Rugged J-Pole Antenna Design

Overview

    The J-pole antenna is one of the most widely used antennas in amateur SIGINT/EW radio due to its low cost and ease of construction & installation.  Its design dates back to the early 1940s when it was trailed behind blimps (Zeppelins) for radio communications during the Eurosavage's annual little scuffle.

    The antenna is called the "J-pole" because its elements are in the shape of a "J", duh.  The antenna elements consists of a main radiating 1/2-wavelength vertical element which is fed by a (non-radiating) 1/4-wavelength matching stub.  It is actually designed to be fed with a balanced transmission line (i.e. not coax), but there are tricks to overcome that.  The radiation pattern is very close to that of a common dipole, but is skewed slightly due to interactions between the main element and the matching stub.

    This article will cover the construction of a rugged, portable J-pole antenna that was designed to be used with FRS radios operating in the 465 MHz band.  J-pole antennas are useful & scalable for any frequency between 50 and 900 MHz.  The gain of this particular style antenna will only be around 1 dBd, based on EZNEC models.  Testing shows it to give approximately the same performance of a 1/2-wave dipole.  Contrary to what others may say, this antenna has no gain when constructed with only a single main radiating element.  Increasing the main radiating element to 5/8-wavelength also does not improve the antenna's performance.

Schematic / Construction

    The best material for this antenna is 1/2-inch diameter rigid-wall copper pipe and fittings used for plumbing.  3/4-inch and 3/8-inch diameter pipe can be adapted when more strength or less weight is required.  All the pipe and fittings must be properly fluxed and soldered for the antenna to work.

The copper pipe should be cut using a tube cutter and not a hacksaw.  This will help maintain the fine dimensions required for the element lengths.

    Every copper piece that is to be soldered should be rigorously cleaned with fine grit emery cloth.  After cleaning the connection, apply a very thin coat of solder flux.  Solder will only stick to the copper with the solder flux.  Avoid using too much flux.

Soldering the connection is the most critical step.  There are two main points you should follow when soldering copper pipe.  First, avoid using too much solder.  It will look messy and is really pointless as the mechanical/electrical strength comes from the metal-on-metal connection and not the solder.  Second, avoid getting the solder connection too hot.  Use a slowly rotating propane torch to heat the entire area to be soldered.  Solder will also only flow where there is heat.  Too much heat can weaken the solder connection and copper pipe.








Schematic is from this excellent website: http://www.packetradio.com/JpoleCALC.htm

Pictures

Example components used in the construction of a portable J-pole.  From the left, is a large air chamber.  This makes a perfect mounting pole.  Next, the white parts, are threaded PVC couplers.  WTF?  Yes, if you isolate the J-pole antenna from the mounting pole, the antenna's radiation angle will be much closer to the ground.  This isolation is optional, as the PVC parts significantly reduce the ruggedness of the antenna's construction.  The rest of the components are standard copper pipe fittings (elbow, T, end caps, threaded adapters) and a length of rigid-wall copper pipe.

Example picture showing the components soldered together.  The PVC couplers should be glued to the copper.

Example picture showing the components of a rugged (non-isolated) J-pole.  This design is more suited for portable operations.  The radiation pattern has a higher "take off" angle, but the antenna will still perform flawlessly in almost all situations.

The removable main radiating element should only be used for low frequency (VHF) antennas.  At UHF and higher, it will harm the antenna's performance - and it's not really needed as the element lengths are not too long.

You'll need to make element feed tabs to connect the coax to the antenna.  An easy way to do this is to flatten copper pipe hangers, and cut off the ends with the holes.  When soldered to the antenna's elements, the holes are perfect for mounting hardware, or for soldering the coax directly.

At VHF and lower frequencies, you can use copper split-ring hanger clamps.  This will make adjusting the antenna's SWR much easier.  Connect the coax leads to the hanger clamps, and slide them up and down to get the best SWR.  Then apply solder into the threaded hole to secure the clamp into place.

Closeup view of the feed tabs soldered to the antenna's elements.  Measure from the center of the tabs when performing the feed length calculations.

Example of the coax feed connection using brass hardware.  This should only be done at VHF or lower frequencies.  Add ring terminals to the coax leads.

A neat trick for fine tuning the antenna's SWR is to add an adjustable tip to the main element.  This consists of brass #10 (32 TPI) hardware soldered to the end cap of the main radiating element of the J-pole.  To do this, first flux and solder the end cap.  Also flux and solder one side of a #10 brass nut.  Heat the end cap again until the solder forms a little pool.  Using a tweezers, place the soldered nut into the pool of solder.  Wait until it cools.  Drill and re-tap the nut with a #10-32 tap.  Add a 3/4 or 1-inch long brass bolt and another locking nut.  Adjust the "tuning stub" while watching the SWR meter as you tune the antenna.

A UHF frequencies, you'll have to solder the coax leads directly to the element feed tabs.  This helps keep the antenna's SWR low.  Use "Liquid Electrical Tape" to seal and waterproof the solder connections and the coax.

Finished J-pole antenna at the test range.  It has a 15 dB return loss (or SWR under 2:1) across the entire FRS band.  Note the coil of coax forming a decoupling loop below the antenna's feed.  This helps prevent RF from flowing on the outside of the coax and distorting the antenna's radiation pattern.  This is an important step.  Just loop about four or five turns of coax tightly right before the antenna feed.