Monthly Archives: May 2017

M0MCX Banana Antenna – an end-fed choke sleeve resonant feedline T2LT antenna design

A new document fully documenting the design of the Banana Antenna has now been released entitled, “Banana, a Half Wave End-Fed Choked Coax Antenna”.

banana-antenna

Banana Antenna

Antenna can be known as – and is similar to:

Sleeve Dipole / Flowerpot Antenna

The Sleeve dipole has traditionally been used by VHF antenna designers by sliding an external metal sleeve over the coax and connecting the sleeve to the braid of the coax so that the antenna appears to be centre-fed with an outboard “sleeve”. Some commercial CB antennas are also made this way. Continue reading

20m band End-Fed Choked Coax Dipoles (T2LT)

WARNING: This post has been replaced with the following analysis and design:

Banana Antenna Design May 2017

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The Resonant Feedline Antenna is also known as:

• Sleeve Dipole (& Flowerpot Antenna)
• Resonant Feedline Dipole (J Taylor, W2OZH)
• Tuned Transmission Line Trap, T2LT (CB folks)

For more about common mode chokes, see this article:

Pictures of this experiment follow including the 10-25 MHz >8K choke follow.

75 ohm to 50 ohm transmission line matching coax stub

Coax Transmission line coax stubs are frequency dependent. Making a stub for one frequency means it WILL NOT work for another frequency. My example is for a 20m Resonant Feedline Dipole, sometimes called a Sleeve Dipole or Resonant Coax Dipole or Tuned Choked Coax Dipole.

So you have an approx 75 ohm impedance antenna and you want to get the best match you can. Take the wavelength of the frequency, multiply it by the velocity factor of your 75 ohm matching coax and multiply again by 0.0815.

For example.

14.225 MHz = 21.089 metres
21.089 * 0.66 (what ever your velocity factor is) = 13.19
Multiply 13.91 * 0.0815 = 1.134m

Therefore, your transmission line coaxial transformer will be 1.134m long which is apparently about 29 degrees around the 360 degree circle.

Data found here: PA0FRI page.

Finally, I discovered MANY pages on eHam and QRZ forums of people asking the same question but most answers are with people answering questions which were not asked – or giving advice how to fix the antenna, or live with it. Why Americans need to argue the toss when others just need answers beggers belief :)