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first test: 7x7 Arrow antenna for AO40 uplink



This morning I tested my new 7x7 RHCP Arrow antenna on AO40.  The temperature was a chilly 14F (-10C) when I set up the camera tripod, wood crossboom, and yagi on the patio.  I ran a 42-foot piece of flexible 9913 coax out the window next to the radio, and stuffed a towel in the cracked window to keep the shack from getting too cold.  I used my old Boy Scout Silva compass for the first time in 25 years to set the azimuth.  To set the elevation, I have a protractor-like device taped to the front of the yagi.  I dial up the desired elevation with a rotating slider, and then adjust the tripod to make the slider level.  This gizmo was originally a "flexibility gauge" for muscle stretching.  The uplink antenna on the patio was pointing through trees, so the receiving antenna (60cm G3RUH dish) stayed on the roof where it could see over the trees.

With the antenna pointed and the coax cable connected, I ran inside the warm house to work AO40.  The downlink signal was normal: S2 noise level and S7 beacon level at MA45 with a squint angle of 16 degrees.  I sent some dits with the FT-847 at maximum power (50W), and heard an S5 signal coming back.  I guess this thing works!  I made 3 contacts with European stations, and didn't notice any fading of my uplink signal.  I guess the antenna's polarization is reasonably close to circular.  LEILA activated one time while I was transmitting, so I backed off the power a little bit.  It only happened once, so I don't know if this little antenna has enough gain to "meet" LEILA reliably.  Perhaps it could reliably "meet" LEILA with a much shorter feedline.

When disassembled, this antenna fits in a 4x22 inch plastic bag.  It takes about 5 minutes to assemble.  Here's a summary of the modifications I did to convert a standard Arrow portable satellite antenna into a 7x7 RHCP yagi:  I ordered a second set of UHF elements from Allen Lowe at Arrow antenna.  He also sent me a small scrap of boom tubing so I could lengthen the boom 2 inches.  The boom needed to be longer because I chose to physically offset the second plane of elements by ¼ wavelength.  The boom is now 39.5 inches (1 meter) long.  I drilled the new orthogonal element mounting holes 6.75 inches behind the existing 7 UHF elements.  I drilled a new hole at the middle of the boom for mounting to the wood crossboom.  The feedline goes to a BNC Tee, which then has 75 Ohm ¼ wave matching cables going to the feedpoint of each driven element.  The gamma matches required considerable retuning after adding the orthogonal set of elements.  The finished antenna has an SWR of 1.3:1.  If anybody is interested, I can email photos of the antenna or details of the matching cables and gamma match tuning.

I plan to set up an AO40 demonstration station at a local hamfest on 24 March.  I asked the AMSAT table to be located adjacent to a south-facing door, so I can run feedlines out the door to an area that has an unobstructed view of AO40.  Outside, a weighted camera tripod will have a 28 inch wood crossboom with the Arrow 7x7 RHCP yagi for uplink and a 60cm G3RUH dish, patch feed, DEM preamp, and TSI downconverter for downlink.  Inside will be a FT-847 and laptop computer running InstantTrack + InstantTune, and 3 sets of headphones.  Now I know the uplink antenna works well, so I should have everything I need for a successful demonstration.  It should be fun!

Wayne Estes W9AE
Mundelein, IL, USA
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