If we assume that the line is not resistive and is capacitive thus having a high SWR...
First we have to get the terms right: the line
isn't capacitive, though the load
impedance attached to it might be.
Transmission lines have a characteristic impedance
: when terminated in this
impedance, the current and voltage are the same all along the line. When terminated
in a different load impedance, the impedance (the ratio of the voltage to the current)
will vary along the line. The effect is that the voltage and/or current (at different points)
are higher then the matched condition: most of the feedline loss is due to increased
current flowing through the same resistance.
Transmission lines are sometimes seen as capacitive at very low frequencies, such as the
AM BC band, where the total length is much less than half a wavelength. In that case
you can analyze short pieces of coax as being capacitive if the load impedance is higher
than the characteristic impedance. But that's just an approximation, and it falls apart
as the coax length gets longer with respect to the operating wavelength. It is rarely
a practical means of analysis for HF antennas.
can anyone explain to me the theory behind why there are "losses" in the feedline WHEN there IS a tuner inline, the SWRs are 1:1 AND the transceiver is putting out full power (since there is no fold back due to high SWR)?
Quite simple: if the tuner is at the shack, the SWR on the coax from the tuner to the
antenna is still operating at a high SWR, regardless of the tuner adjustments and transmitter
output power. The tuner just reduces the SWR on the coax between the tuner and the rig,
presenting a favorable impedance for the transmitter to deliver power into.
There are two main sources of loss in a transmission line: the inherent loss of the matched
line due mostly to the RF resistance of the conductors (which varies with frequency), and the
additional loss in the line due to high SWR, due to the higher currents required to transfer
the same amount of power.
Even when the far (antenna) end of the coax is open or shorted, there will still be some
finite impedance at the shack end of the coax due to losses: for example, with 100 feet
of shorted RG-213 on 20m, the impedance at the shack is about 14+j73 ohms. This is well
within the matching range of many tuners. So you can adjust the tuner and your rig will see
a perfect 1 : 1 SWR and deliver full power output. Since there is no load connected on the
far end of the coax, all that power will be dissipated as heat either in the tuner or in the
feedline. Obviously the losses in the feedline are very high, even though the rig sees a
That's why you can't use the ability to match your antenna with a tuner as any indication
of how well your antenna is working.
If we assume the above, does this have an affect on receive?
The receive case is slightly different since the load that the feedline sees is the input
impedance of the receiver transformed by the tuner settings. But when the receiver
input is close to 50 ohms, conditions will generally be similar for transmit and receive.
That's why we often preset our tuners for best receive signal before keying the
transmitter - it allows us to find approximate tuner settings.