[TowerTalk] Measuring resonance of a yagi element

Robert Harmon k6uj at pacbell.net
Wed Apr 22 10:51:51 EDT 2020


Good luck on getting the files.   They won't give you theiir Eznec files.   

Bob
K6UJ



> On Apr 22, 2020, at 6:38 AM, Chuck Dietz <w5prchuck at gmail.com> wrote:
> 
> 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 at 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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