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- Thread starter JDWilbourn
- Start date
If the FEEDPOINT of the antenna is not a perfect 50 ohm match to the feedline, a particular length of coax acts as a matching transformer in addition to carrying the signal to the antenna.
Stand by for the techno mumbo jumbo.
Stand by for the techno mumbo jumbo.
Rather than keep you waiting, here's some of that "techno mumbo jumbo".
That 18 feet of 'matching transformer' assumes that the input impedance is really something that 18 feet of coax can transform to something close to 50 ohms. It certainly can if the input impedance is a particular value, and assuming that the correct type of coax is used, since the 'type' does determine about that trasnforming thingy. But what if it isn't that 'partucular' input impedance? Or that you've really got the 'right' coax transformer?
- 'Doc
That 18 feet of 'matching transformer' assumes that the input impedance is really something that 18 feet of coax can transform to something close to 50 ohms. It certainly can if the input impedance is a particular value, and assuming that the correct type of coax is used, since the 'type' does determine about that trasnforming thingy. But what if it isn't that 'partucular' input impedance? Or that you've really got the 'right' coax transformer?
- 'Doc
W5LZ said:Rather than keep you waiting, here's some of that "techno mumbo jumbo".
That 18 feet of 'matching transformer' assumes that the input impedance is really something that 18 feet of coax can transform to something close to 50 ohms. It certainly can if the input impedance is a particular value, and assuming that the correct type of coax is used, since the 'type' does determine about that trasnforming thingy. But what if it isn't that 'partucular' input impedance? Or that you've really got the 'right' coax transformer?
- 'Doc
HUH?
Here it is! The perfect example!
You are thirsty (need an antenna). There are a number of choices in the 'pop' machine (bunch of different antennas). There's a problem though, each choice, Pepsi, Coke, Sprite, etc, are a different price (different antennas have a different input impedance). The 'pop' machine can't make change, you have to have the exact change (the antenna isn't an auto-tune one). So, because you don't have a big selection of coins, you can't deposit the exact price for any of the selections. Which means that you will either get a real bargain, or will over-pay for whichever selection that you pick. That 'bargain' doesn't sound bad, but if YOU are the owner of the 'pop' machine, you are gonna come up short when you count the deposits, right? The deposits being what goes to pay bills, and also just happens to be the signal you radiate from the antenna... ... See where I'm going? The only way that everybody comes out 'even' is if the price of your choice (antenna's input impedance) is a set, fixed, price (the coins being the 18 feet of coax.). One size fits all, or you'd better have a lot of change.
- 'Doc
(Okay, maybe it isn't the 'perfect' example, but it's close. You've got a choice; lots of change, different 'types' of coax of different lengths. Or just make the price the same for everything, tuned input. Then it doesn't matter what change you happen to have, or baggy pockets.)
'scuse me while I go take my medication...
You are thirsty (need an antenna). There are a number of choices in the 'pop' machine (bunch of different antennas). There's a problem though, each choice, Pepsi, Coke, Sprite, etc, are a different price (different antennas have a different input impedance). The 'pop' machine can't make change, you have to have the exact change (the antenna isn't an auto-tune one). So, because you don't have a big selection of coins, you can't deposit the exact price for any of the selections. Which means that you will either get a real bargain, or will over-pay for whichever selection that you pick. That 'bargain' doesn't sound bad, but if YOU are the owner of the 'pop' machine, you are gonna come up short when you count the deposits, right? The deposits being what goes to pay bills, and also just happens to be the signal you radiate from the antenna... ... See where I'm going? The only way that everybody comes out 'even' is if the price of your choice (antenna's input impedance) is a set, fixed, price (the coins being the 18 feet of coax.). One size fits all, or you'd better have a lot of change.
- 'Doc
(Okay, maybe it isn't the 'perfect' example, but it's close. You've got a choice; lots of change, different 'types' of coax of different lengths. Or just make the price the same for everything, tuned input. Then it doesn't matter what change you happen to have, or baggy pockets.)
