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Dual Coax with single 102"
- Thread starter Palletburner
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lords
Active Member
"If I am right, Co-phase starts out at 75 ohm and is 12'
for a mobile."
two 75 ohm lines any odd number of quarter wavelengths. 12' would be a half wavelength in polyethylene at the frequency in question and would not perform the necessary impedance transformation.
for a mobile."
two 75 ohm lines any odd number of quarter wavelengths. 12' would be a half wavelength in polyethylene at the frequency in question and would not perform the necessary impedance transformation.
lords
Active Member
really, then why do most shops sell Co-phase setups like this
are they all wrong..lol
all CO-phase is 75ohms ....12' and 18' 1/2 and full wave..
based off any RG 8)
11 meter 40up and 40 down
are they all wrong..lol
all CO-phase is 75ohms ....12' and 18' 1/2 and full wave..
based off any RG 8)
11 meter 40up and 40 down
Just depends on if you are talking about the length from the center to one end, or total length. The 'right' length from the center to one end does need to be an odd multiple of an electrical 1/4 wave length. That length varies because of the velocity factor of the particular 75 ohm coax used. For the average everyday stuff you find, something on the order of 5'11" is an electrical 1/4 wave (assuming 0.66 VF). From there, multiply that length by an odd multiple so that it 'stretches' to where it has to go. That multiplying thingy is for each side, not the total length! 3 x this side and 3 x that side = 6 times the 1/4 wave if you are thinking about total length. Gotta be careful about 'where' you do the multiplying or it won't work right... make sense?
- 'Doc
- 'Doc
laugh your ass off all you want. they sell it because the manufacturer makes it, NOT BECAUSE IT'S DESIGNED FOR ANY PARTICULAR APPLICATION. this is why most hobbyists can't get dual antennas to function properly because they don't have a clue what they're doing and don't understand the principals involved.
what is required to match two 50 ohm loads to a single 50 ohm source is a harness composed of two sections, each consisting of a 1/4 section of line. (1/4 wave transformer) a 1/2 wave line mirrors impedance and a 1/4 wave line inverts impedance. in the case of dual antennas we want to invert the impedances presented by the loads to match them to the source transmitter.
with polyethylene line at .66% VF the length of a 1/4 wave line at cb frequencies is approximately 6' and ANY ODD MULTIPLE.
246 / 27.205 X .66% VF = 5.968' or 5' 11.6"
using foam line @ .78%VF the length is approximately 7' OR ANY ODD MULTIPLE.
246 / 27.205 X .78% VF = 7.053' or 7' 0.6"
so the lengths required for a 1/4 wave transformer based on VF and the frequency in question would be:
6', 18' 30' etc. for polyethylene @ .66% VF
7'. 21' 35' etc. for foam @ .78% VF
since there is no feedline impedance that is close to that value and since we are matching two 50 ohm loads in parallel, (which equals 25 ohms) then two 70 - 75 ohm lines in parallel provides the necessary impedance transformation.
1/4 Matching Section = sqrt ( Z1 * Z2)
sqrt ( 25 * 50 )
sqrt (1250)
35.35 ohms
go google it and if you plan on arguing the point then bring the facts and the math with you.
you're wrong, get over it.
what is required to match two 50 ohm loads to a single 50 ohm source is a harness composed of two sections, each consisting of a 1/4 section of line. (1/4 wave transformer) a 1/2 wave line mirrors impedance and a 1/4 wave line inverts impedance. in the case of dual antennas we want to invert the impedances presented by the loads to match them to the source transmitter.
with polyethylene line at .66% VF the length of a 1/4 wave line at cb frequencies is approximately 6' and ANY ODD MULTIPLE.
246 / 27.205 X .66% VF = 5.968' or 5' 11.6"
using foam line @ .78%VF the length is approximately 7' OR ANY ODD MULTIPLE.
246 / 27.205 X .78% VF = 7.053' or 7' 0.6"
so the lengths required for a 1/4 wave transformer based on VF and the frequency in question would be:
6', 18' 30' etc. for polyethylene @ .66% VF
7'. 21' 35' etc. for foam @ .78% VF
since there is no feedline impedance that is close to that value and since we are matching two 50 ohm loads in parallel, (which equals 25 ohms) then two 70 - 75 ohm lines in parallel provides the necessary impedance transformation.
