Today's Message Index:
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1. 07:56 AM - LAA Rally Sywell (G-IANI)
2. 09:08 AM - Re: Re: VOR Aerials (Greg Fuchs)
3. 09:43 AM - Re: Re: VOR Aerials (Greg Fuchs)
4. 09:43 AM - Re: VOR antennas et all (Greg Fuchs)
5. 09:43 AM - Re: Re: VOR Aerials (Greg Fuchs)
6. 01:22 PM - Re: Re: VOR Aerials (Rowland Carson)
7. 03:56 PM - Re: Re: VOR Aerials (Frans Veldman)
8. 04:24 PM - Re: A good fly in, TEXEL W/E of 12th September (zwakie)
Message 1
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Subject: | LAA Rally Sywell |
UK Owners
Europa will have a large stand at the Sywell show manned by the Factory and
the Club.
Sorry of the short notice but we are looking for a Monowheel to display on
the stand.
The preferred option would be for it to be installed on Friday afternoon and
removed after the show on Sunday. Failing this could anyone fly in their
aircraft early (on one or both days) and leave it on the stand for the day.
The factory will take care of landing entry fees etc.
If anyone can help, please contact me.
Ian Rickard G-IANI XS Trigear, 280 hours
Europa Club Mods Rep (Trigear)
e-mail g-iani@ntlworld.com
Message 2
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Subject: | Re: Europa-List: VOR Aerials |
Hi Frans,
Thanks for the input. Antennas are quite the black art, I think.
Just 4 extra remarks to add to some of your statements:
(responses left long for clarity)
___________________________________________________
>> -From my reading, it appears that the dipole antenna does not HAVE to be
>> straight! The tips can be bent slightly with very little effect, since
most
>> of the radiated energy is closer to the feedline.
>Yes, but bending it will change the impedance. There are two properties
>here: a too long antenna behaves like a coil, too short as a
>capacitance, but if correct, it will present itself as a pure
>resistance. Now, the resistance of a tuned antenna will be around 50
>ohms. But only if it is a normal dipole. Mess with it (bending it, put
>other conductors nearby) and the impedance (resistance) will stray away
>from 50 ohms. Even if the antenna is tuned properly, you will still get
>a bad VSWR because the impedance of the antenna doesn't match the
>impedance of the coax (more correctly, the output/input of the
>transmitter/receiver).
Of course Frans, you are completely right. Bending an antenna will change
its impedance. Looking at the impedance curve diagram for a half-wave
antenna, it shows that the impedance is very high out at the tips, and very
low at the feedline. So I would expect that if most of the antenna is
straight, and only the tips are bent slightly, the impedance change will be
minimal. I am sure there are calculations to be done for figuring that out.
I have seen good reports with tip bended antennas.
Also, it appears that many of the antenna and feedlines on aircraft are not
matched anyway by up to 23 ohms (see below), unless the antenna has a
matching network. It seems to not matter too much though, and this is
considered 'good enough' for aircraft (and line-of-site) usage in much of
the literature.
___________________________________________________
>> -Lowest VSWR should be at 127Mhz, however it will increase at both ends
of
>> the band, 118Mhz and 136Mhz. One of my computations put best theoretical
>> VSWR for a dipole at 1.5. I am not sure if this is correct. Anyone?
>I'm not sure what you mean. The best VSWR is 1:1, although it makes
>little difference if it remains below 1:2.
Let me explain better. I think it was a mismatch calculation between the 50
ohm coaxial cable (that we use) and the Hertz dipole antennas nominal
resistance (digging up my papers, now).
For example:
A transmission line reflection coefficient (lets call it RefC)
is defined as:
( Z(L)-Z(o) ) / ( (Z(L) + Z(o) )
Where: Z(L) is the load, or the antenna
Z(o) is the transmission line impedance
Then VSWR can then be directly calculated, and is defined by:
( 1 + RefC ) / (1 - RefC)
A well made Hertz dipole antenna has a pure resistance of 73 ohms, nominally
(the kind with two copper strips connected to the center conductor and
shield of the coax). The coax cable is 50 ohms impedance.
Thus, RefC = (73 - 50) / (73 + 50 ) =.18699...
Then the VSWR is ( 1 + .18699) / (1 - .18699) = 1.46, or about 1.5
___________________________________________________
>> -Antenna can be impedance compensated, ie a resonant tank circuit between
>> the two antenna elements, set to resonate at the center frequency of
127Mhz.
>> At 127Mhz, the signal will see the added components as an open circuit.
>> Since an antenna will look capacitive if it is short, it will require an
>> inductive reactance to compensate. It will look inductive if the antenna
is
>> long, requiring a capacitive reactance to compensate.
>Yes, but you can't use both together at the same time.
Well, according to this book you can:
In MODERN ELECTRONIC COMMUNICATION by Gary M. Miller, circa 1978,
Go to page 422, and it shows a tank circuit in between the two dipoles, at
the feedline.
To quote:
"With the tank circuit resonant at the frequency corresponding to the
antenna's (half wavelength), the tank presents a very high resistance in
parallel with the antenna's 73 ohms,and has no effect. However, as the
frequency goes down, the antenna becomes capacitive, while the tank circuit
becomes inductive. The net result is a resistive overall input impedance
over a relatively wide frequency range."
