I decided to play with this to see what I could accomplish. This article will describe a 40/30/20m band end-fed half wave antenna, focusing on its matching unit. It is much easier to wait ~2, know that you are going to see about 86% of the temperature change you would see if you waited until time infinity, and just make a simple correction. The overlaid VSWR plots are shown below. 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. It depends on the selection of the inductance and the capacitor values. Ive been thinking about getting an antenna up for 80m, and Ive come across a number of options that interest me and would suit the space I have available realistically no matter what I go for it will be a very compromised antenna, but still much better than none. For the transformer, a T130-2 toroid will be sufficient, on which we wind the seconder with multiple taps, as shown in the below table. Proudly created withWix.com. Subscribe to our newsletter and receive regular updates on new posts and articles. The 1:64 transformer is wound on 3 stacked FT240-52 toroids, this provides same. It will have six turns around a 1.25" diameter PVC fitting that will bump the 10m resonance down about MHz. Of course, if both ends are raised higher, it will produce a more efficiently radiating antenna, but the portable and temporary installation is always compromised by conditions given by a location and its restrictions. This antenna is working for 80,40,20,10 and 6m bands. The original coil is designed for low power portable applications. The SWR figures were as follows: FT240-43 132ft/100pF capacitor used with a load such that the input impedance Zin is approximately 50+j0, Gin=0.02S; Duffy, O. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. 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. This animated illustration shows the harmonically related current conditions that exist on the EFHW antenna wire on various HF bands. The procedure was to run for around 30 minutes starting from the box cool at ambient temperature. I went with a 49:1 unun to match the unbalanced antenna to the unbalanced feed line. Because of the division of power between the 3220 resistor and VNA input, there is effectively an attenuator of -10*log(50/(50+3220))=18.16dB, so |S21| has a component due to this division. That will be the missing resistor in the resistor bridge of the below circuit. The 2 coils are 35uh and 90uh. I checked the inductance again to make sure this hadnt been overly affected, and it remained unchanged at 109.8H, good enough! Toll Free 800-532-2645 636-532-1523 Fax: 636-532-8259. The original coil is designed for low power portable applications. I have seen and read quite a few reports of making this antenna, and some got away no problem with the 110uH when they came to set up and adjust the antenna, others needed to tweak their coil turns. Let's find out by checking the performance of a transformer with no capacitor for single band EFHWs for 20, 30 and 40 meters. The following is the most up-to-date information related to EFHW Compensation Coil Experiment. Ive spent some time on digital modes over the last week and and my signal reports seem very similar to the pre-80m-extension period, which is good, and I am getting out well past 1000 miles on 20, 40m and 80m digital. For our transformer at 100W (==> 71 Vrms into 50 ) , the core cross section for the two FT114s is 0.74 cm2 , consider the lowest frequency as 3.5 MHz and our 5-turn primary, then plugging in the numbers we find Bmax = 0.012 T. Lets consider a much more challenging extreme: 1000W and the 160m band. (30 votes, Rating: 5.00) - Please vote the article with your valuable star rating. So I built myself an improved replacement matching transformer. I settled on a compact secondary using #28 wire. Start by wrapping the wire around the toroid. 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. Research salary, company info, career paths, and top skills for Software Engineer - EDI Having recently built a MTR2 3-band CW transceiver, I was keen to make band switching on a mountain top as simple as possible. PART - 1 : The EFHW antenna as a radiating and receiving element is a decent multi-band antenna with good efficiency on all bands, provided it is driven at its feed-point using a well configured and well-matched driving source. Typical EFHW Antenna Characteristics & Performance, Influence on Performance due to Deployment Environment, Construction Variables and Transmission Line Interface, Optimum Counterpoise length and Coupling Transformer, Influence of EFHW wire orientation on performance, Good Antenna for any Radio Station is vital The importance of antennas for radio communications can never be over-emphasized. With the cooling curve and a good estimate for the heat capacity of the box, now it is just a matter of running some power through the transformer and measuring the the temperature rise from ambient to be able to determine the power that turns to heat in the transformer. But systems that work better increase the prospects of contacts. The conjugate impedance matching arrangement of this nature is far from perfect resulting in considerable reactive currents on some of the bands. If you need to change it, is necessary to recalculate the secondary turns, and the resonance frequency needs to be adjusted also. clear. The blue trace above shows the results of adding a 6-turn 2 coil about 6.5 ft. from the box. The capacitor makes very little difference to the low frequency behavior, but it reduces the input VSWR significantly at the high end. Loss (to mean PowerIn/PowerOut) can be calculated