Coax

 
1/4 Wave bamboo-3
 
80m
 
 

2020

Coax-3 / Bamboo-3

Coaxial cables 1/4 wave long serve a variety of useful purposes in antenna assemblies. Whether it is the phase delay line for a 80 meter Loop Antenna or to make the feed lines for a four-square antenna, the transformer action of a 90 degree long feed line serves many purposes. PA1A making 1/4 75R stub for a 80m loop antenna, Also adapting 1/2" SFC connectors 7/16"fitting COAX-

     
 

Velocity

The most straight forward method of calculating the electrical length of a piece of coax involves first calculating the free space wavelength of the target frequency and multiplying by the velocity factor of the coax.
For COAX-3 the velocity is 85%: cutting 1m

     
 

Transformer

Quarter wave and half wave length piece of transmission lines have special properties. A line quarter wave in length acts like an impedance transformer. A short at one end will appear as an open at the other. Likewise an open at one end will appear as a short at the other. This will happen only at the frequency where the line is 1/4, 3/4, 5/4, etc. waves in length.

Transmission lines a half wave in length come full circle by mirroring the impedance on one end to the other. An open circuit at one end will appear as open at the other. A short at one end will appear as a short at the other.

Note in both figures above the distance a wave travels is shortened by the dielectric properties of the coaxial line. This fact is represented in transmission line characteristics as a variable called Velocity Factor… a unit-less number indicating the fractional speed of light in transmission lines with respect to the free-space speed of light.

(Copyright) kx4o@ www.hamradio.me

 

 
 

Capacitance

It is a good thing Amateur Radio Operators know about transmission line Velocity Factor specifications. Without this knowledge you would be cutting stubs incorrectly. However, some Amateurs assume that just because a piece of coax is used for a stub, Velocity Factor applies. The key to understanding when it does and when it doesn’t is found by following the electric and magnetic fields.

Checking the calcultated and measured capacitance. Both are ~52pF/m. The physical length of a line section is not the same as its electrical length, and they are related by the Velocity Factor (VF) where VF=PhysicalLength/ElectricalLength.

The most straight forward method of calculating the electrical length of a piece of coax involves first calculating the free space wavelength of the target frequency and multiplying by the velocity factor of the coax.

For COAX-3 the velocity is 85%: cutting 1m

 

 

   
 

Quarter wave

The Coaxial Cable Balun has the following dimensions based on calculations: 17,7m
if you are an Imperialist you might convert to inches...

Because this cable is not UV-resistant, I decided to supply a PE water tube around it. Using silicone spray en a lot of forece, finally it worked....

     
       
 

7/16 adjusting

Because I do not have the appropriate connectors, I am using 1/2" - 7/16 pieces

These connectors are to small and therefore using the lathe for drilling them up.

     
       
  Installation 1

Cutting the outer shell
     
       
 

Installation 2

-Remove a little bit of the outer shell with a knife

-Cutting the copper foil with a small tube cutter.

-file away sharp edges and insterting the outer ring

     
       
 

Installation 3

-Carefully bend the foil

-thickening of the inner conductor by soldering

-fit the plug with a dummy and hit the parts together with a hammer

     
       
 

Installation 3

-tighten with open-end wrenches

-First apply Scotchfill anf finishing with Scotch tape with sufficient force
7/16" male on Coax-3 / Bamboo and 7/16" female on Coax-3 / Bamboo

     
       
 
     
 

Measurements

With one end open and the other connected to the antenna analyzer, tuned for a minimum resistance. A short indicates a proper frequency. Shorts will repeat every other quarter-wave.

minimum resistance = 2,2R @ 3620 KHz

     
  -->NEXT PAGE-->