Worldwide DX Amateur Radio Forums - Ham - CB - HF - VHF - UHF

BOOTY MONSTER · #11 10-21-2009, 07:07 PM

Quote:

Originally Posted by Marconi

BM, this has been my experience. If the antenna is tuned perfectly at resonance and shows no reactance, then you will not likely have any transformation in the system and the feed line will appear transparent to the system. Of course the connectors have to be perfect too.

In most cases this does not happen and the feed line will be somewhat reactive. A lot depends on the amount of reactance at your choice of frequency (center) and then the feed line will become a part of the radiating system and then to some degree the longer the line the longer the radiator will appear and the lower the frequency. I am not sure if this condition is linear curve (/)or takes the form of a sinus curve (~).

Line theory is complicated and there is more to this issue, but that is my short story.

so with a less than perfectly tuned antenna the coax can become part of the antennas radiating system and throws the vswr off .... correct ? assuming properly constructed coax .

that makes sense . seems related to how some amps have to have a certian coax length because they are not properly tuned to a 50 ohm load . both examples of coax length affecting vswr fecause or a less than perfect vswr/ohm impedance at the end .
but it does clash with coax length supposedly suppose to not affect vswr .

W5LZ · #12 10-21-2009, 07:51 PM

Booty M',
"but it does clash with coax length supposedly suppose to not affect vswr"

Ah, but that not affecting SWR because of length has a 'catch' to it. That catch is that the load/antenna on the end of that coax is resonant and properly matched to the feed line's impedance. If the antenna's right, feed line length just doesn't matter.

Hang on, it get's even better!
Because reactance changes with frequency, if that reactance is chosen correctly and put in the right place, the further you get away from a resonant frequency, those changing reactances can make the whole thing -appear- to still be resonant and impedance matched. One particular antenna type that comes to mind very quickly that takes advantage of that is a 'Bazooka', and there are others that are common on the CB market, the one's that appear to have a low SWR from daylight to dark, broadbanded like the devil! Why? Cuz they are -not- being 'measured' with a fool proof meter. Think about it...
- 'Doc

Worldwide DX Amateur Radio Forums - Ham - CB - HF - VHF - UHF

Stinger length