what is your vswr at the antenna?

[Shockwave]

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.

Bob, this has helped me to understand what conditions cause impedance transformation however, I'm still trying to figure out the "why" behind it all. It appears that impedance transformation only takes place when the transmission line does not match the source impedance.

I say this because I can change the length of 50 ohm coax and the resistive load at the end of the cable and as long as the cable is 50 ohms, the MFJ was able to accurately read the value of the carbon resistor even when that resistor was 100 ohms.

As soon as the cable was changed to 75 ohms the only length that read the load resistor properly were 1/2 wave multiples. All other lengths caused impedance transformation and change in VSWR. The problem here is I already know I can change source impedance without it effecting the VSWR.

Which brings me to the question, why does impedance transformation take place when the cable is 75 ohms and does not match the source, but not when the load doesn't match the cable or when the source impedance is changed? I see what works now as far as causing an impedance transformation however, I feel no closer to understanding the "why" behind it.

I'd like to assume it's simply because I'm using a 50 ohm MFJ instrument on a 75 ohm cable but this implies we are just tricking the meter and that does not line up with accepted theory that 75 ohm cables can be used to change impedance.

 

[bob85]

are you sure the impedance is not showing to be changing with coax length using the 100ohm resistor and 50ohm coax?

afaik whenever the load does not match the characteristic impedance of the coax with any impedance coax the impedance looking into the coax should change with coax length,

when the load matches the characteristic impedance of the coax and x=0 you have no reflections so no standing waves in the coax,
no standing waves gives a constant impedance along the coax,

when the load does not match the coax, standing waves cause impedance to swing about as you move along the coax mirroring the load @ electrical 1/2wave points and inverting the load @ odd 1/4wave multiples,

here is what mfj say,

"If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving the analyzer to another point on the line will NOT change SWR reading. Impedance and resonant frequency might change from line transformation effects, but the SWR will not change"

 

[Marconi]

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".

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

DB, it is very possible I confused you with someone else. If so, then I apologize. I should have looked back to check who it was...and I didn't do that, sorry.

I posted a thread of my own to help me remember who it was that promised to follow up with some testing of an A99 that I had suggested, but I thought I remembered who it was.

I read your reference above, but my book on Reflections does not have page numbers. The items are list as Chapters and Sections within the chapters.

I do find the topic that you noted in my book at Sec 5.5 "Vertical Radiator Over Earth," and it is basically the same text as your reference.

IMO, the reference you provided is referring to Broadcast antennas that are very large and produce gains at or near unity. Thus the ground effects and the control and elimination of ground losses in such cases...becomes far more important than at 27 mhz with far fewer radials.

This is truly a matter of efficiency as you have noted, but if those differences at 27 mhz are very small, then what does efficiency matter in such cases? I see this as comparing apples to oranges.

 

[Marconi]

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".

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

DB, it is very possible I confused you with someone else. If so, then I apologize. I should have looked back to check who it was...and I didn't do that, sorry.

I posted a thread of my own to help me remember who it was that promised to follow up, but I thought I knew who it was. I didn't look back.

I read your reference above, but my book on Reflections does not have page numbers. The items are list as Chapters and Sections.

I do find the topic noted in my book as Sec 5.5 "Vertical Radiator Over Earth," and it is basically the same as Maxwell's, "Another Look......"

IMO, the reference you provided is referring to Broadcast antennas that are very large and produce gains at or near unity. Thus the ground effects, the control, and elimination of ground losses in such cases...becomes vitally important, and at 27 mhz with fewer radials...this factor would not seem to be nearly as important.

This is truly a matter of efficiency as you note, but if those differences are not detectable, then what does efficiency matter in this case? I see it as comparing apples to oranges.

 

[Robb]

Think it was northern35 IIRC, that was going to do some A99 testing.
I could be wrong - too . . .

Anyway, I think using a SWR meter at the antenna is a great way of doing it. But clearly, not every installation or antenna will always allow for that. So an alternative is needed.

