[TowerTalk] Assembling a Yagi

[Art Greenberg]

Doug,

I'm pretty sure that won't work. The matching network can likely be 
adjusted for a good SWR even if the element lengths are slightly off, 
especially if its one of the directors.

IOW, there isn't necessarily a strong relationship between SWR and 
radiation antenna pattern.

And the whole point of getting the element lengths correct is so the 
radiation pattern is correct.

The EZ-NEC manual more or less says that one can ignore a conductive 
boom at the center of connected yagi elements when the elements are near 
1/2 lambda in length and the boom diameter is a small fraction of that. 
I think that answers my question -- the measurement should be from the 
center of the boom.


On 1/27/23 07:20, Doug Christensen wrote:
> I would assemble it and check where the "dip" is in SWR. This is the only
> real world measurement any antenna tech can make. Be sure to raise the
> assembled array as high as you can and measure where the dip is. Ground
> measurements should be made at least 2 wavelengths away from other
> obstacles.
> 
> 73
> Doug
> WB1E
> 
> On Fri, Jan 27, 2023 at 5:42 AM Art Greenberg <art at artg.tv> wrote:
> 
>> The antenna is a 6 element 6m yagi, VB-66DX. The instructions are a
>> photocopy of a photocopy, I think. The drawing is too small to see
>> sufficient detail.
>>
>> I have an earlier version of this antenna, the 6m "Long John", and for
>> that the elements were indeed pre-cut. There was only a little room for
>> adjustment, that being the depth into the element bracket that the
>> element was inserted.
>>
>> This antenna has a short section of 7/16 dia. tubing at the boom end of
>> each element, and the 7/16 dia. tubing telescopes into that. That change
>> provides much more room for adjustment. I presume the change also
>> lengthens the life of the elements.
>>
>>
>> On 1/26/23 23:00, Jim W7RY wrote:
>>> Most hygain antennas use a predetermined length of tubing at each
>>> element. The measurements are usually  just for the joints after the
>>> first piece of aluminum that is hooked to the boom.
>>>
>>> But, this TH-6 diagram shows the lengths of the pieces against the boom.
>>>
>>> What do the instructions show?
>>>
>>>
>>> 73, Jim W7RY
>>>
>>>
>>>
>>> On 1/26/2023 4:26 PM, Art Greenberg wrote:
>>>> I should proof read before posting. The DE is insulated from the boom.
>>>> Fed with a beta match. Its a Hy-Gain antenna.
>>>>
>>>>
>>>> On 1/26/23 17:24, Art Greenberg wrote:
>>>>> I think I left out an important detail. Elements are all connected to
>>>>> the boom. Even the DE, fed with a "beta" match.
>>>>>
>>>>>
>>>>> On 1/26/23 17:22, Doug Christensen wrote:
>>>>>>   From  center of where the electrical connection starts near the
>>>>>> boom, to
>>>>>> where the element terminates at the outside of the array.
>>>>>>
>>>>>> doug
>>>>>> wb1e
>>>>>> 73
>>>>>>
>>>>>> On Thu, Jan 26, 2023, 12:34 PM Art Greenberg <art at artg.tv> wrote:
>>>>>>
>>>>>>> The instructions for setting element length aren't clear. Do I
>> measure
>>>>>>> from element tip to the side of the boom, or to the center of the
>>>>>>> boom?
>>>>>>>
>>>>>>> I think its the later ...
>>>>>>>
>>>>>>> TNX
>>>>>>>
>>>>>>> --
>>>>>>> Art Greenberg
>>>>>>> WA2LLN
>>>>>>> art at artg.tv
>>>>>>> _______________________________________________
>>>>>>>
>>>>>>>
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>>>>
>>>
>>
>> --
>> Art Greenberg
>> art at artg.tv
>> _______________________________________________
>>
>>
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-- 
Art Greenberg
art at artg.tv