That whole coax length thing...

[C W Morse]

Well I think I figured out what's happening by lengthening your screwdriver antenna you are basically stripping the shield off of coax and by shortening it you are re-shielding all that coax if the cb'er theory holds true.

And to think all that is done with the push of a button, lazy ham guys.op:

Now THAT's the BEST comeback I've seen YET!!! Ya gots me there!!!!!!!!! Thanks! After the day I've had today, I NEEDED that one!! LMAO! (And its done with ONE coax, too!)

CWM

 

[pro151]

I was jest plain skeered to offer up my pinion on coax, but since ol CW dun gone and jumped in with both feet and called BS I will go ahead and fess up to my install sins as well.

I not only run the shortest length of coax possible I will also................










Wait for it........................................................














Here it comes...................................................


















SHORTEN THE COAX ON A MAG-MOUNT ANTENNA!

Oh no, now I have done it. The coax gods will be angry with me and smite me with high ess dubbya arrrs and blow the finals out of all of my reddios.

 

[C W Morse]

"Coax length and Velocity used to tune antennas is bull****"

nobody said anything about changing coax length to tune an antenna . the discussion was about if the length of coax changes what the meter reads . even the OP said he thought it was just fooling the meter .

i personally think the coax was assembled improperly , that can happen even when its new .
can you tell us why his vswr changed ?
i hope this lowly chicken bander didnt disapoint you much by not calling it swr's

ive seen how you love to come down to the cb section and brag about your credentials and beat your chest and try to make us feel stupid ........... but can you at least be on topic when you feed your ego like that ?

But isn't the whole idea to have a RESONANT, EFFICIENT RADIATOR (antenna) at the frequency of interest? It IS MY opinion that coax length is simply HOOEY and and a lot of false effort when one can simply tune the radiating system AT the antenna by shortening or lengthening the whip, removing or adding turns to the coil (if any), and MATCHING the feedpoint impedance to 50 ohms....and THAT doesn't even have to be exact.....using a small shunt coil, a resistor, or a toroid to obtain the match AT the feedpoint. Normally, matching isn't even NEEDED at 27 MHZ, because often the resistance of the center loading coil is enough to do the job of matching the 3 total impedances/resistance in an antenna: feedpoint, coil, and radiation resistances. If your meter is happy, fine and dandy with me. BUT how do you KNOW that you've not got inefficiencies AT the antenna itself if it, too, is not resonant. You can MATCH almost anything----even a barn door (if it's metal). Think not? Take a 10 watt/50 ohm resistor and put it across the terminals of your radio. Transmit. Your meter will show about 1.1 SWR or so, and 50 ohms (if you have an analyzer to measure it. OOOH, what a match)! But you won't have much of a signal! BTW, its called a "DUMMY LOAD"! By paying so much wasted attention to coax length and NOT the antenna, you're loading a mismatch that won't put out what it should. I'm sorry it makes some of us mad, but it is simply the truth! Leave all that VF and coax length stuff alone and tune the ANTENNA, NOT the coax!!!!!!!! IF you want to believe all that false radio guru stuff, be my guest. Most of it is just that: HOOEY!


CWM

 

[BOOTY MONSTER]

thanks for that reply c w morse .

tuning at the antennas feed point would seem to be optimal , but for many folks its not a option and its necessary to use coax to connect the meter to the antenna . from what i can tell the OP was questioning why their vswr changed with a different length of coax . if we are using proper coax assembled properly and we tune the antenna properly by adjusting its stinger and we get an acceptable match why does that match sometimes change for some folks on their meters when they change to a different length of coax if it is also in proper condition ? if its simply fooling the meter or causing it to show a false reading/measurement then how do we know what length is giving the most accurate reading on the meter ?

