|[AMRadio] antenna tuners|
k4kyv at hotmail.com
Thu Apr 20 12:58:06 EDT 2006
>SWR is not all that bad either as long as the transmitter sees a match,
>especially the solid state rigs...And no, high swr
>on a feed line will not cause it to radiate.
>What causes feed line radiation is an unbalance between the two wires in
>High SWR on coax line will cause a little more loss in the feed line due to
>the higher currents involved across the lower impedance of the coax. That
>why when using open wire line that is 400 to 600 ohms, there is much less
>loss. The same power across a higher impedance means less current and less
>current going through the feed line wire means less power loss.
Another factor causing loss with a high SWR is dielectric losses at the
recurring high rf voltage points along the line. At low impedances, it is
the resistive loss in the wire, and at high impedances, it is dielectric
losses that combine to cause signal loss. But SWR is much, much less
critical than most hams have been led to believe.
>An antenna tuner just matches the impedance seen at the transmitter end of
>the feed line to the 50 ohm transmitter output. If there is high swr on the
>feed line, any power reflected from the antenna is not wasted it is just
>re-reflected back to the antenna and eventually gets radiated.
>At HF even fairly high swr on coax lines does not cause excessive loss.
My 160m. vertical uses a 140' run of buried RG-213 from the shack to the
antenna tuner at the base of the tower. I set the L-network to match the
tower 1:1 SWR at 1900 kc and locked it down. The SWR is about 2.5:1 at 1800
kc and the same at 2000 kc. I have measured the rf power input to the tower
using a General Radio antenna impedance bridge and thermocouple rf ammeter,
and at the same DC input power to the final amplifier, I could not detect
any significant difference in rf power at the antenna end of the feedline,
across the entire band. Of course at each point I had to re-measure the
base impedance of the tower, note the rf ammeter readings, and re-calculate
per ohm's law, so I do not guarantee precision measurements, but the
ballpark power was consistent across the band, with no evidence of excessive
loss at the top and bottom edges
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