Posted by
Jim Brown-10 on
Sep 16, 2019; 5:14am
URL: http://elecraft.85.s1.nabble.com/Reflected-power-tp7655160p7655173.html
I disagree, Vic. He said "the return currents are higher." SWR describes
what is going on INSIDE the transmission line -- the differential
current. Common mode current is OUTSIDE the coax (or, in the case of
2-wire line, the difference between the current on the two conductors.
Now, common mode current CAN de-tune the antenna to the extent that the
feedline becomes part of the antenna, but the resulting SWR would be
correctly measured by the LP100A. It might NOT be correctly measured by
the RigExpert, because it breaks up the common mode circuit.
Further -- standing waves on the feedline exist inside the feedline (or
between the conductors), and voltage and when there is SWR, current and
voltage vary along the line in a manner determined by the VF of the
feedline. But in the common mode circuit, the feedline acts as a single
wire with a VF near 0.98, depending on diameter and insulation, and
common mode current and voltage vary along the line just as they would
on any other antenna.
73, Jim K9YC
On 9/15/2019 9:53 PM, Vic Rosenthal wrote:
> He didn’t say common mode current WAS reflected power. He said that common mode current can cause incorrect SWR measurements, and that is true.
>
> Victor 4X6GP
>
>> On 16 Sep 2019, at 7:37, Jim Brown<
[hidden email]> wrote:
>>
>>> On 9/15/2019 6:05 PM, Michael Walker wrote:
>>> Steve
>>> 40 seems to be the killer band for weird things. Dave's comments are the
>>> first place to look. Try also changing jumper cables.
>>> What also could be happening is RF flowing back on feedline in a common
>>> mode. This RF gets measured as return energy and now, since the power is
>>> higher, the return currents are higher and hence the high SWR reading.
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