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goprc(at)live.co.uk
Guest

Posted: Sun Aug 30, 2009 11:55 pm Post subject: VOR Aerials

Hi C

  Can any one tell me the best way and position for mounting a VOR aerial in my

  Europa?

  I put mine inside the fuselage and it has not worked!

  Comments please.

  Thanking you.

  Michael A-A

  G OPRC

 

 

 

 

 

 Use Hotmail to send and receive mail from your different email et='_new'>Find out how.  [quote][b]

paul.the.aviator(at)gmail
Guest

Posted: Mon Aug 31, 2009 4:15 am Post subject: VOR Aerials

Hi Michael,



I put a Bob Archer antenna in the roof of my aircraft and it works

fine.  I originally had it in the floor but the Glideslope & Localizer

didn't work as well.



Regards,  Paul

Ivor Phillips

Joined: 16 Jan 2006
Posts: 253
Location: London UK

Posted: Mon Aug 31, 2009 5:10 am Post subject: VOR Aerials

Hi Michael

It wont be relevant if you have a classic or XS wings already skinned, 

I fitted Dual Bob Archer VOR aerials in the wing tips with a Garmin splitter using RG400 cable, It works fine for all VOR/LOC/ILS.

regards

Ivor

---- Michael Ashby-Arnold <goprc(at)live.co.uk> wrote: 

 	  Quote: 	
		   

 Hi,

 

 Can any one tell me the best way and position for mounting a VOR aerial in my

 

 Europa?

 

 I put mine inside the fuselage and it has not worked!

 

 Comments please.

 

 Thanking you.

 

 Michael A-A

 

 G OPRC

 

 

 

 

 

 

 

 

 

 

 

 _________________________________________________________________

 Access your other email accounts and manage all your email from one place.

gregoryf.flyboy(at)comcas
Guest

Posted: Mon Aug 31, 2009 5:11 am Post subject: VOR Aerials

Paul-



Just for clarification, was that (vor) antenna placed front-ward or

rear-ward with respect to the pilot. 



I am considering putting the glideslope antenna on the top of the fuselage,

just behind the front window, and am wondering what the results have been of

any ground plowed before (for example: prop interference).



Thanks in advance,

Greg



 _________

I put a Bob Archer antenna in the roof of my aircraft 



Regards,  Paul

thomas(at)scherer.com
Guest

Posted: Mon Aug 31, 2009 1:22 pm Post subject: VOR Aerials

Michael,

  

 you might improve reception by taking note of the  polarization of the signal: while Aviation Radio is vertically polarized,  VOR-signals are vertically polarized.

  

 The Europa fuselage is radio-transparent - no need to  mount the antennas externally. That further cleans up the plane.

  

 I assume some wiring problem being the cause of your  NIL-reception as polarisation does not result in such dramatic LOS (Los of  Signal).

  

 BTW, the pilot is a small obstacle for radio waves, too. I  would try to stay away from the engine and instrument panel as there is a cloud  of noise generated.

  

 As Paul suggests: the wingtip is an excellent choice if  you can live with one more connection to be done when mounting the  wings.

  

 good luck !

  

 Thomas, N81EU

 [quote]   ---

frans(at)paardnatuurlijk.
Guest

Posted: Mon Aug 31, 2009 11:49 pm Post subject: VOR Aerials

Carl Pattinson wrote:

 	  Quote: 	
		   Its a while now but our VOR is a copper tape dipole mounted

 horizontally in the bottom of the rear fuselage - about 18" behind

 the rear baggage bulkhead. Not sure of the dimensions but would guess

 a 1/4 or 1/2 wavelength.


	


It should be a 1/2 wavelength.



For what it is worth: I have in the bottom of the rear fuselage also a

1/2 wave dipole. Not straight, but in a V. It is flat in the vertical

plane, but a V in the horizontal plane. The middle of the V points to

the rear, both tips to the front.

