Antenna for the 137 MHz band

The most important is the antenna. A popular cheap antenna is the Turnstile. A homebrew helix is what most of APT stations use but if you are not the Do It Yourself (DIY) one, it will be the most expensive part after the PC and will most likely not perform as good as you expected.

If you are the DIY one, then let me introduce you the Double Cross Antenna (DCA) developed by Jerry Martes using The Cross Concept ©. The Cross Concept is two crossed dipoles fed in phase spaced 1/4 wave length apart, The Simple Cross Antenna. The DC is two simple cross antennas connected togeather. The other pair is fed 1/4 wave later. This antenna is very sensitive toward the horizon and is the ultimate APT antenna when receiving low elevation satellite passes. The image to your right is a NOAA 18 1.7 max elevation pass received using a Double Cross antenna made of 8 mm aluminium pipes. The antenna weight is less than 500 grams and cost less than 15 eur ($20).

Jerry Martes Double Cross Antenna article in the QST magazine February 2008 and Jerry & Patrik GEO article in December.

Double Cross Antenna - A NOAA Satellite Downlink Antenna by Gerald Martes, KD6JDJ

Signal Plotter and Double Cross Antenna by Jerry Martes & Patrik Tast

Double Cross Antenna Kit for the 137 MHz band

Read also document Double Cross Antenna For Beginner (2.8 MB DOC). Browse through more DCA construction images and documents in folder jerry at the User Files section.

If you are using a Quad Helix Antenna read this document QHA (26 kB Doc)

The image below to the left is the ideal Right Hand Circulary Polarized Elevation Plot of the Double Cross antenna generated with the EZNEC antenna software. To the left is the real anenna plot of NOAA 12 recorded portable with APTDecoder software and R2FX receiver.

NOAA 12 maximum elevation 84° on Sunday, 22 July 2007.

Double Cross Antenna (DCA) build instructions

Fred E. Piering has documented his DCA build carefully with a general partlist (available from US Lowres, Home Depot), assembly hints and SWR measurements of the antenna.

DCA Assembly Hints.pdf

For my latest DC I used 8 mm aluminium tubes as dipoles and spaced each dipole 500 mm. I used 20 mm electrical PVC tubes for the "arms" and 3 mm plexi-glass as the center to fasten the arms. The dipoles are from tip to tip 1000 mm. I cut the tubes to 500 mm (eight pieces) and join them with some non electrical conductors, I used PVC.

Phasing harness: Let us call one pair of the simple crosses for the Nort/South (N/S) and the second East/West (E/W). I cut 360 mm of RG58U for the N/S dipoles and 720 mm for the E/W dipoles, 1/4 wave longer in coax. I connected them as shown in the sketch and tilted each dipole to the right (as seen from the top) about 30 degrees so it will form a box.