As an amateur with limited space and budget, I am always experimenting with unusual antenna designs.
In addition to the usual 2 meter and HF interests, I also operate a 10 meter propagation beacon and have always shared my 1/2 size G5RV antenna with the beacon. It is the only wire antenna that will fit into my property and allow me to tune 10-80. I don't have room for more than 1 horizontal wire either.
Sharing an antenna between an HF rig and a beacon that should run 24/7 is a major pain and I have investigated many different verticals but most were cost-prohibitive for me.
The first ham antenna I ever built was my 2 meter J-Pole and it worked flawlessly on the first try and has worked ever since. I always dreamed of a J-Pole for my 10M beacon but knew that at 25 feet long it would take an engineering miracle to build one that would stand up to the wind without guy wires and be within my budget of $0.00.
Staring out the widow at my antenna farm (garden?) I thought to myself 'the antenna itself would be 25 feet high and that doesn't include the pole it is mounted on'. That is when the light bulb above my head came on! 'Why not use the pole itself as the antenna?'
Thus was born this project made from materials I already had on hand.
Update: After using this antenna for a few months on 10m I decided to see if it would tune any other bands and much to my surprise it tunes very well on 12, 15 and 17 meters. I made several 599 contacts to a DXpedition in the south pacific on these bands. It also tunes on 20, 30, 40, 80 and 160 but the performance is poor.
You should be familiar with the construction of a standard copper j-pole antenna before tackling this project.
Here is a diagram of the antenna. It is comprised of a telescopic pole between 30 and 50 feet in length. (mine was 45). The pole itself is the main radiating element (L) with a 1/2" hard copper matching stub (S) off the side. The piece at the top of the stub (detail A) is made of cpvc and is there to keep the tip of the stub from swaying in the wind and changing the tuning of the antenna. The angled piece at the bottom of the antenna is there to support the weight of the copper pipe above it. I also use it as a convenient place to hang the choke. Since it is not an active part of the antenna, it could be made of copper or plastic. Mine is copper.
The feed point of the antenna (F) is also made of cpvc and is easily formed by heating over a stove or with a torch and bending to the shape and size needed.
The ends of the copper tubing where they attach to the pole are formed by smashing the tubing flat and bending them over. They are secured with stainless steel hose clamps.
As you can see in the photos below, I fed mine with 75 ohm coax and used F style connectors but you can use any coax/connector combination you prefer.
I built my antenna to use on my 10 meter beacon and cut it for 28.2215 mHz so my dimensions were as follows:
L = 24.98 feet (299 3/4")
S = 8.29 feet (99 1/2")
F = .81 feet (9 3/4")
Sp = .78 feet (9 3/8")
The L dimension and the F dimension can be adjusted a little bit to fine tune the swr if necessary.
Use the formulas to calculate dimensions for any center frequency you want.
The choke is the big coil of wire at the base of the antenna that you can see in the photos. It is there to keep RF from traveling back down the coax and affecting the swr. My choke is 50 feet of RG-6 wound on a piece of 4" pvc drain pipe. You could use other things also like ferrite beads, a 1:1 balun or simply a chunk of coax rolled up and taped.
You can't see it but the bottom of the pole is stuck into the ground a foot or so and bracketed about 10 feet up. This puts the entire antenna at ground potential which is great for lightning protection and makes for a quiet antenna. A ground rod at the base could also be added for extra lightning protection.
So far, since this prototype has been up, it has been subjected to a 50+ mph wind twice and had very little sway. It is my belief that it would survive a 100 mph wind without guy ropes to steady it.
Shown in the photo above is the detail of the feed point (F). It is 1/2" cpvc pipe formed to the shape shown and has #12 copper wire passing through it to make contact with the pipes on each end. I used an F-style connector, you can use whatever you like or no connector at all by soldering the coax directly to the pipes. The center of the coax connects to the long side (L) and the braid connects to the short side (S) of the antenna.
The photo above shows the cpvc spacer at the top of the tuning stub as shown in detail A. It keeps the copper pipe evenly spaced from the telescoping pole. There is a stainless screw to hold the plastic elbow to the copper pipe so it does not pop off in a high wind while swaying.
Be sure to use cpvc and not the standard schedule 40 pvc. The cpvc pipe and fittings are the same diameter as the copper and fit nicely together.