The 70cm Yagi – $18 (minus coax/connector)
A local contact that I made on the ISS, KF4GTA, suggested that I try out working some FM “birds” when he checked into the Colquitt County Ham Radio Society’s friendly net this week. He mentioned SO-50 and AO-27, and so far I’ve worked the SO-50. At first I didn’t think that I had the right equipment, but then I remembered that my FT-60 was a dual band transceiver that did both 2m and 70cm. Shortly after that realization I was working towards working the birds.
First, I downloaded HamSatDroid on my Samsung Tab and my Motorola Razr. I then added my position and updated the Keps. I noticed there was an AO-50 pass around 10:30pm EST so I started preparing.
Second, I grabbed my FT-60 and my Arrow 146-4 (which is not optimal for 70cm, but it would work in a pinch). I connected that to my radio and then I added the satellite frequencies to it.
Third, I then had to re-enter the frequencies because I knew I needed to adjust for Doppler. I’ve never really had much interaction with Mr. Doppler except for the occasional ambulance, so I was pleased to see how well the Doppler effect worked on a 70cm signal. I have the downlink ranging from 436.805 to 736.785.
I made my first contact using the Arrow 2m antenna, but I instantly forgot the call sign. I also had a very weak audio from the downlink so that is when I started thinking about building a small directional antenna. The parts that I gathered were:
- a stick of 1/2″ sched. 40 pvc pipe (only need about 20″)
- one 1/2″ pvc tee
- three 1/2″ pvc crosses
- eight 1/2″ hose clamps (This is why it is $18…these are expensive)
- a tape measure
- coax with your favorite (or necessary) connector
Next, I needed to cut four pieces of the 1/2″ pipe. I needed three pieces that were 4-3/8″ long and then a final piece that was about 6″ long for the handle. Then I used one of the 4-3/8″ pieces between a tee and a cross, and then one more between that cross and the next cross, and the third between that cross and the third cross. Then the 6″ piece went into the third cross for the handle. I did a quick fit test before proceeding. I was sure to check the element spacing which should be 5-3/8″ once assembled. The tee and crosses add 1/2″ each which gives them an extra 1″ which is why I cut the tubes 1″ shorter.
Next, I drilled a hole in the first 4-3/8″ tube right after the handle so that I could feed the coax through the handle. Then I fed the coax through before splicing it up. I’m not going to glue my antenna since it easily disassembles in case I need to store it in a small space.
- Reflector = 13″
- Driven element = 12-1/2″ (two 6-1/4″ pieces)
- Director 1 = 11-9/16″
- Director 2 = 11-1/2″
Be sure to mark the centers of the reflector and directors so you can line them up on the center of the PVC. I went ahead at this point and mounted the reflector and directors using the hose clamps. They will bind a little, but that will not affect performance.
I took the driven element halves and sanded the yellow coating off so that I could mount the coax to them (to get a good metal to metal connection). Then I stripped the coax and mounted the ground and the center conductors to their respective element halves.
If you plan on using this antenna to transmit, I would suggest doing two things. Checking the SWR is the first and most important thing to do. I have not done this since I only plan on using it as a downlink. The second thing to do is to use a hairpin match to lower the SWR if needed. If I ever get this antenna analyzed, I’ll come back and edit this post.
One important addition (or subtraction) is to clip the corners of the tape measure pieces to lessen the chances of getting cut or poked. The thin metal is super sharp and care should be taken regardless of whether the corners are rounded or not.