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G8DLH wrote:Ok - a bit of a basic one this - but relevant, nonetheless . . .
TX requires a 50-ohm load. I have a half-wave dipole, whose feed impedance is 75-ohms.
Do I use 50-ohm coax - and live with the mis-match at the antenna end ?
Do I use 75-ohm co-ax - and accept the mis-match at the TX end ?
Does the mis-match really matter anyway ?
And if it does, should I use a matching device? If yes, what ?
Your thoughts on this, please.
Al / G8DLH
pallen wrote:... A given transmission line has its least possible RF loss per unit length when it is terminated in its characteristic impedance.
John Cuthber wrote:Power losses in a transmission line increase as SWR increases, because current and voltage down the line both oscillate above and below the values they would have with zero SWR and P=I^R and P=E^2/R. The nonlinear power relationships that power losses have to both current and voltage cause the increased losses in the high current and voltage sections of line to exceed the reduced losses in the low current and voltage sections, causing the net power loss to increase with increases in SWR.
John Cuthber wrote:Power can be lost from RF transmission lines in these ways:
It is true that RF line series resistance tends to relatively low and that conductor-to-conductor line RF leakage resistance tends to be relatively high in most practical RF transmission line applications throughout the HF portion of the spectrum. Even so, nearly all the line loss that does occur is due to those two causes and power lost due to those two causes increase with increases in line standing wave ratio.
pallen wrote:Most baluns have 1 to 4 impedance transformation ratios, such as from 75 to 300 ohms. However, a simple 50 to 75-ohm impedance transforming balun can be made by winding separate primary and secondary windings on a toroidial core. The impedance ratio of a transformer is equal to the square of its turns ratio, so a primary to secondary turns ratio of 1 to 1.225 will transform from 50 to 75 ohms. That exact ratio can't be achieved without using more turns than an HF transformer with a toroidial core should have, but a 1 to 1.2 turns ratio is practical and will transform 50 ohms to 72 ohms, which is plenty close enough.
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