in an earlier post about a month ago i saw someone say that 109 inches was about average lenghth for a mobile antenna 11meters. my question is is that the lenghth of the whip and shaft?? or the whip shaft and coils?? someone fill me in cause im confused!!!! lol just out of curiousity i measured my antenna and total length of shaft was 36inches, the whip was 63 thats only 99inches!!!!????? so wheres the extra 10inches?????
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antenna lenghts
- Thread starter excavator701
- Start date
it's in the coil...
a full 1/4 wave whip is 102" plus a 4" or a 6" spring at the bottom. The spring could be a solid piece as well, but don't confuse a spring with a coil.
Other shorter antennas use a coil somewhere, either at the center or the bottom. Sometimes the coil is a large open air coil or other times it is housed in a plastic case...
a full 1/4 wave whip is 102" plus a 4" or a 6" spring at the bottom. The spring could be a solid piece as well, but don't confuse a spring with a coil.
Other shorter antennas use a coil somewhere, either at the center or the bottom. Sometimes the coil is a large open air coil or other times it is housed in a plastic case...
so are you saying that the coil can be made to any size then the differance would have to be added or subtracted elsewhere to get the proper overall lenght?????????????
excavator701,
A 1/4 wave at 27.185 Mhz is ; 234 / 27.185 = 8.6 feet or 103.3 inches (give or take an inch, slightly long but not by much). That's because the inductance and capacitance reactances of a conductor that long make it resonant at that frequency. If that's too long for convenience, then it can be shortened, reducing the amount of inductive reactance of the conductor. But! If you add a coil to that shortened conductor which has the amount of inductance that you 'cut off', then you have a resonant antenna again. Doesn't really matter a lot where that inductance comes from, length of wire, a coil, or whatever, if the amount is the ~correct~ amount, then the thingy is resonant.
Don't confuse the length of wire making up the coil as having anything to do with the length of the antenna, it doesn't. Stretching that coil out into a straight length of wire, then adding it to the shortened antenna just isn't going to make it work right. The length of the wire making up the coil, the length of the coil and it's diameter determines how much inductance the coil will have. (There's a formula for figuring the diameter and length of a coil of a particular sized wire but I don't remember it off hand and I'm too lazy to go look it up. Oh, the size of the wire used makes a difference too, by the way... already said that didn't I? Oh well, it's been a long day.)
Cain't 'nuthing be simple, can it?
- 'Doc
A 1/4 wave at 27.185 Mhz is ; 234 / 27.185 = 8.6 feet or 103.3 inches (give or take an inch, slightly long but not by much). That's because the inductance and capacitance reactances of a conductor that long make it resonant at that frequency. If that's too long for convenience, then it can be shortened, reducing the amount of inductive reactance of the conductor. But! If you add a coil to that shortened conductor which has the amount of inductance that you 'cut off', then you have a resonant antenna again. Doesn't really matter a lot where that inductance comes from, length of wire, a coil, or whatever, if the amount is the ~correct~ amount, then the thingy is resonant.
Don't confuse the length of wire making up the coil as having anything to do with the length of the antenna, it doesn't. Stretching that coil out into a straight length of wire, then adding it to the shortened antenna just isn't going to make it work right. The length of the wire making up the coil, the length of the coil and it's diameter determines how much inductance the coil will have. (There's a formula for figuring the diameter and length of a coil of a particular sized wire but I don't remember it off hand and I'm too lazy to go look it up. Oh, the size of the wire used makes a difference too, by the way... already said that didn't I? Oh well, it's been a long day.)
Cain't 'nuthing be simple, can it?
- 'Doc
the 102" steel whip in addition to the spring and ball mount produces a total length of approximately 108".
246/27.185 = 108"........
246/27.185 = 108"........
yes, but the medium makes the wave go faster and shortens the wave length, so the length is smaller. The "Velocity Factor," for Cu anyway, is about 0.9512. That is why we use 234/F.
But I believe there are other reasons to have the whip longer than 103.3...W5LZ said reactances?
But I believe there are other reasons to have the whip longer than 103.3...W5LZ said reactances?
yeah, like in the case where the total length is 103.3 inches at which point the antenna would exhibit capacitive reactance at the desired (27.185) frequency, because it's too short.
i never install a 102" whip without the heavy duty ball and spring. i do not consider them "options" as they constitute a portion of the resonant length of the antenna. at a length ranging anywhere from 107.5 - 108.5 inches the 246 formula serves best when using this particular combination of materials. since i consider a 1.5:1 SWR to be essentially flat especially if the combination of R and j leaves Z somewhere in the proximity of 50 ohms, i have encountered no problems when installed in compliant mounting locations. i also prefer to err in the direction of longer than shorter, particularly seeing where many of these are mounted in locations where surrounding metal and other objects in close proximity to the base of the whip tend to introduce additional amounts of capacitive reactance requiring the antenna to be longer than it would be otherwise had not the condition of the mounting location above existed.
furthermore there are other factors to be considered besides material composition when attempting to refine any small percentage discrepancies in the remaining 5% involving the VF.
i never install a 102" whip without the heavy duty ball and spring. i do not consider them "options" as they constitute a portion of the resonant length of the antenna. at a length ranging anywhere from 107.5 - 108.5 inches the 246 formula serves best when using this particular combination of materials. since i consider a 1.5:1 SWR to be essentially flat especially if the combination of R and j leaves Z somewhere in the proximity of 50 ohms, i have encountered no problems when installed in compliant mounting locations. i also prefer to err in the direction of longer than shorter, particularly seeing where many of these are mounted in locations where surrounding metal and other objects in close proximity to the base of the whip tend to introduce additional amounts of capacitive reactance requiring the antenna to be longer than it would be otherwise had not the condition of the mounting location above existed.
furthermore there are other factors to be considered besides material composition when attempting to refine any small percentage discrepancies in the remaining 5% involving the VF.
