I am getting back into Ham Radio, inspired by the incredible response to the Codec 2 project from the Ham community. In my home office, I had trouble listening to the local 2m repeater due to all the high speed digital kit I have. So I decided to put up an external antenna.
To save some coax I just soldered a scrap of speaker wire cut to 457mm (ish) to the end of my coax, rather than using the coax inner. I used a 40mm PVC tube as that was what I had laying around, with 4 turns of coax for the RF choke. For an initial test I taped the antenna and choke to the outside of the PVC tube with electrical tape.
The SWR indicator on my FT-817ND was dead flat (perfect) between 144 and 148MHz. Signals received from the repeater are pinning the signal strength meter, and no more digital noise. Not bad for 10 minutes work! Nice antenna design John!
I put the antenna inside the PVC tube, fitted the end cap and popped it up on my roof. The grey box below is a pre-production Mesh Potato co-located on the same mast, used for my local mesh network and to experiment with Village Telco hardware and software.
How it Works
Here is my explanation of how this antenna works. I might be wrong, but it was fun thinking about it.
A regular dipole has two pieces of wire a quarter wavelength long, fed by some coax in the middle:
Now, imagine rotating the dipole so one of the wires is next to the coax:
Now the clever part. If we remove the wire, RF currents will flow along the outside conductor of the coax. So the outside of the coax will form one side of the dipole, while the inside continues to act as a transmission line.
Now if we add an RF choke a quarter wavelength along the coax, it will stop the RF currents at that point, forming the other half of the dipole. Without the RF choke the RF currents would keep on flowing down the coax, and that half of the antenna wouldn’t be the correct length. So above the choke, the coax outer is an antenna, beneath the choke it’s just a plain transmission line:
A few turns of the coax forms the choke. Too many turns and the effects of the inter-winding capacitance will be larger than the inductance and it won’t be an effective choke. John has a table of the self resonant frequency of various turns and PVC pipe diameters near the bottom of this page.
I have also seen this type of antenna described a resonant feed-line (RFD) antenna in my 1995 ARRL handbook, but scaled for the HF (3-30MHz) bands. All the dimensions are just scaled up for the longer wavelengths, including the RF choke. Because of it’s length (e.g. 40m at 3.5MHz), the antenna is strung horizontally instead of mounting inside a PVC tube.
Wifi Router Sleeve Dipoles
This got me thinking about Wifi router antenna design. Many routers use a sleeve dipole design, here is a picture of the internal construction from this Marty Bugs article:
Very similar to our end fed flower pot design, scaled down to a quarter wavelength at 2.4GHz. But where is the RF choke? And why do we need the sleeve, why not just use the coax outer? Here is my explanation….
The RF choke in the flower pot presents a high impedance to RF currents, which stops current flowing further down the coax outer. However RF chokes are more difficult to build at 2.4GHz, e.g. a choke made from coiled coax would have a self resonant frequency too low to be useful.
So rather than a high impedance in series, how about a low impedance in parallel with the coax outer? This is the function of the sleeve. For 2.4GHz energy, it “looks” like a short circuit, diverting the RF current away from the coax outer.
The sleeve is a quarter wave section. Through the magic of transmission line transformers it forms a transformer or balun. It’s operation is decribed here on Wikipedia. I must admit this infinite to zero impedance transformation sounds like magic to me. I need to read up on transmission line transformers again to fully understand this. But for now I will just accept that it presents a low impedance for the RF currents, preventing them flowing down the coax outer.
A perfect earth has a low impedance, so I guess “electrically” the sleeve looks like a very nice ground plane. So perhaps we are effectively forming a quarter wave vertical antenna over a perfect ground plane.
I am writing all this down to explain it to myself. If any one else has alternate explanations, or I have made any errors, please feel free to correct me!