To 7/8 or NOT to 7/8, that is the equastion

[222DBFL]

What are your numbers there NB?? I shouldn't have an issue getting to you. Conditions have been getting better for sure. And yes I do love my Icom 746!! The only thing it has is the TXCO crystal installed. Keeps the radio rock solid on freq on SSB!! Everyone around here asks me for radio checks to make sure they are on freq. LOL! At any rate yes, 38lsb is normally where I hang, but you already know I can go anywhere on the 11m band. And would love to make some more WWDX contacts!! I'll PM you when I am on and see if you can get on as well. At any rate, have a good one and look forward to speaking with you on air!!
2OR222 Daytona Beach, FL., saying 73 & God Bless,
Sean.

 

[222DBFL]

And thanks for the mic tip. I have not looked at the SM8 mic yet. Time to hit flea bay!!!

 

[Marconi]

What I like is setting the antenna for X=0/1, 1.0:1 & R=50-52ohms, THEN after it's up in the air, seeing the same readings at the radio end of the coax.
- I find if I cut the coax to multiples of a 1/2 wave, ( making sure to use the Velocity Factor in my calcs) I'm not disappointed.

NB, based on your 1/2 wave tuned jumper idea above...what would you expect your matching results might be if you used a tuned 1/4 wave or a 9' foot jumper to tune your antenna instead of the tuned 1/2 wave you suggest? I'm not expecting you to give me specific results...just a general idea of what happens here...either way you do it.

I've not had a chance to mess with the antenna itself NB. It's either too windy, raining, or I have been working!! At any rate I did add a 9ft coax jumper with a Huber Suhner surge arrester and a couple of n-male to SO239 adapters. Now readings at radio are as follows at 27.400.
R=48-49
X=2
VSWR= 1.0:1.

222, your readings are very good. I may have asked this before. Did you tune your antenna with your analyzer directly at the feed point, did you use a short coax jumper, or did you use a 1/2 wave tune jumper to tune your NV4K like NB suggested above?

What was the length of the coax you used to tune?

Did you take an analyzer reading at the radio-end after the antenna was tuned, raised, and using a different length of coax for you working feed line?

With my dummy load which is a 100 watt 50 ohm Bird dummy load I see the following readings with the MFJ attached directly to the load, only having an adapter inline from Din Male to N-male.
R=49
X=0
VSWR: 1.0:1.

What does this test and results suggest to you?

 

[222DBFL]

I used a 9ft coax jumper to tune the antenna at the feedpoint. I then used only the 60ft main run from antenna directly to analyzer and had the same readings at 27.400.
R=50
X=0
VSWR: 1.0:1.
This was before I added the other coax jumper that I don't know the length of into the equation. I then raised my antenna and tool readings again at radio. This is where my issue started.
R=59
X=0
VSWR: 1.1:1.
I then added another 9ft rg8x coax jumper and a surge arrester and 2 n-male to SO239 adapters near the radio. This brought the readings back down to the following.
R=48-49
X=0
VSWR: 1.0:1.
I have not since had the opportunity to go back to the main 60ft run and see what the readings are there. All coax being used is Andrew lmr400 minus the 9ft coax jumper from surge arrester to radio which is rg8x. I've tried several different lengths of coax added at the surge arrester. 3,6,9,12, and 18ft coax jumpers were added. VF was not taken into account. Just cut all coax to multiples of 3ft. Used lmr400. The only coax jumper that makes the system happy is the 9ft rg8x coax jumper. I know it sounds weird, but I have even went as far as trying a different 9ft rg8x coax jumper and it didn't have the same effect as the one I'm using. The readings seem best with the coax jumper I have in place.
So at this point I am leaving it the way it is until I can get to the main 60ft run and check the readings there as like I stated above I did use this coax at the radio end before routing coax and this makes it too short to reach my radio room. So this was the reason for adding the extra length of coax I already had in place from the sirio tornado setup. All that has changed from that setup is the main 60ft run that was new was used. And also I am not sure how long the piece of coax from the 60ft main run is. The coax is all routed and dressed in so its not easy to measure. I could do it with some string I'm sure. But like I said I want to check the resadings at the 60ft main line before I start messing with the gamma match.
I have and am using a 1/4 wave length coax jumper (rg8x), for the last piece of coax that makes the readings at the radio happy, but I want to ask is this the same as using a 1/2 wave coax jumper?? If not can you elaborate as to why? I appreciate all the replies. Thanks much.

 

[222DBFL]

I used the dummy load at the mfj meter as a base line for all other tests. I know that the dummy load is correct as I have 3 and all 3 read the same.
R=49
X=0
VSWR: 1.0:1.
By doing this I was able to establish a good baseline reading as to how accurate my mfj meter really is. Looking at the readings from the dummy load and the way the antenna shows at the radio are almost exactly the same currently. This is why I used the dummy load as to create a verified base line of what the MFJ meter shows with the dummy load connected directly to the MFJ meter. I hope this makes sense. I will keep updating as I get to trouble shooting the entire antenna system as there is more than just the antenna that can and will affect the way an antenna might tune. Be it an impedance mismatch or just the way the coax is routed. I will get to the root of the issue, but I am going to go back from the radio and start my trouble shooting this way. Until then. Again, thanks for all the info and responses.

