An AIM in the shack can do all that for you without ever leaving the rig
and it will do it to two or three decimal places. An accuracy that is
way beyond being useful. .
The difference between 50 ohms and real life coax is close enough that
it really doesn't need to be taken into consideration.
The AIM can show you impedance bumps, SWR, loss, return loss, Z, R,
etc. Even with a dipole the AIM will display changes in parameters
caused by the wind moving the antenna, trees, and any nearby objects.
Save the initial traces, add an antenna and overlay the traces to see
changes.
The AV640 vertical is a great working antenna, but is very sensitive to
nearby antennas. I found a 40 meter dipole over 50 feet distant made
substantial changes as shown by the AIM. It gives far more information
than SWR, but for most of us, the SWR as seen in the shack is good
enough for who it's for.
I knew the real SWR was at the antenna, but for me and my rig the SWR as
seen in the shack was what was important.
A tuner in the shack only changes what the rig sees, which is usually
good enough.
True, if you want every last watt out of the antenna that the coax can
deliver then it needs to be a perfect match at the feed point.
Unfortunately a simple antenna like a dipole is unlikely going to be 50
ohms,
However, there is likely more loss in a tuner at the feed point between
the 2:1, or possibly 3:1 SWR, points than in the mismatch.
In my case, I do use good, low loss coax, with exceptions. Most runs
from the rigs to the common point ground (CPG), the Polyphasers, the
6-pack, the grounding at the bottom and top of the tower, the rotator
loop and the antenna may result in as many as 10 connectors in over 200
feet. The loss in all those connectors does add up, but on 160 through
10 the total is still minuscule (even with UHF connectors).
OTOH I even use RG-8X (on 40) to the sloping dipoles.
If I ran QRP, I might be more concerned, but then, I'd not be running
SO2R with all the switching and connectors.
73
Roger (K8RI)
On 6/1/2016 Wednesday 7:29 PM, Roger D Johnson wrote:
Real coax is seldom exactly 50 Ohms
Real coax has a small amount of reactance
Real coax has loss
If you don't do a SOL calibration at the antenna end you're just
guessing!
73, Roger
On 6/1/16 1:20 PM, Hans Hammarquist via TowerTalk wrote:
When you want to measure the "real impedance" at the antenna, isn't
it important that you have a feeder (coax) that has the correct
Zo=50 ohms all the way?
I guess that the bridge will not read a correct antenna Z if the
coax is not perfect.
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73
Roger (K8RI)
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