Of course, using a tuned 1/2wl of coax for measuring an antenna cannot be practically done by everyone. Imagine a ham with a multiband antenna trying to use this method? Just would not work! But for a CBer that tunes a length of coax for - say 27.205mhz - and uses it to tune an antenna; that can work out. Especially if one has the extra coax to waste for a piece of testing and needs to do it occasionally or even often.

I have and use the MFJ-259B and really don't even use any of the standard SWR meters. But for those that do use a simple meter; then having all of the other available tools to do the job right. Could that include a tuned-length piece of coax?

 

[Marconi]

are you sure the impedance is not showing to be changing with coax length using the 100ohm resistor and 50ohm coax?

afaik whenever the load does not match the characteristic impedance of the coax with any impedance coax the impedance looking into the coax should change with coax length,

when the load matches the characteristic impedance of the coax and x=0 you have no reflections so no standing waves in the coax,
no standing waves gives a constant impedance along the coax,

when the load does not match the coax, standing waves cause impedance to swing about as you move along the coax mirroring the load @ electrical 1/2wave points and inverting the load @ odd 1/4wave multiples,

here is what mfj say,

"If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving the analyzer to another point on the line will NOT change SWR reading. Impedance and resonant frequency might change from line transformation effects, but the SWR will not change"

Bob, I think your remarks are right on point for this subject. However I also think this idea is often found to be difficult to comport with end fed radiators.

 

[The DB]

IMO, the reference you provided is referring to Broadcast antennas that are very large and produce gains at or near unity. Thus the ground effects and the control and elimination of ground losses in such cases...becomes far more important than at 27 mhz with far fewer radials.

I disagree with the idea that he was talking about broadcast antennas only, although he is definitely referring directly to ground mounted quarter wavelength antennas. The .pdf I referred to earlier and the book you looked it up in (I have it as well, I just used the .pdf because it is free to everyone) were both written for ham radio operators, and while there is a ham band near the broadcast AM band, what was talked about applies to every ground mounted quarter wavelength antenna, and to an extent every quarter wavelength antenna that has the potential for ground losses (or losses in general). This includes mobile quarter wavelength antennas, which are notorious for ground losses.

This is truly a matter of efficiency as you have noted, but if those differences at 27 mhz are very small, then what does efficiency matter in such cases? I see this as comparing apples to oranges.

The example I'm sure you have seen me use is for a full length quarter wavelength antenna, where X=0 and R=50 we have 28% in losses (happy to show you the math behind it). If you instead have X=0 and R=36 then you have 0% in losses but suffer from an SWR close to 1.4:1, which is in the range of 2.8% power reflected (but, importantly, not necessarily lost).

The principles that govern full length quarter wavelength antennas are the same weather you are talking about 1 MHz or 1 Ghz antennas. The only things that change as the frequency changes is the size of the overall antenna.

All that being said, I still don't have a good answer to tell you how to figure out what R should be for a given shortened antenna aside from as the antenna gets shorter R at resonance goes down (unless there is some other corrective matching involved), this is why I stick with the full length quarter wavelength antenna for my examples, their R is a well known value. As measured at the antenna lowering the R value while staying at resonance (either via bonding the vehicle or adding more radials to a ground mounted antenna or whatever) has the same effect on efficiency.

I suppose you could create an antenna system that shows the actual R for a quarter wavelength antenna (at least very close), then simply put in a shortened version of the antenna for the same frequency for comparison. This should give you an accurate R for said antenna.


The DB

 

[The DB]

Of course, using a tuned 1/2wl of coax for measuring an antenna cannot be practically done by everyone. Imagine a ham with a multiband antenna trying to use this method? Just would not work! But for a CBer that tunes a length of coax for - say 27.205mhz - and uses it to tune an antenna; that can work out. Especially if one has the extra coax to waste for a piece of testing and needs to do it occasionally or even often.

I have and use the MFJ-259B and really don't even use any of the standard SWR meters. But for those that do use a simple meter; then having all of the other available tools to do the job right. Could that include a tuned-length piece of coax?

