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That whole coax length thing...
- Thread starter The Jerk
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something is confusing me here.
captain kilowatt, did you say that 234 is the number to use for electrical half wavelengths?
think that 234 is the number to use for quarter wavelengths.
velocity factor of LMR400 is .85
234/27.205(.85)=7.3112 feet
468/27.205(.85)=14.6223 feet
just trying to clarify for others that may run across this thread.
LC
captain kilowatt, did you say that 234 is the number to use for electrical half wavelengths?
think that 234 is the number to use for quarter wavelengths.
velocity factor of LMR400 is .85
234/27.205(.85)=7.3112 feet
468/27.205(.85)=14.6223 feet
just trying to clarify for others that may run across this thread.
LC
thanks for starting this thread Jerk
i had to laugh when i read this, just sounded so wrong to me. LOL
LC
Thanks for catching that.
Today I made a few 1/4 wave stubs from coax cable as an experiment in making a coaxial band-reject filter and the formula for 1/4 waves was stuck in my head. YES I did make a mistake when I said that 234/F X Vf was for a halfwave. :headbang I corrected that in my post on the last page. The correct formula is indeed 468/f X Vf. As stated before a little one way or the other will not affect things very much, at least not enough to worry about in this hobby.Some days it just doesn't pay to get out of bed. Tomorrow I don't think I will.
B
BOOTY MONSTER
Guest
i used 492 . it was explained to me that 492 divided by frequency times velocity factor gives a electrical half wavelength of coax . i multiply the four numbers after the decimal point by 12 to give me the inches for that length .
i dont know why i forgot this link/thread till today ............
http://www.worldwidedx.com/cb-antennas/30479-10k-tunes-1-3____102-3-0-a.html
468 is for making 1/2 wave dipoles .... i think .
hehehehe ^5 jerk
i dont know why i forgot this link/thread till today ............
http://www.worldwidedx.com/cb-antennas/30479-10k-tunes-1-3____102-3-0-a.html
468 is for making 1/2 wave dipoles .... i think .
hehehehe ^5 jerk
Whatever works I guess. :bdh: In the end I just cut according to the formula I use, solder one coax connector on and use the MFJ-269 and a pair of cutters to trim for exact length before soldering the other connector on. I have yet to cut a piece too short to begin with.
Thanks for catching that.
Some days it just doesn't pay to get out of bed. Tomorrow I don't think I will.
The VF thing.....that's why said I simply ignore VF. It doesn't make enough difference to worry about. So I don't even figure it into things. Besides, by the time you get it into the air, it's all gonna change again anyway due to surroundings, etc. I just get kinda amused at how people can get so entangled in figuring out "formulas" to the "nth" degree when it doesn't make a hill of beans big enough to cause a f***!
:tongue:GF
CK, can you give us some details as to how you use the analyzer when you trim a 1/4 wave jumper, and what the analyzer shows you when you find the sweet spot?
I use a T connector and a dummy load.
Connect the T connector to the back of the meter. Screw on the dummy load and check your SWR's, they should be flat. Then screw your coax to the T, leaving the dummy load and SWR calibrations the same. Then start cutting the other end of your coax for a exact match.
You will be surprised how much your coax actually affects the meter!
The VF thing.....that's why said I simply ignore VF. It doesn't make enough difference to worry about. So I don't even figure it into things. Besides, by the time you get it into the air, it's all gonna change again anyway due to surroundings, etc. I just get kinda amused at how people can get so entangled in figuring out "formulas" to the "nth" degree when it doesn't make a hill of beans big enough to cause a f***!
GF
I disagree Jerry. Some coax cables have a Vf of only 66% which can make all the difference in the world when using tuned feedlines There may not be much differance between 102 inches and 105 inches but there is a huge differance between 102 inches and 67 inches. That's an error of 50%. Pretty sigificant by any standards.
The best question is why do some people swear they see a difference in SWR with coax length and other are sure there is none? The answer will surprise you guys that see a change with length. When SWR changes with coax length this is a key sign that there is a mismatch between the antenna and the 50 ohm coax. It indicates there are standing waves on the coax line and that's why the impedance changes with length. In this case it takes an electrical 1/2 wavelength section of coax to show the true antenna impedance.
Had the antenna been tuned at 50 ohms to match the coax, any length of coax would indicate a good SWR match because the impedance is constant anywhere along the coax. If different lengths of coax effect your SWR, you need to adjust your antenna and it just may take using the right 1/2 wavelength of coax (calculated with velocity factor) to find the real match.
Had the antenna been tuned at 50 ohms to match the coax, any length of coax would indicate a good SWR match because the impedance is constant anywhere along the coax. If different lengths of coax effect your SWR, you need to adjust your antenna and it just may take using the right 1/2 wavelength of coax (calculated with velocity factor) to find the real match.
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