Over on a thread titled “Different Kind of 3-Meter Antenna,” Ken Norris posted a link to a fascinating article “A Small Loop Antenna.”
http://www.antennex.com/hws/ws0201/nonmag.htm
Over on a thread titled “Different Kind of 3-Meter Antenna,” Ken Norris posted a link to a fascinating article “A Small Loop Antenna.”
http://www.antennex.com/hws/ws0201/nonmag.htm
But I am writing now about Figure 1 from that paper, which describes a “Coax Vertical Antenna.” This all by itself is a different kind of 3-meter antenna of a type I haven’t seen discussed.
The significant thing about it is how the RF signal, imposed on the outer shield of the coax, is fed at the top down through the center conductor to ground.
The physical arrangement of this antenna also, I would think, could be described as a form of “transformer.”
What thoughts might you have about the Coax Vertical Antenna?
Coax Vertical Antenna
Very interesting! Well – I think it’s for
receiving, and not in the AM BCB.
But, I’ll have to read about it a couple
more times.
Low noise RECEIVING can be very hard to
do. My DX listening on the longwave
162 to 285 kHz broadcast band taught me that.
I’ll be back.
Bruce, MICRO1690/1700
Coaxial Vertical=Bazooka Vertical
Developed originally for military use, the Bazooka Dipole was use as a broadband coaxial antenna. Original patent link:
http://www.pentodepress.com/receiving/patents/2184729.pdf
By using one half of the antenna, we can use this design as a vertical. I suggest these links with an explanation of the antenna and construction details. I use a coaxial vertical (dipole) for my Part15 FM station. I get a solid signal on about 3-4 milliwatts output out to 250 feet. It’s made of 6 feet of RG-58 coax, a rat-tail antenna, a snap-on ferrite choke and some connectors.
http://www.iw5edi.com/ham-radio/?the-vertical-bazooka-antenna,18
http://searchmobilecomputing.techtarget.com/definition/coaxial-antenna
Coaxial antennas, like the Bazooka provide for impedance transformation over a broad range of frequencies due to the distributed capacitance along the length of coax. Coaxial antennas will work only on fundamental and third harmonic frequencies above which the capacitance of the antenna causes a direct short to ground. These type of antennas are used primarily 160 meters (1.9 MHz) and up in frequency (160 meters = 77 or 154 foot antenna).
Collinear vhf and uhf antennas are often made from scraps of coaxial cable to produce multiple inductors and capacitors, stacked one on top of another to produce a gain array antenna. Here is a link for this antenna:
http://www.rason.org/Projects/collant/collant.htm
How Interesting
Great, Marshall, that you are familiar with this information. If I understood you to say that the design is good starting at 1.9mHz, isn’t that just inches above the end of the AM band?
Therefore it ought to provide yet another viable antenna to test with.
Use in AM Band
The challenges of this antenna type and design are mostly mechanical.
1) The physical vertical (1/4 wavelength) antenna would be approx. 70+ feet long/high. (-not usable for Part 15 AM BCB)
2) The weight of the antenna and associated hardware would cause the antenna to tear itself apart in the wind. Most of these antennas are commercial built with a special stranded center conductor RG-58 because of its light weight. Solid center conductor coax would work-harden in the wind, break and fail.
3) The vertical version of this antenna still requires an extensive ground “radial” system to work properly.
4) The antenna’s length requires it to be either 90 degrees or 180 degrees of a full 360 degree wave length, minus the coaxial cable velocity factor. AM BCB: (not recommended)
90 degrees = 70+ feet
180 degrees = 141+ feet
Amateur radio does use Bazooka antennas on 160 meters. However, the saying goes like this, “If your wire antenna is not long enough or big enough not to blow down in a wind storm, its not perfect.” These are not broadcast antennas. They are by design either communications antennas or image reflection (radar) antennas due primarily to their very broad bandwidth. Most Bazooka-type antennas are used where construction mechanics do not affect the reliability of the system.
As a final note: a vertical Bazooka antenna for the FM BCB would be around 30 inches long. Construction mechanics challenges are almost non-existent for the FM BCB.
My Big Idea …
… was to make a disconnected loop from 1″ soft copper tubing, install a loading coil attached to one end but not the other, wound on 4″ PVC drain pipe. The other end is PVC fittings attached to the input end of the coil, which will insulate it from the open end. IOW, it’s actually an electric antenna, instead of magnetic.
But I think eddies in the coil field may goof up the signal in the open end. Maybe I should put a couple oversize capacitor plates between … one closest to the coil as ground, and the other like a hat, electrically attached to the open end. Vary the distance between them to tune.
Anyway, the idea is to create a viable useful antenna which could be used on an apartment deck or where a stick antenna wouldn’t be allowed. Polish it up and make it a decorative art piece 😉
But yes, the thing in the pictures is for SWL.
Worth the Time
I think it would be worth time time and material to experiment with this design idea. Some considerations:
-Resonance would involve the voltage and current in the antenna to be in phase or 180 degrees out of phase. The phase relationship will be determined by the values and particular components used to tune the device. 90 and 270 degree tuning circuits may not help very much.
-A non-resonant antenna will work, however the poor efficiency and the effects of harmonic radiation will increase…greatly. Out of band interference becomes a factor here.
-Can any experimenter determine the input impedance of the antenna? What equipment is required to determine these values? How does the transmitter react to this antenna load (SWR, tuning circuit losses)?
-Construction mechanics become a significant factor in the antenna’s reliability. What components are used? How are the components set and anchored in the tuning device and antenna? Does the physical size and length fall within the Part 15 limits?
-This device can be made to radiate. But, will this device be a significant improvement over traditional single conductor 3 meter radiators?
-Can this device be made to radiate in all directions maximizing the potential listening audience? Does the antenna’s directionality supply a significant benefit?
-All intentional radiators are, in the end, simply R-C-L circuits with characteristics similar to a simple tuned tank circuit; the individual values and mechanics of each set of components adding or subtracting from the antenna’s efficiency.
These are just some of the considerations along the road of the experimental antenna.