Monday, February 21, 2011

2M + 70cm Open Sleeve Vertical Dipole


Johnny Pedersen (LA3AKA)

I was playing around with the MMANA Antenna analysis software and wanted to design a 2m/70cm vertical antenna. I tried different antenna models, J-pole, half wave dipoles, Ground Planes …. I then remembered a chapter in the 18. edition of the ARRL Antenna Handbook covering Open Sleeve antennas to make Broad band antennas. I thought that this might be useful for making a dual band VHF/UHF antenna.

0x01 graphic

The Antenna is planned built using 6mm aluminium rods. According to MMANA this will with a distance of 3.2cm between driven element and sleeve elements give an Feedpoint Impdance of 75 ohms on 2 meter and 50 ohms on 70cm. One nice thing with this antenna is that you get a good gain on 70 cm (approximately 3dB over a g Gain:

A Magnetic Loop Antenna for Shortwave Listening (SWL)

Now that we’re on the downward slope of sunspot cycle 23 (2004) you may have noticed that some of your favorite broadcast stations don’t come in as strong as they did a few years ago. This is especially apparent on weaker DX stations. The whip on your shortwave receiver used to be sufficient to pull in some good DX, but now you find yourself looking for something better.

Maybe you have been thinking, or even have already tried, putting up a wire antenna. This may be a great solution if you live in a reasonably quiet area, noise wise, and your shortwave receiver doesn’t easily overload in the presence of strong signals. Perhaps you live in an apartment or are situated where installing a wire antenna is simply not feasible. Or maybe you’re looking for something that offers a bit more mobility so you can take it into different rooms of your house. Consider the small single turn magnetic loop antenna if any of the above situations apply to you.

The small single turn magnetic loop (SSTML) antenna consists of a single winding inductor, about 3 feet (1 meter) in diameter, and a tuning capacitor. A second loop, which is one fifth of the diameter of the large loop, is connected to the feedline and this small loop is positioned in the large loop on the opposite side of the tuning capacitor.

Magnetic Loop Antenna

More at……

Simple 1/4 Wave Ground Plane


If you are just starting out or have the desire to build an antenna here is a simple and fun project. This antenna is perfect for those hams living in the primary coverage area of the .075 repeater. The radials can be made of no. 12 copper wire. The vertical radial (A) should be soldered to the center connector of the SO239. The four base radials (B & C) and (D & E) can be soldered or bolted to the SO239 mounting holes using 4-40 hardware. The four base radials then should be bend downward to a 45 degree angle. The antenna can be mounted by clamping the PL259 to a mast or even passing the coax through a 3/4 ID PVC pipe and compression clamping the PL259. Either way let your creativity flow. If you plan on mounting it outside experience teaches to apply RTV or sealant around the center pin to keep water out of the coax.

Make each radial a 1/4 wave of your desired frequency. Sometimes it helps to add a little extralength to the radials. This will give you some snipping room when you adjust the SWR.
example calculation:

Freq (mhz)    A (inches)    B&C/D&E (inches)
146 mhz            19-5/16            20-3/16

A Tree Friendly 2 Meter Halo Antenna


Having purchased an all-mode, all-band (160m - 70cm) transceiver, I became curious about what 2-meter weak signal operations have to offer. I have a 5/8th over 5/8th vertical collinear antenna hanging in a tree at some 30 odd feet high, but I never heard anything on it, except on FM. The reason for that, I learned, is most 2-meter weak signal operations take place using horizontal polarization. Cross polarization is good for about 20 dB attenuation, which easily translates into the difference between perfectly good copy and inaudible signals. So I decided I needed a horizontal polarized antenna.

As is usually the case with antennas, there are a bazillion designs to choose from and none of them really fulfills all your requirements. I do not have a mast or tower, and I love to use trees for supports, so I wanted something that I could hang from a tree branch. Since I have no means to rotate the antenna, I required that the new antenna have an omnidirectional radiation pattern. It didn't have to be the best performer, because I just wanted to get my feet wet in this new mode of operation. There are few designs that would fit that bill. I settled on the Halo antenna because of its small footprint. This is important because larger designs would require a longer branch, with sufficient clearance in all directions, to hang from. The Halo I describe here has a diameter of only about 12 inches and can be hung virtually anywhere in a tree.

Halo stands for "HAlf wave LOop". The antenna is in fact nothing else but a half wavelength dipole with the legs bent in the shape of a circle. However, the ends do not meet, (especially near the end of the month) so technically it's not a loop. This loop can be fed with coaxial cable using a gamma match.

The Halo is certainly not a new design. Laurence M. Leeds and Marvel W. Scheldorf obtained a patent for this antenna in 1943. You can find their design at the U.S. Patent Office under Patent Number 2324462. Click on the "Images"-button to view the patent. You'll need a special browser plugin to access the patent. See the U.S. Patent Office website for more information on this.

Most Halo designs you find on the internet have moving parts. Often they require some sort of tuning capacitor and have a capacitor in the gamma match along with a slider construction that connects the gamma arm to the radiator. I prefer a design without moving parts so that the antenna doesn't get detuned easily when a bird decides the antenna makes a good resting place. I found the design that I describe here in a German antenna book "Antennen Buch" by Karl Rothammel, Y21BK.

