The last few years have seen a resurgence of interest in the end fed half-wave wire EFHW antenna. It turns out that we can easily measure the effective parallel secondary capacitance by looking at the primary self resonance when the secondary is open. Consequently, lets pick the lowest frequency 7,000MHz that we need to be able to tune the inductance to, for each of the taps on the inductance. Tap and primary high-side are soldered together near the middle of the secondary winding; the ground side of the autotransformer is located over the low-end of the secondary turns. Thirty turns #28 are wound on the two FT114-43 cores. clear. And since I am interested in building EFHW antennas for a single band or a few bands, how will that impact my design? It is not necessary to calculate the above; the results are sown in the following table. Having recently built a MTR2 3-band CW transceiver, I was keen to make band switching on a mountain top as simple as possible. This can then, fed with coaxial cable, in many cases be . Dutch radio amateurs in particular have been active in this area: John, PD7MAA's blog - http://pa-11019.blogspot.com/2012/04/149-transformer-for-endfed-antennas-35.html Jos van den Helm's (PA1ZP) article published in Radcom: "A 3- or 5-Band End Fed Antenna", February 2016, Radcom, published by the RSGB in the UK. There could be any number of reasons for that, where I put the bends in the wire, the location of tree branches, etc. These inductances are required in determining the right capacitance values, of which two values are important: the minimal and the maximum capacitance values. The cooling rate is more difficult to calculate because it involves how heat moves through the structure, but it is easy to measure. 2006. The commonly used primary compensation capacitor does not work very well for this winding method which keeps the leakage inductance as low as it does. A half-wave at the lowest band of interest will also work well on all harmonically-related bands which has become particularly attractive to those wanting a quick portable wire antenna. Wideband RF components ultimately succumb to the non-ideal behavior of materials and assembly methods. With tension on the antenna wire, you can just roll the coil up/down the wire to try different positions as you are optimizing for a particular band. Klubunknl folyamatos a tagfelvtel. There's seemingly conflicting information out there, such as on QRZ the consensus seems to be to either use no choke on the coax, or choke it some ways away from the feedpoint - WA7ARK recommends .07 wavelength, which is about 9' on 40M. The Counterpoise is a short, typically one twentieth (0.05 * ) of the wavelength. The other end, attached to the transformer and antenna analyzer, is about 1 meter above the ground. It is not necessary to calculate the above; the results are sown in the following table. Since it is a EFHW and not a non-resonant end fed, the wire length for 40m is about 67 feet. EFHW Antenna Frequency Compensation Lead 5,144 views Jan 20, 2019 188 Dislike Share Save Steve Ellington 9.92K subscribers 8 inches of wire attached to the antenna post can lower the frequency. Intro EFHW Compensation Coil Experiment Steve Ellington 10.3K subscribers Subscribe 29K views 4 years ago The little coil helps place the EFHW resonance points where we want them. When you are discussing a distributed capacitance, it is difficult to know exactly what you are measuring. Although it is simple to build and is a multiband antenna, it has several attributes that . Hence, there is no optimum single-fit solution. I have an EFHW and the SWR is acceptable on all the bands with the exception of the WARC bands which is to be expected since the harmonic relationship is different. It is quite difficult to satisfy both requirements of achieving proper impedance ratio and resonance of the tank circuit at each tap of the inductance. The negative effects of local QRM on an, How does antenna radiation occur? Below about 7 MHz, transmission losses increase as the core temperature increases. Impressive article and so well done that I may give it a try. In the problematic upper bands, the resonant wire significantly improved the in-band SWR compared to the resistively terminate box (purple). Above 7MHz, there is very little temperature dependence at all. The antenna is effective and the materials and construction are of a very high quality. Wow, Gary! Wrap the first wire around the toroid 27 times (I used a black wire for this). (Tvir s tvbeszl gpek s kszlkek) Ksznm. The tests here were using a dummy load on the transformer, and that did allow confirmation of the design and expected loss at 3.6MHz. The biggest uncertainty in these measurements was the actual power level used. Note: More extensive experiments including operation at HF frequencies higher than 20 meters, harmonic versus fundamental operation and transformer efficiency is posted in this blog. We need to measure the inductances of the toroid with the tapped coil. My concern with the KN5L design is mostly about voltage breakdown through the enamel insulation where the secondary crosses the primary ground. No capacitor was installed. When the transformer was mounted in its box and the output connected to an antenna connector feedthrough, the observed capacitance increased dramatically to about 6.0 pF. It requires a set distance to hang both ends of the antenna, and that is not