'scuse me while I go take my medication...
unless you have a way to measure the input impedance of the antenna in its final mounting location the question cannot be answered. if the input impedance is identical to the characterisitc impedance of the feedline used then any random line length will provide the necessary results. if the input impedance measured at the feedpoint of the antenna is anything like the values i've seen in the last two installations i saw this morning on a 76 Pete and an older model chevy pickup truck then 18' of quality RG8M or RG8X cut to exactly 18' from plug to plug will work well. so well in fact that no tuning of the antenna was required. in these two instances the input impedance at the antenna was anywhere from 32 - 38 ohms resistive with varying small (single digit or small double digit) amounts of capacitive reactance present.
the fact is that these particular values appear in the majority of antenna installations so often that the 18' line has become a mainstay for passenger vehicle and tractor antenna work.
no swr at or above 1.2:1 was seen in either of the two installations across 40 channels. the antenna used in each of them was the PowerStik QS504. the above information applies to any antennas exhibiting the range of resistance and reactance values mentioned.
the fact is that these particular values appear in the majority of antenna installations so often that the 18' line has become a mainstay for passenger vehicle and tractor antenna work.
no swr at or above 1.2:1 was seen in either of the two installations across 40 channels. the antenna used in each of them was the PowerStik QS504. the above information applies to any antennas exhibiting the range of resistance and reactance values mentioned.
DXman
Yes, that's 3100 degrees F. Nine yrs of hard work.
JDWilbourn,
I just installed a system for a guy here yesterday.
I used 15' of coax for his SS whip, it matched out just fine using a antenna analyzer.
That 18' thing is just so manufacture's have enough coax included with their antenna's so it will reach the length of most car's and trucks. Some may require more and some maybe less.
I could be all wrong on this or I could be some what right, but that's just the way I see it...
I just installed a system for a guy here yesterday.
I used 15' of coax for his SS whip, it matched out just fine using a antenna analyzer.
That 18' thing is just so manufacture's have enough coax included with their antenna's so it will reach the length of most car's and trucks. Some may require more and some maybe less.
I could be all wrong on this or I could be some what right, but that's just the way I see it...
DXman, the way you see it is wrong and unless you know for sure, its best to say nothing at all. I know this sounds harsh, but we have tried hard to educate people about coax lengths here and this forum is probably the ONLY ONE to get it right. Giving opinions only clouds the subject. There are others on here who do this too and it only keeps rumors alive.
freecell explained it correctly.
freecell explained it correctly.
Alright then, I have been using a 9' coax with a masterfully built flea market $5 Twig made from some kind of composite that has no wire or coils on it. Is Wrong? lol I have been using a stock Cobra 25 that has never been into. I just bought a Uniden PC76XL that is peaked and tuned and suppose to be a very loud and clear radio, off Ebay. I want to put the right Antenna and coax in the truck. In my other post I was asking about opinions of firestik or Wilsons because in my mind it would be better to manulaly tune them but still be less expencive than say the wilson 1000,2000 or 5000. Most of the truckstops I have stopped at dont offer the 1000 and I am sorta a cheap scate. The 9' black coax is good or should I go ahead and get the 18' clear coax? Is the bigger the ohm number better? LOL a million more question will come to mind when I click the submit button.
you want to only use 50ohm coax.
what there saying is most antennas are not 50ohm at the feed point were you conect the coax. so you can either retune the antenna or trick the radio into seeing a 50ohm load by messing with the coax length. also that length can and will change with differnt types of coax because of velocity factor and such.
what there saying is most antennas are not 50ohm at the feed point were you conect the coax. so you can either retune the antenna or trick the radio into seeing a 50ohm load by messing with the coax length. also that length can and will change with differnt types of coax because of velocity factor and such.
hey fl the word trick or fool while very often used is also missleading in this instance,
the coax actually transforms the impedance when it is not terminated at both ends with its own characteristic impedance,
its not fooling or tricking its transforming
the coax actually transforms the impedance when it is not terminated at both ends with its own characteristic impedance,
its not fooling or tricking its transforming
bob85 said:hey fl the word trick or fool while very often used is also missleading in this instance,
the coax actually transforms the impedance when it is not terminated at both ends with its own characteristic impedance,
its not fooling or tricking its transforming
:shock:
Rich