 

[Marconi]

222, your 9' foot tuning jumper was not a tuned 1/2 wavelength. Do you think your working coax length or the 60' foot coax works out to be multiples of a tuned 1/2 wave length at 27.405 mhz?

 

[222DBFL]

I'm not sure about that Marconi. The coax was directly from the factory cut this length for use for in or outdoor cell sites for the GPS antennas. The length of 60ft was never actually used. Always what reached point A to B was used and the rest either scrapped or used for another job.
Does this make sense???
I understand what you're saying but I don't think that the coax I have setup in totality is actually a 1/2 wave length of the given freq of 27.405. I guess I would have to know the true length of the coax line that is just after the main 60ft run. Like I said, all the coax I have attained are 60ft or shorter runs of coax. I didn't pay for any of it. And I have never had this issue before. I've always used what worked from point a to b.
Do you beleive the 60ft coax is a 1/2 wavelength of 27.405??
I don't think it is. But hey I could be totally wrong. A 1/4 wave length jumper for 27.405 using lmr400 is approx. 7.62ft.
I used the formula of L= 246 div. by 27.405, then mult. by VF which is 85% for said coax which is Andrew CNT400 (LMR400 equivalent). So with this said, if we multiply this by 2 this would be a 1/2 wavelength for 27.405mhz, which would be 15.25ft. So with this I can multiply by 2 more and have a full wave length for said freq. which would be 30'6". This would place my coax jumper of 60ft out of line as well as the total I get is 61ft. Would 1ft of coax make a difference??
And if this were the case then I really didn't tune my antenna correctly if a 1/2 wavelength coax jumper is needed for tuning correctly. And I would like to know why? Thanks for the replies and I know I have opened up a can of worms here, so let the flaming begin LOL!!

 

[222DBFL]

I also want to ask this as well. How can a person tune an antenna with a 1/2 wavelength for tuning of an antenna, then use a 50ft or 100ft main run. Doesn't line up with the 1/2 wavelength rule being used for tuning. What I am asking is why would I need to tune the antenna with a specific length just to null and void this length with a 50 or 100ft run of coax that normally is used for most installs of base stations. Or does everyone do it different??

 

[Marconi]

Do you beleive the 60ft coax is a 1/2 wavelength of 27.405??

No, but it is so close that I wonder if the little difference matters.

I don't think it is. But hey I could be totally wrong. A 1/4 wave length jumper for 27.405 using lmr400 is approx. 7.62ft.

You are correct.

I used the formula of L= 246 div. by 27.405, then mult. by VF which is 85% for said coax which is Andrew CNT400 (LMR400 equivalent). So with this said, if we multiply this by 2 this would be a 1/2 wavelength for 27.405mhz, which would be 15.25ft.

This is how I do the math too.

So, 60' / 15.25' = 3.93 which is very close to 4 electrical 1/2 wave multiples, and again this probably doesn't matter.

You did good.

 

[222DBFL]

Thanks Marconi. Much appreciated.
So I ask this, I used a 9ft coax jumper for tuning. LMR400 Times Microwave coax, PL259 on one end and an N-male connector on the other. By doing the math above even a 1/4 wave jumper would only be 7.62ft. Again, this is off from the 9ft one I used but it worked just fine. I know that the MFJ meter is fine as I used 3 different 50 ohm dummy loads to check readings at 27.400mhz. All 3 showed the same reading,
R=49
X=0
VSWR: 1.0:1.
This was my baseline for tuning and I ended up at 27.400 with
R=50
X=0
VSWR 1.0:1 at the end of the 9ft coax jumper that I used. This is pretty well in line with the dummy load. And the 60ft coax jumper showed the same readings at the radio end.
Like I said, I am far from an expert and am learning by trial and error here so what I see may be different than what others may see due my QTH. At any rate I am pretty sure of my issue and how I need to truly resolve it, but I know in order to do it correctly I will start my trouble shooting at the radio and work my way back to the antenna. Hopefully we will get to the end of the 60ft run and find the readings to be like they were when first tuned, but I have my suspicions still. Will know better when I do trouble shoot it and will keep posting about it as well. And again, to all, thanks for all the input and keep it coming please!! I am not afraid to take a scolding here so don't be afraid to speak up please. The more input the merrier. And again, thanks.
73 and God Bless.
Sean.

 

[Marconi]

I also want to ask this as well. How can a person tune an antenna with a 1/2 wavelength for tuning of an antenna, then use a 50ft or 100ft main run.

222, I attached below 4 pages of the book "Reflections II" by Walter Maxwell, W2DU.

IMO this might help explain how line length effects a working feed line.

With this said, there are distinctions to be noted in these words that should not be ignored. So be sure and watch and try and understand the exceptions also noted for these rules.

