70cm QFH Antenna

This antenna is a work in progress for me. Your results may vary.

All dimensions from https://jcoppens.com/ant/qfh/calc.en.php

This is designed for 3mm wire (14awg). I made the mistake of using THHN wire for the one in the image, which has thick insulation so it is electrically longer than the calculations... so I had to shorten the green arms quite a bit to get it to the frequency it's supposed to be.  The dimensions below are just a starting point and as with any antenna build you'll need to adjust it.

This is designed to attach to a 3/4 inch PVC pipe. for a 433mhz antenna the distance between the top and bottom rods on the long loop is 236.1mm. Your pipe needs to be a bit longer than this for those rods to go through it and have room on top and bottom for and end cap on the top to keep water out of the wiring, and mounting hardware on the bottom, etc.

center tube guide adds 2.5mm to each end of the tube. (from end of tube to center of wire hole)

Use hollow fiberglass garden stakes (amazon) that are ~8.5mm diameter

for 433mhz Cut:

larger loop:

91.6mm tubes x2 (top and bottom of larger loop. the wires go through these tubes)

86.6mm tube x1 for the center of the larger loop (center tube guide goes on each end of this tube and the wire goes through the hole in the printed guide.

this tube will be the same length as the top and bottom tubes once the center tube guide is added. This middle tube will be installed 90 degrees from the top and bottom tubes to support the center of the larger loop on each side.

the top and bottom tubes should be spaced 231mm apart for the centers of the tube holes.

smaller loop:

86.6mm tubes x2 (top and bottom of smaller loop. the wires go through these tubes)

81.6mm tube x1 for the center of the smaller loop (center tube guide goes on each end of this tube and the wire goes through the hole in the printed guide.

this tube will be the same length as the top and bottom tubes once the center tube guide is added. This middle tube will be installed 90 degrees from the top and bottom tubes to support the center of the smaller loop on each side.

Print:

Since this is intended to be outside it is best printed in ASA, which is UV and heat resistant. PETG is probably the next best choice, followed by ABS.

center tube guide x4 (one on each end of the 2 center spacer rods)

top support

top lid

center support

bottom support - the extended version gives a bit more length to attach it on top of a mast of PVC.

3/4" PVC - 168.014mm long piece

Assembly:

On the TOP piece the 2 loops are at almost the same height. This means that they must be cut in 2, leaving enough space to connect the wires and coax together inside.

The side with the coax hole in it is where the SHORT loop goes. We'll call the side with the coax hole the FRONT.

The CENTER piece is slid onto the pipe with the longer end pointed UP. The hole closest to the top is for the SHORT loop and should be facing left and right if you're looking at the antenna from the front. The hole that lines up with the bottom of the piece is for the LONG loop and should be pointing toward you if you're looking at it from the front.

The BOTTOM piece is slid onto the pipe with the notched end facing DOWN. The notch itself serves no purpose other than to differentiate the ends of the bottom support and to give you a reference point. This bottom notch should line up with the coax hole in the top piece.

The holes closest to the bottom are for the long loop so it needs to line up with the long loop side of the top piece (facing left and right).

These positions assure the loops are correctly spaced.

The distance between the long loop rods on the top and bottom should be 231mm from center to center.

Once you have verified that it is oriented correctly and the total length is correct, glue the top and bottom support to the pipe. The fit should be tight enough that the type of glue isn't that important. I used E6000. Make sure the top and bottom pieces are lined up square with each other while the glue is still wet!

The CENTER support should be adjusted so that the top of that piece is about 24mm away from the edge of the top piece.

The top section is where the feed point is. It was designed big enough to give you plenty of space to solder and assemble it without too much trouble. The hole in the front is sized for RG-8x or similar sized coax, but smaller coax should work fine.