[TowerTalk] Measuring resonance of a yagi element

[Michael Tope]

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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