Interesting guys. I have had up a full size Telrex monobander (2 el) since 1995. It has 66 ft full size elements on a 14 foot boom. It has been a killer since it went up. One of the best investments I have ever made. It is 6 ft above an M2 rotary for 80/75 at 112 ft.
73, John, W4NU K4JAG 1959 to 1998 Sent from my iPhone > On Mar 28, 2018, at 8:29 PM, Steve Babcock <[email protected]> wrote: > > Hi Bill, > (If you don't mind, I have shared this to the reflector since I have rx’d > other similar questions) > > I have lately been using 4NEC2 (free and very good program: > http://www.qsl.net/4nec2/ <http://www.qsl.net/4nec2/> ) > My model is not a “standalone” 40m yagi, but includes the 80m Yagi on the > same boom. (the 40m Yagi shares the boom with the 80m Yagi) > > However, I would suggest you build you own model anyway and play with the > Reflector tuning and observe the F/B, gain, impedance and BW. It is very > informative. You will also realize that when the peak F/B approaches the > resonance point the BW will get real narrow. Also, you will see that when the > reflector is tuned that short, the Yagi will actually reverse direction below > the FB peak since the reflector begins to act like a director. Thats why you > need bandswitching. > > The most important thing about building your own model is you can then > determine the resonant point of the reflector when the driver is isolated. In > the model, insert about 1e6 ohms load at the driver element, and then do a > sweep and measure the resonance. Then, when you build the Yagi on the tower, > isolate the driver (just open it up) and use your analyzer to tune the > reflector for resonance at the measured qrg as per the model results. Then > tune the driver and include the necessary matching network based on the > impedance data gained from the model. > > go here to my webpage…everything is explained for the coil build here: > http://www.qsl.net/ve6wz/CC_coil.html <http://www.qsl.net/ve6wz/CC_coil.html> > > For detail on general coil construction and the tuning methods I used, see > what I did with the 80m Yagi here: > http://www.qsl.net/ve6wz/intro.htm <http://www.qsl.net/ve6wz/intro.htm> > > 73, de steve ve6wz > > >> On Mar 28, 2018, at 4:25 PM, Bill Cotter <[email protected]> wrote: >> >> Hi Steve, >> >> I was intrigued with your note below on improving the XM240 antenna. I have >> an older Cushcraft 40-CD that I would like to improve the F/B and possibly >> the gain, but I only care about a bandwidth between 7000-7050kHz. >> >> I would like to know how you designed and built the Hi-Q coils to replace >> the stock coils. And, if you would share your design dimensions for maximum >> F/B, or the model file so I can recreate it down in the bottom CW segment. >> For tools, I have YO, AO and EZnec modeling software, plus a rig expert >> analyzer. >> >> I love this lightweight antenna and want to take it to the next step. BTW: I >> have toyed with the idea of building a W6NL Moxon for 40M, but I'll do that >> from scratch with more substantial materials. Getting this 40-CD back up is >> my short term goal. >> >> Thanks in advance, >> >> 73 Bill N4LG >> >> >> At 12:27 AM 3/28/2018, you wrote: >>> The large BW and 50 ohm feed point of the XM240 is partly due to the >>> loading coils, but mostly because of the reflector element tuning. >>> With any Yagi, as the max FB QRG gets closer to the minimum SWR qrg >>> (resonance), the BW will crash. Cushcraft designed this Yagi to provide an >>> easy match and a big BW, but sacrificed both gain and FB. Specifically, if >>> the reflector is tuned for max FB at or above the design min SWR frequency >>> the bandwidth will be very narrow. This is because the elements are very >>> tightly coupled. Also, the feed point impedance will be very low. >>> The XM-240 has the reflector tuned much below the design centre SWR min >>> frequency. In other words, if you shortened the reflector element of the >>> XM-240, (but left the driver unchanged) the SWR would be very high because >>> the impedance would be much below 50 ohms and the bandwidth would be very >>> narrow, but the FB and gain would be improved. >>> >>> I rebuilt my XM-240 with hi q coils, but I also shortened the reflector to >>> maximize gain and FB based on NEC modelling. I require a helical hairpin to >>> match to 50ohms, since the feed point impedance is around 25 ohms. The >>> bandwidth is so narrow I have built band-switch boxes at each element, each >>> with 4 relays to add inductance to cover all of 40 m. This is the same >>> system used on my coil loaded short 2el 80m Yagi. >>> The XM240 is s proven performer even with the lossy coils, and like any >>> commercial product, simplicity and universal appeal (broad bandwidth) will >>> always lead to a compromise. >>> >>> De Steve Ve6wz. >>> >>> >>> From Babcocks iPhone >>> >>>> On Mar 27, 2018, at 7:25 PM, <[email protected]> <[email protected]> wrote: >>>> >>>> I should have said lossy loading coils may contribute to this exceptional >>>> bandwidth. >>>> >>>> John KK9A >>>> >>>> -----Original Message----- >>>> From: [email protected] [mailto:[email protected]] >>>> Sent: Wednesday, March 28, 2018 01:24 >>>> To: '[email protected]' >>>> Subject: re: [TowerTalk] XM240 SWR plots >>>> >>>> Lossy traps may contribute to this exceptional bandwidth. >>>> >>>> John KK9A >>>> >>>> W7ZZ wrote: >>>> >>>> The XM240 will have an SWR of 2:1 or less over either the CW or SSB portion >>>> of the band if tuned according to Cushcraft's dimensions. The "mid" setting >>>> is intended to straddle the high end of the CW band and the low end of the >>>> phone band. I have one at 85 feet, measured for the MID dimensions, and it >>>> covers the SSB portion of the band beautifully but the SWR starts to rise >>>> quickly below the phone band. Your mileage will vary due to height above >>>> ground and local issues. >>>> >>>> >>>> >>>> 73, Doug W7ZZ >>>> >>>> _______________________________________________ >>>> >>>> >>>> >>>> _______________________________________________ >>>> TowerTalk mailing list >>>> [email protected] >>>> http://lists.contesting.com/mailman/listinfo/towertalk >>> _______________________________________________ >>> >>> >>> >>> _______________________________________________ >>> TowerTalk mailing list >>> [email protected] >>> http://lists.contesting.com/mailman/listinfo/towertalk >> >> >> --- >> This email has been checked for viruses by Avast antivirus software. >> https://www.avast.com/antivirus >> > > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
