lml@lancaironline.net Mailing List Archive

John,
The 406 antennas don't require a ground plane. Yes, it's true that having a ground plane will make the radiation pattern have a lower radiation pattern in the direction above and away from the ground plane, but when you crash you don't crash perfectly upright. As a matter of fact, it's highly likely that you won't be level. The 406 multi freq antenna supplied by ARTEX has a trap built in so it resonates on all three frequencies. I measured the SWR on all three frequencies and were all below 1.5:1. Circular polarization is basically a combination of vertical and horizontal component so if the antenna is something other than straight up and down or totally horizontal then the gain will be best. The Columbia has a wire mesh on the entire outside of the fuselage (and wings) and the antenna is approved inside the fuselage. In an airplane with a glass tail such as the IVP, that is where you would want it.
As far as the VHF/UHF portion being vertically polarized that is somewhat true. The only time you will use this frequency is when someone is nearby, looking for you. Their directional antennas aren't totally vertically polarized since they are usually hand held, and even if your antenna was mounted veritically on the ramp, it will not be in that position when you crash. Don't worry too much about antenna orientation.

Ron

----- Original Message -----
From: John Cooper
Sent: 05/07/14 07:53 AM
To: Lancair Mailing List
Subject: [LML] Re: Antennas for 406 MHz ELTs
Ron and Tom--

But don't the "whip" antennas supplied with the 406 MHz ELTS require a ground plane? The antenna that comes with the ACK E-04 is clearly dedigned for external mounting on a metal skin. See: http://www.aircraftspruce.com/pages/av/elts_zack/whipantennaE04.php What I am looking for is either a quarter-wave "rubber duck" antenna that mounts directly to the ELT, or preferably a copper foil half-wave dipole that I can bond to the inner surface of my aircraft.

Being a Ham Radio operator in a former life, I know that a quarter wave whip antenna is cut for one specific frequency and performance is degraded at all other frequencies unless some kind of "trap" or impedance-matching device is installed. Theoretically, a quarter-wave antenna for 406 MHz should be somewhere around 7" long. For 121.5 MHz it should be around 23" long. Clearly the whip antennas that come with the ELT units are cut for some intermediate frequency which is a compromise that works "OK" at both 406 and 121.5.

And what about polarization? The 121.5 and 243MHz signals should be vertically polarized like your COMM antenna(s), but technically the 406 MHz should be circularly polarized which would either require a helical antenna or two crossed dipoles. Of course the polarization would depend upon whether your aircraft was upright or not :-(
Subject:
Re: [LML] Re: Antennas for 406 MHz ELTs
From:
Ron Galbraith <cfi@instructor.net>
Date:
5/6/2014 7:40 AM
I just installed an ME406 ELT.  The antenna is around 12-14"long thin whip and is for 121.5/243.0/406mhz.  Installs easily inside a glass airframe. 

Ron