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Does coax length really matter when tuning?
- Thread starter 53 bayous
- Start date
Coax should always be kept "short as possible". Alot of people belive that coax run's should be in mulltibles of 3'...This is just not true. Shorter the better. Trying to set the swr, by changing jumper lingths, will just fool the meter into thinking the swr are set right. What mater's, is what the meter is seeing at the end of the antenna coax.
Like if your running a amp, you will have it set-up like this:
"Radio-amp-antenna." So you want the radio and amp to see a swr reading from the coax coming from the antenna. This is where you want to set the swr. Not from where the coax hooks to the radio.
Use a mfj-259B antenna anilizer, hook it up to the coax running to your antenna, and set the swr. Then when you get it tuned to where ya want it. Your antenna is set. If your radio meter reads different, thats because those meters in radios are not that accrate.
Now if you change the antenna coax to a different lingth, you will need to re-set the swr again. OR if you change antennas, or go to a different type of coax, re-set the swr.
Like if your running a amp, you will have it set-up like this:
"Radio-amp-antenna." So you want the radio and amp to see a swr reading from the coax coming from the antenna. This is where you want to set the swr. Not from where the coax hooks to the radio.
Use a mfj-259B antenna anilizer, hook it up to the coax running to your antenna, and set the swr. Then when you get it tuned to where ya want it. Your antenna is set. If your radio meter reads different, thats because those meters in radios are not that accrate.
Now if you change the antenna coax to a different lingth, you will need to re-set the swr again. OR if you change antennas, or go to a different type of coax, re-set the swr.
If you are only operating one band I can see cutting to a certain length I presume. If you operate HF 1.8-29MHZ that wave-length tends to be long for 160 meters...
I always used the shortest length and its always worked for me. I don't run any amps other than my guitar and bass amp. I reckon folks don't care too much for my monotone voice characteristics. I would probably receive a few bumps with a rubber hose if I were to run any amplifiers.
I always used the shortest length and its always worked for me. I don't run any amps other than my guitar and bass amp. I reckon folks don't care too much for my monotone voice characteristics. I would probably receive a few bumps with a rubber hose if I were to run any amplifiers.
Auh yes !! That good ole coax vehical controversey !! ....the ones that seem to know say the size of the stuff really shouldn't matter ?(That's what she said ,but she was lying all along) (but I personally think it does,especially in a vehical) .....Mismatch this and mismatch that !! Alrighty then !! ......Coax in cars and trucks have always been different for me over the years (no duals as of yet) ....I've used a Wilson 1000/5000 for 15 years off and on ....now I run a very small car (Geo,not a lot of ground plane there but enough) Wilsons come with 17 or 18 ft of coax ....in this car of mine I got a 2.5 reading with that size )-: Hmmmm ?(and yes I did try the longer stinger and no change) I cut it down to 10ft with mini 8 and she dropped down to 1.1 Hmmm ? mismatch ? Hell if I know but it worked and it worked through the amp just fine and dandy like !! I got a new amp there after ...it would stick on the high side of the amp ....Hmmm ? I went from a 3 foot jump to a 6 foot and it still did it !! then I tried a 12 foot jumper and it stopped !!! Alrighty then !! but I could see some strange power readys on the meter (less power with this 12 foot jumper for some reason or other ? weird I thought to myself) .......OK ...I said to myself ...lets add up all the coax that I have now ...12 ft jumper /10 ft from the antenna ...22ft total .......Alrighty then ! I got a fresh 24 foot of Belden 95X mini 8 and soldered it to my roof mount connector of my wilson 5K .....Auh yes !! 1.1 around the band !! Amen Hellaluyah !!! back to my 3 ft jumper from radio to amp but no lose of power this time on the meter !!! and no sticky pooh on my high side of my 4 X 454's !! .....so there you have it ....I run a total of 27 ft of mini 8 coax in my small little back pocket car and it works very well .....I honestly do not understand the theroy of relativity here when it comes to all of this ?? but I do understand SWR to a degree and reflective power there after ......my crap is doing fairly well to say the least as far as all that goes ....Mismatch this and mismatch that ??? false readings here and false readings there ??? could well be ??? Hmmm ......but in my case, it's working and it's working well ....and for a guy like me .....if it works ...I NEED NOT FIX IT !!! I suppose what I don't know could well hurt me but to err is human. "I JUST DIDN'T KNOW,WHAT I DIDN'T KNOW" ........."LEAVE IT TO BEAVER" ......I only thought I did. Switch Kit
Dang switch... your coax is longer than mine...
