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mfj 259 right at the mobile antenna
- Thread starter bighammer
- Start date
Then again if we lived in a perfect world we could have a perfect antenna, coax, radio but then we wouldn't need them we could read each others mind wonder what Joe is doing in Rome right let me concentrate on some DX. (Look into my eyes look very deep into my eyes) :shock:
( Hey when someone reads mine will you let me know where I left it can't seem to find it anywhere ) :wow
( Hey when someone reads mine will you let me know where I left it can't seem to find it anywhere ) :wow
As stated, and as most people assume that they know what is meant if something isn't said, the results of this 'test' are far from common. That means one of two things. Either the information given is incorrect, or the assumptions made by people are incorrect. I normally take for granted that the stated information is correct, and that I've made an incorrect assumption about something (why would someone say something that isn't exactly true in a situation like this?). Hmm, which brings up a third possibility (since I missed out on the 'popcorn' suspicious, disappointed mind, you know?).
There are two parts to a situation like that, the 'how' it's done, and the 'why' it's done, which results in the stated results. The 'how' isn't all that difficult to figure out. Sometimes the 'why' is, though. Right now, I'm sort of working on the 'why' thingy. That can have at least two reasons, one is sort of malicious, the other cuz it's fun. I'd much rather 'lean' towards the 'fun' reason than the other one. Why? Cuz it's more fun! (and because I went back and re-read the original post)... All puns intended.
- 'Doc
[Z = sqr(35 x 75) = 50]
Where's my popcorn!?
There are two parts to a situation like that, the 'how' it's done, and the 'why' it's done, which results in the stated results. The 'how' isn't all that difficult to figure out. Sometimes the 'why' is, though. Right now, I'm sort of working on the 'why' thingy. That can have at least two reasons, one is sort of malicious, the other cuz it's fun. I'd much rather 'lean' towards the 'fun' reason than the other one. Why? Cuz it's more fun! (and because I went back and re-read the original post)... All puns intended.
- 'Doc
[Z = sqr(35 x 75) = 50]
Where's my popcorn!?
so what is happening?
looking at the original thread hammer claims coax must be cut for the frequency of use,
he claims the antenna is 50ohms x=o with the mfj at the feedpoint but he still needs to cut the coax to get the impedance at the coax input to also read 50ohms,
it sounds like either his antenna is not 50ohms x=0 or the coax is not 50ohms,
how does a 50ohms resistive load get transformed to 60ohms unless the coax is not 50ohms or the mfj is wrong???
i have not seen this "bighammer" effect myself when using 50ohm coax and a 50ohm resistive load..
looking at the original thread hammer claims coax must be cut for the frequency of use,
he claims the antenna is 50ohms x=o with the mfj at the feedpoint but he still needs to cut the coax to get the impedance at the coax input to also read 50ohms,
it sounds like either his antenna is not 50ohms x=0 or the coax is not 50ohms,
how does a 50ohms resistive load get transformed to 60ohms unless the coax is not 50ohms or the mfj is wrong???
i have not seen this "bighammer" effect myself when using 50ohm coax and a 50ohm resistive load..
ok, i too am assuming that what is posted here are the actual readings bighammer saw.
(BH, you wouldnt be messing with us here to get a rise out of certain individuals would you?!LOL)
when master chief said that the ground losses encountered in a vehicle installation had an effect on the antenna's impedance, i thought maybe this was why bighammer saw 50 ohms at the antenna.
so, if i understand you guys, if the ground losses had somehow added up to whatever was needed to raise the antenna's impedance to 50 ohms, that it would not be a purely resistive load, and the X factor would not be 0?
i am not trying to debate anyone, i am trying to learn, and that is the reason behind asking this:
can someone explain the theory behind my 102" whip install?
i have a 102" whip, using the correct mount and spring, connected through a length of RG8x, (i dont know how long, probably about 10 feet) to an SWR meter, then with a 3' jumper to the radio.
SWR reads about 1.3 on ch.1, 1.5 on ch.40, and about 1.2 on ch.20.
these readings stay about the same when the antenna is hooked right to the 148gtl using its internal SWR meter.
readings also stay the same when running my KL300p inline.
dont remember the jumper lengths i used, but all my jumpers are either about 3', 6' or 12'.
seems to me that my system is prety close to 50 ohms, but i dont know what impedance a 1.5 to 1 SWR implies.
