I say get an MFJ ANALYZER and tune the FRIGGIN ANTENNA!!!!!!!!!!!!!!!

sdorry just could not resist..

Bobby

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I KNEW somebody would come up with the 'holier than thou' thing! Still, I was simply trying to point out that so many of the CB guys are fed this myth of "coax" length, BELIEVE it unquestioningly AND take high offense if this myth is questioned!!! OTH, when you read the ham forums, you rarely run across such a discussion. However, some of us will point out various manuals and publications that would appear to support the "coax length' myth. What they DON'T tell you is, these discourses are, for the most part, applicable to specific antenna designs, beams, verticals, phased arrays and other systems that actually are designed to use the feedline as PART of the radiating (or counterpoise as the case may be) system.
For a normal radio installation, the "formula" for coax length is "a length directly porportional to the distance from the radio to the antenna"!

CWM

now on a "set" of antennas be they co-phased francis amazers for 11M on a big truck or my stacked 2M halos length of the 75 ohm coax in your "co-phase harness" DOES matter......correct???


when i put up my halos i had made each "leg" of my harness an odd multiple of 1/4 wave

in my particular case i used 3/4 wavelength on my stubs to have plenty of harness to reach the halos after separating them 1/2 wave

did i do it correctly or did i waste my time??

im not trying to be a smartass here,,,this is a genuine question

scott

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Hey, "holy one", , what's you opinion on Art Bells big loop.

I have googled it, but is it static that made him have 400V, pretty much constant, on his huge antenna. He seemed to have no real answer..

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nevermind

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This is the book that i bought maybe 15 years ago that started to change my mind on so many myths from cb shops and truckdrivers (which i am one). Best 20 something dollars i spent.

I was allways told to avoid multable 1/4 wave lenths of coax when feeding a dipole.

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Bioman,
No, you are not doing it wrong.

Some facts.
The length of feed line used can be used for impedance transformation. That's good in that there aren't a zillion 'sizes' of coax to match whatever an antenna's input impedances. It's bad in that that impedance transformation can, and sometimes does, produce voltages/currents that will harm the feed line doing the transformation. It's a very quick-n-dirty way of doing it. Emphases on the 'dirty' part. There really are 'better' ways of doing it. Not simpler, or easier, but better, the idea being to get the most signal to the antenna to be radiated without having a lot of signal lost in the process. Right? Notice I didn't say coaxial feed line? Any feed line can be used for this "adjustable length for impedance transformation" thingy, not just coax, which just happens to be the most common, and certainly easier to use in mobile applications (reason for it's invention, WW2, aircraft).

If the impedance seen at one end of the feed line is the same as on the other end of the feed line then there's no 'restrictions' as to length of that feed line, it'll work just fine. (For practical purposes, if what's 'seen' is at least 'close', it's doing as well as can be expected.) There are particular lengths that have certain characteristics that make them usable for particular things. But that means that those impedances on both ends of that section of coax are not the same. If you can make those impedances the same, then the feed line doesn't have to do something that it wasn't designed to do in the first place and probably can't handle in some circumstances.
What about when those impedances are matched just fine in two separate antenna systems and you want to combine those two systems into one system (dual antennas). Think parallel circuits, usually half the impedance is the result if both of those systems are the same. How about using a "step up" impedance transformer? For coax, since both are unbalanced, and you wanna go to another unbalanced feed line, how about an 'Unun' (unbalanced 'balun' sort of)? Hmm... won't find them on the shelves anywhere, so just use 75 ohm coax as is typically done. Or just make the @#$ thing!

This is getting way off of topic so I'll quit. Sorry 'bout that.
- 'Doc

CW Morse while trying to understand your original post it is still way over my head. Are you saying regardless of coax length, whether it be 10' or 36', the antenna will still tune the same via the whip length? The SWRssssss would still tune down to the lowest point regardless of the coax length?

What is funny is when I was told to make coax lengths in multiples of 18' for my truck, it didn't matter what length coax I used for my base!! I set my Vector to 131 inches or what ever the length was and it had a perfectly low SWR reading.

If the actual feedpoint impedance of an antenna (mobile or fixed station) is 50 ohms, and your coax has a characteristic impedance of 50 ohms, then you can use any length of coax without problems. Many if not most mobile antennas do NOT have feedpoint impedances at or even near 50 ohms. 25-30 is typical. Here is where the coax length can make the transmitter feel more comfortable.

However, since SWR is the ratio between feedpoint impedance and coax impedance, you'll still have the SWR and the resultant loss ON THE FEEDLINE. The transmitter's happy, even though there may be a significant impedance difference between the feedpoint and the coax.

How to cure it? Get the antenna closer to 50 ohms.

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