A few comments, amateur radio vs CB

[Captain Kilowatt]

[FONT=Arial, Helvetica, sans-serif][SIZE=-1][FONT=Arial, Helvetica, sans-serif] Come to think of it, I wonder how many people trimmed their coax using all those generally useless formulas (except in certain specific cases), had a WAY out of resonance antenna, and mysteriously had finals to POP with no explanation? But, according to their super-duper meter, they had GREAT SWR[/FONT][/SIZE]

[SIZE=-1][FONT=Arial, Helvetica, sans-serif]CWM[/FONT][/SIZE]

[/FONT]

Ummmmm......Jerry..........if they had a great SWR then the finals would not go POP! now would they. Regardless of how far off resonance the antenna was,if the SWR presented at the radio was low due to coax trimming or whatever, then the radio sees a low SWR.The antenna efficiency however is very low.It's really no differant than using a tuner in the shack. The radio sees a low SWR and the antenna still works like crap.

 

[hookedon6]

Well that rig does 1kw avg. power with miniscule relfected wattage, somewhere around .5 watt so I'd say that the coax length and counterpoise are in total harmony from 26.555 to 27.900 in AM mode.

this is exactly what this thread is about: misconceptions
a 50 ohm resistor will show ZERO reflected wattage.

try this with your 18 foot coax that you cut to get a 1:1 vswr...... put TWO good meters in line, one at the radio and the other one at the antenna feedpoint....... do they read the same?.... which one is right,..... which one is wrong?

 

[bioman]

Coax length is very important when dealing with phasing lines.It introduces the proper delay,or none at all and just transforms impedance, when dealing with multi antenna arrays. Coax length from the radio to the common point of the phasing lines does not matter in the slightest. Using coax length to "tune" an antenna system is a bandaid solution at best.
.

yeah thats what i meant.....

the "harness" is essentially a "Y" with a single straight 50ohm coax from the common point long enough to go across my roof down the side of the house through the wall and to the 225RD <i believe in the 80-85 foot range although i didnt measure ...i ran it where to i needed cut it and put an end on it > but i have the LEGS <arms??> of the "Y" cut to 3/4 wave of 145MHZ with each one going to each one of my pair of halos which are separated by 1/4 wave <again i used 145 as my baseline freq>

glad to know i wasnt wasting my time

scott

 

[mackmobile43]

One of the things I notice about the differences in the two is not only the different "culture" of them, but a vast difference in the technical approach to the radios themselves. Without getting into a vast discussion on one of these subjects, I do notice the constant attention of CB ops to so-called coax length and an obsession with what they call SWR's (ssssssssssss) whose plural pronunciation (more often than not) indicates a lack of basic understanding of what SWR (SINGULAR!) is!
On the CB boards that I casually read, there is almost an OBSESSION with this "coax length" issue whereby the operators seem to be "tuning" their antennas by trimming their coax to a certain length while completely IGNORING the REAL way to set an antenna so it will resonate on a particular band or frequency! Many of these "experts" proudly launch into a vast discourse on velocity factor (VF) and complex formulas that seem more intent on impressing the reader than actually achieving RESONANCE.
So what IS this velocity factor thing? Simply put, it is the speed at which an RF signal travels thru a medium. It can be AIR, WATER, WIRE, or a CABLE (same thing as wire, really). But that is ALL it is. PERIOD!!!! IT HAS LITTLE TO DO WITH 'TUNING' OR RESONATING AN ANTENNA! Worrying about it and including it in a bunch of formulas is utter horses---!!!!!
I see this formula that goes something like 492+ or - VF/frequency in mhz-yada yada yada= so 'n so coax length! HA! It, for the most part, and for a CB or any OTHER antenna, IS-UTTER-HOOEY-AND HORSE-DUNG!
The question is: can you "tune", or SHOULD you "tune" your CB antenna by trimming your coax to a certain length? NO! So what SHOULD you do?
For a normal installation that has adequate metal, enough to serve as the other half of your antenna (remember that that whip sticking up on your car IS only HALF of your antenna system) you should first assure that the whip is in the clear of obstructions (let it "see" over the car). Then you should loosen the set screw for the whip (for coil type antennas) and slide it UP or DOWN in the collet while observing the SWR (NOT SWR'sssssssssss). Another hint is to straighten out a hanger and use it as a substitute for the whip if the stainless whip doesn't let the antenna "tune". This way, you don't ruin a good whip by accidentally cutting too much off! :headbang Run whatever length of coax you need to the radio and fergeddaboudit! Can you tune antennas cutting coax? Yes and no. What you are doing is fooling your SWR bridge into reading a 50 ohm match and a "good" swr. BUT! The antenna IS STILL MISMATCHED BECAUSE IT HAS NOT BEEN SET TO A "TRUE" AND ACCURATE RESONANCE!!!! What you have is an inefficient antenna. Remember those little 5 watt "dummy load" things that ALWAYS show a "good SWR no matter what? That's because they are nothing more than a RESISTOR with an 50 ohm impedance!!!! In effect, your antenna, because it is being "tricked" by the coax (which IS 50 ohms, BTW) becomes, more or less, a sort of resistor that exhibits varying kinds of radiation because it has not been "set" to a "true" SWR.

