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Thread: ELT Antenna Location

  1. #1

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    ELT Antenna Location

    Any recommendations as to the best location for an ELT antenna in a PA-18. I am considering locating the ELT itself in the rear of the top extended baggage.

    Thanks

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    mvivion's Avatar
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    Quote Originally Posted by bubb2 View Post
    Any recommendations as to the best location for an ELT antenna in a PA-18. I am considering locating the ELT itself in the rear of the top extended baggage.

    Thanks
    Be sure to read up on the requirements for mounting. Frankly, a LOT of ELTs don't meet that criteria. Needs to be REALLY stiff, almost no flex.

    As to Antennas, on a fabric plane the issue can be providing a ground plane for the antenna. Ideally, I'd prefer to see the antenna mounted close to the vertical fin, in hopes that if the plane winds up upside down, that vertical fin MIGHT prevent the antenna from being damaged.

    MTV
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  3. #3
    wireweinie's Avatar
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    What Mike said. Read the install instructions.

    Specifically for antennas, any rod or whip antenna is 'vertically polarized'. This means it is designed to transmit (ELT antennas ONLY transmit) from a vertical position, either pointed straight up or straight down. If you could look down at the tip of the antenna the signal would look like the circular ripple created when you drop a pebble into water. it's a concentric circle that starts at the antenna and grows out from there. If you move the antenna to another position, such as sideways, the antenna still sends the signal out the same way. So, in that position the signal pattern is also sideways, so now it looks more like a figure eight, when viewed from above.

    Also, metal dampens/deflects the signal so don't place an antenna inside a fuselage with a metal frame and/or metallic coatings (silver dope). Just don't do it.

    All antennas need a ground plane. This is a metal surface that the antenna fits in the center of. On a fabric aircraft, I like to use a piece of stainless window screen glued to the inside surface of the fabric, at the mounting location. Fit the antenna through the fabric, through the center of the ground plane material, and secure with a fabricated bracket, to the fuselage tubing. Traditionally ELT antennas have been mounted on the battery box access cover on Cubs. As they are going away, I like to mount them just forward of the vertical stab. Like it was stated above, if the aircraft flips over, there should be some protection for the antenna.

    If you ever hear 'good enough' used to describe the installation of safety equipment, it's wrong.

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  4. #4

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    Also, metal dampens/deflects the signal so don't place an antenna inside a fuselage with a metal frame and/or metallic coatings (silver dope). Just don't do it.

    All antennas need a ground plane. This is a metal surface that the antenna fits in the center of. On a fabric aircraft, I like to use a piece of stainless window screen glued to the inside surface of the fabric, at the mounting location. Fit the antenna through the fabric, through the center of the ground plane material, and secure with a fabricated bracket, to the fuselage tubing.



    Web[/QUOTE]Recommended size of the screen?

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    wireweinie's Avatar
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    Bigger is better, but on a Cub it's usually a compromise due to the layout of the structure. Try for at least 10" in all directions from the antenna base.

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    Quote Originally Posted by wireweinie View Post
    Bigger is better, but on a Cub it's usually a compromise due to the layout of the structure. Try for at least 10" in all directions from the antenna base.

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    Cub crafters mounts them under the fabric in the aft fuselage . Passed all tests for FAA and nothing to break off.

    Sandy
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    wireweinie's Avatar
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    Quote Originally Posted by arcticairalaska View Post
    Cub crafters mounts them under the fabric in the aft fuselage .

    Sandy
    I won't and I never will. Placing an antenna inside a metal structure WILL degrade the signal. Look up the definition of a 'Faraday cage'. So placing the antenna inside a ferrous metal structure is not a wise idea.

    As far as Cub Crafters doing it, I'm not the oldest guy here but I'm old enough to know that just because the faa says it's ok does not mean that it is, in fact, ok.

