Connecting the Radio to the Sky

Connecting the Radio to the Sky
In this column we will look at a Herringbone log periodic antenna and another LEO satellite antenna. Photo A depicts such an antenna. A log periodic antenna such as the one in photo B works very well over its design range. However, when you look at the third harmonic —that is, run a 145-MHz log periodic at 435 MHz —you get the pattern in figure 1. Ever look at most of your larger outdoor TV antennas? The longer elements are usually swept forward like the ones shown in photo C. There is a very good reason for these swept elements. TV Channels 2-6 cover the frequency range of 56 MHz to 88 MHz. TV Channels 7-13 cover 172-216 MHz. This puts Channels 7-13 within the third harmonics of Channels 2-6. Fifty years ago it was found that if you sweep the elements of a log periodic forward about 30 degrees, the third harmonic pattern really cleans up, as in figure 2. In short, a 145-MHz-band log periodic on 435 MHz is going to give you a pattern like the one shown in figure 1, but if you sweep the elements forward, you now get the pattern in figure 2 at 435 MHz. For years I have tried to make simple log periodics, but the booms are the tricky part. On a log periodic the booms are a parallel transmission line, much like 300-ohm twinlead, but with an impedance in the 100-125 ohm range. With the right density of elements, this 100-125-ohm impedance is pulled down to 50 or 75 ohms, depending on your application. Get the impedance of the boom right, get the density of the elements right, and you can directly solder 50-ohm coax to your antenna and still get a good SWR. Construction For the boom I used 1/2" x 1/2" square hardwood about 18 inches long. For the boom conductors I used .7-inch wide strips of PC board. The PC board was coated with construction adhesive and clamped together overnight. Again, the boom is a transmission line and the impedance is important. With wood and fiberglass boom materials you want the final dimensions to look like figure 3 —that is, a 1:1 ratio for the width and separation. You can experiment with different materials for the boom and transmission lines, but again, you want to keep that width/separation ratio about 1:1. Elements For the element material I used bronze welding rod. Itstrong and easy to solder. Some kind of template to help hold the 30-degree angle is helpful, but with some big pliers you can bend the elements at the base a bit to help line up the elements after your solder job. The second and front elements would have the left side on top and the right side on the bottom. The number one mistake I have seen in log periodic construction is getting the top and bottom elements out of sequence. On a proper log periodic the phase of each element reverses. Sometimes the transmission line is reversed at each element; in my case I just alternated the elements top/bottom as in photo C. Also see photo D. In photo E you can see the coax attachment is pretty simple. The coax center conductor goes to one side and the coax shield to the other side. A few cable ties, or even duct tape, can hold the coax in place. This antenna is a design in progress and I plan to give the AMSAT lads an opportunity to fully test the prototypes at the Dayton Hamvention® demonstrations in May. So look for future tweaks to this antenna, but the prototypes really are looking good. Not into the Low Earth Orbit (LEO) satellites? Ita great dualband repeater antenna, as you can see in frequency Plot 1. Need something really portable? Build the antenna using bare # 10 household copper wire for the elements. Now you can bend the elements against the boom for a very compact antenna. Then, when needed, just bend the elements sort of straight and to a 30-degree angle again. Itnot pretty, but it is very compact and portable. You can usually wad up the elements about a dozen times before they have to be replaced. The original antenna was tested by WB0TEM on the antenna range at the Central States VHF Society Conference. The 144-MHz gain was 5.3 dBd; 435 MHz was 7.6 dBd. For the marketing lads, this would be 7.4 dBi at 144 MHz and 9.7 dBi at 435 MHz. Can you come up with some really innovative ways to build or use these antennas. How about some feedback? Let us know what creative ways you have modified this design. From the Other Side of the Atlantic This next construction article is a bit more self-serving. Last October I attended the RSGB (Radio Society of Great Britain) Convention at Milton Keys and had the opportunity to upgrade my UK Intermediate license to Advanced, making me eligible for my new call, G8EMY. Oh, I can give a lot of examples of how