It should be noted that increasing the counterpoise beyond 1m made no noticeable improvement. Essentially, the antenna was not energized. Inverted-V. Large Horiz Loops. To minimize the chance of common mode currents on the coax that can cause the coax to become part of the antenna a balun is sometimes used. Figure 10 - Proper Feed - 1m return (counterpoise). Your feed line is essentially the counterpoise, so a separate one is not needed. I believe this accounts for some of the failures that have been described in ham reports that were supposedly corrected when a 1/4 wavelength radial was added. As far as completing the half wavelength antenna is concerned there is no difference between vertical and horizontal orientation! MFJ-1717. The coupler was tuned to resonance on the bench first using a resistive load. 2). The following data are for the far field radiation patterns and 3D color views of an End Fed Half Wave Inverted-V antenna with the high point in the center at 40 feet and the ends about 10 feet over ground.. Magnetic Loop. However, I am skeptical about how much of these ground (or surface) wave currents, if any, return back to the feed point and contribute to the feed point losses. Let's use 35 Ohms for an example. QRPGuys Portable No Tune Half Wave Antenna – $20. According to Moxon [1], because of the very high impedance (i.e., very low current) at the end of a half wavelength antenna, only a small counterpoise (1m @ 14 MHz) or a few pF of capacitance to ground is required to return the current. The two resistances will add up to a total feed point resistance of 70 Ohms but the efficiency of the system will only be 50%. Here are some other examples of the end fed half wave antenna. Electronic devices and circuit theory (robert boylestad)(1) 10). Electronic devices and circuit theory (robert boylestad)(1) Download. Feeding the antenna at the center is by no means a requirement however. The end result is a distribution of current that is at a maximum at the center and a distribution of voltage that is at a maximum at the ends (fig.1). 18). This dipole got its name because of its length. With an efficiency of 98.6% I don't see any reason to lay down an elaborate radial system for a 1/2 wave length vertical from an efficiency point of view. Bashing listings is not allowed; violators will lose section privileges. I had taken measures to prevent as much coupling as I could to the shield of the coax (no direct connection, fig. Turns Ratio = √(Za/50) The following data are for the far field radiation patterns and 3D color views of a sloping End Fed Half Wave antenna with the feed point near ground and curving upward to 40 feet at the far end. In other words, it was modeled as two half wavelength wires end-to-end and the length of each was kept the same but adjusted to obtain a purely resistive match. then, Testing My Mini 64:1 Half-Wave End-Fed Antenna Transformer. The test above proved that this is not possible. However, in the real world it is possible to connect the return side of the LC circuit to the ground side of the feed line as shown below (fig. It is often commented that the End Fed Half Wavelength Antenna needs no "counterpoise" or radials to work. On a scale of 1 to 5, this kit is rated at 2 depending on your experience. 1,181 3,813. MFJ-1716S Dual Band Flexible Duct HT Antenna - SMA . For permanent home use you may also want to try a conventional L-network consisting of an inductor in series and a capacitor to ground on the antenna side. With an “end fed halfwave” where you use a voltage transformer it is most convenient for the earth / counterpoise side of the transformer secondary usage. A standard recommendation (see QST, March 1936, p. 32, "An Unorthodox Antenna") is an 84' long end fed and a 17' long counterpoise (6.5' for 20m). Typically, a parallel tuned circuit is used at the end of the antenna with the feed line link coupled or tapped to the coil in the circuit. At this point the antenna worked well. 11). Any improvement that you make to this lossy ground will improve this situation. A longer "counterpoise" offers nothing to the performance and this can be verified by computer modeling. Note that a perfect match occurs when the vertical is about 0.47 wave lengths tall. I hope that I have shown that feeding a half wave length antenna on its end without an extensive ground system or "counterpoise" is practical and it really works. The 0.05 wavelength "counterpoise" length is what I also determined empirically as to being the ideal length for the counterpoise. If you adjust your coupler into a resistive load on the bench first and then adjust the antenna for a proper match then you should have a resistive match that will minimize current through the coupler and through the "counterpoise". MyAntennas End-Fed Half-Wave Antenna for 80 – 10 Meters Bottom Line The EFHW-8010-1K provides an easy-to-deploy antenna that can be operated on all HF bands, except 60 meters, with a low enough SWR to be used without an antenna tuner on most bands. Mobile Loop. Energy can be fed anywhere along its length and the impedance will increase as the feed point moves away from the center (more voltage, less current). If you do plan on operating over a large range of frequencies then the "counterpoise" requirement increases but a match can usually still be obtained by retuning the coupling circuit. At first thought this would seem to be impossible yet many people have done it very successfully. 19). So far this discussion has had the antenna floating in space for the most part. A BNC female connector is supplied for connection to your radio. In the past wide band HF loop antennas have not been cheap, normally costing $300+ dollars from manufacturers like Wellbrook. I first adjusted the LC circuit using a 4.7k Ohm resistor (fig. The user supplies the 1/2λ wire for attachment to the captive S.S. hardware. MFJ-1717S High Gain … MMANA Modeling. I have successfully used 100W without any problems with this coupler. MFJ-1715S Superflex ThinDuck HT Antenna with SMA Male Connector. 9:1 for Za = 4050 ohms In some cases simply the stray capacitance of the LC circuit to the environment and feed system will provide enough counterpoise as shown here (fig. MMANA Tut 4. I am always learning so you may see this page change from time to time as I learn more and correct errors. December 19, 2014 No comments. It should be noted that in these cases adding the often recommended 1/4 wavelength radial will not necessarily alleviate the problem either. 