75ohms?
anyway, whats the deal with my system, i thought i knew what was going on before, but now i dont think i actually do.LOL
sorry BH, i hijack your threads for my own personal gain, its just what i do.LOL J/K!
loosecannon
(BH, you wouldnt be messing with us here to get a rise out of certain individuals would you?!LOL)
when master chief said that the ground losses encountered in a vehicle installation had an effect on the antenna's impedance, i thought maybe this was why bighammer saw 50 ohms at the antenna.
so, if i understand you guys, if the ground losses had somehow added up to whatever was needed to raise the antenna's impedance to 50 ohms, that it would not be a purely resistive load, and the X factor would not be 0?
i am not trying to debate anyone, i am trying to learn, and that is the reason behind asking this:
can someone explain the theory behind my 102" whip install?
i have a 102" whip, using the correct mount and spring, connected through a length of RG8x, (i dont know how long, probably about 10 feet) to an SWR meter, then with a 3' jumper to the radio.
SWR reads about 1.3 on ch.1, 1.5 on ch.40, and about 1.2 on ch.20.
these readings stay about the same when the antenna is hooked right to the 148gtl using its internal SWR meter.
readings also stay the same when running my KL300p inline.
dont remember the jumper lengths i used, but all my jumpers are either about 3', 6' or 12'.
seems to me that my system is prety close to 50 ohms, but i dont know what impedance a 1.5 to 1 SWR implies.
75ohms?
anyway, whats the deal with my system, i thought i knew what was going on before, but now i dont think i actually do.LOL
sorry BH, i hijack your threads for my own personal gain, its just what i do.LOL J/K!
loosecannon
lords
Active Member
W5LZ said:
paying attention to the big bold only doc..why can't it?(and not the bold popcorn
)I don't know and just wondering why?
and then this???
the antenna is 50ohms x=o with the mfj at the feedpoint(using 3 in of coax to the mount(WOULDN"T THIS CHANGE IT) but he still needs to cut the coax to get the impedance at the coax input to also read 50ohms
if the antenna is 50 Ohms why would even messing with the coax change it?
and why if it is 50ohms would you want to mess with it..remember its been stated all through this forums(I think) that
coax length doesn't matter..
maybe I can't understand normal thinking as with freecells posts
so this is why i have changed my attitude I never said I was good with antenna measuring do hickys...lol seeing though freecell was making a point in a post and i jumped the gun about coax length and I was wrong,because the dam coax was 18' not 12' in a post
now wouldn't cutting the coax on a mobile be the same as raising the stinger or lowering the stinger?
loosecannon,
An SWR meter can only tell you there is a mismatch, not whether it's (+) or (-). So assuming an input impedance of around 30 ohms for the whip, then, Z = sqr(30 x 50), or about 39 ohms, which is about a 1.3:1 SWR. Change that input impedance just a little, since all installations are different, and you'll see the readings you are getting.
Another way to 'fudge' a quick answer for SWR and impedances is to divide the 50 ohms by the SWR figure. 50 / 1.2 = 42 ohms. But, you also have to do it the other way around since you don't know the sign (+/-) of the difference and, 50 x 1.2 = 60 ohms. Both the 42 ohm and the 60 ohm will show SWR of 1.2:1.
An SWR meter is sort of like a scale that only tells you that you don't weigh the right amount. Doesn't say if you are too fat or too skinny, just that you aren't 'just right'. Odd sort of scales, you say? That's absolutely right! But then an SWR meter is sort of an odd thing too.
An SWR meter can only tell you there is a mismatch, not whether it's (+) or (-). So assuming an input impedance of around 30 ohms for the whip, then, Z = sqr(30 x 50), or about 39 ohms, which is about a 1.3:1 SWR. Change that input impedance just a little, since all installations are different, and you'll see the readings you are getting.
Another way to 'fudge' a quick answer for SWR and impedances is to divide the 50 ohms by the SWR figure. 50 / 1.2 = 42 ohms. But, you also have to do it the other way around since you don't know the sign (+/-) of the difference and, 50 x 1.2 = 60 ohms. Both the 42 ohm and the 60 ohm will show SWR of 1.2:1.
An SWR meter is sort of like a scale that only tells you that you don't weigh the right amount. Doesn't say if you are too fat or too skinny, just that you aren't 'just right'. Odd sort of scales, you say? That's absolutely right! But then an SWR meter is sort of an odd thing too.
lords
Active Member
An SWR meter is sort of like a scale that only tells you that you don't weigh the right amount. Doesn't say if you are too fat or too skinny, just that you aren't 'just right'. Odd sort of scales, you say? That's absolutely right! But then an SWR meter is sort of an odd thing too.
good way of putting it...lol
good way of putting it...lol
lords,
About the 'not close to 50 ohms' thingy for a 1/4 wave antenna. Because that's just the way it is. '50 ohms' is very, very seldom what any antenna's input impedance is. Doesn't matter what kind of antenna you're talking about, or what "resonant" length. [Watch the quotation marks! They can tell you more than you'd ever believe!]
In this instance, the quotation marks indicate that since the antenna length is resonant, it has '0' reactances, or x = 0 on an MFJ analyzer. Which is the definition of 'resonance'. Resonance doesn't imply that the resistive element of impedance, the 'R' on that meter, is going to be 50 ohms, or any particular number.
