QFH Antenna for Weather Satellites on 137MHz
Long ago while on Meridian I built a "Tall Narrow QFH" antenna. Just Google QFH or QFA, there are many designs. The Tall Narrow was a good performer but awkward to build. Finding the 8mm elbows was hard and they were expensive. It was also tricky soldering them in place while trying to wrangle the copper tube.
So, given my previous experience, I looked at other designs. One I used was here, the other was here.
As I had some 12mm copper tube and PVC I decided to use ideas from both. I used the copper tube for the horizontal elements and RG-58 coax for the spiral connections.
Read them both and take note of their methods.
The TinHat all copper antenna used:
As I has 50mm PVC pipe, I used that instead of the 32mm used by them. This has the advantage of making a more stable platform for the supports, and room to work when making the connections at the top.
I used a 6mm drill for pilot holes, and then step drills. Be careful with the 12mm holes, you want the tube to be tight.
Drilling the holes. I mucked up the first one as the templates slipped out of alignment. Start with a centre line to work from. Make the templates to top, middle and bottom and tape them in place. Top has four holes in the same plane, about 20-30mm from the top. Mark the holes at the top B1, S1, B2, S2 for Big an Small loops. The middle support is 90 degrees from the top and bottom and the Big support is lower than the Small. I used 22mm PVC but electrical conduit would be fine. Make the middle template so that the two supports just miss each other, each side of the mid-point.
The lower loop is in line with the top, so again mark the holes B1, B2, S1, S2. I used 570mm for the distance from top to bottom (Big loop) and 520mm for the Small loop as per the diagram above.
I got a 50mm floor flange for the PVC pipe to support the upper elements, and fixed them in place with copper saddles.
I soldered the coax to the copper tubes, but as it is a big heatsink, it took some time and the results aren't pretty. It's probably easier to use small crimp rings to attach them, as I did for the coax feed to the elements.
Drill two holes about 20mm apart about 20mm below the flange. This is for the 4-turn balun. Feed the coax through the top hole leaving a few inches to work with. Wind the four turns and feed it back though the lower hole and out the bottom of the support. Look at the diagrams on the tinhat site.
Make two wire jumpers about 10cm long and put small ring crimp connectors on, making sure they are well fixed. Strip a small section in the middle for the coax feed to be soldered on.
The jumpers go from Big to the neighbouring Small in a CLOCKWISE direction.
I actually cut about 25mm off this.
I then drilled angled holes in the end of the PVC supports for the coax, then another hole for a cable tie to hold them in place.