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ELT Antenna Length

J

Jasperfield

Registered User
NC
I'm about to show my electronic ignorance.

I have a new 406 Artex ELT for my project (which will replace the new AmeriKing unit I bought two years ago...which is now for sale). The Artex antenna is a few inches longer than the AmeriKing. I would like to use the shorter AmeriKing antenna rather than the longer Artex one. Both ELTs are 406 and 121.5. The shorter antenna is a nicer unit.

Will the shorter antenna work about like the longer one; And what do you (who know) think?

Thanks
Jasperfield
 
Antennas are made to work on a specific frequency or frequencies. Any antenna made to work with 406mhz will function. The only hangup is some of the install manuals will tell you that you can ONLY use their antenna. Just don't install an 121.5 mhz antenna.

Web
 
Thank you, Web, for answering...I had hoped you would respond.

For such a simple airplane there's an awful lot of things that aren't so simple.

Jasperfield
 
Parts get changed when broken. Lol.
But you are correct, as usual. Electrically, any of them will work.

Web
 
The ELT antenna broke off my last airplane (OK, OK, it had a little help!).
I was gonna scrounge up a replacement, but then I realized hat I had two com antennae but only one com radio.
I figured that 121.5 was within the antenna design range (118 > 136 , so I hooked the ELT up to the unused one.
Seemed to work OK.
 
I does wok pretty well for 121.5 mhz but not for 406 mhz.

Web

hotrod180 said:
The ELT antenna broke off my last airplane (OK, OK, it had a little help!).
I was gonna scrounge up a replacement, but then I realized hat I had two com antennae but only one com radio.
I figured that 121.5 was within the antenna design range (118 > 136 , so I hooked the ELT up to the unused one.
Seemed to work OK.
Click to expand...
 
So, an antenna for a 406 ELT is tuned more for 406 MHz, and is "just okay" for 243 and 121.5?
 
txpacer said:
So, an antenna for a 406 ELT is tuned more for 406 MHz, and is "just okay" for 243 and 121.5?
Click to expand...

The antennas for the 121.5 ELT's were designed for just that one freq, which falls within our VHF com band. The antennas that are designed for 406 mhz ELT's are designed for both 406 and 121.5 mhz. Having the incorrect antenna (designed for another freq) generally does not cause damage to the transmitter but it will result in diminished power out. Just ask a ham operator or a boat guy about setting up and tuning antennas. So connecting the Com antenna to a 406 ELT will result in decent power out on 121.5 mhz (because it's designed for that freq) and lower than normal power out on 406 mhz (because it's not designed for that freq). Have someone throw a test set on it and read the power out.

Web
 
Standard 'dual band' antenna design these days for monopoles is to make an end-fed 3/4 wave antenna for 406, resulting in an 18 inch or so whip, which will also be 1/4 wave resonant to about 135 Mhz. so, a loading coil is put 6" up to retune the whip to 121.5, but because the inductance is at the max voltage and minimum current point for 406, it doesn't effect tuning for 406 much. Both are fed at 50 ohms, works fine, lasts a long time....
 
How much will the 406 signal degrade if the antenna is mounted inside the fuselage of a Cub? Some ELTs allow this installation, so it shouldn't be too bad.
 
Here we go again....the ELT to satellite link has about a 35 db fade margin...do you want to use it all up with your antenna installation? My motto is: let your fade margin get used up with variables that you CAN'T control....in a strict answer to your question, it will vary between 0 and 35+ db depending on what position you end up in, and the angle to the satellite. I know that in a cub, the probability of ending up on your back is high, so it behooves you to mount your antenna where it isn't going to get crushed. I went to a lot of trouble to mount mine back on the top of the fuse back by the tail, so it would be protected by the vertical stab. Interior mounting of an antenna is never a good idea, and only justified by preventing damage to the antenna from the crash...I'd do it in a heartbeat if the cub was made of plastic.....
 
I figured this has been discussed before, but I haven't had much luck with the search function, lately.

So, mounting the antenna on the rear by the vertical fin, can you get an effective ground plane on that small of a fuselage section?

By the way, to you guys that know the real answers to this stuff, thanks.
 
txpacer said:
I figured this has been discussed before, but I haven't had much luck with the search function, lately.

So, mounting the antenna on the rear by the vertical fin, can you get an effective ground plane on that small of a fuselage section?

By the way, to you guys that know the real answers to this stuff, thanks.
Click to expand...

Nobody has the "correct" answer to your question, because nobody knows in what position your aircraft will wind up after an accident. That was the response I got from an engineer with Artex when I asked that question.

It is important to understand that the 406 signal is much stronger than the 121.5 signal. That works because it is a burst signal, not continuous, like 121.5.

I mounted my 406 antenna inside the fuselage, and I'll take my chances there. It uses the back side of the baggage compt. as a ground plane. Not optimum mounting perhaps if the plane is on its gear, but I figure that antenna has a good shot at surviving almost any arrival.

YMMV

MTV
 
Also, whip and rod antennas are made to radiate outward from the antenna, similar pattern to the concentric circles you get in water when you drop a rock in. That means they work best when oriented straight up or down. Not so well when they are horizontal.

Web
 
wireweinie said:
Also, whip and rod antennas are made to radiate outward from the antenna, similar pattern to the concentric circles you get in water when you drop a rock in. That means they work best when oriented straight up or down. Not so well when they are horizontal.

Web
Click to expand...

Web, true, but the exception is that here, we're trying to communicate with satellites.....and they may be overhead or on the southern horizon. Again, there's no "perfect" orientation, really.

MTV
 
Agreed at no 'perfect' solution. My theory is to work the odds in my favor as much as possible. I can't control what happens in a crash, but I want my equipment to start out in optimum condition.

Web
 
fobjob said:
Interior mounting of an antenna is never a good idea, and only justified by preventing damage to the antenna from the crash.....
Click to expand...
One of the owners of an aircraft avionics manufacturer told me that I could mount a VHF antenna inside a tube and fabric fuselage without any loss of performance. I asked him specifically of this because I wanted to maintain an authentic exterior look of an antique Stinson.
 
skywagon8a said:
One of the owners of an aircraft avionics manufacturer told me that I could mount a VHF antenna inside a tube and fabric fuselage without any loss of performance. I asked him specifically of this because I wanted to maintain an authentic exterior look of an antique Stinson.
Click to expand...

It works as long as it's non metallic. Otherwise it forms a loose Faraday cage which works to block some/lots of power of the out put signal. Aluminum dope can work the same way.

Web
 
I think Nimpo measured the loss, it was 10db, as I recall, through aluminized fabric...
"So, mounting the antenna on the rear by the vertical fin, can you get an effective ground plane on that small of a fuselage section?" I mounted it to a .035 'tent' that was 8 inches deep and bridged the two upper longerons. A more than adequate counterpoise.
20150711_131817.jpgThe bend on the left was to give extra clearance to a trim indicator cable. Positioning the top surface to be exactly flush with the fabric was an effort.
 

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