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[W5LZ]

bob85,
I don't think you said anything wrong. Used in that way the coax can do a transformation of impedance. The point though, is that since the input impedances of most antennas are not always the same, that a "standard" transformer length of 18 feet just won't always make the resulting impedance what it should be. So, a 'standard' 18 foot length isn't the 'cure all' it's cracked up to be.
Another part of the using coax as an impedance transformer thingy is that there are sometimes quite high voltages envolved in that transformation. Those high voltages can/will affect the coax being used as a trasnformer, the worst case being arcing. Those high voltages do not result only from using high power, a fairly low power can play @#$$ with those coax transformers. Coax was just never made to handle that sort of thing.
- 'Doc

This is turning into another one of 'those' debates and none of it is 'new'. I quit.

 

[C W Morse]

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 (quote)

CB RAdio has its own special rules about coax length! :!: Doncha know! ROFLMAO!

CW Morse

 

[freecell]

"The point though, is that since the input impedances of most antennas are not always the same, that a "standard" transformer length of 18 feet just won't always make the resulting impedance what it should be. So, a 'standard' 18 foot length isn't the 'cure all' it's cracked up to be."

........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.



"high voltages do not result only from using high power, a fairly low power can play @#$$ with those coax transformers. Coax was just never made to handle that sort of thing."

that's pretty funny stuff considering that low impedance series matching circuits operate at proportionately low voltage and high current levels and visa versa for high impedance circuits, but you're right, your spiel is getting old.

low impedance = low voltage / high current
high impedance = high voltage / low current

maybe something to worry about with say an end fed 1/2 wave vertical (high impedance) but not the least in an end fed 1/4 wave vertical (low impedance).



"CB RAdio has its own special rules about coax length!"

"what the feedline does: every transmission line is also an impedance transformer. unless there is a perfect match between the antenna feedpoint impedance and the characteristic impedance (Zo) of the feedline, the complex impedance (R ± jX) will vary all along the line, with values repeating only at 1/2wl or 180° increments."

L. B. Cebik, W4RNL....an amateur radio operator.

 

[skirtchaser]

Uh....yeah, Did you get the memo on the T.P.S. reports ?

 

[C W Morse]

"The point though, is that since the input



"CB RAdio has its own special rules about coax length!"

"what the feedline does: every transmission line is also an impedance transformer. unless there is a perfect match between the antenna feedpoint impedance and the characteristic impedance (Zo) of the feedline, the complex impedance (R ± jX) will vary all along the line, with values repeating only at 1/2wl or 180° increments."

L. B. Cebik, W4RNL....an amateur radio operator.

SO! This means, according to the "coax length gospel", it means that if I must have 18 feet of coax for my mobile on 27 MHZ, then I must have exactly 59.0771149+ feet of coax for 3.915 MHZ in there, right? After all, if the coax "transforms the impedance" at 27 MHZ, then it must do the SAME thing for 3.9 MHZ. Ergo, all I have to do to operate down there is to have 60 feet of coax and it will work!


(Don't be upset, I'm sort of pulling your leg a bit!)


CWM

 

[Captain Kilowatt]

"The point though, is that since the input impedances of most antennas are not always the same, that a "standard" transformer length of 18 feet just won't always make the resulting impedance what it should be. So, a 'standard' 18 foot length isn't the 'cure all' it's cracked up to be."

........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.



"high voltages do not result only from using high power, a fairly low power can play @#$$ with those coax transformers. Coax was just never made to handle that sort of thing."

that's pretty funny stuff considering that low impedance series matching circuits operate at proportionately low voltage and high current levels and visa versa for high impedance circuits, but you're right, your spiel is getting old.

low impedance = low voltage / high current
high impedance = high voltage / low current

maybe something to worry about with say an end fed 1/2 wave vertical (high impedance) but not the least in an end fed 1/4 wave vertical (low impedance).



"CB RAdio has its own special rules about coax length!"

"what the feedline does: every transmission line is also an impedance transformer. unless there is a perfect match between the antenna feedpoint impedance and the characteristic impedance (Zo) of the feedline, the complex impedance (R ± jX) will vary all along the line, with values repeating only at 1/2wl or 180° increments."

