In modern communication systems, shortwave radio systems are the only communication systems that are not subject to network hubs and relay conditions. Once war breaks out or disaster strikes, various communication networks may be damaged, but the anti-destruction ability of any other communication system is far less than that of short-wave communication systems. Today we are with the radio uncle BG5WKP to see the production and installation process of an inexpensive end-feed half-wave short-band antenna.
An inexpensive antenna in the HF shortwave band
Text: Dan Maloney
The right to use the shortwave (HF) band must be obtained with sufficient radio knowledge and pass the relevant radio level examination, and at the same time, there must be radio equipment that is several orders of magnitude more expensive than the $30 portable walkie-talkie.
But while HF shortwave equipment can be expensive, not everyone has to spend a lot to get through. And, since antennas often affect or disrupt the connectivity of radio amateurs, we will look into a simple but versatile antenna design that can be adapted to support outdoor connectivity from large, powerful base stations to portable QRP (low power): end-fed half-wave antennas.
Antenna manufacturing is the ultimate goal of all radio amateurs. I understand that designing antennas can achieve the electronic functions you want, and there is a lot of engineering to make sure they can withstand the tests of the outside world. I think the latter point of antenna construction is more appealing to me personally. Getting the antenna to survive wind, snow, sun, and rain is an interesting challenge, so I tend to spend more time thinking about the mechanical design of the antenna rather than thinking about the RF aspect that someone has already solved.
So I set out to find the right antenna for my situation. Perhaps the easiest antenna to build is the classic half-wave dipole. They have two units, each a quarter of the design wavelength, that radiate from the central feed point, which is where the coaxial cable feeder is connected. Of course, there are a lot of details and complexities, but for me, the basic problem is the central feed point. My radio room is located in the far corner of my house, so it is difficult to install an antenna like this without a long feeder, which introduces unacceptable signal loss. In addition, the dipole for the 80-meter band will be 40 meters end-to-end, which is difficult to adapt to in my narrow suburbs.
For my purposes, end-feeder half-wave (EFHW) antennas are a good choice. It sounds like it really is: a bundle of half-wavelength wires fed in from the end (40 meters in my case, so I can work in the 80-meter band). But it's not as simple as cutting off a 40-meter-long wire and connecting it to your station. The problem is that the impedance of the antenna changes as the feed point moves away from the center. When the feed point reaches the end of the wire, the impedance increases all the way to about 2500 ohms, which is indeed a very poor match for transceivers that expect a 50 ohm load.
To solve this problem, the EFHW antenna needs a transformer to match the impedance. When used to match the impedance between a balanced antenna (e.g. dipole) and an unbalanced feeder (e.g. coaxial cable), these are called balun "baluns". However, in this case, both the antenna and the coaxial cable feeder are unbalanced, so the transformer I built is technically "unun". Whatever you call it, it's very easy to build.
My 49:1 matching transformer is already installed and ready for outdoor connectivity. I considered filling the whole thing with epoxy, but decided not to.
I followed the excellent guidance provided by Steve Nichols (G0KYA) to wind up my 49:1 autotransformer. It's basically just a big ferrite ring core, which I bought from eBay, but there are a lot of options on Amazon, with a couple of solenoid coils. My core is the FT-240-61, which means it has an outer diameter of 2.4 inches and is made of Type 61 material. I used 18 AWG solenoid wires on the windings. While winding, I noticed that the paint layer on the electromagnetic wire was cut by the edge of the ferrite core. After covering the toroid with a cloth friction band to cushion the edges slightly, I rewire it. This way it is by no means superfluous to be designed to handle the output of a 100W transmitter.
Like I said, a lot of the hassle I've done with this transformer has to do with making it work mechanically. I mounted it in a rugged plastic electrical enclosure and provided stainless steel fittings for connecting antenna wires and ground connections. I also installed hoop bolts to bind the antenna wires. A high quality SO-239 socket (for feeder connection) and a 100 pF high voltage capacitor (better matched on higher frequency bands) complete the transformer. Fortunately, the antenna should cover a frequency band of 80 m to 10 m.
The way I put the pulley on the ground on the tree. The effect will be better on trees with smoother bark.
Luckily, this tree of mine is only 150 feet deep, and it is blessed and cursed by the very tall, sturdy American Ponderosa Pine. My lot length and the location of the trees allow the use of full 40-meter-long wires in an "inverted L" type configuration. My plan was to tilt the wires from the transformer as upward as possible in the first tree and then extend it horizontally to the anchor point in the second tree.
This sounds much easier than it actually is. While many hams were lucky enough to hang antennas from wires towering high above the branches, my pine trees were all trimmed off the lower branches, and the first living branch was above 40 feet (12 meters) above the ground. I chose a "smarter job" approach and came up with the idea of using a PVC tube as a putter to basically push a loop of rope onto a tree. Although the roughness of the American turquoise bark was constantly stuck to the nylon rope and PVC pipe as I added the profile, it was actually effective enough to keep the anchor point about 25 feet (7.5 m) above the ground, but much higher than I could possibly have climbed up with my 24-foot ladder, but the risk of falling was greatly reduced.
The anchor point is set in another tree using a similar method, which has attracted a lot of attention from neighbors. People should always take the opportunity to do some "ham friendly" activities, and I assure the neighbors that I will not disinfect their children or interfere with their TV viewing. However, taking karma as an example, the tree I was working on decided the next day to cut down a dead branch that fell and damaged my neighbor's Durango. It obviously came from a much higher position on the tree than where I worked, but it still attracted the strange eyes of some neighbors.
Shake release at the anchor end. Note the safety wire through the spring hole in case it breaks.
One of the most important parts of using trees as anchors for long-line antennas is tree swing. Trees make a lot of circles, and if you tie a wire tightly between two trees and don't allow them to move with the wind, it can have a big impact. However, you want your wires to remain more or less tight, as their shape affects its performance. There are two ways to solve this problem, and I chose to use clothesline pulleys at both anchor points. At the midpoint, the wire passes through the pulley. At the end anchor, wire is tied to a sturdy nylon rope via a dog bone insulator. The rope went through the pulley, through the hard spring down through the trunk of the tree, to the anchor point. When the trees shake, the antenna can be extended or retracted by a few inches without sagging or breaking.
I must admit that the installation is not actually complete yet. The antenna really should be properly grounded, and I'd love to knock a grounding rod near the transformer. However, the main power supply in my home and the main power supply in the entire community is buried directly under our rear fence, which is a high-risk job. Despite having excellent underground location services, I'm reluctant to test its accuracy with my precious self. So I'm going to wait for a decent place.
Still, I couldn't resist trying the antenna, so to avoid RF entering the radio room, I circled the feeder 10 times around the hollow choke and connected it. I haven't done QSO yet, but using the WSPR (Weak Signal Propagation Reporter) system, I was able to reach four continents in 24 hours on the 80m, 40m, 30m and 20m bands.
Hair-trigger: WSPR reports 24 hours at 80 meters, 40 meters, 30 meters and 20 meters.
Here comes the uncle:
Isn't it pleasant to make your own antenna at home on the weekend?