1/4 Matching Section = sqrt ( Z1 * Z2)
sqrt ( 25 * 50 )
sqrt (1250)
35.35 ohms
go google it and if you plan on arguing the point then bring the facts and the math with you.
you're wrong, get over it.
lords
Active Member
all CO-phase coax is 75ohm
i don't care how long it is...
you do get the point i have right from the start don't you Math man?
the REST
deleted by me ...not worth a arm chair fest
i don't care how long it is...
you do get the point i have right from the start don't you Math man?
the REST
deleted by me ...not worth a arm chair fest
"I ain't going to whine over it I use tunable antennas"
that has absolutely nothing to do with what is being discussed.
it takes a 1/4 wave matching transformer to match two 50 ohm loads in parallel to a 50 ohm transmitter, pure and simple, regardless as to whether the antennas are "tunable" or not.
the facts are that the ability to understand the operation of a dual antenna system (or any antenna system for that matter) involves an understanding of the mathematics involved. you asked the question and it's been answered correctly.
a pair of 12' foot lines of 75 (poly or foam) IS NOT A 1/4 WAVE TRANSFORMER @ 27Mhz.. and performs poorly when used with dual antennas.
as far as i know THIS IS A TECHNICAL FORUM. you're obviously not motivated to understand the how and the why of any of this so in that case just run with the laymens translation that i have already provided and don't bother yourself with the rest.
75 ohm feedline in lengths of 6' and 18' in a polyethylene composition (or 7' and 21' when using foam) are all that's required to match dual antennas installed on most vehicles, tractor rigs included.
all dual antenna matching harnesses ARE NOT 75 OHMS........
Freightliner Cabover Antenna Matching
Freightliner Cabover Antenna Matching
that has absolutely nothing to do with what is being discussed.
it takes a 1/4 wave matching transformer to match two 50 ohm loads in parallel to a 50 ohm transmitter, pure and simple, regardless as to whether the antennas are "tunable" or not.
the facts are that the ability to understand the operation of a dual antenna system (or any antenna system for that matter) involves an understanding of the mathematics involved. you asked the question and it's been answered correctly.
a pair of 12' foot lines of 75 (poly or foam) IS NOT A 1/4 WAVE TRANSFORMER @ 27Mhz.. and performs poorly when used with dual antennas.
as far as i know THIS IS A TECHNICAL FORUM. you're obviously not motivated to understand the how and the why of any of this so in that case just run with the laymens translation that i have already provided and don't bother yourself with the rest.
75 ohm feedline in lengths of 6' and 18' in a polyethylene composition (or 7' and 21' when using foam) are all that's required to match dual antennas installed on most vehicles, tractor rigs included.
all dual antenna matching harnesses ARE NOT 75 OHMS........
Freightliner Cabover Antenna Matching
Freightliner Cabover Antenna Matching
yes, RG-62, 95 ohms....and you can parallel two lines to cut the impedance in half.
actually the variety of differing impedances that feedlines are available in affords quite a bit of matching flexibility when you think about it in these terms.
actually the variety of differing impedances that feedlines are available in affords quite a bit of matching flexibility when you think about it in these terms.
lords
Active Member
freecell ,
all of what your saying is correct..but if you look at the first post
the question was duel coax but in a further post he did say leave one antenna off so this makes it 75ohm Co-phased coax not 2 50ohm pieces of coax running into a T connector...
so instead of running me through you book BS which is 3/4 wrong in the first place...
so if it was like you applied it
your wrong...mr tech (mush have you licence..acting snobby like you are)
you can pass that BS by noobs but not by me so give me a break...don't answer this ..i already think your a book fool anyway...so don't bother
the facts are that the ability to understand the operation of a dual antenna system (or any antenna system for that matter) involves an understanding of the mathematics involved. you asked the question and it's been answered correctly
lmao great sales pitch!!!!!
were talking about coax going to it!!!