L. B. Cebik, W4RNL....an amateur radio operator.

What was so "funny" about what Doc said? I think you really need to put the ego aside Freecell. I just don't understand why you always have to see fault with some peoples posts.What Doc said was TRUE but perhaps it was not 100% relative to the case presented here.BTW let's not talk about anybody's spiel getting old.We all have a few skeletons in the closet. :roll:

 

[freecell]

"perhaps it was not 100% relative to the case presented here."

it's not relevant at all.

 

[Captain Kilowatt]

Relevance and truth are differant things.How many other threads are on this board with irrelevant posts that simply get overlooked without being made fun of? I think you know what I mean and let's leave it at that.

 

[JDWilbourn]

OK :? Now that yall know I'm not radio savey, what is yalls opinion of what coax, 9' (less to hide) or 18', should I be running in my truck with a Uniden 76 and wilson fiberglass antenna tuneable tip?

 

[freecell]

coaxial feedline power rating data applies to the location of the current loop nearest to the generator end of the cable. cable attenuation stated in terms of dB per 100 feet applies only to a very small fraction of a wavelength at that loop. the remainder of the cable is at a lower temperature and therefore has a lower attenuation. when there are high standing waves on the cable its temperature varies with distance from the generator as a series of alternating hot and cool spots. the distance between hot spots is 1/2-wavelength with cool spots occurring at all odd 1/4 wavelengths. that also presupposes that we are not calculating the length of the line in order to introduce a creative mismatch by terminating the line at some point capable of reconciling the mismatch between the line and the antenna or the generator. this is what this thread is about.

no high power operation was mentioned and the resultant swr values attached to the resistance and reactance figures are of no significant consequence to include any of this in response to the original posters question and there are no high values of standing waves present on the line. even though power handling capacity in watts is inversely proportional to SWR there isn't enough of a mismatch in any of the examples presented to make it an issue, particularly not in a low impedance circuit with a finite line length.

"Now that yall know I'm not radio savey, what is yalls opinion of what coax, 9' (less to hide) or 18', should I be running in my truck with a Uniden 76 and wilson fiberglass antenna tuneable tip?"

again, that will depend on the input impedance of the antenna in question in the desired mounting location. if the antenna input impedance is in the neighborhood of 50 ohms with negligible reactive content then any length of line will do, it simply doesn't matter at that point. absent the necessary information no particular line length can be recommended. that being the case and without an antenna analyzer handy your best bet is to install a tuned 1/2 wave line between the antenna and the radio and use the swr meter built into the 76 to get an idea of the feedpoint impedance value via swr measured at the transmitter. swr can be translated to values of impedance (resistance and reactance) rather simply.

opinions here are irrelevant and meaningless.

what is it that you think you got wrong now bob?

 

[bob85]

when somebody gives me a :shock: it makes me take a second look at what i typed lol,
sometimes i see what i did wrong sometimes i dont,
i stand by what i said ( transforming ),
i am always willing and trying to learn.

 

[Hamin' X]

OK :? Now that yall know I'm not radio savey, what is yalls opinion of what coax, 9' (less to hide) or 18', should I be running in my truck with a Uniden 76 and wilson fiberglass antenna tuneable tip?

Just go with the shortest length that will fit your installation, tune your antenna for the lowest SWR and enjoy. Even if your antenna has an impedance as low as 30 ohms, your SWR will only be 1.67 to 1 and will not hurt your performance to any noticeable degree.

There are those here that will say different and that this will damage your amp, or radio. This is nonsense, as any well designed piece of radio gear will easily tolerate this amount of mismatch. If it won't, get your money back.

Rich

 

[FL Native]

y0 bob they are definately way out line questioning you. 8)

DAMN.......... :?

 

[Hamin' X]

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 point of my " :shock: ", was that it is not changing the feedpoint impedance at all. Rather it is allowing the radio to "see" an acceptable load, so that it will put out full power. The mismatch at the antenna still exists.

The "transforming" length of coax is only part of the total antenna system and it is the antenna system that ends up being 50 ohms.

Rich

 

[FL Native]

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.

...



i stand by my statement.

tune the antenna and you wont have to worry about coax length.