137 MHz Quadrifilar Helix

A web page by Mr. George Goodroe (K4CFP) at http://www.qsl.net/kf4cpj/qha/ presents specifications and construction details for making a circularly polarized NOAA weather satellite receiving antenna. The steps below describe my implementation of Mr. Goodroe’s antenna. There is nothing original in these steps. Please refer to the referenced article for definitive information.

Step 1. Make a template (strip of wood at bottom of photo) and cut copper tubes. Dimensions are:  Four 90 mm (top of mast), two 190 mm (bottom of mast through tubes), two 903 mm and two 1002 mm elements. The tubing is 3/8 inch diameter copper available from Home Depot or Lowes.

Cut pieces and template

Step 2. Measure locations and drill 3/8 inch holes in PVC mast for tubes. Spacing between top tubes and closest bottom tube is 900 mm. From top tubes to bottommost tube is 1000 mm. The short 3/8 inch OD tubes should fit snuggly through the drilled holes. (See photos accompanying steps 5 and 6 below).

PracticeSmall jumperStep 3. Practice drilling and tapping a hole in a scrap piece of tubing. Screws do not need to be self-tapping. Copper is soft enough that almost any screw will do—I used screws from an old computer case. When satisfied, drill and tap a similar hole near one end of each of the four short top tubes. Sand the area around the holes for a good electrical connection.

Step 4. Make a couple of short jumpers. Ring terminals in these illustrations are #18-22. Wire is #18 stranded.








Bottom tubesPrepare coax at topStep 5. Push/pull the two bottom tubes through so that equal lengths protrude on each side. Do not hammer the tubes, as this might compress the copper making it difficult to attach elbows.

Step 6. Drill a hole for the coax somewhere near the top (not critical). Push the coax through the drilled hole and feed the end out the top. Attach ring terminals to the coax. Push in the four top tubes with tapped screw holes facing up.







Connect coax and jumpersStep 7. Fasten the coax center conductor and one end of a short jumper to one of the tubes. Fasten the coax shield and one end of the other short jumper to the opposite tube. Connect the free ends of the two jumpers to the unconnected tubes clockwise from the previously connected end of each jumper.

When step 7 is complete, verify all the connections using an ohmmeter. Connect one meter lead to the coax center conductor at the distal end and then touch each of the top tubes with the other meter lead. Repeat this procedure with the coax shield. Cap the top of the mast. This part is done.







BalunElement positioning and shapeStep 8. Wrap four turns of coax around the mast and tape at ends to hold the turns in place. I used Scotch Super 88 vinyl electrical tape.

Step 9. Attach elbows and elements. Position and bend elements to mock-up the antenna’s final shape.













Step 10. Disconnect one end of one elbow at a time. Sand and apply flux. Reconnect the tube and solder to elbow. Recheck shape and proceed to next elbow. A total of 8 elbows (16 joints) should be soldered.

Step 11. Fabricate a base or mast attachment for the antenna. I used a rectangular piece of wood and mounted an SO-239 socket on it. Of course, the coax can be continuous from the antenna, without installing a connector at the base. The photo below shows the finished ‘table-top’ antenna ready for action on the balcony.

Ready for Action


Initial Testing

It is not necessary to put the antenna on an analyzer, but since I have an analyzer (RigExpert AA-230 Zoom), why not see what it says. The graph below shows results of this test.

SWR


NOAA Satellite Signal

With my setup, 137 MHz satellite signals fade in and out during each pass, producing pieces of map, but not full maps. I think this is due to nearby physical obstructions, trees and houses, the balcony roof etc. Some passes are better than others. Passes to the west (balcony side of house) are generally better than passes on the east. The SDR# image below shows a moderately strong signal from NOAA-15 as received on December 28, 2016. The satellite’s longitude was approximately 85 west. (Audio sample)

NOAA-15 Signal (SDR#)

The sliver of map corresponding to the strongest part of the NOAA-15 signal was clear, but during the entire pass, the signal was more out than in. When the signal was weak, WXtoImg traced dark lines across the map.

Slice of Weather Map

I have not yet succeeded in recording a clear full scale map. The best partial map received thus far is the one shown below from NOAA-19 on Dec. 31, 2016:

NOAA-19 December 31, 2016

The WXtoImg program sometimes displays a warning:

Warning message from WXToImg

I think this message simply asserts that the audio signal is not good enough to decode. The signal-to-noise ratio is certainly low at times. Volume should be okay. IF bandwidth seems broad enough.


References

1.       http://jcoppens.com/ant/qfh/calc.en.php
2.       http://metsat.gogan.org/ant_qha.htm
3.       http://www.qsl.net/kf4cpj/qha/
4.       http://www.teske.net.br/lucas/2016/01/qfh-antenna-and-my-first-reception-of-noaa/
5.       http://www.gregorystrike.com/2010/05/16/quadrifilar-helix-antenna-137-mhz/