If the coax at a certain length was so good at radiating a signal, then WHY would we even NEED the antenna?

Finally, the only time that coax should be a "certain" length is in certain "phased" setups or when the ground (or counterpoise) is not sufficient. (Remember the above comments about the vehicle being the OTHER half of the antenna system?)

One of the glaring differences in amateur operations and CB is the LACK of the obsession with coax length with respect to CB. Look at the amateur discussion boards where you find very little discussion of "coax length" or the same preoccupation with it vs CB boards. While there ARE some situations where coax may be required to be certain lengths (some special folded dipoles, verticals, etc), it is generally very unusual.

Here's ONE amateur who has NEVER resorted to "cutting" coax to a certain length in order to resonate an antenna system unless it was an integral part of the mfr's design! (ME!)

73

CWM

So how's you swrs been hanging lately CWM?

 

[mackmobile43]

this is exactly what this thread is about: misconceptions
a 50 ohm resistor will show ZERO reflected wattage.

try this with your 18 foot coax that you cut to get a 1:1 vswr...... put TWO good meters in line, one at the radio and the other one at the antenna feedpoint....... do they read the same?.... which one is right,..... which one is wrong?

Um I never said my coax was 18' long and as a matter of fact the coax length on that rig is exaclty 24'.

The point that CWM did not understand is that the antenna pictured was fully to the rear of the vehicle thus the amount of counterpoise the antenna was seeing was minimal but none the less satisfactory so the antenna and the radio are a perfect marriage.

 

[jazzsinger]