all of what your saying is correct..but if you look at the first post
the question was duel coax but in a further post he did say leave one antenna off so this makes it 75ohm Co-phased coax not 2 50ohm pieces of coax running into a T connector...
so instead of running me through you book BS which is 3/4 wrong in the first place...
so if it was like you applied it
your wrong...mr tech (mush have you licence..acting snobby like you are)
you can pass that BS by noobs but not by me so give me a break...don't answer this ..i already think your a book fool anyway...so don't botherthe facts are that the ability to understand the operation of a dual antenna system (or any antenna system for that matter) involves an understanding of the mathematics involved. you asked the question and it's been answered correctly
lmao great sales pitch!!!!!
were talking about coax going to it!!!
When I use to do mobiles, we used RG-71 100 ohm coax too. We found it only at military surplus. It seemed to us to work pretty well, but I'm not sure we really knew what we were doing. The construction of this coax did not allow them to hold up well in this installation under a truck, but we felt they did work. They were prone to getting water in the harness.
We just tuned the harness by trimming to the best SWR and then installed it with two SS 102" whips attached to pickup rear bumpers. They worked out to make a pretty short harness, but they were just wide enough to go between the antennas. They were not long enough to use with mirror mounts however.
We did use a little math after getting the first cut @ 468/freq as the base number. We did not figure VF in our math. We figured the trimming took care of whatever the VF was. As I recall we always came up long and had to trim a bit to end up at the beginning frequency.
We just tuned the harness by trimming to the best SWR and then installed it with two SS 102" whips attached to pickup rear bumpers. They worked out to make a pretty short harness, but they were just wide enough to go between the antennas. They were not long enough to use with mirror mounts however.
We did use a little math after getting the first cut @ 468/freq as the base number. We did not figure VF in our math. We figured the trimming took care of whatever the VF was. As I recall we always came up long and had to trim a bit to end up at the beginning frequency.
lords
Active Member
6', 18' 30' etc. for polyethylene @ .66% VF
7'. 21' 35' etc. for foam @ .78% VF
whats missing there hmmmmmmm 12 and 14 what you don't like them numbers??
RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
so many people using this and your calling them all wrong..
tisk tisk....
7'. 21' 35' etc. for foam @ .78% VF
whats missing there hmmmmmmm 12 and 14 what you don't like them numbers??
RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
so many people using this and your calling them all wrong..
tisk tisk....
it only appears that i'm wrong there because you can't read.
i was talking about 1/4 wavelength matching sections not 1/2 wave lines. 1/2 wave lines mirror impedance and 1/4 wave lines invert impedance.
you don't use a 102" on one side of a dual antenna harness without an antenna in the other side. generally speaking, dual antennas cannot be configured using rg58 for the harness in any but the most exceptional instances. (see links) all dual antenna harnesses are not 75 ohms. 12' of 75 is not a 1/4 wave transformer at 27 mhz.. 12' of poly and 14' of foam is a 1/2 wave line but a 1/4 wave line is required for the proper matching of dual antennas, not 1/2 wave.
6', 18' 30' etc. for polyethylene @ .66% VF
7'. 21' 35' etc. for foam @ .78% VF
the lengths above are for 1/4 wave lines........
coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
these lengths are for 1/2 wave lines. any other questions?
i was talking about 1/4 wavelength matching sections not 1/2 wave lines. 1/2 wave lines mirror impedance and 1/4 wave lines invert impedance.
you don't use a 102" on one side of a dual antenna harness without an antenna in the other side. generally speaking, dual antennas cannot be configured using rg58 for the harness in any but the most exceptional instances. (see links) all dual antenna harnesses are not 75 ohms. 12' of 75 is not a 1/4 wave transformer at 27 mhz.. 12' of poly and 14' of foam is a 1/2 wave line but a 1/4 wave line is required for the proper matching of dual antennas, not 1/2 wave.
6', 18' 30' etc. for polyethylene @ .66% VF
7'. 21' 35' etc. for foam @ .78% VF
the lengths above are for 1/4 wave lines........
coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
these lengths are for 1/2 wave lines. any other questions?
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