IMO we see here that Maxwell correctly gives us something on the one hand and takes it away on the other. I think this resource is the kind of stuff that CB ideas are often taken from, but I sometimes sense the full text is not always fully considered in the reading, so we often just get CB BS instead.

Doesn't line up with the 1/2 wavelength rule being used for tuning. What I am asking is why would I need to tune the antenna with a specific length just to null and void this length with a 50 or 100ft run of coax that normally is used for most installs of base stations. Or does everyone do it different??

222, I haven't been convinced yet that we need to use a 1/2 wave line for tuning or for a working feed line. It is my opinion we will likely see as good match using any other random length line. So, IMO line length matters little.

I read Maxwell's information here to suggest there might be detrimental issues using a resonant tuned working feed line...Common Mode Currents on line I3 in the attached image. 222, maybe you can test this idea and post your results.

So, I ask this question, if we are tuning a new antenna and the load is not matched to the feed line showing a mismatch, what will a tuned 1/2 wave feed line in this case show us that a random feed line won't?

I don't know if the following is real scientific theory or not, but I've always been lead to believe...that any length of a working feed line will look transparent (show a perfect match anywhere along the line and at the radio end of the line, if the antenna match at the feed point is resistive (prefect) at the frequency of choice.

The only exception for this is some attenuation...due to resistance and added line length. Again, read about this specifically in the attachment below in the first paragraph of Section 21.3 the first page. Better still read it all, over and over.

These are my opinions.

 

[222DBFL]

I totally agree with the what is being said and that is a good read if you read all of it and also understand all of it!!
If all of the system is a 50ohm system, whether it be an antenna or dummy load, the length of coax being used shouldn't matter as long as the entire system is matched at 50ohms. Correct me if I am wrong with saying this. And thus any length of coax can be used for tuning. Only when a mismatch occurs does it create a situation in which coax length will matter. Am I on the right track or way off here guys??? Please any reponses are welcome!! Thanks much.

 

[Marconi]

I totally agree with the what is being said and that is a good read if you read all of it and also understand all of it!!
If all of the system is a 50ohm system, whether it be an antenna or dummy load, the length of coax being used shouldn't matter as long as the entire system is matched at 50ohms. Correct me if I am wrong with saying this. And thus any length of coax can be used for tuning. Only when a mismatch occurs does it create a situation in which coax length will matter. Am I on the right track or way off here guys??? Please any reponses are welcome!! Thanks much.

I agree with you here 222, but I could be wrong.

 

[The DB]

So, I ask this question, if we are tuning a new antenna and the load is not matched to the feed line showing a mismatch, what will a tuned 1/2 wave feed line in this case show us that a random feed line won't?

Assuming there are no common mode currents present, a tuned electrical half wavelength feed line will faithfully recreate the conditions for X and R at the load (antenna). Any other point along the feed line will have differing values for X and R. If you are limited to SWR, any point along the feed line will show the same SWR, so if you have such a limit, an electrical half wavelength tuned feedline will do you no good.

Taking this a step further, many people think that simply because the X and R variables change that SWR will also change, when in fact, the relationship between X and R variables and SWR is such that on a half electrical wavelength of feed line every possible value combination of X and R for a given SWR will be present, and thus SWR is maintained, no matter where on the feed line the measurement is taken.

Further, if you know the electrical length of the feed line at the given frequency, it is possible to calculate what X and R are at the load itself.

All of this is not taking attenuation into account which on a half electrical wavelength of feedline will generally be very small, if it is noticeable at all.


The DB

 

[The DB]

I totally agree with the what is being said and that is a good read if you read all of it and also understand all of it!!
If all of the system is a 50ohm system, whether it be an antenna or dummy load, the length of coax being used shouldn't matter as long as the entire system is matched at 50ohms. Correct me if I am wrong with saying this. And thus any length of coax can be used for tuning. Only when a mismatch occurs does it create a situation in which coax length will matter. Am I on the right track or way off here guys??? Please any reponses are welcome!! Thanks much.

This is correct, if everything in the system is 50 ohms then the system will show a perfect SWR match. The name for such a match is Z0 (pronounced zee zero) if memory serves. I'm pretty sure its in the book Marconi referenced. If you are only using SWR, you are fine, it doesn't matter where you are on the feed line, you will see the same thing, unless you have another issue to deal with.

However, if you are using X and R measurement to tune the antenna, it is possible that with some feedline/antenna impedance combinations that something you would expect, for example making an antenna longer which will increase the X variable on a 1/4 wavelength antenna, may actually make X appear to go down on your analyzer. Because of this unless you have a known reference, i.e. direct access to the feed point of the antenna or a reproduction of said feed point by using an electrical half wavelength feedline, simply watching the X and R variables doesn't tell you very much. That being said, you can generally get close, all you have to do is get R closer to 50 and X closer to 0. But then you can do the same thing with just an SWR meter...

That is of course assuming that those measurements are the measurements you want, and many people think that is always the case. There are, however, times that that will not get you the best performing antenna tune...


The DB