One of the things I notice about the differences in the two is not only the different "culture" of them, but a vast difference in the technical approach to the radios themselves. Without getting into a vast discussion on one of these subjects, I do notice the constant attention of CB ops to so-called coax length and an obsession with what they call SWR's (ssssssssssss) whose plural pronunciation (more often than not) indicates a lack of basic understanding of what SWR (SINGULAR!) is!
On the CB boards that I casually read, there is almost an OBSESSION with this "coax length" issue whereby the operators seem to be "tuning" their antennas by trimming their coax to a certain length while completely IGNORING the REAL way to set an antenna so it will resonate on a particular band or frequency! Many of these "experts" proudly launch into a vast discourse on velocity factor (VF) and complex formulas that seem more intent on impressing the reader than actually achieving RESONANCE.
So what IS this velocity factor thing? Simply put, it is the speed at which an RF signal travels thru a medium. It can be AIR, WATER, WIRE, or a CABLE (same thing as wire, really). But that is ALL it is. PERIOD!!!! IT HAS LITTLE TO DO WITH 'TUNING' OR RESONATING AN ANTENNA! Worrying about it and including it in a bunch of formulas is utter horses---!!!!!
I see this formula that goes something like 492+ or - VF/frequency in mhz-yada yada yada= so 'n so coax length! HA! It, for the most part, and for a CB or any OTHER antenna, IS-UTTER-HOOEY-AND HORSE-DUNG!
The question is: can you "tune", or SHOULD you "tune" your CB antenna by trimming your coax to a certain length? NO! So what SHOULD you do?
For a normal installation that has adequate metal, enough to serve as the other half of your antenna (remember that that whip sticking up on your car IS only HALF of your antenna system) you should first assure that the whip is in the clear of obstructions (let it "see" over the car). Then you should loosen the set screw for the whip (for coil type antennas) and slide it UP or DOWN in the collet while observing the SWR (NOT SWR'sssssssssss). Another hint is to straighten out a hanger and use it as a substitute for the whip if the stainless whip doesn't let the antenna "tune". This way, you don't ruin a good whip by accidentally cutting too much off! :headbang Run whatever length of coax you need to the radio and fergeddaboudit! Can you tune antennas cutting coax? Yes and no. What you are doing is fooling your SWR bridge into reading a 50 ohm match and a "good" swr. BUT! The antenna IS STILL MISMATCHED BECAUSE IT HAS NOT BEEN SET TO A "TRUE" AND ACCURATE RESONANCE!!!! What you have is an inefficient antenna. Remember those little 5 watt "dummy load" things that ALWAYS show a "good SWR no matter what? That's because they are nothing more than a RESISTOR with an 50 ohm impedance!!!! In effect, your antenna, because it is being "tricked" by the coax (which IS 50 ohms, BTW) becomes, more or less, a sort of resistor that exhibits varying kinds of radiation because it has not been "set" to a "true" SWR.

If the coax at a certain length was so good at radiating a signal, then WHY would we even NEED the antenna?

Finally, the only time that coax should be a "certain" length is in certain "phased" setups or when the ground (or counterpoise) is not sufficient. (Remember the above comments about the vehicle being the OTHER half of the antenna system?)

One of the glaring differences in amateur operations and CB is the LACK of the obsession with coax length with respect to CB. Look at the amateur discussion boards where you find very little discussion of "coax length" or the same preoccupation with it vs CB boards. While there ARE some situations where coax may be required to be certain lengths (some special folded dipoles, verticals, etc), it is generally very unusual.

Here's ONE amateur who has NEVER resorted to "cutting" coax to a certain length in order to resonate an antenna system unless it was an integral part of the mfr's design! (ME!)

73

CWM

completely baffled by bullshit !!! :headbang:headbang

The best way to feed an antenna is with a multiple of an electrical wavelength of coax,NOT to tune the antenna but to repeat the impedance at the antenna feedpoint to the remote location of the swr meter as an electrical halfwave in coax is an impedance repeater.in an ideal situation swr should be measured right at the antenna feedpoint but it isn't always practical so using an electrical halfwave allows for more accurate measurement remotely,swr will read lower than it should if using crap quality lossy coax due to cable return losses,if changing coax length alters swr you most probably have common mode currents on cable.

The biggest difference between cb'ers and hams is cb'ers don't need to sit a test to talk boll@cks and bullshit.:headbang:headbang:headbang

the formula for working out electrical halfwaves in coax is 300/27/2 x velocity factor which is usually .66 for solid polyethylene mil spec rg cables,for low loss foam cables the velocity factor usually ranges between .78 and .80,this gives the answer in metres,to convert to feet for you guys in the good ole US of A multiply answer by 3.28.

you can substitute 27 for whatever frequency is centre of your antennas operating bandwidth,ie for US 40ch cb band substitute 27 for 27.210.

"this doesn't apply to antennas where coax is deliberately used as part of matching or radiating system".

even better resonate antenna with a vector network analyser for zero reactance,which most probably won't coincide with lowest swr,then tune impedance mismatch with a matching network,but antenna analysers ain't cheap.

