The DIY Scanner Discone

By KJ4DGE

I rent a room in a area of Alexandria far enough away from Metro DC to be a hindrance to listening to VHF/UHF. Couple that with being in a valley, and next to the woods and it's a real RF hole. My one advantage is I am on the second floor. All that's means is the built in antenna on the Pro-2004 does not work well at all. I looked this morning on the net at various types of homebrew antennas and thought I have little room, little materials, few tools, what can I do with those issues? First I wanted a broadband antenna that would fit the range of the 2006. It had to be small enough to fit in the window, pull in at least 2 meters through 800 MHz and be able to modify it as time goes along.

I looked a Bi-conical antennas which are small and great for UHF but big if scaled down to VHF. Also I had few materials on hand other than electrical tape, some wiring and a hunk of coax. Well I also had an empty liter bottle of Coke. So I set off thinking what better a form to build on? It's flexible, easy to work with, can have a base which can be filled with anything for weight. So off I went drawing what it might look like and within 2 hours I had a working receive antenna that does good. In thinking about the length of the elements I considered the basic top length of 17 inches, the spokes at the hat for 4.6 inches and the radials between 6,7, 8, 9 and 10.5 inches. I was not trying to be that exact. Since each radial acts as an independent element I figured the lengths about right for the bands I wanted to cover. But first I needed solid wire. Looking in the closet I found some wire coat hangers. I cut those first and scrapped off the paint. Next I removed the jacket from the coax braid and inner conductor for the length of the liter bottle, then punched a hole in the cap. Now looking at this mess I thought how do I keep the bottom half separate from the top hat and vertical element? The cap is the feed point for the vertical and top spokes while the outer part of the bottle is the connect point for the lower elements. It's not pretty.

Here are the parts:

Next I punched a small hole in the bottom of the bottle and a small one near the flange for the copper braid. I snaked both through and ran the braid out and around the outer surface of the bottle holding it in place with a piece of electrical tape. Next I worked on the top element and spokes. First the spokes need to have some conductive element where they all meet. I found the inner liner from some chewing gum and used that. I notched a small section of the inner conductor and ran a wire around this back to the cap top.

Stripping about inch from the flex electrical wire I next connected it to the braid and let it hang. But before all that I filled the bottle with some copper pennies for weight.

You could use sand I suppose. I did not want to fill it up as that would affect the antenna I am sure. I secured the lower elements to the side of the bottle with clear shipping tape. The reason for this is the lower elements are suppose to be out at a 45 degree angle but for now I just wanted to be able to sit it in a window for testing.

Ready for testing!

Ignore the 1:1 homebrew balun in the pic above that was for the HF rig, it works too! Here is the antenna in the window for testing.

Now you are asking does it work. Here are some real-world observations. The US Park Police still uses analog in DC downtown on 166.925. Before I could hear nothing now I can hear them. Other VHF area FD's (Prince William county VA and Bethesda MD) comes in just fine. But where it really shines is 800 Mhz. Does it pick up HAM bands? Have been listening to the Brandywine repeater with QSO's along with a auto ID from the NVFM Clubs 224.100 machine this morning so far. I am sure if I let out the lower elements at the angle they are suppose to be at, the improvement would be perhaps even better. It also would not fit in the window. I am toying with the idea of a rod through the center of the bottle and sticking it out the window once that is done. It certainly has improved the listening for me and cost me 46 cents in pennies! This is a part of the hobby that I really find the most fun, which is building homebrew antennas. Sort of looks like a strange big vacuum tube! :)

Epilogue:

Testing results 5/7/15. -- Really amazing to me at least. I pulled all the radials off the braid and stuck 4 wire picture hangar wires for the ground plane while monitoring 162.450 NOAXX weather for this area as it's a fringe copy here. With the new radials the signal was gone. So I reconnected the "ugly wires" to the braid and it came back. Next I took an aluminum pie pan and cut the center out to fit around the bottle notching every few inches to use as a guide for the ground radials. Now it looks like a weird vacuum tube with a skirt :-)

Wow! What a difference. Now the NOAA station is a good copy. I also removed the top spokes altogether. Signals I thought I would never hear came popping in. The MVARC repeater on 146.655, which I can only hear outside and then if I move around, came in along with a great copy on the input of 146.055. Here are the other results:

224.100 NVFM repeater, solid signal.

155.685 Prince Georges FD, solid copy

378.100 "Giant Killer" SAM command post Andrews AFB, solid signal.

All VHF air frequencies, 118-136 MHz come in loud and clear.

So for an experimental antenna made from junk lying around, a couple hours of work and a couple mods, I am very pleased with the results! And then there is the simpler approach. This antenna has been around a long time. It is simple ground planes made from coax and coat hangers. 21 inches of the inner conductor as the vertical and the braid wrapped around the bottom support for the ground for the 3, 21 inch wire coat hangers. Lastly the light weight wood pieces nailed together with tinny nails and tape, counter weight and stuck out the window. Cost: 0. Results? Aircraft I have never heard before, solid 2 meter signals, solid US park police, solid everything. Key here is getting it outside!

KJ4DGE

Have a great summer!

KJ4DGE