RADIO HABANA CUBA
THE CBQ ANTENNA: Part 1
A LOW-COST, EASY-TO-BUILD, COMPACT QUAD ANTENNA
FOR THE 27 mHz CITIZENS' BAND
BY ARNIE CORO
Host of "Dxers Unlimited"
SEND YOUR COMMENTS, QUESTIONS AND IDEAS DIRECTLY TO ME AT: firstname.lastname@example.org
This is a typical weekend project... It will BOOST your CB set's receiving and transmitting by almost 6 dB... and that's a lot. When transmitting, it means that your power will be practically QUADRUPLED in the direction that the antenna is pointing to. Receiving, the CBQ will provide not only GAIN, but also will help reduce interference coming from other directions. QUAD ANTENNAS are nothing but FULL WAVE LOOPS, to put them in simple terms.
For 27 mHz, the QUAD DRIVEN ELEMENT is practically a one wavelength loop, laid out in a SQUARE. So each side of the loop is 0.25 wavelengths long, that is, 1/4 wavelength... OK?
Now to make that LOOP UNIDIRECTIONAL, you simply add another similarly shaped element, which in the case of the CBQ is slightly longer than 0.25 wavelength on each side, so that it will work as a REFLECTOR. The DRIVEN ELEMENT has to FACE in the direction that you want to receive or transmit. That's the "theory"... Now, practice:
Antenna's design frequency: 27.000 mHz
Reflector's overall length: 11.44 m. or 37 ft, 6 in.
Each side of the reflector is then 2.86 m. or approximately 9 ft, 4 in.
The reflector element is a completely closed loop, so you must solder the wire to complete the electrical circuit.
Driven element, overall length: 11.17 m. or 36 ft, 8 in.
Each side of the driven element is 2.79 m. or 9 ft, 1 in.
Separation between Reflector and Driven Element: 1.00 m. or 3 ft, 4 in.
(Read below for perfect matching by moving the reflector back and forth.)
You connect the feedline (more about this later) to the driven element. Depending on HOW you connect the feedline, the CBQ sends and receives either vertically or horizontally polarized radio waves.
If you want to communicate with mobile units, with vertically polarized antennas then you must use VERTICAL POLARIZATION. If your main interest is to communicate with other fixed stations using horizontally polarized antennas, then you feed your CBQ for horizontal polarization, which by the way will offer the added advantage of a substantial noise reduction, as human-made interference is mostly vertically polarized! The CBQ is fed directly with 75 ohm TV coaxial cable.
You obtain a nice match by displacing the reflector element along the antenna's support boom until you obtain the minimum STANDING WAVE RATIO. If you are not familiar with that term -- SWR or Standing Wave Ratio -- never mind. You can end up with a nice CBQ using the separation between elements given above, which is 1.0 meter, or 3 ft.,4 in.
With that separation, the antenna will provide a reasonable SWR, but if you really want to make it match perfectly to the coaxial line's 75 ohm impedance, then you must follow the adjustment procedure... i.e., moving the reflector to and from the driven element until the SWR meter shows a 1 to 1 ratio, a perfect match. BUT... never mind if your match goes only to 1.5 or 1.6 to 1; that is not significant at the typical power levels involved in CB radio.
The SWR meter used must be one designed for 75 ohms, as a 50 ohm impedance SWR meter will not provide the correct information.
If you are a perfectionist -- and some CB operators are! -- then you can add a very simple coax balun to your CBQ... Simply wind 5 turns of your 75 ohm coax, using a diameter of about 10 centimeters, tape them together or, better yet, strap them together with plastic straps. This forms a simple balun that stops radio frequency from flowing on the outer braid of the coax.
Erect your CBQ at no less than 5 meters above average terrain...that's about 16 feet, and if you want to use the antenna to communicate in different directions, install a simple TV rotator. The CBQ is light enough to be handled by a standard TV antenna rotator.
The spreaders used to keep the wire loops into a square form can be made of many different insulating materials... The ideal way of doing it is with fiberglass poles. They don't need to be very thick, as the overall length for the CBQ is fairly reasonable.
The BOOM material can be aluminium tubing... 50-mm or 2-inch aluminium will be ideal, as it need be only about 1.3 meters or 4 feet long... You do need a little extra boomlength to be able to hold the spreaders properly. Mechanically, the CBQ is not that all complicated, but it does take a little work to figure out how to mount the spreaders properly. I use two pieces of angle iron or heavy angle aluminium on which the spreaders rest for a certain distance, keeping them well aligned.
To make the antenna vertically polarized, you must interrupt one the VERTICAL SIDES (either one will do) right at the center, and connect the coaxial feedline there.
To make it send and receive horizontally polarized waves, you must break one of the horizontal sides, in this case the LOWER HORIZONTAL SIDE is the obvious choice.
The 27 mHz center design frequency provides good overall performance on the whole CB band, with slightly higher SWR at the band edges.
Just e-mail me at: email@example.com
In Part 2: Adding more elements to the basic design...
02 March, 1998
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