The V serves a couple of purposes: A straight dipole is not sensitive in

the direction of the tips if it would be straight. So, if you would put

it parallel to the length axis of the ship, it would be sensitive to the

sides, but not to the front and rear of the ship. If you would turn it

90 degrees, it wouldn't fit as the fuselage is not wide enough for a

straight 1/2 wave dipole. By shaping it in a V, it becomes sensitive for

all directions (with a slight bias to the front of the ship) and it will

fit nicely in the rear fuselage.

Keep in mind though that if you do this, the dimensions of the dipole

will change. I used an antenna analyzer to tune it properly for the

VOR-band.

Sure I know that the front of the ship generates some noise, but this

noise will get into the radio anyway. I do not expect humans and the

engine to be serious obstacles for the radio waves. Both are quite small

compared to the wave length.



An antenna in the wing tip could be a nice idea, but maintenance of the

coax cable (they do age) would be troublesome, and wing tip strobes (as

I have) will spoil the advantage, as strobes produces lots of noise. A

V-shaped antenna in the middle of the ship will null out noise from the

strobes as they are symmetrical to the antenna from that position.



Have not been flying with it, but if I reach that stage, I will let you

know whether it works or not.



Frans

gregoryf.flyboy(at)comcas
Guest

Posted: Tue Sep 01, 2009 2:36 am Post subject: VOR Aerials

I did this and a little bit of research not long ago, but don’t have the paperwork next to me, so from memory, but if I make a mistake for Gods sake, correct me rather than pass on bad information::  

   

Note that this is for the communications band, but VOR band is similar in concept, except that it is is horizontally polarized, instead of vertically polarized. I believe the glideslope is horizontally polarized, as well.  

   

wavelength = v/f, where v=velocity of light and f=frequency.     

   

V=3*10^8 (m/s), and f=127Mhz (cycles/sec)  

   

Hence wavelength = 3*10^8/127*10^6 (127Mhz is center of the band for audio communications) = 2.3622 m/cycle  

   

The dipole is set for half wavelength:    

   

Half wavelength would be 1.181m, so each leg of the dipole is  0.59055m, or 23.25 inches (for dipole in free air). Actual length will be somewhat smaller (15% plus?), to be adjusted with a VSWR meter, but this is a starting point.  

   

This dipole is oriented vertically: center conductor of coaxial cable connected to the copper strip oriented vertically up, and coax ground connected to copper strip oriented down. It does not need a ground plane, as in a Marconi style of antenna.   Metal covered planes will use the Marconi style. Composite aircraft can use them too, if they have ground plane or ground plane strips laid down that are roughly the length of the antenna element. One advantage of a Marconi style antenna is that it (along with a proper ground plane) simulates a full wavelength antenna while using only a half-wavelength physical height. The theory goes deeper, since this is a counterpoise method, which either creates its own ground plane, or is supposed to interact with the real ground plane. I am not sure which is true. If it needs to interact with the real earth ground plane, then planes(airplanes) may loose some of the Marconi advantage, since they are so far away from the real ground. These antennas are used in the tops of skyscrapers, effectively, though.  

   

Other notes for those with enquiry minds (like me):  

   

-The bigger the diameter of the antenna conductor, the broader the bandwidth of response (and a slight decrease in gain, most likely).  A small diameter wire will tend to be highly tuned to one frequency. I think that the copper tape will simulate a bigger diameter, and that is good, I suppose.  

   

-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?  

   

-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.   

Hence at the lower end of the band (118Mhz), the antenna will look short to the signal, and the inductance in the tank circuit will compensate somewhat for the VSWR issue, since its reactance lowers, while capacitive reactance increases.  

At the high end of the band (136Mhz), the physical antenna will look long to the electrical signal, and the capacitance in the tank circuit will compensate for the frequencies higher than the center, since its reactance will lower, while the inductive reactance increases.  

I would try setting Xc (capacitive reactance) equal to Xl (inductive reactance) at both ends of the band, when calculating the tank circuit.  

   

I should probably leave the above topic out, since for all practical purposes, the high frequency ferrite cores placed on the end of the coax (antenna side) will reduce the VSWR, since it presents a lossy environment for energy lost in the bouncing signals in the coax, and then there is no need for impedance compensation. This is how the Europa style Antenna is, and is perfectly adequate, especially for line of site signals, and is the easiest to implement.  