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    I installed mine right in front of jack screw, if you enlarge the photo you can see the ground plane I attached to the spine and top longerone, its on with just a pressure fit held in place by maybe 4 pc4 screws but mostly the fabric. On a previous bird I used 1" copper tape in a radial pattern to extend the ground plane but it rattled a bit. When I got inspected for a replacement airworthiness certificate I remember one of the FAA guys commenting that was a good idea to attach the 'door' with screws not a hinge and quick tabs that way its an inspection panel and not a door and doesn't require more paperwork. The front door is a door for the rear of the extended baggage
    Staying alive in an airplane has a lot more to do with mastering ourselves than mastering the aircraft.

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    I mounted the antenna on the top wing root fairing (on the right wing, outside of plane)).

    Then I mounted the ELT aft of the right-side fuel sight gauge (inside the plane). The remote switch and buzzer mounted in front of the sight gauge.

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    Right in front of the fin. I know mine works since I set it off one day in parking. The phone call came quicker than I expected.
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    Mounting the antenna straight up is one thing but when was the last time you saw a wreck that crashed in a way that left the antenna straight up? Not trying to start an argument here, just curious as to the logic train that brought this concept into the station.

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    wireweinie's Avatar
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    Quote Originally Posted by stewartb View Post
    The phone call came quicker than I expected.
    Lol. It's always a surprise. They have a few minute response time mandate and I'm pretty sure they meet it.

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    wireweinie's Avatar
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    Quote Originally Posted by Mr. Ed View Post
    Mounting the antenna straight up is one thing but when was the last time you saw a wreck that crashed in a way that left the antenna straight up? Not trying to start an argument here, just curious as to the logic train that brought this concept into the station.
    The antennas are 'vertically polarized', meaning they are designed to transmit with the whip or rod straight up or straight down. No logic train, just a design fact.

    As for the position of the antenna after a crash, ya' play the best odds. End up on the belly and the antenna is straight up. End up on your back and the vertical stab should keep the antenna off the ground. Some operators place the antenna straight down on the tailboom on Bell 206's so I've actually tested them and they work just fine.

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    Or like a handful of years ago when a 182 went on its back in a bog in Galena, antenna on top like every Cessna, and the 406 signal was received. None of us knows for sure what the transmit limits are. We do our best to maximize our chances.

    I believe my installation manual says the antenna can be located internally but must be vertical. Don’t remember exactly. Don’t care. I like it external. Given that lots of guys still uses 121.5? Clearly not everyone is concerned about beacon effectiveness.

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    I like the idea of the dipole antenna mounted inside the fuselage. Dipoles are more efficient than groundplane antennas. The cub fuselage is not a faraday cage, but a number of tubes a near the resonant frequency or multiples of the resonant frequency so that they suck up the radiant energy coming from the antenna. But an intact antenna inside a open but bent cage is better than an antenna stuck in the mud or torn off the aircraft structure.
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  16. #16
    BC12D-4-85's Avatar
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    This might be worth a look for an internal antenna option> http://www.advancedaircraft.com Some comparative signal field strength testing after installation would be interesting.

    A homemade setup> https://peter-ftp.co.uk/aviation/mis...a4ULs_rev3.pdf

    Gary

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    Bill Rusk's Avatar
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    Here is a link to info on how and where I did mine. Might help.

    https://www.supercub.org/forum/showt...l=1#post686221


    Bill
    Very Blessed.

  18. #18
    wireweinie's Avatar
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    A pattern of conductive materiel surrounding an antenna is the definition of a Faraday cage. Even if it was not designed/built for that purpose. Will an antenna inside a steel tube fuselage covered in metallic (silver) dope transmit a strong enough signal? Possibly. Do ELT manufacturers allow for internal antennas? No, with one exception. If you have a non conductive structure, such as wood, fiberglass, etc, you can. But that's because non conductive materials will not dampen a signal. Even then the antenna needs to be oriented vertically and needs a sufficient ground plain to work correctly.

    As for the antenna in the mud or broken off, I can't help you if that happens. But this is aviation. We increase the odds in our favor but nothing is %100 guaranteed. If that antenna was placed in a good spot such as in front of the vertical stab and it ends up broken off, then chances are the ELT will not help anyways.