difficult that test is for an American, much less a Texan. There is no question pool —a study guide, but not a question pool. Therefore, you have to learn how the British mains power system is wired, different bands, and British methods in wording questions. Many questions are worded with double negatives. Many questions have all four answers correct (all of this was not an option), and you have to pick the correct”answer. They also have schematic symbols for components that we do not have. And, of course, you never run a tube amp grounded grid; you earth the grid of your valve. I did have a bit of fun with the local Texas WOUXUN dealer when I asked him if he had ever seen the talkie in photo F. He said, of course!”until he looked more closely. This is the 4-meter/2-meter dual-band model. WOUXUN has to ship the antennas separately; they are too long to fit in the normal box. Maybe a 50/146-MHz model is in the works for the U.S. market? Next Up The WA1ZMS experimental 4-meter beacon has been heard in Europe. With multi-hop Es and the rise in the F-layer, cross-band QSOs are a real possibility. Over 30 countries now have the 4-meter band, and commercial antennas are few and far between. Also, while U.S. hams cantransmit on 70 MHz, we can listen, and cross-band QSOs are quite legal. In a way this is a reverse of the 1980s, when U.S. hams would talk to Europe on 50.110 MHz, and Europe would come back on 28.885 MHz. I worked several UK and Irish stations this way. This time we still transmit in the 50.100-50.125 MHz DX portion of the 6-meter band, and listen at 70.100 MHz. Now all we need are some simple 70-MHz beams. These antennas are taken from the 6-meter beams of a few months ago, but just are a heck of a lot easier to build. Again, a simple dipole element is used as the driven element. With careful placement of other elements, the 72-ohm dipole is loaded down to 50 ohms and directly fed with coax. Note that the 2-element Yagi of just the driven element/director (DE/D) has more gain, is physically smaller, and has a better front-to-back ratio than the driven element/reflector. Just about all the European antenna projects emphasize the DE/D combination for their usually smaller ”for outdoor antennas, but this has never really caught on in the U.S. The DE/R does have better bandwidth, but for the 4-meter band we are looking at pretty much a spot frequency. One down side to using a simple dipole for the driven element is that a lot of the RF energy tends to run back down the outside of the coax. This is a problem with transmitting; the rig gives you RF bums when you get near the mic or touch the case. At this time U.S. hams can only use this antenna for receive of 70-MHz signals, but that RF on the coax goes two ways. Stray RF, one of the few TV stations still on VHF low, and noise from digital appliances are picked up by the coax and work their way back to your receiver. A balun on the coax will take care of this noise problem, and there are two types of baluns that are pretty easy to build. The Noise Balun What we need is an RF choke so the coax itself is coiled into a choke as shown in photo G. Use RG-58, RG142, or another small coax for the last few feet of your coax run. I d stay away from RG-8X for this application. In the hot sun the foam dielectric tends to turn into gum”and the center conductor moves around, often touching the shield and shorting out the coax. Just make three or four loops out of the coax about 4 inches in diameter. You can hold the coax loop in place with cable ties, electrical tape, or even garbage-bag ties. Ferrite Balun A ferrite core increases the inductance of a coil. A ferrite bead on the outside of the coax increases the inductance of that bit of coax. Two or three large ferrite beads that fit snugly over the coax should do it. The clamps on the beads are easy to use, but the plastic holders are not UV resistant and tend to fall off after a few months. Ibeen there and done that! Some electrical tape, or better yet, heat-shrink tubing, will protect the plastic holders that clamp around the ferrite. Construction The boom is 1 1/2" or 2" wood. The elements are 3/16th" bronze welding rod. However, almost any rod material in the 3/16-to 1/2-inch diameter range can be used. Itkind of hard to solder to aluminum, and yes, those guys at hamfests selling aluminum solder sure make it look easy, but Inever been able to get it to work like that. So for aluminum elements you will need to come up with your own way of attaching the elements with some kind of clips or solder lugs for the coax. Good DXing! Discounts for a site look here:  

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