always given. Select components with good insolating materials and properly chosen, relatively high voltage rated parts. The vector sum total of all currents flowing on various physical antenna wires plus other currents like CMC on the transmission line or parasitic induced currents on masts or nearby objects must always be equal to zero. The VSWR plots below show results from the same transformer tested on the bench with minimal output capacitance, with a primary compensation capacitor, either 100pF or 27pF included, with the transformer installed in its aluminum box including antenna connector feedthrough, and with the transformer with an 18 clip-lead on the antenna output. They will be adjusted for resonance at or close to the CW portion of the bands. Lets do a quick calculation to see how much an issue is this output capacitance. Can a diode be used to rectify signals smaller than its threshold voltage? End Fed Half Wave (EFHW) Antenna Upgrade Part 1 - The Matching Transformer. The second smallest number I measured was 2.7pF, derived from the self resonance at 2.72 MHz with the magnetizing inductance (measured at 25.8 H) with the secondary open and the output wire folded to a small 1/2 length. . I came up with a very simple way to build these coils on the antenna wire that proved quick and easy to change and test, involving small piece of PVC pipe and a piece of Velcro shown below. During portable operations, I noticed that I forgot the toggle switch in the tuning position, just to discover that no one calls me back to my relentless CQ-ing. The goal of the temperature characterization is to determine how much temperature difference (core to ambient) there will be for a given level of power dissipation. Once that's done, the instructions say "adjust the coil as necessary" to bring the other bands into resonance. There is lot of controversy around the EFHW antennas. Update: Full size 132ft EFHW For one month I replaced the 66ft wire and loading coil and replaced it with a full 132ft of wire, connected to the FT240-43 balun. Even at only 100W, the RF voltage on the high Z output of the transformer can easily be >1300 Vpp, so this is not an idle concern. Start by wrapping the wire around the toroid. Typically the IC-751A would read about 60W on its power bridge, the Siliconix would show >50W on the 50W scale and 45W on the 500W scale, so there is ~20% uncertainty. The technical storage or access that is used exclusively for anonymous statistical purposes. Subscribe to our newsletter and receive regular updates on new posts and articles. HA5CBM Mikls, Finally the tank circuits specification is complete, which is sown below. Thermal gasketing between the cores and the aluminum heat sinks could help a lot, as would a single piece of aluminum rather than the stack up of thin sections I used. For calculating the length of a half wave in metres, I use 141 (MHz) for wire elements. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. I am already seeing interesting things with it, but not enough measurements to make any conclusions yet. But I would caution its usage for higher power than 10-15W. This totally un-rigorous calculation just gives a quick idea of how much you have to be careful. The parasitic reactances near the feed-point also vary between installations. The wire as I strung it up seemed to resist having a good SWR for the 20m band. However, I think I made a good choice with a Hammond 1590A diecast aluminum box. The End Fed Half Wave antenna or the popularly known EFHW antenna has been around almost ever since the inception of HF radio. It performed well on both the 2m and 70cm bands. I did struggle with the C in pF calculation..I seem to be getting hung up on the correct decimal placement..for example, for the 40-meter Capacitor requirement, I get .000000000258: C pF = .000001 / [ ( 2 * 3.14 * 7 MHz) ^2 ] * 2 uH 1) More primary turns and primary inductance improve the transformer efficiency and low end performance. (30 votes, Rating: 5.00) - Please vote the article with your valuable star rating. It matches the impedance fairly well to the half-wave antennas high impedance at one of its ends. Thanks again Jim. I drilled a couple of holes at each end of the former, a small one for the wire to pass through and a larger one for a bolt and wingnut for wire attachment as I wanted the end result to be portable across different antenna projects. The previous table already contains these measurements. |k"^N>F.AnJkJsq'PV}o0I. The SWR for 30 meters, 1.05:1, was the lowest of all three antennas since the transformer was tuned for impedance match at 30 meters. So, how to construct an end-fed half wave antenna? Lets call this element the LoadAttenuator. I got the, http://pa-11019.blogspot.com/2012/04/149-transformer-for-endfed-antennas-35.html, End Fed Half Wave (EFHW) Antenna Upgrade Part 2 - The Loading Coil. The overlaid VSWR plots are shown below. Winding the wire manually was a bit tedious but once done I temporarily held the wire with cable ties and checked the inductance. Another completed tuner, which is more compact then the pervious one shown, also it is lighter, more suitable for portable operations. Ive been wanting to put together a decent multi-band antenna for a long time and finally settled on an end fed half wave antenna. A 100pF silvered mica was connected in shunt with the transformer primary.
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