    You're a big boy, you do what you feel is correct. But as an 'electrical guy' my job is to help customers get the best equipment/installation possible. I will not accept 'good enough' with safety related equipment.

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  19. #19
    sjohnson's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    This might be worth a look for an internal antenna option> http://www.advancedaircraft.com Some comparative signal field strength testing after installation would be interesting.
    ...Gary
    I installed one of these in my vertical stabilizer as a comm antenna. Works fine. Signals are relatively weak directly forward and down (blocked by the firewall). Still, it seems work as well as most Cub installations, as nearly every installation results in some compromise and weak coverage areas.

    406 MHz ELTs transmit on two frequencies (406 and 121.5), and have monopole antennas designed specifically for them. The Advanced dipoles are unlikely to work well for 406 ELTs.
    The modern equivalent of the phrase "lions led by donkeys" is "innovation led by lawyers".
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  20. #20
    wireweinie's Avatar
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    Lol. I didn't even catch that.

    You are correct. A com rated dipole would work for the 121.5 mhz signal as that frequency is within the design range of the antenna. But the 406 mhz signal is in an entirely different band.

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    sjohnson's Avatar
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    Just to provide a contrary view, I mounted my 406 ELT internally, using the baggage compartment as a ground plane and very close to the ELT, with the antenna tilted quite a bit.

    Of course it isn't ideal for transmission, but IMHO, it is more important to keep the antenna attached to the ELT in an accident, than maximizing transmission efficiency in flight. As demonstrated by the success of internal comm antennas, the powdered aluminum layer on fabric is not all that conductive and doesn't make a debilitating Faraday cage. I'm pretty sure that the Husky also has an internal ELT antenna.
    The modern equivalent of the phrase "lions led by donkeys" is "innovation led by lawyers".
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  22. #22
    BC12D-4-85's Avatar
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    Those dipoles can be cut down to resonate on 406 but might require a matched signal splitter or ? to also feed a 121.5 antenna from the same coax. The ELT factory antenna setup takes care of that. But....if concerned about impact damage why not mount a second ELT antenna on the belly fed by a splitter?

    Gary

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    Mine is on the metal battery hatch behind the passenger (turtledeck).

  24. #24
    wireweinie's Avatar
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    Its handy to put it there but with the 'newer' Cubs and after rebuilds a lot of those covers are going away.

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    Quote Originally Posted by sjohnson View Post
    Just to provide a contrary view, I mounted my 406 ELT internally, using the baggage compartment as a ground plane and very close to the ELT, with the antenna tilted quite a bit.

    Of course it isn't ideal for transmission, but IMHO, it is more important to keep the antenna attached to the ELT in an accident, than maximizing transmission efficiency in flight. As demonstrated by the success of internal comm antennas, the powdered aluminum layer on fabric is not all that conductive and doesn't make a debilitating Faraday cage. I'm pretty sure that the Husky also has an internal ELT antenna.
    Ditto
    Remember, These are the Good old Days!

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    mvivion's Avatar
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    Heres the deal, though: You can install your ELT wherever it makes you feel good, and it will work perfectly right up until it’s activated. After that, it’s a bit late to move it.

    I only see one “avionics type” engaged in this conversation. I guess I’d follow his recommendations. Or, don’t crash.

    MTV
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  27. #27
    wireweinie's Avatar
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    Quote Originally Posted by mvivion View Post
    Or, don’t crash.

    MTV
    Highly recommended.

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    JimParker256's Avatar
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    Here's a neat experiment in support of WireWeinie's "faraday cage" argument: Find a nearby "big box" store that was built with "tilt-up concrete walls." Those walls are poured with rebar laced together throughout, and when the tilt them upright and tie them together in the corners with steel beams, and install a metal roof, they form a pretty darn good faraday cage. Take a look at your cellphone, and note the number of bars of coverage you have outside, versus inside the center of the store... You could have 5 bars of coverage outside the store, but when you walk in, the signal drops dramatically (in most cases, it goes to almost nothing), and the further you go inside the store, the weaker it gets. That's a faraday cage in action...
    Jim Parker
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  29. #29
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    I recall a number of stories about ELTs being activated for various reasons other than crash situations. I believe MTV related one type at OshKosh, and there is an example above.

