Can I roll my own ELT antenna? Or must I install the mfg supplied antenna?

[hplevyak]

Folks,
I'm building a GlaStar and currently working on the ACK Avionics E-04 ELT installation. I would like to install the ELT antenna inside the composite fuselage. The ACK E-04 supplied antenna calls out the need for a ground plane when used on a composite structure and state it has to be 48" in diameter. (Foil tape elements 24" long (six of them). And I just don't have that space.

As an alternative, I'm thinking of putting in a Jim Weir style copper dipole "Double V" antenna. I'd build it using copper foil with one of the V elements having each leg 22.5" long (121.5 MHz), and the other V having leg lengths of 7.3" long (406MHz). I'd place this vertically on the side wall of the tail tunnel.

So my question is....Can I legally do this and still meet the requirements of the TSO and FAR?

The ACK E-04 install instructions do state the following:
"Other antennas may be used provided they meet the minimum VSWR requirements as noted in the specifications section of this manual and meet COSPas/SARSAT approval for use with the model E-04 ELT."

I'd sure appreciate some guidance on this.

Thanks,
Howard (GlaStar builder....deep in wiring and all installing all things electrical)

[WLIU]

Aircraft Spruce sells at least one antenna that matches the description of what you would like to fabricate. In the fine print, this dipole antenna is stated to be suitable as an ELT antenna for composite aircraft.

http://www.aircraftspruce.com/catalo...?clickkey=5356

So since you need to make the FCC happy, in addition to the FAA, I will suggest that if you fabricate your antenna, hook up an SWR meter and a hand held radio, and do a quick test on any aircraft freq to prove that you meet the required SWR, and take a photo for your builder's log, you are good to go. Alternatively, you could purchase and install the AC Spruce unit.

Best of luck,

Wes
N78PS

[steveinindy]

I would think it would be a lot easier to just buy and install the embedded antenna from Aircraft Spruce. Unless you have a way to massively improve the performance of the antenna by making one of your own, why not spend that time on something else would be my argument?

[FlyingRon]

Jim's antenna design is likely to be superior to the one that comes with the ELT. You can measure the SWR if you want, but frankly, if you measure and construct it carefully, it's going to work just fine. As for legality, on your homebuilt, there's no reason not to do it Jim's way. The FAA doesn't care. The FCC doesn't care.

The D is silent in Jim's name.

[steveinindy]

Jim's antenna design is likely to be superior to the one that comes with the ELT. You can measure the SWR if you want, but frankly, if you measure and construct it carefully, it's going to work just fine.

I don't know enough about electronics to know if it would or not. I just figure that one would have better things to do with their time.

[CraigCantwell]

Mock the antenna up on the a/c then buy a local ham friend a dinner and get him to run it on his antenna analyzer.... Trim or adjust for an exact match and then install.

[WLIU]

Actually, the FCC does care. But practically speaking, they won't care until you crash and your antenna creates interference problems with other spectrum users as they look for your dead body.

The above post is a great idea. Since one state purpose of homebuilding is education, this is an opportunity to learn something from a local ham radio enthusiast. And while you are checking your ELT antenna, you can check your comm antenna(s), which you really do care about.

Best of luck,

Wes
N78PS