   

-From my reading, it appears that the dipole antenna does not HAVE to be straight! The tips can be bend slightly with very little effect, since most of the radiated energy is closer to the feedline.  

This knowledge could be handy for tight spaces. To use this info to advantage, I am planning on installing a glideslope made out of copper tape, following the curvature of the Europa fuselage top (inside the fuselage, of course), and located just behind the front main window. This should take out some of the ‘parasitics’, such as ME as an antenna parasite (hmmm…).  Mostly it is to keep the antennas away from each other.  

   

Hopefully this information will be useful for someone. It is the type of info I have been craving of late.  

   

   

Remember, I have never taken antenna theory before now, and have never built an aircraft antenna yet, so I am only just learning (work in progress), so let me know of any additions or better explanations, or if I have made a mistake here. From reading the responses on the server, I know there are other technical types out there, so If there is a frown, let us know J Also, Although the copper tape antenna is omnidirectional, I assume it has slightly more radiant power either orthogonal to the plane of the tape, or does it have slightly more radiant energy parallel to the plane of the tape? (if anyone knows)  

   

   

Greg  

   

     

   

  

     

________       

  

From: owner-europa-list-server(at)matronics.com [mailto:owner-europa-list-server(at)matronics.com] On Behalf Of Carl Pattinson

 Sent: Monday, August 31, 2009 3:17 PM

 

   

    

My Physics is a bit rusty but i am sure there are those that would know the formula for aerial length. v = f  X wavelength (I think) though im not sure what v should equal ?  

    

   

    

Hope that helps.  

  [quote]    

---

frans(at)paardnatuurlijk.
Guest

Posted: Tue Sep 01, 2009 3:11 am Post subject: VOR Aerials

Greg Fuchs wrote:



 	  Quote: 	
		   I did this and a little bit of research not long ago, but don't have the

 paperwork next to me, so from memory, but if I make a mistake for Gods sake,

 correct me rather than pass on bad information::


	


Ok, I will give a few corrections:



 	  Quote: 	
		   Half wavelength would be 1.181m, so each leg of the dipole is  0.59055m, or

 23.25 inches (for dipole in free air). Actual length will be somewhat

 smaller (15% plus?), to be adjusted with a VSWR meter, but this is a

 starting point.


	


It is indeed a starting point. There are many things that can influence

the final length. Especially if you bend the antenna in some way.



 	  Quote: 	
		   This dipole is oriented vertically: center conductor of coaxial cable

 connected to the copper strip oriented vertically up, and coax ground

 connected to copper strip oriented down. It does not need a ground plane, as

 in a Marconi style of antenna.   Metal covered planes will use the Marconi

 style. Composite aircraft can use them too, if they have ground plane or

 ground plane strips laid down that are roughly the length of the antenna

 element. One advantage of a Marconi style antenna is that it (along with a

 proper ground plane) simulates a full wavelength antenna while using only a

 half-wavelength physical height.


	


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.



 	  Quote: 	
		   The theory goes deeper, since this is a

 counterpoise method, which either creates its own ground plane, or is

 supposed to interact with the real ground plane. I am not sure which is

 true. If it needs to interact with the real earth ground plane, then


	


It doesn't need to interact with the real earth ground plane.



 	  Quote: 	
		   -The bigger the diameter of the antenna conductor, the broader the bandwidth

 of response (and a slight decrease in gain, most likely).  A small diameter

 wire will tend to be highly tuned to one frequency. I think that the copper

 tape will simulate a bigger diameter, and that is good, I suppose.


	


Correct.



 	  Quote: 	
		   -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.



 	  Quote: 	
		   -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.



 	  Quote: 	
		   I should probably leave the above topic out, since for all practical

 purposes, the high frequency ferrite cores placed on the end of the coax

 (antenna side) will reduce the VSWR, since it presents a lossy environment

 for energy lost in the bouncing signals in the coax, and then there is no

 need for impedance compensation.


