| Author |
Message |
    
Kruser
New member
Username: Kruser
Post Number: 1
Registered: 4-2005
| | Posted on Saturday, April 09, 2005 - 2:55 am: | 
|
Ok, for years I have heard that cutting coax or adding coax does nothing for the actual SWR of an antenna system. It only fools the meter. I won't argue that this is true. But, if a meter is what you are using to find the lowest SWR, then do we want a fooled meter? What is the difference between a fooled meter and not fooled meter? Do you just tune for the lowest reading and say the rest is the foolisness? Maybe I need to stay at Holiday Inn Express or something! Any thoughts out there? |
Wendy
Member
Username: Wendy
Post Number: 99
Registered: 6-2004
| | Posted on Saturday, April 09, 2005 - 8:39 am: |
|
The hurricane damaged my antenna. I put up a new antron 99 antenna and coax (the big heavy kind).
My swr's were all out of wack couldn't get them down. My sadelta mike squealed like crazy. Several feet of extra coax cable was added and the swr's were good and the mike squeal was gone. I know nothing except to push the button and talk. Someone else does anything else for me. |
Hotwire
Intermediate Member
Username: Hotwire
Post Number: 270
Registered: 1-2005
| | Posted on Saturday, April 09, 2005 - 9:02 am: |
|
When that happens it means that somewhere something is wrong. Most of the time you will have a bad ground or your antenna is way off tune. If all is well you should get the same reading regardless of coax length. |
Redwolf_145
Junior Member
Username: Redwolf_145
Post Number: 15
Registered: 4-2005
| | Posted on Saturday, April 09, 2005 - 2:49 pm: |
|
i don't agree... I Beleave Coax has everything to do with your SWR.... And How Do you know if your SWR is Done... Cause if your meter is fooled then there would be no way to tell if it is really down... |
Bruce
Senior Member
Username: Bruce
Post Number: 2449
Registered: 9-2003
| | Posted on Saturday, April 09, 2005 - 3:56 pm: |
|
Redwolf
Coax is a transmission line VSWR is the MISSMATCH at the antenna. TRUE you can use your coax to help match things but the fact remains the VSWR is still there. FIX THE ANTENNA and your meter will show the change no matter how much coax you use. |
Dx431
Senior Member
Username: Dx431
Post Number: 1005
Registered: 2-2002
| | Posted on Saturday, April 09, 2005 - 5:08 pm: |
|
I will have to agree with Bruce on this one. |
Racer_x
Intermediate Member
Username: Racer_x
Post Number: 296
Registered: 2-2005
| | Posted on Sunday, April 10, 2005 - 2:23 am: |
|
I agree with Bruce but add this - RF propogates through coax just like it does through the antenna and through the ether. Because of this there are points of high and low current, and high and low voltage. That is how it it possible to use coax as a matching transformer, and it's also how you get different VSWR readings with different lengths of coax.
To get a true representation of the antenna's match at the radio's end of the coax, the coax must be in 1/2 wavelength multiples. For folks that use different bands (amateur radio operators) this is not practical, but for CB operators it can be practical and even useful. |
Scrapiron63
Advanced Member
Username: Scrapiron63
Post Number: 820
Registered: 12-2001
| | Posted on Sunday, April 10, 2005 - 10:35 am: |
|
Y'all still didn't answer his question about the happy or unhappy meter. And how do you set up an antenna without using some length of coax and a meter, and does that coax come in play.
You can use an Analyzer, but still need some length of coax.
I bought a new MFJ 259 analyzer a few years ago, keep it untill after helping a ham friend set up his antennas, he wanted it worst than I did. And really for just working with antennas for one freq like the CB band, a 10 dollar swr meter will work fine.
The reason I mention the analyzer, I would use it to set up base antennas, for myself and friends, with a piece of coax long enough to be away from the antenna so not to interfer, that piece which I still have is 36 feet. I was told long ago that coax worked better in multiples of 3, if that is right or wrong, I don't know but it's worked for me, happy meter syndrome maybe.
After setting an antenna up like this, almost always when it was installed on the tower or whatever with the length of coax the owner had installed, the swr readings would change, sometimes for the worst. We could trim the coax and improve the match and make the meter/analyzer happy, or work on the antenna matching device to do the same, which was hard to get to sometimes.
So which is better, and if you change the antenna matching device, are you not matching it to the coax.
Guess I should add also, the instructions for one antenna I remember, the PDL II, advises using multiples of 4 feet for coax, so they thought coax length comes in play. |
Rover
Intermediate Member
Username: Rover
Post Number: 160
Registered: 2-2004
| | Posted on Sunday, April 10, 2005 - 9:18 pm: |
|
The correct way to "tune" an antenna is AT the antenna itself. If the antenna is resonated (tuned to the proper frequency) and its impedance is matched AT the feedpoint, the antenna will work.
I know that people will try to argue me down on this, but there is simply no reason to diddle with coax in order to "match" an antenna. Think of it this way. First, remember that if coax length is specific to 27 MHZ (3, 6, 9, 12, or 18 feet, etc), then the rules that govern this apply to ALL frequencies. Thus, referenced to a 1/2" wave dipole, at a frequency of 1.850 MHZ, and according to well-used antenna theory, I must have
TWO HUNDRED & SIXTY FEET of coax to "match" the antenna! Now that presents a logistics, practical, and cosmetic problem. Where will I KEEP this 260 feet of coax? How do I account for anomolies related to where I must mount the antenna in relation to the station? How about multi-band operation? Does it mean that I have to have an antenna for each frequency and 8, 10, 15, 20 separate coaxes, each cut to a "certain" length? I'd be tripping over all those coaxes.
Thankfully, I DON'T have to do this, and the reason is, for the most part, coax length is smoke and mirrors. In my ham station, for example, I am using a 120' dipole and an A99 for all-band coverage by way of an LDG tuner (I don't like tuners, but that is another story). I have two feedlines (one is ladderline) into the house and neither are cut to a "certain" length.......unless you count the length it took to get from the antenna to the radio and a "certain" length;)
IMHO, trimming coax is a waste of time. If you want to trim coax to a certain length for 27 MHZ, and it works, do what you wish. Chances are, it would have worked with a random length and you would have finished the job a hour or two earlier! ;)
73 |
Racer_x
Intermediate Member
Username: Racer_x
Post Number: 306
Registered: 2-2005
| | Posted on Monday, April 11, 2005 - 6:06 pm: |
|
Rover - You miss the point. Coax CAN be used as a matching transformer and it CAN be used to show an accurate VSWR at the radio end of the coax, but it's not necessary.
At CB frequencies the length of a 1/2 wave section of coax - depending on velocity factor - is 11 1/4' to 13 1/2'. That's not so long that it would effect the signal or be inconvenient. And it's cheaper than a tuner. |
Rover
Intermediate Member
Username: Rover
Post Number: 165
Registered: 2-2004
| | Posted on Monday, April 11, 2005 - 10:34 pm: |
|
And the only reason for using a tuner is for a multitude of frequencies. A tuner isn't the best way, but sometimes compromise in order. When you have to be able to operate on just about ANY HF frequency at a moment's notice, then the tuner comes into play. I don't LIKE tuners, but sometimes ya gots to do whatcha gots to do! 
73 |
|
