12-17 Meter Dual Band Yagi
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| Feb 6th 2021, 18:14 | |
DataJTotal Topics: 0 Total Posts: 0 |
I have completed a Homebrew, dual band, 2 element, trapped yagi. The driven element consists of two elements, insulated from each other. (TH3) Is constructed in a likewise manner. I have constructed a "Hairpin", or "Beta" style matching system, as the impedance for this unit is on the order of 20-25 ohms; needs to be 50 ohms to match the RG8 coax. I have been able to get the match, according the the MFJ-259C, down to the best match as R49 X1, with SWR 1.0, on 17 meters. On 12 Meters I haven't been able to get any kind of matching......I think the problem is the length of the 12 meter elements. My design started out as a 12/17 Rotatable Dipole and I decided to add an additional element and pick up some gain. If anyone has any experience in doing something like this, your feedback would be appreciated. TNX Jim Gameson - K0JMD Mission, Ks. |
| Feb 8th 2021, 12:26 | |
W9IQTotal Topics: 0 Total Posts: 0 |
Jim, It would be helpful to know the spacing and the length of the elements and their sections. What type of traps did you use on the new element? Is the new element configured as a director or a reflector? What is the brand/model number of the base rotatable dipole? With regard to the 12 meter lack of a match, did the antenna provide a match on 12 meters when it was a single element antenna? If so what was changed besides adding the new element? Do you have a proper common mode choke by the hairpin match? - Glenn W9IQ |
| Mar 8th 2021, 20:21 | |
DataJTotal Topics: 0 Total Posts: 0 |
Hi Glenn - - Sorry to be so long in replying - - I didn't know there was any reply. The base Dipole portion of this antenna started out as described by AD5X, Phil Silas, who built a Rotatable Dipole for 12/17 meters. (Here is a link to his article: http://www.ad5x.com/images/Articles/Trap%20Dipole%201217RevA.pdf As I went along, I decided to turn it into a two element Yagi and pick up a couple of decibels gain - - after all, it only required a second element and a boom. My traps are designed after my Hy-Gain Th-3 antenna, currently in storage in my garage. I turned nylon stock, 1 1/8" in diameter to fit into a 7/8" ID aluminum tube, wound various turns of 14 gage wire and anchored to aluminum tubes at each end with metal screws. Then I made the capacitor from a 12" long x 1 1/2" aluminum tube; machined an aluminum fitting to connect one end of the tube to the driven side of the element and the capacitor. The insulated fitting on end of the trap faces the outboard or "Tip" end. (Otherwise the capacitor will not function - - the polarity has to be correct). This assembly was then tuned using an MFJ-259C with a pair of 100 ohm composition resistors soldered in parallel, and also soldered on to a PL-259 coax plug. The two resistors yielded 49.5 ohms - - close enough for 50 ohm match. The MFJ was turned on and the wire at the end of the resistors was connected to the Trap end which has the aluminum fitting. Next it is just a matter of turning the dial on the analyzer until it provided the resonant frequency of the traps. Both 12 meter traps were resonated at 24.425 mhz. (The traps are never tuned to resonate on your operating frequency - - if so, they become a fully tuned circuit, (Tank Circuit), and the RF energy will burn them up). The boom dimension is 7 feet wide, each driven element originally set at 9'4" to the conductive end of the trap. Both sides are set up this way. My matching system is a Hairpin Match, 48" long and 8" wide. The matching system is adjusted by loosening nuts on the muffler clamp and moving the sliding aluminum bar along the outboard portion of the boom, either inward or outward, depending what the MFJ Analyzer is indicating. When using the MFJ Analyzer, the driven element changes Impedance as the frequency shifts. There are numerous frequencies in which the analyzer yields 50 ohms and swr at 1.0 and "X" at 0. I have been able on several occasions to get it to resonate at 24.930 MHZ, 50 Ohms Impedance, "X"=0, SWR 1.0, leave for a few minutes and come back to check it and it is way out of tune. The antenna will not re-tune no mater what I do to adjust it. Completely mind-boggling; it should be repeatable. I did build a 10 meter mono-band beam back in 1978 which used a gamma-match, just a few tweaks and it worked perfectly, and, I thing was a better performer than my TH-3 on 10 meters. So, sorry to be so lengthy, but want to make sure I didn't leave anything out. I am thinking about trying out the VNA Analyzer and see if it works any better. The MFJ-259C drifts....not as badly as my Heathkit 1960 GD-1B Grid Dip Oscillator, but the MFJ is not very stable. Any ideas would be appreciated. 73's, Jim Gameson - K0JMD It did resonate yesterday at 24.930, and also at 18.130...it was perfect 50 Ohms, "X" = 0 and SWR 1.0. During the course of this project, I found several articles on Yagi Tuning; most state not to try tuning them on the ground, (mine is about 4' in the air), but either set them on a 6' ladder and go from there. I do have an 8' 2x4 that I have been contemplating to try. Also need to do one thing this after is to ground the boom to an earth ground. |
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