Basic Design

The design of this antenna is very simple and straightforward. It basically consists of a half wavelength piece of copper tubing bent into a circle. Between the ends of the tube there needs to be a gap of at least 1 3/16". This is to minimize capacitive coupling between the ends. This antenna is fed by a coax feed line through a gamma match. The gamma match is constructed from 6 1/4" #4 or #6 copper wire. This wire is bent into an L shape. The short end of the wire is soldered on the inside of the loop at the point where the long end of the gamma arm aligns with the halfway point of the loop. See below:

Basic 2m Halo Design



Interested in ultra low cost 2 meter antennas that are easy to build using cheap parts; that require no tedious matching and adjusting; that are almost invisible; that are portable, compact, quickly assembled; and that can be converted into a beam? These antennas are somewhat based on the "V" designs in other projects on this site.

They include the Ultra-simple wire version in figure 1
The Table Top version in figure 2
The 2 element beam version in figure 3

Fig. 1 Ultra-simple "wire" version above made on an SO-239 connector.Designed for hanging from any handy support and can be hung from trees, used inside motel rooms or as a "stealth" antenna.

Fig 2. Table top "wire" version above using a dowel or other simple base.Upper and lower elements must be self supporting. Use aluminum or copper tubing. Disregard the reference to the upper insulator in figure 2

Fig 3. Yagi or Beam version above

This is a variation of the designs above.By adding the extra reflector element about 16 inches behind the driven element and increasing it's length to 20 inches each side (5%), some gain can be realized! According to the article, this version had not been tested but should work with a bit of experimentation. It's no more than a standard dipole with a reflector added to come up with a 2 element yagi with all elements bent forward at a 90 degree angle.


In all of these designs, please note that the center conductor from the coax connection is connected to the element in the "down position". According to the article from which these designs were taken, this helps in adjusting swr!

Simply change the angle and or trim each half a very small amount for best swr. Remember on these antennas that the driven elements have to be insulated from each other and also their support.

The beam version can be made in a "T" shape with an insulated boom between the driven element and reflector and the "T" portion for the support mast. Small diameter PVC would be a good choice.You will have to use your ingenuity for the mounting of the elements to the support so the antenna will maintain the approximately 90 degree configuration. Experiment.

An alternate version of each antenna can be built with all elements either vertical or horizontal instead of in the form of a sideways “V”.These designs can be used from HF up thru 440 or above with a little experimentation.Just dig out that old formula you should have learned for a starting point for the lengths......468/freq = half wave dipole (driven element) and add 5 percent to the length for the reflector.

The spacing should be a little less than .25 wave lengths from driven to reflector.(According to the article, using a director and driven element arrangement would cause problems with a poor match and the spacing would be a lot closer.)Using an MFJ 259b or equal would help with tuning the antenna for your particular choice of frequency, but if you're not that lucky, then just use the old swr meter and very low power while testing. As always, start with longer elements and trim down. It is very difficult to add length!

HF VHF UHF active antenna electronic project


A very simple and efficiency active antenna electronic project can be designed using this electronic schematic circuit that is based on transistors. This active antenna electronic project is useful for a wide range of RF frequencies covering three RF bands HF , VHF and UHF . This simple active antenna is designed to amplify signals from 3 to 3000 MegaHertz, including three recognized ranges: 3-30Mhz high-frequency (HF) signals; 3-300Mhz veryhigh frequency (VHF) signals; 300-3000MHz ultra-high (UHF) frequency signals.

This HF VHF UHF active antenna contains only two active elements : Q1 (which is an
MFE201 N-Channel dual-gate MOSFET) and Q2 (which is an 2SC2570 NPN VHF silicon transistor). Those transistors provide the basis of two independent, switchable RF pre-amplifiers. Two DPDT switches play a major role in this circuit , switch S1 used to select one of the two pre-amplifier circuits (either HF or VHF/UHF) and switch 2 is used to turn off the power to the circuit, while coupling the incoming RF directly to the input of the receiver.

S2 is useful to give to receiver nonamplified signal access to the auxiliary antenna jack, at J1, as well as the on-board telescoping whip antenna.This circuit must be powered from a simple 9 volt DC power circuit ( or a 9 volts battery) and is very useful for use as an indoor antenna .

HF VHF UHF signal booster active antenna electronic project

HF Dipole


A very basic program for calculating the length of each leg of a 1/2 wave wire dipole antenna. Program good for 1 - 500 MHz, although intended for MF - HF useage. This app does nothing more than the standard 468/freq (MHz) type calculations. It was written for DOS many years ago and ported to Windows. The output shows the 1/2 wavelength and 1/4 wavelength design wire length in feet and meters. This app is probably of no help to experienced antenna designers.

Style: GUI, File size: 46K, zipped, 22K.

Update : Minor improvements made Feb 9, 1999

Current Version is:  2 / 9 / 1999

Download the file