On 4/22/20 1:38 AM, Michael Tope wrote:
Tom,
The proximity of the element to ground will shift the resonance, so
you'll need to raise the element you're testing high enough above the
ground that this detuning is no longer a significant source of error.
You can use EZNEC to estimate this height.
Depending on the feedpoint configuration of the driven element, you may
be able to use it as a test control to evaluate different coupling
schemes for the reflector test. If you have something like the Nano-VNA
or one of the various other low-cost VNAs that support short-open-load
calibration, you could calibrate at the end of a 50 ohm coax cable (one
long enough to reach the element under test when it is raised high
enough above the ground to minimize detuning) and then measure the
resonant frequency of the driven element. That would be your control
standard.
You could then jumper across the driven element feedpoint to mimic the
continuous reflector and then setup your "non-invasive" coupling scheme
for comparison against the VNA test.
Clever idea to get a feel for the test uncertainty.
Whatever you do, it's going to be a lot of work if you want to get a
result that is more accurate than the simulation with EZNEC combined
with careful measurement of the final reflector assembly length. In the
EZNEC simulation, just how sensitive is the performance to reflector
length? Are we talking a few inches or fractions of inches? Are you
worried about modeling errors or errors in the physical measurement of
the overall element length when put the pieces of tubing together?
This is the most important part - the model can easily tell you the
sensitivity. In fact, that's one of the useful things about modeling -
it can tell you if the design is "picky" - if your 40m Yagi has element
lengths that are 1/2" critical, I'd wonder about whether the design
allowed for movement of the elements in the wind, and with gravity
drooping them.
It's a bit tricky to relate the resonance as found in the model to the
resonance expected measuring it. Let's say you replace the driven
element feed point with a 50 ohm load, and you move the excitation to
the reflector. You can run the sweep in the model, and get a resonance.
And you could even run your model at stepladder height, or Yagi pointing
vertical. But how sensitive is the resonance to soil properties. Most
of us don't have a helicopter to suspend our antenna from a 377
ohm/square rope.
The slope of X is pretty steep around resonance - parasitic L or C from
your measurement system will throw off the "frequency where X is zero".
If you are worried about validating the modeling process, you could
first do that with the driven element (where it is easy to make the
measurement) by comparing the predicted resonant frequency with the
measured resonant frequency. If there is good agreement, then you could
use the modeling results to build the reflector with confidence and just
skip the more difficult electrical measurement of the reflector.
To be honest, if I measured numbers different than the model, and the
model is of reasonable fidelity, I'd trust the model more than the
measurement - especially given the complexity of measuring it without
parasitic effects. I have pictures in my mind, now, of a Yagi with
shorting relays and nanoVNAs in the middle of each of the elements, all
mounted on a really tall tower, etc.
People DO measure the resonances (plural for a reason) on a trapped
yagi, mostly because its a good check that you put the correct trap on
the correct element. Sometimes, the traps all look the same, but are
different inside. Or, as a quick check that you got the lengths of the
pieces between the traps correct. Or that you put the right element in
the right place. Yes, people DO assemble antennas with the elements in
the wrong order, and a lot of feed systems will clamp on a element that
is too long or too short just as well as the one that's just right.
The antennas with multiple interlaced elements (C3, for instance) are
pretty easy to get mixed up. But you don't need to measure resonances
to check it. You need a tape measure and the list of lengths and
positions. I suppose I can imagine a field day scenario where someone
had marked all the elements with tape or paint (1 band for director, 2
for driven, 3 for reflector, different colors for different bands) and
the tape fell off or paint faded.
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