 

[Captain Kilowatt]

yeah thats what i meant.....

the "harness" is essentially a "Y" with a single straight 50ohm coax from the common point long enough to go across my roof down the side of the house through the wall and to the 225RD <i believe in the 80-85 foot range although i didnt measure ...i ran it where to i needed cut it and put an end on it > but i have the LEGS <arms??> of the "Y" cut to 3/4 wave of 145MHZ with each one going to each one of my pair of halos which are separated by 1/4 wave <again i used 145 as my baseline freq>

glad to know i wasnt wasting my time

scott

Nope,not at all. You did right.

 

[Captain Kilowatt]

completely baffled by bullshit !!! :headbang:headbang

The best way to feed an antenna is with a multiple of an electrical wavelength of coax,NOT to tune the antenna but to repeat the impedance at the antenna feedpoint to the remote location of the swr meter as an electrical halfwave in coax is an impedance repeater.in an ideal situation swr should be measured right at the antenna feedpoint but it isn't always practical so using an electrical halfwave allows for more accurate measurement remotely,swr will read lower than it should if using crap quality lossy coax due to cable return losses,if changing coax length alters swr you most probably have common mode currents on cable.

The biggest difference between cb'ers and hams is cb'ers don't need to sit a test to talk boll@cks and bullshit.:headbang:headbang:headbang

the formula for working out electrical halfwaves in coax is 300/27/2 x velocity factor which is usually .66 for solid polyethylene mil spec rg cables,for low loss foam cables the velocity factor usually ranges between .78 and .80,this gives the answer in metres,to convert to feet for you guys in the good ole US of A multiply answer by 3.28.

you can substitute 27 for whatever frequency is centre of your antennas operating bandwidth,ie for US 40ch cb band substitute 27 for 27.210.

First note the bold part above: You are correct.They can do that without taking any tests. :laugh:

Second whether you agree with CW Morse's methods of expressing himself, what he has posted is NOT bullshit and IS in fact 100% true. (Did I just defend Jerry? :blink I agree that using 1/2 multiples of coax will repeat the antennas impedance back to the TX or meter but it in NO way is necessary to do so. You said "The best way to feed an antenna is with a multiple of an electrical wavelength of coax,NOT to tune the antenna but to repeat the impedance at the antenna feedpoint to the remote location of the swr meter as an electrical halfwave in coax is an impedance repeater".Why would you feed an antenna with multiples of 1/2 wavelengths of coax if the purpose was not to tune the antenna and have a faithfull SWR reading? It does make it more conveinient to know what the true SWR is but it gets a bit rediculous to throw in an extra 40 feet of coax just to meet that requirement on 160m. The proper length of cable to use is what ever it takes to connect the antenna to the TX. Ideally the antennas would be tuned before being installed in the final configuration. In my case I have electrical 1/2 wavelengths of cable made up for each ham band for the sole purpose of tuning the antenna. Once the antenna is tuned I replace the 1/2 wave length of RG-8X with whatever length of Belden 8214 or Heliax it takes to reach the radio.Never had the SWR change a bit after replacing the 1/2 wave cable with a random length after tuning.

 

[BOOTY MONSTER]

C W MORSE , the formula i mentioned b4 wasnt being used to tune the antenna . it was to try to get the most accurate swr reading possible as QRN mention in his last post .

so are you saying us cb'ers dont need to worry about SWR'sssssssssssssssssssssssss ????
do we even need to bother checking ot tuning our SWR'ssssssssssssssss ????
can we just throw our SWR meters away ????
do our crappy (compared to ham amps) cb amps need lower SWR'ssssssssssss than our crappy (compared to ham radios) cb/export radios need ????
whats the highest acceptable SWR'sssssssss for our crappy equipment on our crappy little section of the freq. spectrum ?????
does it hurt anything to have the lowest possible SWR'sssssssssss ????

also . on a slightly differnent note but still on topic .......
how come (on cb amps anyhow) sometimes they wont work properly with certian coax lengths or seem to work best with certian lengths ? ive read many post about folks having problems with amps keying using a 3 foot jumper but going to a 6 foot or 9 foot jumper gets everything working properly .
WASSUPWITDAT ??????

