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.
Another thing to consider is getting an extra section of aluminum tubing
that is the exact same size as the center section of the reflector. This
piece of tubing would be sacrificial. You could cut it in the center and
insulate the two halves. Then you could substitute this insulated piece
for the continuous center section of tubing being careful to keep all
the dimensions the same. It wouldn't have to be super robust
mechanically since presumably you won't be running this test in 100 MPH
winds with 3/4" radial ice on the elements. It would just have to be
strong enough survive being raise up in the air. You could probably get
by with something like an old broom handle and couple of hose clamps.
With this substitute center section, you could do a straightforward
impedance measurement with the VNA.
If you think the element to boom clamp has a significant effect on
tuning, you could include the clamp and a short piece of tubing that is
the same OD as the boom.
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?
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.
All my experience with Yagi's is assembling to the manufacturer's
dimensions. I haven't done any design projects where I went from
simulation all the way through to assembly, so I could be wrong, but I
was under the impression that most Yagi builders (at least those
building mono-banders with full size elements) are going straight from
simulation to physical assembly without making these sorts of individual
measurements. It would be interesting to hear what experienced builders
of high performance Yagi's have to say on this matter.
73, Mike W4EF...............
On 4/21/2020 6:02 PM, Tom Hellem wrote:
I would like to measure the self-resonant frequency of the reflector on a
home made 20 meter yagi. It seems obvious the best way would probably be to
open it up at the center and make it into a split dipole element and hook
up my analyzer. However I would like to avoid cutting it and dealing with
splicing it back together again. Can anyone suggest another way to
accomplish my objective? The reason for doing this is because it is
apparent from modeling in EZNEC that the
length of the reflector is critical to the performance parameters of the
antenna, e.g. gain, F/B, bandwith, etc., and it looks to me like it doesn't
take much of a change in the length of the element
Thanks in advance for any replies.
Tom K0SN
Missoula, MT
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