Brodhead Pietenpol Association

Original Posted By: Timothy Willis
Subject: Pietenpol-List: Antennas-- more

Original Posted By: Jim Ash
Tim...I've pretty much exhausted my efforts at trying to explain the pros and cons ofsome of these designs. I don't mean for that to sound short, just that it takesa lot of effort to attempt to explain this science in a clear manner via text...in person we could cover some ground pretty quick, but for me to keep poundingthe keys... well, I'd rather direct you to a couple of textbooks and manuals,all of which would also prepare people for FCC testing... i.e. ham radiostuff. Maybe someone else might chime in. Honestly, if a guy can afford to spend the bucks, the antenna that Mike Cuy showedus is sounds like a reasonable good option. Aside from the price, it is 1.adaptable to a variety of installations, and 2. apparently a decent performer.When I say, "aside from the price", keep in mind that is coming from someonethat doesn't see a problem in fabricating a decent antenna out of stuff I alreadyhave laying around. If I didn't have much of a clue of these things, I'dlikely just shell out the extra scratch and be done with it. There are a fewitems on my project that I will be doing exactly that. Just for grins, here is an antenna I built about 10 years ago when I was in flighttraining. At the time, I lived about 20 miles north of Tulsa InternationalAirport and wanted to listen to the radio chatter from the house. At that distance,the rubber ducky offered intermittent reception at best, and that wasif I were outside. So, I constructed this simple vertical dipole from PVC andspeaker wire. For those unfamiliar, I simply split the center conductor andshield of my RG-58 coax and connected (soldered) the speaker wire directly tothese leads (see crumby Paint Shop Pro drawing... circa 2003) To hold these elements spread apart, and vertical, I simply used electrical tapeto secure the wires to a length of 1" PVC and stuck it in the air (about 5 feetoff the ground)... making for a vertically polarized dipole. As you can see, the length of each is element is 21.5" long. For anyone doingthe math, that puts the fundamental frequency around 130MHz, not the 122.5MHzI suggested we shoot for in my previous post. Why? Well, I didn't plan to navigatewith this antenna, so I didn't care about the lower portion of the airband... my interest was 118-137MHz, so I picked a nice round number near the middleof the voice portion of the band and hit the calculator... 234/130MHz=1.8feet...nice round numbers, right? If I multiply 1.8*12 I get 21.6 inches...21.5 was close enough for this project, and since I was sharing this informationwith a friend at the time, I proceeded to keep it simple. Here is a shotI took for that friend to illustrate the overall size of this simple antenna...pretty small eh? I know, I know... I look way too serious in this photo. Ha!In case that's not geeked out enough, here is a flame thrower I built for a wirelesscommunications class project back in 2003. I am normally into HF communications,which means BIG antennas and BIG power, but in order to keep thingssimple (small) and level the playing field, our instructor asked everyone to designand build an antenna for 144MHz... the 2M amateur radio band. I'll cutto the chase and simply describe that this a 5-element cubical quad. A quad isa full-wave loop antenna... meaning that the elements are one full wavelengthlong, not a half wave, or a measly quarter wave (as we are talking about withour air band antennas). A quad is efficient, quiet (low noise signature) andprovides high gain. The main drawback, especially in the HF world, is thatthe are very large and have a high wind loading... i.e. they get blown aroundor load up with ice and crumble easier than other designs. How good is this design?It smoked everyone in the class, including my instructor. The next closestantenna was my instructor's TEN element yagi... picture an old TV type antennawith the elements stacked one in front of the other. For you other geeksout there, the numbers that matter... 9.3dBd gain. Three s-units over a dipole!I was able to trip a repeater in Choteau, Oklahoma, roughly 40 miles awayon ~3 watts of power with the antenna on a 10' mast. The trick? Fine tuning(SWR was well under 2:1 for the entire band, and 1:1 at our test frequency...146MHz), and low loss feed line... I used bury-flex LMR400, which only costme about .1dB of attenuation at these frequencies. Oh, just for the record,there was a well built J-pole in the mix that belonged to my lab partner... weak!But then again, I don't think anyone wants to mount a quad to their airplaneor contact an airport in the next state! Any questions? LOL!--------Mark ChouinardWings, Center Section and Empannage framed up - Working on FuselageRead this topic online here:http://forums.matronics.com/viewtopic.p ... ______Date: Sun, 25 Mar 2012 01:04:22 -0400 (EDT)