Hi Tom,
I don't think you are taking the right approach. I don't think you can just
"tune" one element of a Yagi antenna. You need to measure all the element
sections and placements including the element mounts and run them through a
computer design program. Changing any dimension will affect the other
dimensions to some degree. If it is a commercially sold Yagi, the file will
already be available and can be used to optimize the antenna.
Chuck W5PR
On Wed, Apr 22, 2020 at 3:38 AM Michael Tope <W4EF@dellroy.com> 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.
>
> 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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