"The SWR AS SEEN BY THE RADIO will change HOWEVER THE SWR AT THE ANTENNA WILL NOT CHANGE. The coax acts as an impedance transformer as as the length of the coax changes, the impedance transformation ratio also changes."

if we just cant connect our vswr meter to the antennas feed point and we must have a section of coax inbetween them what length gives the mose accurate ratio ??

these are questions to try to understand what been posted in this thread . thanks

 

[W5LZ]

"if we are using proper coax assembled properly and we tune the antenna properly by adjusting its stinger and we get an acceptable match why does that match sometimes change for some folks on their meters when they change to a different length of coax if it is also in proper condition ?"

Because if you adjust the length of the antenna for best match with a particular length of coax, you are doing two things. First, you are also adjusting that antenna's resonance since length determines that. An SWR meter doesn't know what resonance is, it can't measure it, it can't tell you anything about resonance.
That SWR meter can only tell what the gross impedance match/mismatch is. If you are doing that reading from the transmitter end of the feed line, then you are actually reading the combined impedance of the feed line -AND- the antenna. That meter can't tell the difference between just the antenna's impedance, or just the feed line's impedance, it can only tell you what the combined impedance of both antenna and feed line is. So, if you change that feed line's length, you've also changed the combined impedance of the feed line and the antenna. It will -always- work that way UNLESS the antenna's input impedance is the same as the characteristic impedance of the feed line. That's why it's always better to do that antenna impedance matching AT the antenna rather than with the feed line. If the antenna is really 50 ohms and no reactance, which will mean resonant AND an SWR of 1:1 (or close), the length of the feed line makes no difference. You've eliminated the feed line as a variable.
Since it's sort of difficult to put that SWR meter at the antenna's input in most cases, you can use a particular length of feed line that is 'invisible' to RF at the frequency of use. That particular length of feed line is an 'electrical 1/2 wave length'. Which is where the velocity factor of the feed line comes into play. That E1/2 wave length not long enough to reach the antenna? Use whole multiples of that length, same difference as far as RF is concerned, it's like it really isn't there and the meter is hooked directly to the antenna's input.
So how do you find the right length for that "invisible" electrical 1/2 wave length of feed line? You have two choices. Do the math, or, use a meter that can tell you how long it should be. That meter will never be an SWR meter. Sorry, it just can't do it.
If it ain't one thing, it's another! Ain't it??
- 'Doc

 

[The Jerk]

Again, I did not do this to tune the antenna...I just figured the Belden coax was a little better quality and swapped it in. I didn't really "touch" anything else, including the length of the stinger on the 5000. I basically reinstalled my system exactly how it came out, short changing that piece of coax, and now my SWR is "constant" for lack of a better term.

I figured it would stir up a hornets nest...I for one don't believe the length thing, the coax just happened to be that 12-foot length that I installed (and for those that care, it WAS the shortest length possible I could use).

 

[mackmobile43]

Again, I did not do this to tune the antenna...I just figured the Belden coax was a little better quality and swapped it in. I didn't really "touch" anything else, including the length of the stinger on the 5000. I basically reinstalled my system exactly how it came out, short changing that piece of coax, and now my SWR is "constant" for lack of a better term.

I figured it would stir up a hornets nest...I for one don't believe the length thing, the coax just happened to be that 12-foot length that I installed (and for those that care, it WAS the shortest length possible I could use).

If yours had the same stock coax as my 5k it's not likely the belden is any better coax, most likely what happened is you wound up with a better termination of the coax on both ends.

 

[The Jerk]

This was on a through-roof mount purchased separately...RG-58 A/U.

 

[mackmobile43]

This was on a through-roof mount purchased separately...RG-58 A/U.

Ok, mine came with a nice 8x mini in black of coarse.

 

[Captain Kilowatt]

thanks for that reply c w morse .

tuning at the antennas feed point would seem to be optimal , but for many folks its not a option and its necessary to use coax to connect the meter to the antenna . from what i can tell the OP was questioning why their vswr changed with a different length of coax . if we are using proper coax assembled properly and we tune the antenna properly by adjusting its stinger and we get an acceptable match why does that match sometimes change for some folks on their meters when they change to a different length of coax if it is also in proper condition ? if its simply fooling the meter or causing it to show a false reading/measurement then how do we know what length is giving the most accurate reading on the meter ?