There is no need to use a tuned half wavelength of feedline for SWR as SWR does not change on a given feedline except for attenuation (in the absence of common mode currents). Unless you are using something more complex than an SWR meter (such as your MFJ analyzer) a tuned half wavelength section is really irrelevant.


The DB

 

[The DB]

I say this because I can change the length of 50 ohm coax and the resistive load at the end of the cable and as long as the cable is 50 ohms, the MFJ was able to accurately read the value of the carbon resistor even when that resistor was 100 ohms.

are you sure the impedance is not showing to be changing with coax length using the 100ohm resistor and 50ohm coax?

afaik whenever the load does not match the characteristic impedance of the coax with any impedance coax the impedance looking into the coax should change with coax length,

when the load matches the characteristic impedance of the coax and x=0 you have no reflections so no standing waves in the coax,
no standing waves gives a constant impedance along the coax,

when the load does not match the coax, standing waves cause impedance to swing about as you move along the coax mirroring the load @ electrical 1/2wave points and inverting the load @ odd 1/4wave multiples,

here is what mfj say,

"If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving the analyzer to another point on the line will NOT change SWR reading. Impedance and resonant frequency might change from line transformation effects, but the SWR will not change"

To show what bob85 is referring to I ran a scan of a 75 ohm test load at the end of a 50 ohm coax.

Shockwave said he changed out the feedline to different lengths to account for different wavelengths of feedline. I, however, am using a different method to achieve the same goal, I am opting to change the frequency itself as opposed to changing the actual feedline. In the end the two methods have effectively the same end results, however, with my VNA this is the easier method to use.

The two graphs are are the same, except for different data points for the information at the right. Due to the transformer action of the feedline over a range of frequencies you can see how R varies based on the varying wavelength/coax length.

One last chart just to confirm a point made in this and other threads.

This is the same chart as above only with the SWR line added. As you can see the change in SWR is barely noticeable across the chart, even with rather large variances R and X comparatively.


The DB

 

[wavrider]

The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?

The real SWR page.

Might open some eyes. or perhaps more questions.

 

[The DB]

The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?

The real SWR page.

Might open some eyes. or perhaps more questions.

Sweet...

Wait...

This looks familiar, didn't someone post something similar earlier in this very thread?

The source I use is here:

The Real SWR Page - Explained by WC7I - Will high SWR burn up my radio?

The DB

 

[wavrider]

SWR and TRANSMITTERS - What is SWR all about? Making Sense of SWR Readings

Look at the chart and se the difference between 1:1 and 5:1 vswr on the receive end.

 

[wavrider]

That was posted a day or so ago, I have slept since then.

The point I was trying to make is coming to light now.

Do not need to worry to much about the VSWR at the antenna, it is the Antenna system which needs to be tuned.

Of course you tune the antenna as well while tuning the system.

Using 50 ohm coax as a transmission line the length is not critical.

On the other hand depending on what frequency you are working length of ladder line does influence performance of the antenna system.

In the case of my Mosley PRO67B 10 40 meter yagi, I would be pulling what hair I have left out if I tried to tune each band to a perfect VSWR reading.

Also 1% loss of efficiency will never be seen nor heard on the receiving end of a transmitted signal.

Sweet...

Wait...

This looks familiar, didn't someone post something similar earlier in this very thread?




The DB

 

[Marconi]

I disagree with the idea that he was talking about broadcast antennas only, although he is definitely referring directly to ground mounted quarter wavelength antennas. The .pdf I referred to earlier and the book you looked it up in (I have it as well, I just used the .pdf because it is free to everyone) were both written for ham radio operators, and while there is a ham band near the broadcast AM band, what was talked about applies to every ground mounted quarter wavelength antenna, and to an extent every quarter wavelength antenna that has the potential for ground losses (or losses in general). This includes mobile quarter wavelength antennas, which are notorious for ground losses.

