Waveguide Antennas  

The Downpipe Antenna

The downpipe antenna was born after seeing the success of the Trevor Marshall Waveguide (WG) design, but after frustration at trying to buy 'small' lengths of the required ALU tubing, at realistic prices. In Australia, the minimum quantity is 6 m - you normally need less than 1 m. A 6m length will not fit in a car, and costs AU$160.

However, 95 x 45 x 0.4 mm ZincAlum downpipe is readily available in any Australian hardware store, and a 1.8m length costs less than AU$ 9. While this material is much more 'flimsy', it turned out sufficiently strong enough that it did not distort when the slots were machined (my main concern), and was plenty robust enough to mount solidly with V-Clamps. The material itself is designed to be used outside - so corrosion is expected to be minimal.

While trying to understand the theory of the WG antenna better, I learned:

The Cookbook

Ingredients

1. 1 length (approx 1.8m)  of 95mm x 45mm ZincAlum downpipe.
2. Female-pin N-Connector, Panel Mount, Screw-type
3. 40mm of 1.5-2mm dia solid copper wire
4. Tube of caulking compound
5. 1ea 3mm (1/8") Aluminium rivet
6. V-Clamps, for mounting
7. 1 roll of UV-stabilized, microwave-transparent tape. 50mm (2") wide

Tools

1. Router, with 6.5 mm bit, or
   Nibbling tool (hand operated, or electric)
2. Hacksaw
3. Rivet gun
4. Drill, with 3mm (1/8") bit
5. Set Square (for nice 90 degree faces)

Method

1. Note that while the downpipe has a nominal size, it is manufactured so that one length fits inside the next. The material will either slowly change size from one end to the other, or will be deformed at one end.
2. Decide from which end you will work. The one with the straightest cut is a good choice. Mark TOP with marking pen.
3. Mark BOTTOM at the approximate location of the bottom of the air column. This is approx. 815mm from the TOP.
4. Workout the AVERAGE LARGE ID of the air column. Take a few OD measurements between TOP and BOTTOM. Decide where a good average point would be. Measure the Ave OD (e.g. 95mm). Measure the material thickness (e.g. 0.4mm). Calculate the Ave ID as (Ave OD) - (2 x thickness) (e.g. 95 - 2 x 0.4 = 94.2 mm)
5. Calculate your resonant frequency wavelength. Download this Excel file. Select the Wavelength Calculator tab. and enter the Ave. ID in the Large ID cell shown. Note the calculated Lg (your wavelength), and Small ID. Confirm that your tubing has a Small ID that is LESS than the number calculated.
6. Calculate the Dimensions for your antenna. Select the Antenna Dimensions tab in the Excel file. The wavelength (Lg) calculated in the step above should be automatically transferred to the correct cells in this spreadsheet.
7. Square-off the Top end. Use a set square and a file/grinder.
8. Mark and cut your downpipe at the TOTAL LENGTH value from the Antenna Dimensions spreadsheet. (e.g. approx 910-920 mm for an 8-slot antenna).
9. Mark the position of all slots. Cut the 6.5 mm slots with your router or nibbling tool. If making a 360 degree antenna, the slots on the back are positioned such that you can see through both slots from front-to-back.
10. Mount the N-Connector. Mark the position. Drill and mount temporarily. Remove.
11. Make the feed. Solder a length of copper wire into the solder bucket of the N-Connector. Cut so the length of the copper wire extends 31mm from the end of the metal shield of the N-Connector. See the Brick Antenna Design for details.
12. Cut bottom-reflector mounting slots. Mark the bottom of the Air Column on the SMALL sides (only). Use a hacksaw to cut through the SMALL SIDES ONLY of the antenna at the bottom of the air column. The two resulting slots will be the thickness of the hacksaw blade.
13. Make the Bottom reflector. Use an off cut to make an L-shaped reflector, that slides through the two slots (step above). It should protrude about 1 mm on the far side.
14. Drill hole for rivet. Drill a hole for the rivet so that the bottom reflector will be held in place. Note: Keep the reflector as flat/straight as possible, to maintain antenna performance. DO NOT RIVET IN PLACE YET.
15. Make the Top reflector cap. Use an off cut to make a 'cap' that fits neatly over the top of the antenna. Note: Keep the reflector as flat/straight as possible, to maintain antenna performance.
16. Drill V-clamp mounting holes. These V-clamps go in the bottom 100 mm section below the bottom reflector. See Trevor Marshall's design for details.
17. Clean all metal chaff from inside the antenna.
18. Mount the N-Connector/Feed assembly. Caulk around edges to make waterproof.
19. Attach Top Reflector cap. Do not rivet...as you don't want protrusions inside the antenna cavity. Just fix in place with some caulking compound - which will also waterproof the top.
20. Attach Bottom Reflector.  Slide bottom reflector in place, and rivet on one side. Caulk around the two slots to make waterproof.
21. Cover slots with tape. See UV-Tape for more details.

Mounting

UV Tape

Link Tests

Independent Tests

The tests reported here are well documented, and confirm the 2dB on-axis gain difference seen in My Tests (below)

My Tests

The three WG antennas mentioned in the Introduction, were tested as follows:

Results

Price

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