On 10/2/2015 8:14 AM, Jeff wrote:
On 02/10/2015 07:47, Jeff wrote:

However, in Wayne's situation, the length of the coax is only 15'. On
14MHz, that's just over a quarterwave (taking the velocity factor into
account). But even with a horrendous SWR, how much loss does this length
of 213 coax have? It might be a lot more convenient to do all the
matching in the warmth and comfort of the shack.

You still have to bear in mind that the 20-j130 load gives something
like a 20:1 mismatch at the interface between the coax and the load.

This means that only about 17% of your froward power will reach the
antenna on its 1st trip up and down the coax, the other 83% will be
reflected back down the coax towards the ATU and suffer the loos in the
coax again.

This will be repeated again and again, with 83% of what is re-reflected
being re-reflected again, and so on. So you can see that even with a
small loss in the coax if is amplified by the multiple trips up and down
the coax. This higher the VSWR the more power is re-reflected to undergo
multiple losses.

Jeff

Having done a few quick calculations; with a vswr of 20:1 at the
antenna, and a feeder having a loss of 0.5dB, and an ATU matching at the
Tx end, the total power wasted in the cable with a 100W transmitter is
over 55W, with only about 45W actually reaching the antenna.

Even reducing the cable loss to 0.1dB results in about 20W being lost in
the cable.

Reducing the VSWR reduces the loss significantly.

What is the reflection coefficient at the ATU/feedline interface?

--

Rick