low pass filter or low swr?

[Shockwave]

Sorry Bob but the "cb myth" here is that changing coax length fixes the problem. If there is an impedance mis-match with a device you must correct the problem not add coax. This topic usually comes up when someone adds an amplifier and the SWR goes up. The band aid fix is always fiddle with jumper length. That's all I am saying Bob, i'm no X-pert. Take care.

Bob was quoteing Midnight Special in that first sentance and does not agree with what was stated there. He knows the mismatch would still be present.

 

[bob85]

people keep getting confused with impedance looking into a mismatched coax and vswr along the same mismatched coax,
we must have gone over this a dozen times on this forum, it should be the top sticky in ham and cb antenna sections since it is the most misunderstood topic.
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just because you can manipulate impedance and measured resonant frequency looking into your coax by using the transformer action of coax terminated with an impedance other than its own characteristic impedance that does not imply that you are changing vswr by the same mechanism,

vswr in coax free from cm currents on the braid gives a gradual reduction in vswr the further from the load you place the meter due to loss in the coax,
you can place the meter anywhere in the coax to take readings, the meter reads the vswr at that point in the coax,
there are no magic lengths that give a more accurate reading due to any transformer or repeater action of a mismatched coax such as the often suggested electrical 1/2wave of coax,

1/2waves of coax repeat the terminating impedance looking into the coax at the radio end, it has nothing to do with vswr on coax free from cm currents other than the gradual reduction in vswr due to coax loss the further from the load you place the meter,

inserting an electrical 1/2wave of low loss coax including the line section length of the meter has the advantage of allowing you to remove the meter & jumper once you are done without upsetting the conditions looking into the coax from the radio end,

odd 1/4wave multiples of coax invert the terminating impedance, it does not change vswr on coax free from cm currents,

providing you do not have cm currents on the braid all you need to know to determine vswr at the antenna
( within the accuracy of the meter ) is the vswr at the point of measurement and how much loss you have in the coax between meter and antenna,

don't take my word on it, go read birds detailed white paper or walt maxwells reflections for an explanation in laymens terms,
if that's too much like hard graft just read mfj's instructions for analyzers,
never take what you read on forums as gospel, go look it up and do your own experiments,

http://bird-electronic.com/en/Resou...-Measurements-Using-In-Line-Power-Meters.ashx

http://homepages.ipact.nl/~pa1are/tuner/reflections.pdf

http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf

you cannot manipulate vswr by playing with your coax length unless you have common mode currents on the braid a faulty meter or a meter not calibrated to the coax characteristic impedance,

when you have cm currents on the braid changing coax length can change vswr significantly,

since common mode impedance is seen in parallel with the load you are actually changing the terminating impedance ( antenna + braid cm impedance ) by playing with coax length which changes vswr,

it is not fooling the meter,

you are not using transformer action of mismatched coax to change vswr,

when the load is not matched to the coax that does not cause vswr to change with coax length ( unless you have cm currents on the braid )

vswr is not a ratio of transmitter to antenna impedance,

coax is not a two conductor cable, it has 3 conductors including the outside surface of the braid,

the magnitude of standing waves inside the coax are determined only by the ratio of the coax characteristic impedance to load impedance.