It is called an 'Impedance-compensated dipole'
Now, just because it is in a book, does not mean it is right. Being that it
is a highly tuned tank circuit, you may be right-O-right-on. It may be much
too rejective of frequencies around the resonant frequency the tank is set
to. Band spreading resistors in parallel will only progressively short out
the antenna. Possibly, it may be for a more limited frequency range. I think
the burden of proof will be on why the compensation will work (for aircraft
bandwidth), rather than why it won't.
I will try to look into this further...
___________________________________________________
>>Nope. If you use a ground plane, the antenna is 1/4 wavelength. A dipole
>>is two 1/4 wavelength stacked on top. The bottom one acts as a ground
>>plane. There is no advantage in using a "real" groundplane or 1/4
>>wavelength element. In composite aircraft you have the choice, in metal
>>airplanes you are bound to the groundplane model.
Yes, of course that is right. In a metal airplane, most people would use a
1/4 wavelength antenna to keep the wind drag down due to the external
antenna, and their airplane skin would be used as the ground plane (Like a
Marconi Antenna).
The point I was trying to make here, but did not quite describe it well
enough, is that the overall height of the Marconi antenna is only half the
equivalent overall height of the Hertz antenna. That might be where an
advantage is.
Since 1/2 wavelength is better than 1/4 wavelength, of course 1 wavelength
is better than 1/2 wavelength, for antenna heights.
In a composite aircraft, a half wavelength Marconi actually might be able to
fit, since it is the same height as a 1/4 wavelength Hertz-style Dipole.
In the Europa, the only place a 1/2 wavelength 'straight' Marconi antenna
would fit easily, would be in the tail fin (same place where we put the
Hertz dipole). Unfortunately, the only ground plane for this antenna would
be forward into the fuselage, so this would be far from omni-directional,
and would not work. Now, if the tip could be slightly bent (and with proper
adjustment), one might be able to put one in the mid-section of the
aircraft, with a ground plane on the bottom of the fuselage. I 'might' try
this at some point, but will probably stick with the Hertz dipole we all are
accustomed to, maybe with a few minor adjustments.
A side note: I believe that one of the books I was reading stated that a
1/4 wavelength Marconi acts as a 1/2 wavelength Antenna with the earth as
the ground plane. Something about an 'image' wave coming from the earth
itself, equivalent to another 1/4 wavelength of antenna height. That is, it
has the same efficiency in delivering the signal to the receiver, as a half
wavelength antenna. This would probably work just as well with a 1/2
wavelength vertical antenna (with earth as ground plane) being the same as a
full wavelength vertical antenna. Now, talk about an efficiency increase
from good old mother earth! This might be only when using the earth as a
ground plane, and may only apply at the 'bouncy' lower frequency bands
(being a Marconi and all), or it might depend on how far the antennas
artificial ground plane is from earth, so that it can interact with it as a
'counterpoise'. I will have to research more, to be sure.
___________________________________________________
Best Regards!
Greg Fuchs
Message 3
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Subject: | Re: Europa-List: VOR Aerials |
If I could take a stab at this:
The information I have about loop antennas, is that they are normally much
smaller than a wavelength, but they do have broadband response. Also they
are bi-directional, not omnidirectional. I suppose that they could be set up
in the tail to transmit and receive in the forward or aft directions, but
they would not then work laterally. They are used in direction-finding
applications, such as the old style DF steer, and ADF.
Possibly, you might be thinking about a Folded Dipole? If you take a normal
dipole, and connect the free ends with a longwire, then shape it into a
collapsed rectangle with donut-style ends, you would have a Folded Dipole.
It looks a bit similar to a loop. A folded dipole has the same radiation
pattern as a regular dipole. Unfortunately, it also has a high input
impedance, of 288 ohms. To make it work with the current Radios and
Transmission lines, would mean sacrificing a lot of radiated energy from the
antenna, so I don't believe it would be a viable option.
-Greg
>Is a full wave loop installed in the tail an option here, feed point on
the floor or roof to create the horizontal polarization
>craig
Message 4
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Subject: | VOR antennas et all |
Good ideas, Ferg, am liking all of the alternatives.
But you have stimulated my right brain a bit. Why should we settle with our
little 5 (or so) watt radios with only 100 mile ranges.
How could pilots from all countries communicate together? I guess we could
use those ideas from the local Amateur Radio club, and put in Ham band
radios into our aircraft! That will just fit the bill. I am sure they would
jump at the idea of installing those radios, too.
Then all we have to do is wait for the right ionosphere conditions at the
right time of the day, then we turn off a few of our accessories for the
extra power, turn on the third alternator, then .. Ahh well.
Someday I guesstimate, it will be hard for me to keep running from the ham
shacks.
Cheers,
Greg
_____________________________________________
Cheers,
I note the level of discussion about antennas rising again, so will
make my pitch for an alternative...
...