21). End fed wires are possible using a 49 :1 balun and 33 to 72 feet of antenna wire. 5) there doesn't seem to be much of an issue. For a link coupled antenna this would all work fine. lengths from the antenna as antenna system loss or not is up to you. Amateur Gear eBay Auctions Ending Soon Amateur Radio Equipment … Posted date: January 19, 2021 in: Antenna. This is not to say that the antenna cannot be used at other frequencies. An Off-Center-Fed (OCF) dipole with total span of 84+17=101 ft, with the feed point at 1/6th of the total span (101 / 6 ≈ 17 ft = 17%) from one end ( = 83 / 17 OCF). The coupler can be adjusted to compensate for any reactance that may be present due to other lengths but it is around this length where the whole antenna system is the most stable and provides a purely resistive load to the coupler. Figure 17 - Lumped LC Circuit Replaced with 1/4 Wave length Stub (End Fed Zepp). The direction of radiation is the Blue trace on the polar graphs. Receive only. Depending on the gauge wire you opt to use, the radiating element may be stored wrapped around the device for storage and transportation. The graph below (fig. For the 1/4 wave length vertical with 35 Ohms of feed point resistance, the input power will be divided between the ground resistance and the feed point resistance. In this case many people say that a large radial system consisting of up to 120 radials are required just as is required for a 1/4 wavelength vertical (see below). In practice the impedance at the end of an end fed half wavelength antenna is on the order of 1800 to 5000 ohms. The ground plane is assumed to be perfectly electrically conducting (PEC). For VHF, anyone who has ever used the old AEA Hot Rod end fed half wave antenna for an HT know the drastic improvement it made over a 1/4 wave antenna. Stock availability and shipping volume can delay shipping. There have been many heated arguments between folks who really only differed in whether they sided with the practical aspects of using this antenna or the sterile theoretical description of the antenna (fig. I want to improve an existing end fed wire in an attic used only for shortwave listening. Being able to feed the antenna from an end may make it a good solution for many support configurations. 16). End fed wire on a telescoping fishing pole. Mobile … When the antenna was properly installed I adjusted its length along with a 1m long counterpoise to obtain the same 50 Ohm resistive impedance without having to retune the circuit (fig. However, I now believe that the way to minimize common mode current in circuits where the feedline may not be totally isolated is to use a resonant half wave length antenna adjusted to provide a resistive load to the coupler. The far end hangs down. The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a quarter wave parallel transmission line stub of Lecher system form. The AC Sweep Type will be left on Linear. Figure 3 - Impedance versus "Counterpoise" Length for an End Fed Half Wave Free Space Antenna, Figure 4 - Two Half Wave Length Conductors End-To-End for Determining Resonance in Modeling. Even though there should be no limitation as to what frequency this antenna can be used on my discussion here is mainly related to HF since this is where this type of antenna is most practical. It is designed as a highly portable wire antenna, easily set up as an inverted V, horizontal, sloper, or in the case of higher frequencies, a vertical radiator. What I want to do is cut the antenna wire in half so as to make a simple dipole and then feed that dipole with ladder line straight to the radio. pgs. MLA-30 Active HF Loop Antenna. 8:1 for Za = 3200 ohms The length is 43 ft fed into a vintage tube receiver using 75 ohm TV coax. I use this type of antenna extensively in the field with my little single band QRP rigs. I don't expect anyone to take this information as gospel. There may have been some stray capacitance but there was not enough to "complete the circuit". This does not mean that an elaborate ground is required for an end fed half wave length vertical antenna to "have something for the antenna to push against" or to "complete the circuit" as is often said. MFJ-1717 High Gain Dualband Flexible Duck HT Antenna - BNC . Figure 8 - Moxon's Ideal End Fed Half Wave Length Antenna (with a bit added by AA5TB). Use the formula below to determine the turn ratio. The end fed 1/2 wave antenna, much like the 43 foot vertical, has become a cult phenomena. The dipole can be straight or bent. So how is this possible? In the graph below I have plotted what MININEC predicts for the impedance of an end fed half wave antenna versus the "counterpoise" length. MMANA Tut 3. This is the feed impedance and SWR of an end fed 80 meter half wave with an ideal 49:1 transformer and good stable counterpoise or ground. MMANA Tut 2. Based on the predictions above a 6:1 to 8:1 turns ratio will be close enough. 0.47 free space wavelength) was determined by modeling the length required to obtain resonance (jX = 0) when the "counterpoise" was the same length. Figure 5 - Ideal End Fed Half Wave Length Antenna. For this reason a bunch of (>1/2 wave length) radials should make an improvement on ground wave field strengths or on very low elevation sky wave paths by lowering the pseudo-Brewster angle. Of course this pattern will repeat itself out for many In circuits where the feedline is physically isolated (link coupled, fig. In reality, most couplers will be able to tune out the reactance to achieve a much lower SWR but you should try to operate at the point where the antenna's load is resistive, especially if the feed line and coupler are the "counterpoise" as in figure 15. It is important to use a counterpoise. The closer any antenna gets to the ground the more current is induced into that lossy ground. Figure 11 - Improper Feed - No counterpoise at all, no worky. Feed Impedance. RF signals are electromagnetic waves that consist of an … If you design a coupler to feed an impedance of 2450 ohms (7:1 turns ratio), then the expected SWR for a vertical monopole operated against ground is shown below (fig. As the "counterpoise" itself approaches a half wavelength, the impedance approaches 5000 ohms. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until an impedance match is obtained. If you take this a few steps further you can see how the antenna evolves into a classical J-Pole antenna that is often used at VHF with good success (fig. Horizontal is 90°. There is just no need to unduly require it to "complete" a half wave length vertical, whether fed at the end or anywhere else.
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