The example I'm sure you have seen me use is for a full length quarter wavelength antenna, where X=0 and R=50 we have 28% in losses (happy to show you the math behind it). If you instead have X=0 and R=36 then you have 0% in losses but suffer from an SWR close to 1.4:1, which is in the range of 2.8% power reflected (but, importantly, not necessarily lost).

The principles that govern full length quarter wavelength antennas are the same weather you are talking about 1 MHz or 1 Ghz antennas. The only things that change as the frequency changes is the size of the overall antenna.

All that being said, I still don't have a good answer to tell you how to figure out what R should be for a given shortened antenna aside from as the antenna gets shorter R at resonance goes down (unless there is some other corrective matching involved), this is why I stick with the full length quarter wavelength antenna for my examples, their R is a well known value. As measured at the antenna lowering the R value while staying at resonance (either via bonding the vehicle or adding more radials to a ground mounted antenna or whatever) has the same effect on efficiency.

I suppose you could create an antenna system that shows the actual R for a quarter wavelength antenna (at least very close), then simply put in a shortened version of the antenna for the same frequency for comparison. This should give you an accurate R for said antenna.

The DB

I agree this article does not spell out specifically that it was for Broadcast, but the suggestion in examples sited is just that IMO. Maxwell also uses several references and one in particular #20, by G.H. Brown, Lewis, and Epstien, entitled: "Ground Systems as a Factor in Antenna Efficiency," as being the bases of the classic work which lead to FCC standards for AM Broadcasting ground-radial systems. I think this article was also posted here on WWDX in a thread on counterpoise that HomerBB started. I made the remark, because I seem to remember the counterpoise thread.

I also qualified my statement on "efficiency" by suggesting the theory did apply to all frequencies, but due to raising 27 mhz antennas for CB, the effects of adding more radials like they did in Broadcast work, was likely overkill and would not justify the expense when used at higher frequencies. This fact was also referred to in Maxwell's remarks, and I talked about it in my previous post in this thread.

I might try an Eznec model or two to try and show what I think might happen when adding a bunch of radials to a raised 1/4 wave antenna, say 40' feet, as compared to an antenna at BC frequencies, but my Exnec program is limited to 500 segments, so I see if it is possible to use close to 100 radials. IMO, there will probably be some differences to be noted, but I doubt those differences will be worth the added effort and expense compared to only a few radials at 27 mhz. This is the only point I was trying to make db.

Otherwise, I don't think we disagree too much.

 

[The DB]

I agree this article does not spell out specifically that it was for Broadcast, but the suggestion in examples sited is just that IMO. Maxwell also uses several references and one in particular #20, by G.H. Brown, Lewis, and Epstien, entitled: "Ground Systems as a Factor in Antenna Efficiency," as being the classic work which lead to FCC standards for AM Broadcasting ground-radial systems as the bases. I think this article also posted here on WWDX in a thread on counterpoise that HomerBB started. I made the remark, because I remembered the counterpoise thread.

I also qualified my statement on "efficiency" by suggesting the theory did apply to all frequencies, but due to raising 27 mhz antennas for CB the effects of adding more radials, like they did in Broadcast, was likely overkill and would not justify the expense for higher frequencies. This fact is also referred to in Maxwell's remarks, and I talked about the subject in my previous post in this thread.

I might have to do an Eznec model or two to try and show what I think happens when adding a bunch of radials to a raised 1/4 wave antenna, say 40' as an example. IMO, there will probably be some differences to be noted, but I doubt those differences will be worth the effort and expense. This was my only point db.

I agree that raising an antenna an appreciable amount off of the ground changes things as far as efficiency goes. There are a reason very few elevated antennas come with more than four radials. Generally when I am applying losses to an antenna system I am referring to mobile antennas as that is the most commonly used quarter wavelength (and shortened quarter wavelength) antennas used at CB frequencies today. To add to that a vehicle in general is not anywhere near lossless, weather it is acting like a groundplane or a counterpoise (traditional broadcast use of the word here as Cebik and Laport have interpreted the term).


The DB