"The SWR AS SEEN BY THE RADIO will change HOWEVER THE SWR AT THE ANTENNA WILL NOT CHANGE. The coax acts as an impedance transformer as as the length of the coax changes, the impedance transformation ratio also changes."

if we just cant connect our vswr meter to the antennas feed point and we must have a section of coax inbetween them what length gives the mose accurate ratio ??

these are questions to try to understand what been posted in this thread . thanks

I'm a bit late responding to this and i see Doc gave the answer about the length but here is my $0.02 worth. An electrical half wavelength of coax cable will NOT transform impedances therefore if you use an electrical 1/2 wavelength of coax between the antenna and the SWR meter the SWR meter will read the true and actual impedance that the antenna is presenting. Since I like to mess around with antennas while building them and NOT mess around with them after they are installed I have several pieces of coax cable made up that are electrical 1/2 wavelengths on each of the ham bands to allow a much easier time adjusting them. Several years ago Radio Shack was selling off all their RG-8X cable at 5 cents/ft so I bought a couple 500 ft reels to use as jumpers etc. I made up cables that were electrical 1/2 wavelength long on 80m, 40m, two for 10m ( I join them together when tuning on 20m) and one for 6m. The 6m and 10m cables join together for 15m as the tuning is close enough. I use these cables between my antenna and either an SWR meter or more often the MFJ-269 antenna analyzer and once tuned for a perfect match I can install the antenna at the final position and run ANY length of cable back to the radio and the SWR will be exactly the same as it was when I tuned it. I have done this countless times over the years and it has never failed me. The "magic" length of cable for determining the TRUE impedance is 468/freq X velocity factor. It works every time.

 

[BOOTY MONSTER]

"if we are using proper coax assembled properly and we tune the antenna properly by adjusting its stinger and we get an acceptable match why does that match sometimes change for some folks on their meters when they change to a different length of coax if it is also in proper condition ?"

Because if you adjust the length of the antenna for best match with a particular length of coax, you are doing two things. First, you are also adjusting that antenna's resonance since length determines that. An SWR meter doesn't know what resonance is, it can't measure it, it can't tell you anything about resonance.
That SWR meter can only tell what the gross impedance match/mismatch is. If you are doing that reading from the transmitter end of the feed line, then you are actually reading the combined impedance of the feed line -AND- the antenna. That meter can't tell the difference between just the antenna's impedance, or just the feed line's impedance, it can only tell you what the combined impedance of both antenna and feed line is. So, if you change that feed line's length, you've also changed the combined impedance of the feed line and the antenna. It will -always- work that way UNLESS the antenna's input impedance is the same as the characteristic impedance of the feed line. That's why it's always better to do that antenna impedance matching AT the antenna rather than with the feed line. If the antenna is really 50 ohms and no reactance, which will mean resonant AND an SWR of 1:1 (or close), the length of the feed line makes no difference. You've eliminated the feed line as a variable.
Since it's sort of difficult to put that SWR meter at the antenna's input in most cases, you can use a particular length of feed line that is 'invisible' to RF at the frequency of use. That particular length of feed line is an 'electrical 1/2 wave length'. Which is where the velocity factor of the feed line comes into play. That E1/2 wave length not long enough to reach the antenna? Use whole multiples of that length, same difference as far as RF is concerned, it's like it really isn't there and the meter is hooked directly to the antenna's input.
So how do you find the right length for that "invisible" electrical 1/2 wave length of feed line? You have two choices. Do the math, or, use a meter that can tell you how long it should be. That meter will never be an SWR meter. Sorry, it just can't do it.
If it ain't one thing, it's another! Ain't it??
- 'Doc

in the words of gomer pyle ..... "THANK YOU , THANK YOU THANK YOU" !!