8)
Well at least until I operate 160 meters mobile...
8)
Well at least until I operate 160 meters mobile...
Geekster said:
Here's one that few have tried (and failed) to answer. I do not believe in the coax length thing at all. Nada. Nicht, Nyet. Here's why. I run an antenna called a Screwdriver which is basically a continuously tuning all-band HF antenna. It tunes from 3.5 thru 30 MHZ, fluidly moving from band, frequency to frequency with the press of a switch (see the thread "Carolina Cyclone".)
ALL antenna theory is the same meaning that the same electrical principles that apply to 11 Meters apply to 80 Meters.
So that would mean that I would have to have a switch and a
"certain" length of coax for EVERY band I operate--which is about 10 of 'em! Ergo, I would have a piece of coax about 60' long for 80M, then about 32' feet for 40 Meters, then 16' feet for 20 Meters, about 12' for 17 Meters, nearly 10 feet for 12 Meters, and 9 feet for 11 Meters......AND several others for operations on FEMA, etc! WHERE would I PUT all that coax in a little pickup? :shock: But I operate on ANY HF frequency I am authorized from 3.5 to 30 MHZ with ONE coax that *may* be about 7 feet long since the radio is remoted to the toolbox.
The mobile amp takes about 10 inches!!! The SWR is about 1.2 on average on ALL bands! Can't beat it. It tells me that "coax length" is NOT what "tunes" an antenna.
CWM
and the fact that you use the type of antenna that you do precludes you from dealing with any of what we're discussing here.
?????????
And why is THAT? I get it. CB radio is special and RF rules apply ONLY to CB radio, right? N O T! IF coax must be a "certain" length for 27.205, then it "must" be a certain length for 3915 KHZ. Coax length applies in ONLY a few special instances.
1) when there is insufficent ground or ground integrity
2) when you are dealing with non-metallic vehicles.
3) when the coax itself acts as the counterpoise and or ground.
In option # 3 the antenna is actually a dipole turned on its end where the coax is actually the entire OTHER half of the antenna.
IF the vehicle body has sufficient ground.
IF there is sufficient ground intregrity to secure an approximate 50 ohm match. (DC ground is often NOT the same as RF ground).
Then the ANTENNA itself can be resonated by lengthening and shorting the whip or changing the loading coil turns, and the FEEDPOINT match can be achieved with capacitance/resistance
via coils, toroids, or applicable matching device.
EXCEPTION: the "co-phasing" so popular with truckers.
All other "normal" HF antenna installations fall under these same rules. If 27 MHZ has to have "18 feet", then 3915 KHZ would have to have 120 feet (using a reference of 1/2 wave).
If it applies to one, it applies to the other. And the screwdriver is the quickest and truest way to debunk all this "I GOTTA HAVE 18 FEET of COAX NO MATTER WHAT". The durn thing would not work if what the "coax length" gurus say were true.
There are plenty of CBers that have found this out; it ain't just
"me"!
CWM
And why is THAT? I get it. CB radio is special and RF rules apply ONLY to CB radio, right? N O T! IF coax must be a "certain" length for 27.205, then it "must" be a certain length for 3915 KHZ. Coax length applies in ONLY a few special instances.
1) when there is insufficent ground or ground integrity
2) when you are dealing with non-metallic vehicles.
3) when the coax itself acts as the counterpoise and or ground.
In option # 3 the antenna is actually a dipole turned on its end where the coax is actually the entire OTHER half of the antenna.
IF the vehicle body has sufficient ground.
IF there is sufficient ground intregrity to secure an approximate 50 ohm match. (DC ground is often NOT the same as RF ground).
Then the ANTENNA itself can be resonated by lengthening and shorting the whip or changing the loading coil turns, and the FEEDPOINT match can be achieved with capacitance/resistance
via coils, toroids, or applicable matching device.
EXCEPTION: the "co-phasing" so popular with truckers.
All other "normal" HF antenna installations fall under these same rules. If 27 MHZ has to have "18 feet", then 3915 KHZ would have to have 120 feet (using a reference of 1/2 wave).
If it applies to one, it applies to the other. And the screwdriver is the quickest and truest way to debunk all this "I GOTTA HAVE 18 FEET of COAX NO MATTER WHAT". The durn thing would not work if what the "coax length" gurus say were true.
There are plenty of CBers that have found this out; it ain't just
"me"!
CWM
yes coax length matters..
it works like this...
the shortest and least you can use the better..
do not persay add coaz to get to a 1/4 or 1/5 or full wave or any such thing..
that is rubbish...
use only what is needed leaving a few inches to tops a foot more then needed..
then tune SWR to lowest you can..
