WICEN Vic: Flower Pot Antenna - Online Calculator - Beware!

[John & Bronwyn]

Since the Alpine Rally I have been investigating why a mobile whip may 
not work properly when mounted on a pole, and how to fix this. This has 
involved variants of the flower pot design. More on this anon.

Along the way I was reminded that Graeme VK3BXG drew my attention to a 
website which supposedly calculates the dimensions for a flower pot 
antenna (here: 
https://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.html 
)

This website uses the cable velocity factor as an input to calculate the 
length of the cable for the coil (which is actually a self-resonant choke).

Unfortunately, /*there is no relationship between the cable velocity 
factor and any dimension of the coil */(or the rest of the antenna, for 
that matter).  So the website cannot be correct. //

Also, while the length of wire is the main effect which determines the 
coil self-resonant frequency, */there are other factors involved which 
must be included/* to get a sufficiently accurate result.

John Bishop VK2ZOI (sk) who developed the flower pot, specifies that the 
self-resonant frequency of the coil should be about 3% to 4% below the 
operating frequency for best results.

So how to get the coil correct?

You could use the attached designs, which have been successfully 
constructed by many WICEN members.

If you really need to use a coil with different dimensions (such as 
different size pipe), use the coil calculator here: 
https://hamwaves.com/inductance/en/index.html#input

It is a bit complex, but is well worth coming to grips with. Once you 
have entered the required dimensions in the 'Input' section, scroll down 
to the bottom of the Results section to Table 8, the Self-Resonant 
Frequency. That's the result you need.  Vary the coil dimensions to get 
the self-resonant frequency in the target range.  Incidentally, I use a 
wire diameter of 4mm (the diameter of RG58 shield) and overall diameter 
of 5mm (needed to obtain the length of the coil from the number of turns 
close-wound).

For those more adventurous, if you keep scrolling past Results, there is 
a FAQ section with a summary of the research behind this calculator.  
And towards the bottom are the actual formulae used.

And finally, if you are curious about the phenomenon of coil 
self-resonance, Dr David Knight G3YNH has written a research paper about 
it, here: https://g3ynh.info/zdocs/magnetics/appendix/self-res.html .  
(Dr Knight's work is also included in the calculator).


Cheers all, and Happy New Year!


John VK3ZRX
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