Chuck and Ryan's Build Tips

[ceslaw]

PBY:

The same tie downs and hardware were used as on the top aft panel included with the kit, but the tube clamp is a 5/8” size, part number MS27405-5 rather than the ½” used at the top aft panel. I don't recall for sure (since it has been seven years) but I suspect they were ordered from Aircraft Spruce.

The extra tie downs were ordered from Cubcrafters since I wanted all of them to match. Pete at CC was very helpful.

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[Dang. Looking at this picture I see it needs to be tightened up a bit! No threads showing.]

[Rick Bosshardt]

Yves,
looks like you are making good progress!
Rick

[PBY Catalina]

Yves,
looks like you are making good progress!
Rick

Hi Rick!
In covering phase...
I'll make a little report soon.

[Rick Bosshardt]

Cool!
Ill be back in Switzerland next April... will she be flying by then??

[PBY Catalina]

Cool!
Ill be back in Switzerland next April... will she be flying by then??

Mmmm maybe not... but I'll have my old L4/J3 to have fun !

[Rick Bosshardt]

That would be fun too! Ill see if I can break away for a day from family stuff.

[ceslaw]

ELT Antenna Mismatch. A discussion on another thread revealed an issue with the function of the ELT antenna mounted horizontally on the aft end of the extended baggage compartment. Thanks to PBY, Andy and Dave for discovering the issue.

Problem: The horizontal antenna is mounted on a bracket that raises the base of the antenna a couple of inches above the vertical baggage compartment which is intended to serve as the ground plane. This results in a mismatch between the antenna and the ELT at 121.5 MHz which may prevent the Kannad ELT from passing its self-check test in some instances and markedly diminish the emitted signal.

The goal: A solution that can be easily duplicated at minimal cost with no substantive changes to the Cubcrafter’s design.

Solution: Adding a single quarter wave radial 23 ¼” inches long will result in a near perfect match at 121.5 MHz solving the problem.

1. The base of the antenna must be mounted in the ‘low’ location described in a post seven years ago. This will provide the needed vertical space for the radial. https://forum.cubcrafters.com/showth...ll=1#post10882

2. A quarter wave radial was cut from piano / music wire to 23 ¼”, the same total length as the antenna. A half inch ring terminal for 16G wire was used, crimping and then soldering the wire in place. The base of the wire was bent at a 90-degree angle at the crimp to provide more space at the top. Heat shrink was added at the top end.

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3. The top of the wire was secured with a standoff ¾” in length. A section of pitot tube material and a wire tie was used. THIS DISTANCE IS CRITICAL. Varying this length, even a half inch one way or the other, will result in a much higher SWR.

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4. A straight edge simulating the location of the fabric confirmed clearance.

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Discussion: A quarter wave antenna typically requires a suitable ground plane to function. By mounting the base of the antenna a couple inches above the extended baggage compartment the antenna was not properly tuned adversely impacting its function and potentially the self-test procedure with the Kannad ELT. The two inch standoff results in a high SWR of nearly 5 to 1. The goal is 1 to 1. Anything over 3 to 1 is unacceptable.

Adding a quarter wave radial effectively ‘tunes’ the antenna providing a proper match at 121.5 which was confirmed with the antenna analyzer. The SWR of 1.1 is near perfect meaning virtually none of the transmitted signal is being reflected back down the coax line. The impedance shown is an excellent match for the 50-ohm RG-142 coax.

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The question arises: Is a large ground plane better than a single radial? Some would argue yes, but in this application, it likely does not make much difference. A quarter wave antenna mounted on a proper ground plane is omni directional, which is a good thing. But for an ELT one has no idea what its orientation will be if it is ever activated in an emergency. Further, in this application the antenna is mounted horizontally within a steel frame, so the usual radiation patterns should not be expected. A single radial tends to give the antenna a small amount of directivity, but that is not likely to make a difference for the same reasons. The most significant factor in improving the function of this antenna mounted in a horizontal position within the frame is a good match, which this simple solution achieves.

406 MHz issues. The long radial addresses the 121.5 MHz matching issue, which is what the Kannad self-test procedure checks. The Kannad self-test does not check the 406 MHz function.

I do not know how the RAMI dual band antenna is ‘loaded’ at 406 MHz. It may be designed to function independent of the ground plane. Nonetheless there is no harm in adding a quarter wave radial. Accordingly, a 7-inch-long quarter wave radial, made with the same material as the 23 ¼” radial, was fabricated and is visible in the picture opposite the long radial.

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According to Cubcrafters, the 406 MHz function was live tested in coordination with SARSAT and was deemed “acceptable.” Adding the 7” radial can only improve its function.

The next project will be relocating the ELT to improve its internal GPS reception.

[Andy]

What was the 121.5 MHz SWR with the ELT antenna mounted per CubCrafters installation drawings? (on bracket, central location, no radials)

[ceslaw]

Andy:

4.5 to 1 SWR without the radial.

[Andy]

4.5 to 1 SWR without the radial.

No significant change between the CubCrafters standard central location and the lower location adopted by some builders?