    Real world examples of antennas working, signals being received. Copy installations like theirs and we should be ok. I bet theirs were by manufacture instructions.

    If an antenna comes off in a crash, and you are able- do your best to reattach, even if not perfect some signal is better than none.

    Lastly, if everything was perfect we would not need ELTs.
    I don't know where you've been me lad, but I see you won first Prize!

  30. #30
    BC12D-4-85's Avatar
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    Remember those old yellow EBC-102A ELT's? Mounted them somewhere in the cockpit with attached flexible whip antenna. They seemed to work ok especially when installed near a window or skylight. Different frequency than the 406's tho, and made when satellites were scarce.

    Gary

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    Quote Originally Posted by BC12D-4-85 View Post
    Remember those old yellow EBC-102A ELT's? Mounted them somewhere in the cockpit with attached flexible whip antenna.
    one of these coupled with a PLB (personal locator beacon), would be an acceptable solution for some.

  32. #32
    wireweinie's Avatar
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    How so?

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    Quote Originally Posted by aktango58 View Post
    I recall a number of stories about ELTs being activated for various reasons other than crash situations.
    I once had one activate when I tossed it on the kitchen table at the end of a long day. I had been disassembling a PA 16 project I had bought and put the ELT on the table. The Battery hadn't been replaced in 15 years so I expected nothing. About three in the morning the dog went nuts. The C.A.P are parked outside in their tracking truck. They found me! I had no idea what was going on until I went out to talk to them. I was surprised when they told me they were tracking an ELT - and it was coming from inside my house. I showed them the ELT with the sticker showing a 15 year old battery and they had no problem with it. But it was one of those things that sticks with you.

    ROA

  34. #34
    dabridgham's Avatar
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    People have brought up the idea that ELT antennas ought to be vertically polarized. When we were using 121.5 MHz and searchers were listening for you on their comm radios, which have vertically polarized antennas, this makes sense. Once we go to satellites being the primary receivers of the signal, not so much. The satellite's vertical and yours are seldom going to match up so either the satellites have circularly polarized antennas (best answer) or they just live with whatever they get.

  35. #35
    wireweinie's Avatar
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    Explain how one antenna needs to be vertically oriented and another does not, when they are both vertically polarized. What's your engineering data? What install data are you referring to? How many different systems have you installed?

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  36. #36
    dabridgham's Avatar
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    Assuming you, wireweinie, are talking to me, I'll explain a little more about antennas. First, an antenna isn't intrinsically vertically or horizontally polarized. It's linearly or circularly polarized and if it's circular, then it can be left-handed or right-handed. And then a linearly polarized antenna may be mounted vertically or horizontally (or in between) so the signal it puts out is vertically or horizontally polarized (or in between). If an airplane with a vertically oriented linearly polarized antenna (like an ELT antenna) is lying on its side, it's putting out a horizontally polarized signal.

    If the transmit and receive antennas are cross-polarized, 90 degrees apart for linearly polarized antennas or left vs right for circularly polarized antennas, then the received signal will be very low. If one antenna is circular and one linear, then the received signal will be constant no matter the orientation in polarization of the linear antenna, though it will be half of what it would be for matched antennas.

    Now consider satellites. What's vertical? The obvious answer is that it's a line from the satellite to the center of the Earth. For a satellite that's not directly overhead, that means your vertical and its are going to be different. If your antenna is vertical and its is too, you will be somewhat cross-polarized, how much depends on how far the satellite is from directly overhead. By the way, vertical antennas put out a lousy signal straight up so the situation where your verticals align is the worse case for other reasons. If the satellites have circularly polarized antennas then this problem goes away and the orientation of the ELT's linearly polarized antenna does not matter. Well, it doesn't matter to the satellite. As I said, searchers on the ground or air will also be using vertically polarized antennas so for them it's still beneficial if your ELT antenna is also vertical.

