[TowerTalk] Adding 15 meters to a 204BA

[Leeson]

Yep Stan, you have that derivation right. As you note, for a clamp 
around the boom, the clamp electrical half-length includes 0.3 x the 
boom radius, while the physical length to determine the exposed inner 
section length is just the 2-5/8" length of the tubing insertion. That 
2-5/8" subtracts from the full inner section tubing length to determine 
the exposed length.

Although it has the correct exposed section lengths and clamp equivalent 
diameter, the table I posted shows a clamp electrical length of 3.125", 
0.2" larger. I'm not sure where that would have come from. Fortunately, 
at 20 meter wavelengths that difference hardly seems to have affected 
the outcome of calculations.

Some last notes: In several places I see the cylinder diameter for 
calculating equivalent diameter of the Hy-Gain clamps is (correctly) the 
clamp OD, 1.5" for the #13 clamp. This could be misinterpreted as the 
inserted section tubing size itself, or as the length of the inserted 
section.

When I calculate the Hy-Gain driven element setup, I recognize the 67 pF 
capacitance of the tubing and its insulator, in parallel with the 
element impedance. See Table 9-5 on page 9-9 of my Yagi book.

The original Excel spreadsheets that accompanied the book were updated 
to xls format, with the ones with "F" updated in 1999 to the EIA-222-F 
wind force model. These are posted at 
https://www.dropbox.com/sh/qevnshmw9m3jm88/AABSn1iVhh1Q6qrPEYgwcq83a?dl=0

I was surprised to find the 1995 note I wrote on 204BA mods, still 
online at https://www.qth.com/ka9fox/hygain_204ba_mods.txt

Dave, W6NL/HC8L

On 11/5/25 5:19 AM, Stan Stockton wrote:
> Thanks, Dave!  I had forgotten all about the great book you wrote 30+ years
> ago.  Also, my wife and I are watching a series on TV that we watched 10
> years ago and it's almost like we had never watched it.😀
> 
> So in the chart it shows the equivalent diameter as 2.14" and the length as
> 2.925".  I am assuming from what I read that the half length of the clam
> shell clamp that extends out from the edge of the boom is 2-5/8" with an
> overall length of 7.25" (5.25" + 2") and that the length of 2.925 is
> derived by adding .3" for half the electrical length of the boom diameter?
> Hope that makes sense and is correct?
> 
> Thank you so much for reminding me of the book.  A lot of it is way over my
> head but it's full of practical information as well.
> 
> 73...Stan, K5GO/ZF9CW
> 
> On Wed, Nov 5, 2025 at 12:52 AM Leeson <leeson at earthlink.net> wrote:
> 
>> Modeling of the HyGain element clamp is derived in Chapter 9 of my 1992
>> book, "Physical Design of Yagi Antennas,"
>>
>> https://www.dropbox.com/s/hmhkeofz0igrg1e/Physical%20Design%20Of%20Yagi%20Antennas%20D%20B%20Leeson%20V2.pdf?dl=0
>>
>>   From page 9.3, the equivalent diameter of the HG204BA clamp is 2.14
>> inches. This clamp model is used in K6STI's YO and other Yagi software.
>>
>> Dave, W6NL/HC8L
>>
>> On 11/4/25 5:43 PM, john at kk9a.com wrote:
>>> Maybe easy for you, Stan  I have not done well designing multi-band
>> Yagi's although I have not tried doing so with AutoEZ.  I would be
>> interested in seeing your WEQ file for this.  Hy-Gain uses a large formed
>> aluminum element clamp so you might have to make some guesses for
>> modeling.  Insulated elements are easier.
>>>
>>> GL
>>> John KK9A
>>>
>>>
>>> Stan Stockton K5GO wrote:
>>>
>>> Dave,
>>>
>>> This is pretty easy to do.  Would probably end up being 3 on 20 and 4 or
>> 5 on 15.  The difficult part is inputting exactly what the lengths and
>> diameters you are going to use for each element.  AutoEZ will do the rest
>> of the work using one feedline.
>>>
>>> If you give me the exact lengths and diameters of the first 3 elements
>> on your 20m antenna and same info for what you want to use for the 15m
>> elements, I’ll do it for you.
>>>
>>> No 10 meters?
>>>
>>> Stan, ZF9CW
>>>
>>> Sent from my iPhone
>>>
>>>> On Nov 3, 2025, at 11:40 PM, dave arruzza via TowerTalk <towertalk at
>> contesting.com> wrote:
>>>>
>>>> Has anyone successfully added, non trap, 15 meter elements to a 204ba.
>>>> 73
>>>> Dave
>>>> W1CTN / NC4DARADIO GASTONIA203-906-6209
>>>
>>>
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