HF Digital Messaging - Emergency / Disaster Relief - Interoperative Communications - Ham Radio
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Broadband Antennas: Diagram above: Broadband VertaLoop as described in this article. Wire loop supported by trees, Diamond BB7V Vertical Element with feedpoint resistive matching unit, "ground" radial wire, metal support pole, coaxial cable, ferrite feedline choke clamps, and grounding points. Concepts and Construction of the Broadband VertaLoop Antenna Design and Installation Notes by Bonnie Crystal KQ6XA Background
The Broadband VertaLoop Antenna was developed for use with Automatic Link Establishment to cover HF frequencies from 3.5MHz to 30MHz continuously with low SWR. The antenna presented here is a convenient combination of the Diamond BB7V vertical antenna (modified) and a large irregularly-shaped quad loop of wire, and a wire radial that is used to fine tune the antenna for best comprimise of low SWR on the various frequencies of operation. The objective was to provide improved performance over just a Diamond BB7V alone, or just a quad loop alone. On the higher HF frequencies, the vertical tends to provide good DX performance, while at the lower and middle HF frequencies the the loop provides higher efficiency. Improving Upon the Broadband Resistively-Matched Vertical The Diamond BB7V is a resistively-matched vertical element antenna that normally uses the coaxial cable as the ground reference plane. In this configuration, both the coaxial cable and the support mast are part of the RF radiating element. This article is intended to show how the efficiency of a resistively-matched vertical may be increased, and to provide various design concepts and ideas for a range of broadband antenna types that can be built using the basic resistive-match method. Expand on the Concepts The antenna described in this article may be seen as a prototype antenna, more as a basis for design and use by radio operators seeking to erect broadband antennas. Keep in mind that exact length of the wires, antenna height, and other aspects of this design may be changed to suit the user's application and the environment that the antenna installation is being built for. Advantage of the Large Loop The advantage of the use of a large loop of wire for this application is that the impedance at any given frequency is usually above 100 ohms. This works in concert with the impedance of the resistance in the matching unit to provide the desired low SWR at 50 ohms nominal impedance for use with standard coaxial cable and HF transceivers. Results The installed Broadband VertaLoop Antenna, as built and documented here, provides SWR below 2:1 over 1.8MHz-30MHz range. The antenna system appears to provide approximately +3dB to +20dB of estimated transmit and receive signal strength advantage (depending upon frequency) over the original Diamond BB7V vertical alone as intended by the manufacturer to be installed. This huge advantage has provided a significant improvement in on-the-air performance for this station's ALE operations in the HF frequency range of 3 MHz -30MHz at the 100W to 200W transmitter power level. |
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 Photo above: Looking up at the antenna complete installation on a residential dwelling chimney, with vertical element, bottom loop wire, radial wire, and top loop wire. |
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Photos above left and right: The Broadband VertaLoop being installed. Testing at lower height, being
prepared and tested for SWR, ready for final installation; including
details of the camouflage, the feedpoint connections, and the wires,
cords, and cables. The antenna element has been painted black and the
support pole has been painted to match the color of the residential
structure, in an effort to minimize the appearance of the antenna, and
blend in with the surrounding vegetation.
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| -end of article on Broadband VertaLoop Antenna |
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Autotuner Fan Antennas: |
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| The Autotuner Inverted-V-Fan-Dipole and
Autotuner V-Fan-Dipole Design and Installation Notes by Bonnie Crystal KQ6XA Background Various versions of multi-wire dipole antennas are known and widely used. HF inverted-V antennas called "maypole" antennas, have been utilized with dipoles resonant in the amateur bands. The most common has been the 3.8MHz/7.1MHz resonant version fed with 50ohm coax. Technically, the antenna system consists of two or more dipoles of different lengths arranged radially in inverted-Vee form with a single common feedpoint. There are other configurations possible within the general category of "fan dipoles". Autotuner Problems with Single Wire Antenna Autotuners have been in popular use for both amateur and non-amateur applications, especially when many channels or bands of frequencies are utilized throughout the HF spectrum. Problem frequencies are sometimes found in long single-wire autotuner installations, usually due to combined RF reference plane (tuner ground) and wire resonance resulting in a very high impedance presented to the autotuner. At the problem frequencies, it can take a long time for the autotuner to repeatably find a match, or it may not be possible for it to find an acceptable match. Other problems with the same root cause can lead to excessive RF radiation from the feedline at the transmitter (hot mic syndrome). Sometimes, simply changing the length of the antenna wire slightly is sufficient to move the "problem" to an unused frequency. But changes in the ground conductivity due to rain or other factors can bring the problem back. Multiple Antenna Wires for Fast Autotuning HF-ALE (Automatic Link Establishment) requires fast autotuner action, and the linking functions best when the antenna matching autotuning cycle is completed within a fraction of a second. Application of the multi-wire dipole principle to the autotuner installation provides a solution. In practice, it has been found that there are advantages to certain wire lengths or wire length ratios for autotuner use in the HF spectrum. These ratios of wire lengths present multiple "convenient" lower impedances to the autotuner at any given frequency, enabling it to achieve a matched condition rapidly and repeatably, thereby mitigating "problem frequencies". Autotuner Fan Dipoles in Use for ALE I have developed the two successful versions of an autotuner fan dipole antenna system shown above, through both theoretical and empirical design (trial and error). I am presently using one of these antenna systems on the air 24/7 for ALE, from 1.8MHz to 28MHz. I am using an SG-230 autotuner in this installation, but the principles are the same for most of the common autotuners of similar type. Common Mode Chokes I've set up 3 different SGC autotuner systems at base stations using the common mode chokes (1:1 balun a misnomer) in the control/DC cables/feedlines, combined with a grounding strap to earth. These techniques keep some of the noise from computers and equipment in the station from being conducted into the autotuner's antenna system on receive, and they help choke off RF currents on transmit from going down the cables into the station. In the first two of those installations, severe RFI was eliminated that was present before the "chokes" were installed. In the third, I installed the chokes during the initial installation, and have not removed them to see how much difference they make. Grounding Indeed, many operators are content to simply ground the coax and control cable at the station entrance (good practice). I'm from ye olde school of lightning protection (having built broadcast stations and telephone central switching offices in my earlier career), so you will see additional ground straps present near the antenna in my base station antenna designs. I believe that a direct lightning discharge path to earth ground is a good design starting point for basic lightning protection. I also believe that the possible loss in RF efficiency at some frequencies is worth trading for the added safety that earth grounding at the antenna provides. Temporary Portable Installations For temporary portable installations when no chance of lightning hazard exists, the safety ground strap could be eliminated. The common mode chokes and control feedline ferrites may also be eliminated if no "hot mic" RF feedback or RFI is experienced. Feedback and Field Reports Requested There are other possible combinations of wire lengths and configurations that should function in a similar manner. I am interested in the results of others who are using this type of antenna system or derivatives of it. Feedback or field reports may be sent directly to the HFLINK or HFpack groups. |
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