Length of coax makes no difference in SWR
unless the coax is the antenna (and who would want to do that)
Later
it works like this...
the shortest and least you can use the better..
do not persay add coaz to get to a 1/4 or 1/5 or full wave or any such thing..
that is rubbish...
use only what is needed leaving a few inches to tops a foot more then needed..
then tune SWR to lowest you can..
Length of coax makes no difference in SWR
unless the coax is the antenna (and who would want to do that)
Later
KC,
Yer a man after me own heart! I've been doing this stuff since I was 15 years old and, you know what? I never even HEARD of "coax length" until I heard it from a CBer, and by then, I was 20 years old! I had been putting up antennas and experimenting with them for FIVE years and had not even heard of such a thing. I was like.....HUH?
So you are exactly right; it mattereth not except in some special circumstances. The "formula" for the "right" length of coax is "A length equal to the distance from the radio to the antenna, ideally as short as possible!!?"
73
CWM
Yer a man after me own heart!
I've been doing this stuff since I was 15 years old and, you know what? I never even HEARD of "coax length" until I heard it from a CBer, and by then, I was 20 years old! I had been putting up antennas and experimenting with them for FIVE years and had not even heard of such a thing. I was like.....HUH?So you are exactly right; it mattereth not except in some special circumstances. The "formula" for the "right" length of coax is "A length equal to the distance from the radio to the antenna, ideally as short as possible!!?"
73
CWM
Well I cut my Coax after reading all the info..It was 18" with a SWR of 1.5 . I cut the coax in the 3' increment reccommended to where it now is 12' my SWR increased to 2.0. with a little adjusting it is back to 1.5 . haven't tried another antenna Yet...So what can I say about the Myth. . The ears are working so much better.. Oh the Ant. is a FireStick that I had to cut to lower the SWR
I'm just a truck driver, but from what I've learned in the few years that I have been getting serious about my radio equipment, coax length only matters in certain circumstances.
If you have a ground plane problem or a slight mismatch, then the length of your coax will affect the reading you get on your meter. Look at what I said. It will affect the reading. . .not the actual swr level. When your reading changes significantly after changing coax length, that means you are fooling the meter into seeing a better match. The mismatch is still there. You will see a slight variation in swr reading when using different lengths of coax if your antenna system is fairly well tuned. But, if you have a 2.5 swr and get it down to 1.5 by changing your wire length, that 2.5 is still there. Take care of any grounding problems first, than try tuning the antenna.
If you have a ground plane problem or a slight mismatch, then the length of your coax will affect the reading you get on your meter. Look at what I said. It will affect the reading. . .not the actual swr level. When your reading changes significantly after changing coax length, that means you are fooling the meter into seeing a better match. The mismatch is still there. You will see a slight variation in swr reading when using different lengths of coax if your antenna system is fairly well tuned. But, if you have a 2.5 swr and get it down to 1.5 by changing your wire length, that 2.5 is still there. Take care of any grounding problems first, than try tuning the antenna.
Grounding problems = 0 I checked and cleaned all ground straps I could find. I ran a new ground from the Antenna Mount to the Frame. checked swr after and saw no difference. so I guess the mount was grounded pretty good. I understand about the RF ground not being worth 2 cents. open roof and antenna on drivers side with no trunk . So my ground plane is poor. I could try mounting the antenna on the hood maybe that would help. I am not a perfectionist because I can't afford to be.. 1.5 on Channel 5 thru 8 will get me out of the Woods..lol
spiderman5095 said:
Yep!
You can MAKE a meter see about anything! I can take a 5 watt, 50 ohm resistor and make your radio happy, right? It'll sit there all day and load into that resistor. Uh, it's called a dummy load, BTW. I can also take a small coil a toroid, or a capacitor and make the radio "see" a 50 ohm load and be very happy. Out there where the signal counts, you STILL have a mismatched antenna! I can take a so-called "tuner" and load the antenna; you still have a mismatched load; the radio sees a 50 ohm load, so it's happy. I can give a hypochondriac a sugar pill and tell 'im that it is a "cure" for something
It *may* make the patient happy and he thinks he is really "cured". So if your coax "cures" something, by all means......The coax length thing, I believe, was born on CB, lives on CB, and will remain there as CB "gospel" until the end. TUNE the antenna itself, insure the feedpoint impedance (easier to do these days with the analyzers), and don't give the coax a second thot! If you can "tune" the antenna with the coax, what do you NEED it for?
CWM