On 2019-10-12 8:18 p.m., W5DXP wrote:
From: QRP RX: The loss in the cable due standing wave happens with poor impedance match between receiver/cable for RX and cable/antenna for TX.
A poor impedance match is usually a bad idea for coax but a conjugate match using low loss parallel feedline is often acceptable. For instance, according to TLDetails, the matched line loss for 65 ft. of RG8x at 7 MHz is 0.5 dB. The loss in 65 ft. of 600 ohm parallel line at 7 MHz feeding a 50 ohm load is 0.2 dB even though the SWR on the 600 ohm feedline is 12:1. The loss in the impedance matched RG8x with an SWR of 1:1 is 2.5 times the loss in the impedance mismatched (but conjugately matched) 600 ohm feedline with an SWR of 12:1.
In hopes of making things a little clearer to the newly arrived in
the world of r.f. energy transfer...
Electrical energy, including r.f., one can say can be moved in the
form of current or in the form of e.m.f., the "push" on the energy, the
voltage. (Of course, there is always some of each in any such transfer.)
The reason that 600 Ohm open wire line, above, is so attractive is that
the energy transfer is being made by voltage. Increasing voltage means
decreasing current and so decreasing I2R losses. High powered short wave
stations used open wire technique for that very reason, r.f. energy had
to be "transported" a long way to get out to the vast antenna farms such
stations used. 600 Ohm and 1,200 Ohm spacings were used.
It's the same reason that hydroelectric installations, distant from
the consumer city, "transport" energy in the form of voltage rather than
current. Very high voltage brings it's own set of losses from leakage
across thousands of insulators, corona and other effects. It is the
business of engineers to figure the best balance between the savings
from low current and the losses from high voltage in the design of a
particular long distance power line.
In a radio station where a random length antenna is in use, the
impedance at the antenna feed point can vary hugely. A tuner at the
transmitter can be adjusted to match whatever impedance is seen at the
transmitter end of the open wire line. The standing waves on the open
wire line involve little current and so the losses to I2R, as noted in
the post above, are inconsiderable.
John
at radio station VE7AOV
