On Mon, May 02, 2005 at 08:36:13AM -0700, Lex Mierop wrote: | That's interesting, because I called Horizon asking this specific | question (JR base loaded whip on 50Mhz), and the tech told me | explicitly not to use it. Is this a case where it it's better to | ask for forgiveness rather than permission?
When you lost your plane, who would you ask for forgiveness? Would you forgive yourself? :) Seriously, on the ham band, you can pretty much do anything you want. Your transmitter is supposed to have your call sign on it, no commercial use, one watt power limit. Beyond that, you can pretty much do whatever you want, use any antenna you want, and you're legal. So you can use any antenna you want, and that's just fine. As for `are the antennas the same', well, the same antennas are generally used for 27, 35, 40, 50, 53, 72 and 75 mHz transmitters and receivers, so it seems pretty likely that the transmitters and receivers have whatever matching networks are needed to make the standard antennas (usually they're sized about right for the 72 mHz band) approximately resonant. So common sense tells me that if your transmitter uses the same stock antenna for any module, that you'd use the same rubber duckie antenna as well. But this is just an educated guess -- I haven't actually tested it. Len measured his 50 mHz rubber duckie antenna and found it to be resonant at 62 mHz -- I'm not sure what to make about that. Don't forget that there is probably some wire inside the transmitter that would be included in the length of the antenna (since it's probably not part of a balanced line or coax, but instead just a single wire coming out of the circuit board), and making it longer would decrease the overall resonant frequency. But all the way down to 50 mHz? Not sure. And I'd expect it to be resonant at around 72 mHz rather than 50 mHz anyways, unless his transmitter and/or module was actually designed for an antenna appropriate at 50 mHz. Also, the fact that we're usually holding our transmitters when they're in use changes the properties of the antenna. Though if Len used one of those portable antenna analyzers, and held the analyzer like he would a transmitter, that would probably approximate that pretty well ... Ultimately, there's not much better than real world testing. However, our standard `range check' (i.e. collapsing the transmitting antenna) doesn't work with rubber duckie antennas, so any proper test will probably involve you walking a half mile or so away, and even that's not a perfect test because being so close to the ground will affect the results as well. But it's probably pretty good, as long as you have a clear line of sight between the receiver and transmitter. For a slope plane, especially a foamie, I probably wouldn't worry too much about using a rubber duckie antenna -- it's not likely to get too far away. Ditto for a R/C car. But for a TD plane, especially one that I liked to speck out, I'd probably be very worried about it until I tested it myself. Rubber duckie antennas supposedly reduce the signal by as much as 10 dB -- I don't know how accurate this is, or what exactly it's being compared to, but a factor of 10 reduction in power could result in a factor of 3 reduction in range, and that could give you a range of 0.5 miles (assuming the usual stated range of 1.5 miles), which could be farther than you could fly and still see the plane ... There's lots of guesses and estimates in that last paragraph, but combined they eat up the margin of safety that we enjoy pretty effectively. I guess if your plane really is specked out, and you've realized that you've lost control, you'd have plenty of time to remove your rubber duckie antenna and try the stock antenna again in flight. But if it's 0.5 miles away but with only a few hundred feet of altitude ... I'd hate to risk it. -- Doug McLaren, [EMAIL PROTECTED], AD5RH People don't usually make the same mistake twice -- they make it three times, four time, five times... RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
