A high performance, high gain,wide
bandwidth antenna for VHF and UHF.
A broadband antenna is a must for 440,450MHz. The main problem of most antennas (as well as the matching system) for this band is the ability to achieve a low VSWR over a wide bandwidth while maintaining a high gain. This limits most antennas to a bandwidth of 1 or 2 MHz. By using quagi dimensions for directors and all spacings, the bandwidth of the driven element can be widened to 10 MHz by changing the driven element and the reflector to the double quad design, then bending them at a 90 degree angle. When this is done, all three radiating elements on the driven element reinforce each other at the first director for added gain on both receive and transmit.
Inan antenna system with two quad loops driven at the center, there is an impedance of approximately 75 ohms. When used in an array , this can drop to around 50 ohms , which can be driven with 50 ohm coax directly without any matching device. This helps with the simplicity of the easy-to-build broadband design.
The antenna is made from #10 AWG house wiring because of its stiffness and low cost. The boom is made from wood (I " x 2" pine for 70cm and I " x 3 " for 2m) . DO NOT USE METAL, as it will interfere with the element length and radiation pattern.
Using a #39 bit, drill all the holes in the boom, except the one for the driven element, which does not get drilled completely through. The undrilled part of the boom between the two holes keeps the two wires from touching at the feedpoint , and is also the element center support. (See Figures 1 and 2 below)
The directors must be cut for the higher portion of the band to be used, so as not to act as reflectors at the highest frequency used. When making the directors , cut care-fully, file both ends flat , and then deburr to get the correct length. (A burr on one end of an element can change the length by one MHz or more.)
When mounting the directors,wrap some #18 bare wire around the directors on both sides of the boom, then solder so that the elements will not slide down through the boom.
If the antenna is to be stacked side by side for vertical radiation, the mast can be metal as it will not affect the pattern . For horizontal operation, the mast and supporting structure should be nonmetallic if they are in the field of the radiation pattern.
The support for the front of the driven element and reflector is made of wood, approximately 1/4 " x I" x 12" long . The center of the two wood cross supports is held in place by a nail approximately 2" long, bent over to hold the support from moving . Three notches position the elements in place and two # 18 tie wires hold and support the elements to the supports. This ensures that all three vertical elements are the same distance from the first director.
The coax should go from the driven element back through the reflector, then be looped back to the supporting structure. The mast should be of a non-conducting material : wood, fiberglass , etc. A metal mast will change the gain and pattern of the system.
When the antenna is complete. adjust the VSWR by unsoldering the side of the driven element where it is soldered together, then slide it to increase or decrease the length. When you have completed the antenna, and before installation , coat the wood with a latex-base finish to protect it from the weather.
This antenna is similar to the parabeam (''J '' beam), but uses the quagi spacings and is bent so that the first director is in line with the outside of the driven element.
At the 1991 W3MIE Field Day, site tests were conducted on the 440-450 QUAG-V, and it showed a much-improved gain and bandwidth over a quagi of the same size.
" The VHF Quagi," QST, April 1977.
" Two Meter FM Antenna: Ham Radio, May 1971.
" Multi-element Twin-Loop Array Antennas For VHF/UHF," QST, January 1980.
" 10-Meter Lazy
Quad", QST, July 1968.
Drawings, diagrams, and tables referenced in article:
The author thanks WA3ANA for his
assistance in preparing
this article and building the prototype - WB3JDI for VSWR and
gain tests at the 1991
Field Day site, and K3VLQ and all the others who assisted
with the tests.
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