Chuck and Ryan's Build Tips

[aeroaddict]

Great information, thanks. When building I installed the Artex and the instruction specifically stated the antenna have a 3 foot clearance from metal.

Obviously the CC mounting recommendation does not meet this requirement.

If your are already covered and flying, wouldn't a simple solution be to mount an external antenna on the metal turtle deck?

[Andy]

[QUOTE=aeroaddict;27702]If your are already covered and flying, wouldn't a simple solution be to mount an external antenna on the metal turtle deck?[/QUOTE]

I doubt the turtle deck is large enough to get enough separation between antennas. Also, the antenna would probably be removed if the airplane ended up on its back.

[aeroaddict]

Agree, but both both antennas may not be Tx'ing if the ELT has been activated. Also, antenna location is somewhat subjective in regards to aircraft orientation during emergency landings.

[Daveembry]

What do you think about this from the Installation Manual? Their instructions are essentially what you have done except with more of them. Says length at least same as antenna but what is “foil”? Aluminum foil? Pieces of rigid aluminum? 8 pieces.

I’m still on the part about the antenna needs to be mounted to the ground plane……or at least do as they say or having the shield of the antenna attached to the ground plane. Currently it’s attached to the “mount”….

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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.

Attachment 11865

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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.

Attachment 11867

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

Attachment 11868

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.

Attachment 11869

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.

Attachment 11870

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.

[ceslaw]

Dave:

Using multiple radials of wide copper tape or something similar is a fine idea if your goal is to preserve the omni directional radiating pattern of a quarter wave vertical antenna. Recall that in theory the radiation pattern resembles a horizontal doughnut with the strongest signal perpendicular to the quarter wave vertical antenna. For the VHF COM mounted on the top of a fabric plane this would be appropriate and desirable. Doing so should lead to a proper SWR of 1 to 1.

But the ELT antenna, mounted within a steel frame, which will be activated under uncertain conditions with the aircraft's position not known is a different story. Attempting to obtain an omni directional radiating pattern is pretty much pointless and likely impossible. The goal for the ELT antenna is, therefore, simply obtaining a good match between the antenna and the radio. The single radial accomplishes this goal splendidly. There is really no reason for installing a large ground plane inside the aft end of the plane for the ELT antenna, although that is another way to achieve a better match between the antenna and radio.

Those instructions are good generic tips for a proper installation on the top of the plane, but not realistic for the ELT antenna installed inside the fuselage. Sometimes the simplest solutions work well.

The foil those directions refer to is likely copper foil, which comes in rolls in various widths and is held in place with adhesive. I have used if to establish a proper ground plane on a fiberglass boat. It would work well on a composite / fiberglass aircraft. It would be a bit cumbersome to glue it to the back side of fabric. It is readily available from amateur radio suppliers like DXEngineering.

Don't get hung up on the shield being connected to the ground plane. That is effectively what happens when the base of the antenna is screwed to the ground plane, since is is grounded and the BNC connector is connected to the shield, so they are all tied together.

The addition of the radial effectively is the same as connecting the shield to the ground plane. The radial is the ground plane.

Hope this helps.

[Daveembry]

ok thanks. But……my problem has been with the Kannad self test not liking the standard CC installation using their “mount” instead of mounting the antenna directly to the ground plane. I understand about it being grounded when attached…..but is it really attached directly to the ground plane? Or instead to the mount and it to the ground plane making the antenna actually in front of and not “opposite” in the ground plane?

All my problems were Kannad not liking that installation and repeatedly failing the self test due to antenna problems (3 beeps and led flashes followed by 5 more). This was always cured by removing the “mount” and connecting the antenna directly to the ground plane.

Don't get hung up on the shield being connected to the ground plane. That is effectively what happens when the base of the antenna is screwed to the ground plane, since is is grounded and the BNC connector is connected to the shield, so they are all tied together.

The addition of the radial effectively is the same as connecting the shield to the ground plane. The radial is the ground plane.

Hope this helps.

[Andy]

But……my problem has been with the Kannad self test not liking the standard CC installation using their “mount” instead of mounting the antenna directly to the ground plane. I understand about it being grounded when attached…..but is it really attached directly to the ground plane? Or instead to the mount and it to the ground plane making the antenna actually in front of and not “opposite” in the ground plane?

You have to think of the mount, not only as a mechanical support with low DC resistance, but also as part of the antenna system. The mount makes the antenna electrically longer and also moves the feed point. The antenna, which was designed assuming a ground plane at its base, will have quite different electrical properties when separated from the ground plane by the mount. The properties will also be changed by the steel tube fuselage.

[ceslaw]

All my problems were Kannad not liking that installation and repeatedly failing the self test due to antenna problems (3 beeps and led flashes followed by 5 more). This was always cured by removing the “mount” and connecting the antenna directly to the ground plane.

The addition of the radial which will match the antenna to the radio with an SWR equal to or close to 1:1 will result in a successful Kannad self test. It cures that problem.

[Daveembry]

ok great! So it is the ground plane that is the problem like I thought. Hopefully CC will change the way they mount it to the back of the baggage panel so that the correct ground plane can be established. I’m thinking to keep my original thoughts of making a separate ground plane that the antenna mounts directly on and is installed between the tubes further aft.

The addition of the radial which will match the antenna to the radio with an SWR equal to or close to 1:1 will result in a successful Kannad self test. It cures that problem.

[ceslaw]

Relocating the Kannad ELT. Carbon fiber is an excellent radio frequency (RF) shield which will block a GPS signal. Cubcrafters locates the Kannad ELT, which has an internal GPS receiver, under the pilot’s seat. Although this is not a problem for the RF transmissions which have a remote antenna, it is a problem for the Kannad’s internal GPS receiving antenna. The inability to receive a GPS signal may delay locating the transmission if activated.

The goal: Relocate the Kannad ELT so that the GPS signal would effectively radiate, with minimal changes to the Cubcrafter’s design. Alternatively, one can provide an alternate GPS signal source to the Kannad ELT through its external plug.

The solution: Moving the Kannad ELT from below the pilot’s seat to the baggage compartment will permit effective detection of the GPS signal.

The Carbon Cub has brackets on the top right side of the baggage compartment used for mounting the Amsafe seat belt control unit. Since we do not have that option, it was an ideal location for the ELT.

To match the Kannad mounting bracket to the existing bracket a plate was fabricated from a ¼” thick piece of HDPE (Star Board). A pattern was made to confirm the dimensions.

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The ELT is mounted with the top rather than the bottom resting in the mounting bracket. This required a tiny bit of trimming to assure a proper fit. (Excuse the upside down picture).

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Once strapped in place the ELT was secure and solid.

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Next attention was turned to installing the remote control that comes with the ELT, which regulations require if the ELT is not accessible to the pilot. Because of the need for sufficient space for the rather deep plug, locating it on the right-wing root panel was not an option. Accordingly, it was installed on the seat pedestal where the antenna cable would have emerged if the ELT were mounted under the seat.

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Wiring the two plugs was a time-consuming project. Fourteen feet of triple conductor shielded wire were required. Once the wiring was completed the ELT self-checked perfectly.

Other benefits of this arrangement include the creation of a handy space under the seat to stow little items, like a cell phone, and shortened and more accessible antenna feed line.