I think I'd phrase things a bit differently than a number of posts.
What ever the height, the VNA should give accurate SWR readings for
"that height"
From what I've seen and read, on "the log" all elements are active and
the antenna is quite sensitive to both height and mounting angle. As
Paul said, The closer to the final height, the more accurate the
readings will be for that height.
All antennas are a compromise, or group of compromises. To get gain and
a good F/B on all it's bands takes a BIG and heavy antenna. The Log
make a lot of sacrifices for bandwidth.
The VNA is an excellent tool that when properly calibrated will give
readings to an accuracy far greater than is useful. It's interesting to
watch the Resistance and SWR change on a windy day when you can see
parameters to 3 decimal places. From a practical point we are only
interested in the digits to the LEFT of the decimal.
Each of the two stations here have a VNA wired in so I can adjust the
antenna, or line tuner for a 1:1 SWR, tune the amp into the dummy load,
switch to the antenna, and never put out more than a tiny signal for the
VNA. The only real problems are needing compensation in the dummy load
for the higher bands and the bypass circuit.
73,
Roger (K8RI)
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
Of Doug Ronald
Sent: Friday, September 25, 2015 8:16 PM
To:TowerTalk@contesting.com
Subject: [TowerTalk] Expected VSWR of antenna near the ground
Importance: Low
I have assembled my 19 element LPDA with the front 12 elements about
3 feet off the ground and parallel to the ground. The back elements
are laying on the ground, but electrically connected in
the usual LPDA way. Today, as a check before I erect it, I swept with a VNA,
the antenna from 1 MHz through 32 MHz. The VSWR is horrible!
<snip>
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