Building satellite Cross-Yagi with gamma matches and phasing cable
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| Nov 17th 2020, 18:36 | |
AC0ZJTotal Topics: 0 Total Posts: 0 |
A while ago, I made a cross-yagi 2m antenna for satellite work. I used the “Cheap Yagis By WA5VJB” design with measurements from The ARRL Antenna book, 23rd ed., Table 5.4. I made two crossed yagis, using PVC, 14-2 copper wire and 1/8 steel rods for reflectors/directors. The cross-elements are close together, so for phasing I used the harness described in the Antenna Book, section 17.2.1, shown in 17.13. I soldered/crimped the 75 & 93 ohm cables, feeding into a chunk of 50 ohm RG-8X terminated with a SO-239 connector. The whole thing works great. I’m able to pick up a number of satellites including CAS-4A and B, and the NOAA polar weather satellites. Being that the antenna was thrown together and quite flimsy, I tried to make a similar antenna using the parts from two Arrow 146/437-10WBP dual-band satellite antennas. I took the antennas apart, and used one boom with two sets of cross polarized 2m elements, along with the gamma matches. I used the same phasing harness as I did for the aforementioned “cheap” yagis, except I’m using BNC terminations. The results are terrible! I’m not sure why. I used my MFJ-269 antenna analyzer, and the SWR at 145 MHz was between 2 and 3. The NOAA signals are probably down by 20 dB (eyeballed on SDR spectrum) and I can barely hear the satellites I was previously receiving. The arrow antennas, when connected directly to the MFJ-269 and the phasing harness disconnected, have an SWR of about 1.1 at 145 MHz. I’ve been searching the internet for any notes regarding using a crossed yagi with gamma matches, but can’t find anything that mentions both. I would have thought the 50 ohms impedance presented by both gamma matches, would have meant the same phasing harness would work just fine. Now I’m not so sure. Can anyone tell me, can a phasing harness work with gamma matches, or is there something I’m missing? Thank you! *** Update 11/22/20 *** - I've come to the conclusion that gamma matches that close to each other don't/can't work. The matches were at 90deg to each other, mounted on the boom, staggered by about 1/2 inch (the booms aren't long enough to stagger by wavelength/4). It's an observation, one data point. I went back to using the antennas as they were originally built, with one mast containing elements for 2m and 70 cm. I put up two masts at 90deg to each other, and made a phasing harness for the 2m (Antenna Handbook, pg 17-8, Fig 17.13, and don't forget your lengths must be the electrical length, which is wavelength/4 * velocity factor). Now the swr is close to 1 at 145.5 MHz and I'm able to receive CAS-4A/B. I think I might have some intermittent connection issues but much better results. |
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