i did a little math with 8x coax since thats what i use ... VF = .78
channel 1 E1/2WL = 14 ft. 2 3/4 inches x 4 = 56 ft 11 1/8 inches
channel 20 E1/2WL = 14 ft 1 1/4 inches x 4 = 56 ft 5 inches
channel 40 E1/2WL = 14 ft x 4 = 56 ft 1/8 inch

so basically add a 6 ft jumper with a barrel connector will get about 56 ft 1 inch and be four 1/2 WL on channel 36-37 of the cb band (i talk mostly channel 35&36 am and 38lsb) and allow the meter to give its most accurate reading for my most used channels . how convenient is that

my coax is probally a few inches more or less of 50 ft but it still should help my meter be its most accurate on the cb band . so using electrical half wavelengths of coax (or multiples) isnt a waste of time or coax after all .

thanks for starting this thread Jerk

 

[W5LZ]

One last thing about that velocity factor Booty, it isn't constant or exactly what it says it should be all the time. So, 'ball-park' close is really all you cn count on (unless you go to a #3LL of a lot of trouble). That 'ball-park' close is typically close enough. All things considered, it's more 'bottle-rocket science' than real live 'rocket science'.
- 'Doc

 

[Captain Kilowatt]

One last thing about that velocity factor Booty, it isn't constant or exactly what it says it should be all the time. So, 'ball-park' close is really all you cn count on (unless you go to a #3LL of a lot of trouble). That 'ball-park' close is typically close enough. All things considered, it's more 'bottle-rocket science' than real live 'rocket science'.
- 'Doc

Agreed. In this case close is close enough. Another thing to realise is that when using a single E 1/2 wavelength of cable is when you will have the most accuracy unless you actually tune the cable length with an analyzer before calling it a done deal. I have noticed that when using my antenna analyzer that three 1/2 waves do not quite give me the same length as three times a single 1/2 wavelength, the longer cable has to be slightly shorter than three times the single length. A similar thing occurs when using odd multiples of 1/4 waves when making an antenna. It has to do with the end effect with the antenna and I suspect something similar occurs with the coax cable lengths. Basically what I am saying is that for the most accuracy and it may just be splitting hairs, use the least number of 1/2 waves of cable you can to do the testing.

 

[Marconi]

The "magic" length of cable for determining the TRUE impedance is 234/freq X velocity factor. It works every time.

Captain Kilowatt, is 234, the right value for a 1/4 wavelength? I would use 246 and that is what Booty Monster used. I might use 234 to get a closer length for a real world radiating element over real ground, but calculating a tune feed line or jumper is not affected by end effect. Maybe the total overall length will be affected due to added resistance in a multiple wavelength deal like this, but I don't think end effect has anything to do with jumpers or feed lines.

You are correct however, end effect does play a role for the antenna elements making the length a bit shorter than isotropic by about 5%.

 

[Captain Kilowatt]

Captain Kilowatt, is 234, the right value for a 1/4 wavelength? I would use 246 and that is what Booty Monster used. I might use 234 to get a closer length for a real world radiating element over real ground, but calculating a tune feed line or jumper is not affected by end effect. Maybe the total overall length will be affected due to added resistance in a multiple wavelength deal like this, but I don't think end effect has anything to do with jumpers or feed lines.

You are correct however, end effect does play a role for the antenna elements making the length a bit shorter than isotropic by about 5%.

Honestly since the difference is less than 1 % it makes no difference which you use. After stripping the cable and installing connectors and trying to keep the length exact as well as manufacturing variances in the velocity factor I would think that the results would negate that <1% error. I use 234 because it is simply half of 468 which is used for 1/2 waves. It's not rocket science so close is close enough. For applications that require greater accuracy as in phasing of commercial broadcast towers etc. coax cables are not used. descrete L and C are used to fine tune the systems but as hobbyists the coax cable method works just dandy.