  37. #37

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    Quote Originally Posted by wireweinie View Post
    How so?
    121.5 to fulfill the FAR’s

    PLB so someone can actually find you.

    downsides to this setup:
    must manually activate the PLB (problem here is if you are pinned and can’t get to it)
    PLB gets damaged/lost in the accident
    probably other issues, those are the big ones that come to mind

    upsides:
    cheaper (the unit and no installation costs)
    more versatile (i.e. removable)
    better than only having a 121.5

    with that said - I have a 406, sat phone, and an inreach.

    406 for a rescue
    sat phone to call buddy for prop and struts
    inreach to text the girlfriend

  38. #38
    wireweinie's Avatar
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    I'll use English as we have company;

    Generally speaking, we use two types of antennas on aircraft; Omni directional and directional. As their names indicate, omni antennas are designed to receive or transmit from any clock position. Com antennas are of the omni type. With the antenna properly installed, you can listen from or talk to a station regardless of the direction of the signal with respect to the aircraft. Directional type antennas either transmit or receive a signal in one direction. Unless you are using special mission equipment, we only use direction receiving antennas on aircraft. Nav antennas and ADF antennas are examples of this. When a signal is received, the receiving unit is able to 'see' the direction from the aircraft to that signals source.

    ELT antennas are omni directional, transmitting antennas. We need to have a signal radiated in all clock positions to increase the odds of someone (now days a satellite) receiving that signal. They are also of the quarter wave design. At our level, this means that the antenna length is one quarter of the wavelength of the signal it's designed to work with. Not important to us, but what IS important is that quarter wave antennas require a ground plane to operate properly, so keep that in mind when installing. While it would be awesome to find an antenna that transmitted in a full sphere they don't exist in the aircraft world. Each kind of antenna has strong points and weak points. For us, using rod type antennas, they will transmit the strongest signal when the receiving antenna is oriented the same way as the transmitting antenna. I.e., They antenna at ATC is straight up and if ours is too, then we can communicate well. Have a buddy with a Bonanza with the com antenna in the wing tip? He still has to turn the aircraft 90º to the tower to talk at a distance. This is because the Bonanza's antenna is horizontal to the vertical antenna in the tower.
    To visualize the signal put a donut (or a bagel if you prefer) on a rod antenna. That is what the signal looks like as it is transmitted. Just an every growing donut. (See the attached pic, the rod antenna would be along the Y axis) If you have another rod antenna, oriented vertically, the 'donut' will eventually cross that one and allow the radio to hear the signal. Now take the donut with the antenna in it's hole, and lay it on it's side. The signal still has the 'growing donut' shape, but now it's on it's side. The receiving antenna will have no trouble IF it is perpendicular from the transmitting antenna. But if the receiving antenna is off the tip or the base of the transmitting antenna, there will be almost no signal to be received.
    No, we can't foresee all outcomes of a crash and therefore the final position of the ELT, but the antenna's installed position is designed to give the best chance of survival, to remain operational, after the crash.

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  39. #39
    wireweinie's Avatar
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    Quote Originally Posted by akwing View Post
    121.5 to fulfill the FAR’s

    PLB so someone can actually find you.

    downsides to this setup:
    must manually activate the PLB (problem here is if you are pinned and can’t get to it)
    PLB gets damaged/lost in the accident
    probably other issues, those are the big ones that come to mind

    upsides:
    cheaper (the unit and no installation costs)
    more versatile (i.e. removable)
    better than only having a 121.5

    with that said - I have a 406, sat phone, and an inreach.

    406 for a rescue
    sat phone to call buddy for prop and struts
    inreach to text the girlfriend
    I'll agree with all of that except using something nearly worthless just to fulfill an faa requirement. If you fly around with something in your aircraft, make sure it has a good use.

    Up here the sat phone extremely important!

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  40. #40
    BC12D-4-85's Avatar
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    This is well beyond topic, but if interested read Chapters 22/27 from this book: http://www.w3pga.org/Antenna%20Books...ions%20III.pdf

    It describes the antennas used by Sat phones and satellites by one of the developers Walter Maxwell, plus other radio transmission/reception theory.

    Gary

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