The Great Cantenna Conversation.

[Don Latham]

Lordy!  I just went online to get the lower cutoff frequency vs. waveguide diameter for a circular waveguide.

I consulted two online calculators, ChatGpt 4, and a "new ChatGpt" that v.4 importuned me to try. This called o3-mini "for science".

The proper calculation for the lower cutoff frequency is:

Fc = (1.8412*c) / (2*pi*a)  where c is the speed of light, pi is 3.14159, and a is the RADIUS of the waveguide cylinder.

Note Fc is in cycles/sec or Hz, and c and a have the same length units,  1.8412 is an approximate magic number from Bessel functions. never mind, it's right.

So, what happened?  Chat4 USED THE WRONG FORMULA. it leaves out the 2 in the denominator. Maybe it was stupidly thinking diameter. Maybe sniffing motherboard cleaner. Who knows. Anyway, I will certainly not trust gpt4 for much of anything at least scientific. Maybe o3-mini can be trusted, maybe not, it and the two online calculators were correct for 6 inches diameter at 1.153 GHz.

At any rate,  for us barbarians, the speed of light converts to 11,802,900,000 jnches per second or  11.8029 * 10^9 which is fun. This means that the cutoff frequency is:

<<<<<<<<<<<<<<<<  Fc in GHz  = 6.9174 divided by (waveguide diameter in inches)  >>>>>>>>>>>>>>>>>>>>>.

Thus  our 6 inch cheap pipe cuts off at 1.1529 GHz which is incredibly serendipitous for all us excolonials.

I checked this a couple of ways. It's right.     { In my RSF (rod-stone-fortnight) units, that's 1.39 *10^15 cycles per fortnight. A 1420 MHz signal is approximately 1.72×10151.72 \times 10^{15}1.72×1015 cycles per fortnight according to o3-mini.}

Incidentally, o3 gives us the following advice for a 6 inch guide: " Keep the TE11 mode between ~1.15 GHz and ~1.50 GHz to ensure single mode operation. Frequencies beyond this range could bring in higher modes, disrupting the TE11 mode". Essentially this means that if energy is transferred into higher order modes, it will be lost to our simple probe pickup.

Good advice IMHO.

Don

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Don Latham
PO Box 404,
Frenchtown, MT, 59846
406-626-4304

[Alex P]

Something like This ???

[b lowell cardon]

To those concerned or interested: a nice number for the speed of light is 1 foot per nanosecond. 

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[Lester Veenstra]

Vive la Grace H.

 

Lester B Veenstra  K1YCM  MØYCM  W8YCM   6Y6Y W8YCM/6Y 6Y8LV (Reformed USNSG CTM1)

les...@veenstras.com

 

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[Don Latham]

ayup

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From: "sara" <sara...@googlegroups.com>
To: "sara" <sara...@googlegroups.com>
Sent: Monday, February 3, 2025 4:07:31 AM
Subject: [SARA] Re: The Great Cantenna Conversation.

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