 

[Captain Kilowatt]

on a slightly differnent note but still on topic .......
how come (on cb amps anyhow) sometimes they wont work properly with certian coax lengths or seem to work best with certian lengths ? ive read many post about folks having problems with amps keying using a 3 foot jumper but going to a 6 foot or 9 foot jumper gets everything working properly .
WASSUPWITDAT ??????

Quality control issues with the amp maker. Impedance is not 50 ohms so changing the length of coax brings things into line with what the amp likes to see.If everything was tuned to a proper 50 ohms in and out there would be no need to play with cable lengths. I have NEVER seen an amateur amp for any band that needed a certain length of coax to work properly.

 

[BOOTY MONSTER]

i thought 50 ohm coax had the same impedance for its entire length what ever it may be ? i understand there will be a certian tolerance in manufacture that that allows a lil variation , but not much .
does coax impedance change with length to allow a amp to see a more comfortable/acceptable load ? im thinking it doesnt .
If.........im right , what changes in the coax length to make it play nicer/properly over a length that doesnt ?

 

[W5LZ]

The impedance of coax doesn't change with length unless there's something wrong with it. Why the length matters is how the particular 'length' of a signal (wave length) relates to the length of feed line used. There will be spots where the SWR is low along a length of feed line, and spots where it's high. A cyclic thing. The idea with using particular lengths of feed line is to pick one of those 'low' spots so that the transmitter ends up in it. (An SWR meter has no brains, the user has to supply those, know where those hi/low spots are.) 'Juggling' those lengths to find the 'sweet spot' is certainly one way of handling a mismatch condition, not the best way of handling it but it can help. Best way is to make sure there's no mismatch to start with. Then there's no reason to have to worry about the length of the feed line, it just has to be long enough to reach between two places. The 'bad' part about doing it that way is that typical coax has limits (voltages/currents). Exceeding those limits (actually, just getting close to them) means something is going to not 'like it' and quit working the way it should. Which is better, the 'quick-n-dirty' way of handling a problem, or the 'right' way? That's up to you, ain't it?
- 'Doc

 

[hookedon6]

ok, i'll try to explain it and not be toooo technical.
the point is to make the antenna radiate the best signal, keep that in mind ..... the point is to make the antenna RADIATE the best signal.

it all has to do with voltage and current. many of you use the term "swr", the correct term is VSWR (the v is for voltage and not "variable").

as a radio proudces rf, it is plotted in the form of a sine wave, as voltage vs current. at the radio output(0 degrees) the current is max and the voltage is min, the same again at 180 degrees and again at 360 degrees. (this is where the even multiples of 1/2 wave comes in, but , it has nothing to do with vswr adjustment).
at 90 and 270 degrees the current is min and the voltage is max.

i'm not gonna explain why,.... but an antenna needs current to create an rf wave. the current (not the voltage) through the radiating section of the antenna is responsible for generating the magnetic field, which generates the rf wave. this is why you want the current to be MAX at the feedpoint of the antenna and WHY the vswr reading should be taken there (or mults of 1/2 wl away. to monitor and ensure that you are feeding from a "max current" loop and not a voltage loop and therefore creating the best signal.

so put the meter where you want the max current (ie: the antenna feedpoint) and adjust the antenna for resonance at that frequency.