The type of metal used for the antenna does affect it's 'velocity factor', but not as much as the metal conductor's diameter does.
['C2' - the medium makes the wave go slower, not faster. At least when compairing to a vacuum.]
The length found by using the figure '234' is only a close approximate (slightly long, but when building antennas too long is better than too short, right?). There is no absolute, positively 'perfect' way of figuring antenna lengths by using a 'standard' formula. Just too many environmental variables. But the '234' one is certainly close enough in most cases. (for 1/4 waves)
And, while it wasn't specifically mentioned, you can not find resonance by measuring SWR. Just one more 'worm' to put into the can...
- 'Doc
['C2' - the medium makes the wave go slower, not faster. At least when compairing to a vacuum.]
The length found by using the figure '234' is only a close approximate (slightly long, but when building antennas too long is better than too short, right?). There is no absolute, positively 'perfect' way of figuring antenna lengths by using a 'standard' formula. Just too many environmental variables. But the '234' one is certainly close enough in most cases. (for 1/4 waves)
And, while it wasn't specifically mentioned, you can not find resonance by measuring SWR. Just one more 'worm' to put into the can...
- 'Doc
"The length found by using the figure '234' is only a close approximate (slightly long, but when building antennas too long is better than too short, right?)."
no, slightly short. the 234 formula leaves the antenna too short for resonance at 27.185. you go install a 103.3 inch 1/4 wave and attach a tuned electrical 1/2 wave line to it and look into the other end with an analyzer and/or check the resonant frequency with a grid dip meter and let me know when you see resonance at 27.185....it isn't happening. if you're using the analyzer then sweep it up the band and you'll find it. resonance will be the point where the inductive and capacitive reactances are of equal magnitude and X = 0. if you want to establish resonance you first need to know how to find it.
no, slightly short. the 234 formula leaves the antenna too short for resonance at 27.185. you go install a 103.3 inch 1/4 wave and attach a tuned electrical 1/2 wave line to it and look into the other end with an analyzer and/or check the resonant frequency with a grid dip meter and let me know when you see resonance at 27.185....it isn't happening. if you're using the analyzer then sweep it up the band and you'll find it. resonance will be the point where the inductive and capacitive reactances are of equal magnitude and X = 0. if you want to establish resonance you first need to know how to find it.
it has more to do with what's being measured. at 103.3" you may be seeing an acceptable swr but the antenna is actually resonant in the range immediately above 27.185.
Once learned wrong, it's always wrong...right, the wave goes slower. 
Well, I knew what I meant anyway.
I've found it too long too! I liked the 4" spring on a stud mount. But there are just too many variables, so I keep a 6" spring and some other short lengths of rigid shafts laying around too...
BTW, if your using an analyzer, would you need an electrically tuned 1/2 wave coax anyway? How do you do that with a frequency sweep? Usually I just use a phase stable "jumper"
Well, I knew what I meant anyway.I've found it too long too! I liked the 4" spring on a stud mount. But there are just too many variables, so I keep a 6" spring and some other short lengths of rigid shafts laying around too...
BTW, if your using an analyzer, would you need an electrically tuned 1/2 wave coax anyway? How do you do that with a frequency sweep? Usually I just use a phase stable "jumper"
" i've found it too long too! I liked the 4" spring on a stud mount. But there are just too many variables, so I keep a 6" spring and some other short lengths of rigid shafts laying around too..."
so you're using a 102" whip + 4" spring + 1" stud = 107"........
"BTW, if your using an analyzer, would you need an electrically tuned 1/2 wave coax anyway........?"
if you want the values of resistance and reactance present at the feedpoint repeated at the opposite end of "a line" terminating with the analyzer then an electrically tuned 1/2 wave line will be required to make that happen.
so you're using a 102" whip + 4" spring + 1" stud = 107"........
"BTW, if your using an analyzer, would you need an electrically tuned 1/2 wave coax anyway........?"
if you want the values of resistance and reactance present at the feedpoint repeated at the opposite end of "a line" terminating with the analyzer then an electrically tuned 1/2 wave line will be required to make that happen.
so allow me to reiterate one more time. there are only 2 conditions known to exist that establish the resonance of (a tuned circuit) an antenna. resonance occurs either when both inductive and capacitive reactances are of equal values, effectively cancelling each other out or when the amounts of both types of reactance = 0. (actually this second condition may only occur in a black hole) short of being equipped and able to measure these values you have no idea where (at what frequency) the antenna is actually resonant. an swr meter isn't going to provide you with this information and indeed it was never meant to.
freecell, I was going to post a question about ant tuning but I think you answered my question. If I understand right you cant properly tune an ant. without a ant. analizer like a MFJ 259b for example. If this is not right let me know and I will post what I was thinking about doing. Thanks