The 'something else going on' part means that there are ways of 'adjusting' things so that you can get the desired 'R' and 'x' values to give you a 1:1 SWR (or as close as anybody ever gets to it anyway). There are quite a number of those "ways", some are more desirable than others for various reasons. The 'desirability' sort of depends on the 'why' you're doing it some particular way. The two 'why's that first come to mind are because it's easiest, or, because it produces the maximum amount of radiation from the antenna thingy. Nothing wrong with the 'easiest' way if it produces an 'acceptable' amount of radiation! There again you get into a definition of what's 'acceptable'. I only know of two ways to define that. By experimentation (re-inventing the wheel in the case of antennas), or, by accepting the results of what someone else has already found out by experimenting.
Starting to get a little convoluted (as in wormy) ain't it? Sort of what happens when you start at some point and have to work in 'both' directions at the same time, huh?
I also won't be surprised if it doesn't answer your questions, or helps in how you think about this stuff. It's just an explanation of how/why I said it the way I did. I'm certainly NOT a good teacher, and make no claims that I am.
- 'Doc
I think I need more coffee. Ought'a go good with that popcorn!
About the 'not close to 50 ohms' thingy for a 1/4 wave antenna. Because that's just the way it is. '50 ohms' is very, very seldom what any antenna's input impedance is. Doesn't matter what kind of antenna you're talking about, or what "resonant" length. [Watch the quotation marks! They can tell you more than you'd ever believe!]
In this instance, the quotation marks indicate that since the antenna length is resonant, it has '0' reactances, or x = 0 on an MFJ analyzer. Which is the definition of 'resonance'. Resonance doesn't imply that the resistive element of impedance, the 'R' on that meter, is going to be 50 ohms, or any particular number.
The 'something else going on' part means that there are ways of 'adjusting' things so that you can get the desired 'R' and 'x' values to give you a 1:1 SWR (or as close as anybody ever gets to it anyway). There are quite a number of those "ways", some are more desirable than others for various reasons. The 'desirability' sort of depends on the 'why' you're doing it some particular way. The two 'why's that first come to mind are because it's easiest, or, because it produces the maximum amount of radiation from the antenna thingy. Nothing wrong with the 'easiest' way if it produces an 'acceptable' amount of radiation! There again you get into a definition of what's 'acceptable'. I only know of two ways to define that. By experimentation (re-inventing the wheel in the case of antennas), or, by accepting the results of what someone else has already found out by experimenting.
Starting to get a little convoluted (as in wormy) ain't it? Sort of what happens when you start at some point and have to work in 'both' directions at the same time, huh?
I also won't be surprised if it doesn't answer your questions, or helps in how you think about this stuff. It's just an explanation of how/why I said it the way I did. I'm certainly NOT a good teacher, and make no claims that I am.
- 'Doc
I think I need more coffee. Ought'a go good with that popcorn!
lords
Active Member
na that was good enough ,,I got ya
need more sugar for this coffee though.lol
I was just wondering why he measured R at the feed point with 3" coax instead of using the coax that came with the antenna as a whole...but he was just checking the antenna itself..
need more sugar for this coffee though.lol
I was just wondering why he measured R at the feed point with 3" coax instead of using the coax that came with the antenna as a whole...but he was just checking the antenna itself..
"...just checking the antenna itself."
Exactly right, Lords. For initial antenna adjustments, you need to know what the antenna, by itself, represents. For 27 MHz applications, a three-inch piece of coax used as a jumper from the analyzer to the antenna feedpoint doesn't represent a significant fraction of a wavelength and therefore won't significantly change the results.
Then, once you know what the antenna's characteristics are at the frequency of interest, you can start to determine just how you want to feed it. Two points you need to keep uppermost in your mind when doing this: First, as 'Doc said, a "resonant" antenna is one with X equal to zero; doesn't matter what the R value is. Second, any real-world antenna will be "resonant" (X=0) at only ONE frequency.
The method you determine for feeding this antenna will have to take these points into consideration.
Exactly right, Lords. For initial antenna adjustments, you need to know what the antenna, by itself, represents. For 27 MHz applications, a three-inch piece of coax used as a jumper from the analyzer to the antenna feedpoint doesn't represent a significant fraction of a wavelength and therefore won't significantly change the results.
Then, once you know what the antenna's characteristics are at the frequency of interest, you can start to determine just how you want to feed it. Two points you need to keep uppermost in your mind when doing this: First, as 'Doc said, a "resonant" antenna is one with X equal to zero; doesn't matter what the R value is. Second, any real-world antenna will be "resonant" (X=0) at only ONE frequency.
The method you determine for feeding this antenna will have to take these points into consideration.