is much more than just an antenna.

To get the most efficient system one needs to take into consideration the whole system not just the antenna.

I have yet to see a dummy load of X=0 or 50 ohms. they come close but not perfect.

I do not understand the easy way out? Easy way out of what?

As for efficiency of the antenna. It can be the most efficient antenna in the world. 99.9% efficient. But if you can not transmit into it due to impedance mismatch at the transceiver it is useless.

I agree, you have to look at antennas as the whole system, I don't think anyone here is denying that, especially bob85 who created the thread. But in order to look at the whole system you have to also look at all of its component parts and what they are doing, AND put them in perspective. An antenna is no better than the worst part of the antenna system. In the case of this thread the feedline is definitely not helping the rest of the antenna any, even though we don't really know anything about the given antenna aside from its feedpoint SWR, and we have to figure that out.

Knowing the whole antenna system is important, but what you suggest appears, at least to me, like focusing only on one part of said system, and that is making only the radio happy with a 1:1 SWR at the radio. This is important, but other things affect the radiated signal far more than a reasonably low (2:1) SWR at the radio, for example, antenna efficiency. A 1% change in antenna efficiency will have more of an effect on radiated RF than a 2:1 SWR reading taken at the radio (in comparison to a "perfect" 1:1 SWR reading), and that is assuming the reflections presented by the 2:1 SWR are counted entirely as loss, which we know isn't necessarily true.

Another thing to look at, and ironically part of the subject of this thread is feedline losses, and their effect on the rest of the antenna. If the feedline in question is loosing 70% of the signal the radio transmits into it the radio will be happy over a very large bandwidth, but the antenna itself will only see 30% of the signal.

As I said above, I agree with you on having to look at the entire antenna system (and I think most people who posted in this thread do as well), however, there is far more going on in an antenna system than what the radio thinks it sees. The dummy load example I gave before shows that to be true even if it is only very close to a perfect SWR match.

The DB

 

[wavrider]

Agreed

I agree, you have to look at antennas as the whole system, I don't think anyone here is denying that, especially bob85 who created the thread. But in order to look at the whole system you have to also look at all of its component parts and what they are doing, AND put them in perspective. An antenna is no better than the worst part of the antenna system. In the case of this thread the feedline is definitely not helping the rest of the antenna any, even though we don't really know anything about the given antenna aside from its feedpoint SWR, and we have to figure that out.

Knowing the whole antenna system is important, but what you suggest appears, at least to me, like focusing only on one part of said system, and that is making only the radio happy with a 1:1 SWR at the radio. This is important, but other things affect the radiated signal far more than a reasonably low (2:1) SWR at the radio, for example, antenna efficiency. A 1% change in antenna efficiency will have more of an effect on radiated RF than a 2:1 SWR reading taken at the radio (in comparison to a "perfect" 1:1 SWR reading), and that is assuming the reflections presented by the 2:1 SWR are counted entirely as loss, which we know isn't necessarily true.

Another thing to look at, and ironically part of the subject of this thread is feedline losses, and their effect on the rest of the antenna. If the feedline in question is loosing 70% of the signal the radio transmits into it the radio will be happy over a very large bandwidth, but the antenna itself will only see 30% of the signal.

As I said above, I agree with you on having to look at the entire antenna system (and I think most people who posted in this thread do as well), however, there is far more going on in an antenna system than what the radio thinks it sees. The dummy load example I gave before shows that to be true even if it is only very close to a perfect SWR match.

The DB