Given that, I have always suggested a builder of aircraft who needs
advice or measuring in the radio line contact his/her local Amateur Radio
club and solicit a dash of help in citing or measuring the antenna, both the
Av VHF, Beacon, VOR and assorted accessory freqs.
Ferg
Europa Classic 914 CSprop
Message 5
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Subject: | Re: Europa-List: VOR Aerials |
Yep, thanks Mike
I meant that the math to figure out the antenna length would be the same,
assuming that you used it on the VOR bands. I did not have the VOR band
information with me, so I did not know off the top of my head, what they
were. Now I do, thanks to you!
Regards,
Greg
Greg,
The VOR system operates in the VHF frequency band, from 108.0 to 117.95 MHz,
so that it is below the air/ground VHF R/T communication band from 118 MHz
upwards. The difference between horizontal and vertical polarisation of the
antennas would not be sufficient to prevent interference if they shared the
same band.
You should therefore centre the tuning of your VOR antenna around the
mid-band frequency of 113 MHz.
Regards,
Mike
Message 6
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Subject: | Re: Europa-List: VOR Aerials |
At 2009-09-01 21:23 +1000 craig bastin wrote:
>Is a full wave loop installed in the tail an option here, feed point
>on the floor or roof to create the horizontal polarization
Craig - I guess you are thinking of something what's described here:
<http://www.davemorris.com/Dave/MorrisDFLoop.html>
I note that this description is intended for comms (not VOR) and so
the designer states:
"Placing the gap halfway up the fuselage side causes the antenna to
radiate in vertical polarization. (If you deviate from the plans and
place the gap at the top or bottom, the antenna will become
horizontally polarized, and you will suffer a significant decrease in
signal strength both on transmit and receive, so don't do it!)"
This type of aerial seems to be ideal for a Europa installation,
unless I have missed something. Obviously, though, you couldn't have
both VOR & comms aerials of this type in the fuselage as they would
be much too close together!
regards
Rowland
--
| Rowland Carson LAA #16532 http://home.clara.net/rowil/aviation/
| 1300 hours building Europa #435 G-ROWI e-mail <rowil@clara.net>
Message 7
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Subject: | Re: Europa-List: VOR Aerials |
Greg Fuchs wrote:
> Of course Frans, you are completely right. Bending an antenna will change
> its impedance. Looking at the impedance curve diagram for a half-wave
> antenna, it shows that the impedance is very high out at the tips, and very
> low at the feedline. So I would expect that if most of the antenna is
> straight, and only the tips are bent slightly, the impedance change will be
> minimal.
We are talking about impedance, not resistance. A bend tip will interact
with the rest of the antenna, and the impedance change may be much more.
> Then the VSWR is ( 1 + .18699) / (1 - .18699) = 1.46, or about 1.5
Ok, I see what you mean. This may be right. Keep in mind though that in
a Europa, a dipole will be anything except the theoretical impedance.
There is too much stuff around it. Most of the stuff around it will
lower the impedance, so 50 ohms will be quite close in most cases. In my
ship, it is close to 50 ohms.
> Well, according to this book you can:
>
> In MODERN ELECTRONIC COMMUNICATION by Gary M. Miller, circa 1978,
> Go to page 422, and it shows a tank circuit in between the two dipoles, at
> the feedline.
Well, it is worth some investigation. Never heard about it.
> The point I was trying to make here, but did not quite describe it well
> enough, is that the overall height of the Marconi antenna is only half the
> equivalent overall height of the Hertz antenna. That might be where an
> advantage is.
>
> Since 1/2 wavelength is better than 1/4 wavelength, of course 1 wavelength
> is better than 1/2 wavelength, for antenna heights.
No, this is not true. (Assuming that you mean antenna length, and not
antenna height). A dipole (whether that be two 1/4 elements or 1/4
elements and a ground plane) is the best omnidirectional radiator. 100%
of the energy is radiated. A longer antenna has no benefits. Sure, you
can bundle the signal to certain directions, but that will be at the
cost of the radiation of other directions. Compare it with a light bulb.
Want to get more light somewhere? Can be done, but at the cost of light
at other places.
Now, to explain again: a dipole (as used in the Europa's) is exactly the
same as a 1/4 antenna on a ground plane. In a dipole, one of the 1/4
elements IS the ground plane. It is the same, and the radiation pattern
and efficiency is therefor similar.
I get the impression that you are reading books about short wave
communications. A lot of what you write would be correct for short wave.
But in airplanes, we are talking VHF. And we want an omnidirectional
radiation pattern. A standard dipole is perfect for this.
Further: what is the purpose anyway? The limit is the line-of-sight, and
we can reach that with a standard dipole. Nobody is going to send a
QSL-card or giving awards if you manage to crank out a stronger signal
anyway. The rest is airplane logic: keep it as light weight as possible,
and as simple (reliable) as possible.
Frans
Message 8
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Subject: | Re: A good fly in, TEXEL W/E of 12th September |
A little kick of this thread...
So far appr. 15 Europas have signed in. It's going to be busy around Texel that
weekend with more than 130 sign-ins, but there's always room for more Europas
:D
Please visit www.flyin.nl for details.
Visit - www.EuropaOwners.org
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