in dB as -|S21|-LoadAttenuator-MismatchLoss=18.64-18.16-0.03=0.450dB, or an efficiency of 10^(-0.45/10)=90.2%. The coil is just 12 turns of wire wrapped around a 0.7 inch diameter tube, located about 6 feet past the antenna's feedpoint. In an ideal setting, the center of the antenna will have the lowest voltage, consequently impedance, which is why it is possible to feed it by 50 or 75 coaxial cable. The disadvantage of this switch is that one needs one hand to hold it, another to key down, and yet another to tune the capacitor. I used the 14g enameled wire and wound it on the FT240-43. It may be one or even five kilo-ohms. The blue trace above shows the results of adding a 6-turn 2 coil about 6.5 ft. from the box. It is worth experimenting with its length also. I will publish my findings, but I cannot not promise to complete it until early fall. I was a little long on 80m not surprisingly, so folded back about 40cm of wire leaving about 2.1m beyond the coil. The following describes such a transformer using a Fair-rite 2643625002 core (16.257.2914.3mm #43). The point is to get the characteristic time constant for the cooling, which is the reciprocal of the exponent coefficient that Excel shows for the fitting equation. And not all hf antenna installation are same, with most requiring bit of tinkering. To calculate the impedance ratio, use the following equation: A completed tuner is shown here, which depicts the inductance assembly. <> This antenna is only 15m long using 2 loading coils. In the problematic upper bands, the resonant wire significantly improved the in-band SWR compared to the resistively terminate box (purple). The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. This is indeed a classic antenna that is not only, Introduction to Omnidirectional Antennas In the literal sense, an Omnidirectional antenna is an antenna that radiates uniformly in all directions. The technical storage or access that is used exclusively for anonymous statistical purposes. Show more. Ambient temperature is 20, and the core temperature increased by 5 over 120s @ 20W continuous input. Basu Bhattacharya (VU2NSB). Selecting the variable capacitor that can be varied from this minimum to maximum capacity, will allow tuning to resonance for 40, 30, and 20m bands at each of the switch position of the tuner. Throw one end of the antenna wire, extended with a rope, as high as you can on a three, and walk away with the other end and attach it to the tuner. The transformer is wound as an autotransformer, 3+21 turns, ie 1:8 turns ratio. 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. Be cautious, such voltages may cause a quite painful skin scorch. In my research on End Fed Half Wave (EFHW) antennas I found that nearly all have a capacitor in the primary (rig) side of the transformer. is the initial disequilibrium temperature, is the equilibration time constant, and is the heat capacity. Tuning the antenna to its resonance will result in 50 seen at the output of the SWR circuit. Impressive article and so well done that I may give it a try. Yes, indeed EFHW antennas are very useful for SOTA or other field operation. This reveals we only need to compensate for the reactive component. It tests out well on the antenna analyzer for 3.5 to 29.7 MHz. Most of the time 5 to 20W will be adequate for activation anyway. The computation method is also touched on. The band compensating inductor placed on the wire is another story. 100W on a Wire Antenna - Is it good enough for DX? At this point, I'm leaning towards the EFHW, but any of these antennas seem . (Tvir s tvbeszl gpek s kszlkek) Ksznm. PART - 2 : Most EFHW antenna configurations resort to the use of broadband impedance transformers at the feed-point to attain acceptable SWR on all operational bands. Therefore, installation is quite easy, and its tuner circuit is rather simple as well. Very interesting design. Im very happy with the result. 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. If you want to minimize leakage inductance, the way to do that is to keep the two windings very close to one another. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. These four data points are plotted on the Transmission Losses chart shown below as the yellow squares, all but the 80 meter point coming in a bit lower than expected losses based on the VNA measurements. From the condition, it looked like it had been installed outdoors for a few years, so I got it at a good price. Thanks in advance, good health for you in these times, In case you need to perform calculations, use the formula: Where the frequency f is in MHz and the inductance L is in H. To this end, I weighed the main components of the box and looked up the specific heat capacities to get an approximate overall heat capacity for the entire box. As I reexamined my formula, you pointed it out correctly, there is an error in it. No wire counterpoise is used to reproduce the SOTA setup. As the length of the antenna is around 2/3 of the span of a half-wave dipole on 80m, there are two compromises. The usual problems of having the multiple band resonances all line up appear here with the end-fed design, similar to the same issue with off-center-fed designs. It was difficult to work out where it was coming from but I found that it was more noticeable when it was windy outside. The impedance per unit length of a lumped transmission line is where L and C are values per unit length. We use cookies to optimize our website and our service. Since the tuners location is best if it is handy for the operator, one end of the end-fed antenna is also close to the operator.