J-pole antenna
The J-pole antenna is one of the simplest antennas to construct, and therefore is a very good first antenna to build. This page is to collect our experiences with J-pole antennas. The J-pole is a fairly large antenna, so it is best used at VHF frequencies, although it works on other frequencies as well if you have the space.
See also the original article, also published in QST.
See also Folding J-Pole Workshop.
Theory[edit]
The design criteria begins by stating that the j-pole is an end-fed vertical half-wavelength antenna (also known as an end fed zepp), that is matched to the transmitter output impedance by a series tuned section of transmission line. Most published J-pole designs have been derived by making the series section, or J section, a quarter wave length long.
components[edit]
- half wavelength dipole
- variations might use 5/8 wavelength instead
- some variations add a second half wave section with a phasing section ("super J")
- quarter wavelength transmission line match
- transmission line equation
- velocity factor
- feedline tap between 0 and 90 degrees
- off frequency inductance / capacitance of dipole element partially canceled by capacitance/inductance of matching section resulting in increased bandwidth
- optional balun
- not a balanced antenna; choke balun only prevents common mode radiation on coax
radiation pattern[edit]
- Omni-directional antenna
- sensitive to surrounding metal objects due to lack of true ground plane
- try to keep at least a wavelength or two away from filing cabinets, etc.
- feed polarity helps reduce this
Types[edit]
There are quite a few variations on the J-pole antenna. Here are the ones we've tried.
roll-up emergency J-pole[edit]
This antenna is trivial to build, and is the ultimate in portability for a large antenna. In an emergency, tie a weight to it and throw it up a tree. If your kit contains a bit of ladder line and a soldering iron, this would be easy to construct in the field with no additional parts.
The disadvantage of this antenna over the next few is that it is fairly fragile and narrow bandwidth due to the small diameter of the wires.
Tip: to increase bandwidth of this antenna, use ladder line only for the matching section, and connect a piece of coax to one side for the radiating part of the antenna. The wider diameter of even rg58 coax is enough to cover the entire 2m band.
- assorted transmision line impedances
- 300 ohm TV line
- 450 ohm ladder line
- sample lengths for 2m
copper pipe J-pole[edit]
The copper pipe J-pole sacrifces portability and ease of construction for durabiltiy and bandwidth. If some care is taken to weather proof the feedpoint and perhaps add a thin layer of laquer to reduce corrosion, this makes an excellent permanent outside antenna for 2m or even 6m.
fold-up copper pipe J-pole[edit]
The 2m copper pipe J-pole is really too large and cumbersome to carry around. Adding joints to the longest element and the support pole is a significant aid to transportation with only a small sacrifice in durability.
- Articles
- backpacker's J-pole (PDF)
- QST March 2005: Folding copper J-pole
- backpacker's J-pole (PDF)
The parts list in the above article list non-standard parts not easily available. These parts are a little easier to find. (Thanks, Keith! This list first used for the 2006-05-19 meeting)
| cost | # | description |
|---|---|---|
| 9.62 | 5 | ½” Copper pipe in 19.25” segments |
| 1 | ½” Copper pipe 2.75" segment | |
| 0.58 | 2 | ½” Copper Pipe Couplings |
| 0.54 | 1 | ½” Copper Pipe T Joint |
| 0.33 | 1 | *½” Copper Pipe Elbow |
| 0.96 | 3 | ½” Copper Pipe End Cap |
| 0.54 | 2 | ½” Copper Pipe tube strap |
| 1.26 | 1 | Bungee cord Apx. 7’ |
| 0.98 | #8-32x1 5/8” hook and eye | |
| 0.98 | 6 | #8-32 nuts |
| 0.98 | #8-32x5/8” screws | |
| 0.98 | #8 washers | |
| 0.98 | #8 lock washers | |
| 0.98 | 2 | #6-32x½ bolts and Nuts for Feed Point |
| 0.22 | 2.5”x2.5” Lexan sheet | |
| 1.50 | SO-239 Panel Mount connector | |
| 0.21 | #8 O ring terminals, crimp on | |
| Steal! | A Few Inches of Wire |
tuning[edit]
- Adjust length of dipole element and quarter wave matching section to set center frequency
- diameter of elements sets bandwidth of antenna
- position of feed tap on matching section sets impedance
- tap wires should be as short as physically possible.
- try to match 50+0i ohms
- generally, lower reactance is more important than 50 ohm resistance
- polarity of feed point tap
- polarity of feed is nearly irrelevant
- placing shield on long element and core on sub reduces sensitivity to surrounding metal slightly