this is where a lot of you get confused, an antenna can only be resonant on ONE frequency (although it can be usable on near freqs also if it is broadbanded enough).
now you have a choice to make..... how far away from the antennas resonant frequency are you going to operate? changing freq will change the wl and make the antenna NON-resonant. for a cb antenna any freq change will really be minor, but for an amateur antenna that isn't the case. to go from (say) 6 meters to 80 meters will change the wl from about 18 feet to 250 feet. this is why the amateurs say to "just run the coax and cut it off", ... and then use a tuner or a "trapped" antenna.
yes, in an "ideal world" we all would have a seperate tuned feedline for every freq that we operated on, (and seperate monoband antennas too) but that isn't practical.

to sum it up, (if you don't change the freq) and changing the coax length makes a big difference in vswr, then the antenna is not resonant at that freq. or the coax has a problem.


oh.... this ones for booty monster:
"also . on a slightly differnent note but still on topic .......
how come (on cb amps anyhow) sometimes they wont work properly with certian coax lengths or seem to work best with certian lengths ? ive read many post about folks having problems with amps keying using a 3 foot jumper but going to a 6 foot or 9 foot jumper gets everything working properly .
WASSUPWITDAT ??????"

thats because they don't have band switches and adjustable coils to properly match the amps plate to the antenna load,.... and adding/subtracting coax manually changes the load.

 

[jazzsinger]

double post,sorry

 

[jazzsinger]

First note the bold part above: You are correct.They can do that without taking any tests. :laugh:

Second whether you agree with CW Morse's methods of expressing himself, what he has posted is NOT bullshit and IS in fact 100% true. (Did I just defend Jerry? :blink I agree that using 1/2 multiples of coax will repeat the antennas impedance back to the TX or meter but it in NO way is necessary to do so. You said "The best way to feed an antenna is with a multiple of an electrical wavelength of coax,NOT to tune the antenna but to repeat the impedance at the antenna feedpoint to the remote location of the swr meter as an electrical halfwave in coax is an impedance repeater".Why would you feed an antenna with multiples of 1/2 wavelengths of coax if the purpose was not to tune the antenna and have a faithfull SWR reading? It does make it more conveinient to know what the true SWR is but it gets a bit rediculous to throw in an extra 40 feet of coax just to meet that requirement on 160m. The proper length of cable to use is what ever it takes to connect the antenna to the TX. Ideally the antennas would be tuned before being installed in the final configuration. In my case I have electrical 1/2 wavelengths of cable made up for each ham band for the sole purpose of tuning the antenna. Once the antenna is tuned I replace the 1/2 wave length of RG-8X with whatever length of Belden 8214 or Heliax it takes to reach the radio.Never had the SWR change a bit after replacing the 1/2 wave cable with a random length after tuning.

having accurate swr readings and tuning antennas are two totally different things,so the purpose of having an impedance repeater isn't just for use when tuning,it allows accurate readings after tuning too.or in your words its more conveniant to have accurate readings,or in anyone whos running any amount of power's words its downright essential to avoid destruction of the rf amplifiers transistors or tubes.

duhhhhh,the reason you use a 1/2 wave impedance repeater is so you know your meter is reading accurately,how can you tell if a fault develops later if your meter isn't reading accurately.should your antenna develop a slight impedance change with a random length of coax you may or may not even notice it depending on how the length of coax transforms that change.with a 1/2 wave repeater it will show any changes accurately.

why do you use half wave repeaters to tune your antennas on every band if you don't think it makes any difference? your contradicting yourself.

true if the antenna is perfectly matched and there's no standing waves on the feeder then you can use any random length of coax,how often does that occur nowadays when people are using antennas on broad bandwidths.the further you move from resonance the greater the mismatch will be,very few people use antennas on one resonant frequency.

why would you add an extra 40 feet on 160m? a 1/2 wave in coax at 160m is between about 50 and 70 metres or approx 150-210 feet depending on coax used not to mention this is the cb section so whats 160m got to do with anything? if your trying to tell me your a ham and therefore a superior intellect on radio theory then i'm not impressed.if you were that knowledgeable about radio,antenna's and transmission lines you wouldn't even allow crap like rg8x in your shack.:headbang:headbang:headbang