	


Incorrect. If you look carefully at the electrical diagram of the

antenna, you will see that the outside of the coax is connected to one

of the 1/4 wavelength elements. The problem is, at the end of the coax,

the electrical current is not sure which way it should go: it has two

choices, the 1/4 wavelength element, or the outside of the coax. Part of

the current will choose the outside of the coax, and then discover that

it doesn't end after 1/4 wavelength. It continues to travel over the

outside of the coax, creating havoc with other conductors nearby, and

creaping altogether into your electrical system instead of being

radiated away.The ferrite cores are there to block this route. True

enough, if you have current on the outside of the coax, the VSWR will

get worse. So ferrite cores seem to improve VSWR, but only bad VSWR due

to currents on the outside of the coax. Ferrite cores won't do anything

if the VSWR is bad due to a too long or too short antenna.



 	  Quote: 	
		   -From my reading, it appears that the dipole antenna does not HAVE to be

 straight! The tips can be bend 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).



Hope this is of some help.

Best regards,

Frans Veldman

craigb(at)onthenet.com.au
Guest

Posted: Tue Sep 01, 2009 3:25 am Post subject: VOR Aerials

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

 [quote]    

[b]

willie.harrison(at)tinyon
Guest

Posted: Tue Sep 01, 2009 3:27 am Post subject: VOR Aerials

 	  Quote: 	
		   	  Quote: 	
		  	
	




Can I ask a simpler question re VOR aerials? Is there an optimum location and orientation for the "rubber duck" antennae you get with handheld Navcoms, eg ICOM? These are a bit rubbish for VOR range when used in the cockpit and attached to the transceiver set. Mine needs to be held up close to the perspex before it can get a stable VOR signal. How would it be if the flexible antenna were mounted in the baggage bay floor pointing downwards and connected with a suitable length of coax?

Willie

 [quote][b]

m.j.gregory(at)talk21.com
Guest

Posted: Tue Sep 01, 2009 4:45 am Post subject: VOR Aerials

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  

         

  

From: owner-europa-list-server(at)matronics.com [mailto:owner-europa-list-server(at)matronics.com] On Behalf Of Greg Fuchs 

 Sent: 01 September 2009 11:31

 To: europa-list(at)matronics.com

 Subject: RE: Re: VOR Aerials  

  

   

   

I did this and a little bit of research not long ago, but don’t have the paperwork next to me, so from memory, but if I make a mistake for Gods sake, correct me rather than pass on bad information::  

   

Note that this is for the communications band, but VOR band is similar in concept, except that it is is horizontally polarized, instead of vertically polarized. I believe the glideslope is horizontally polarized, as well.  

   

wavelength = v/f, where v=velocity of light and f=frequency.     

   

V=3*10^8 (m/s), and f=127Mhz (cycles/sec)  

   

Hence wavelength = 3*10^8/127*10^6 (127Mhz is center of the band for audio communications) = 2.3622 m/cycle  

   

The dipole is set for half wavelength:    

   

Half wavelength would be 1.181m, so each leg of the dipole is  0.59055m, or 23.25 inches (for dipole in free air). Actual length will be somewhat smaller (15% plus?), to be adjusted with a VSWR meter, but this is a starting point.  

   

This dipole is oriented vertically: center conductor of coaxial cable connected to the copper strip oriented vertically up, and coax ground connected to copper strip oriented down. It does not need a ground plane, as in a Marconi style of antenna.   Metal covered planes will use the Marconi style. Composite aircraft can use them too, if they have ground plane or ground plane strips laid down that are roughly the length of the antenna element. One advantage of a Marconi style antenna is that it (along with a proper ground plane) simulates a full wavelength antenna while using only a half-wavelength physical height. The theory goes deeper, since this is a counterpoise method, which either creates its own ground plane, or is supposed to interact with the real ground plane. I am not sure which is true. If it needs to interact with the real earth ground plane, then planes(airplanes) may loose some of the Marconi advantage, since they are so far away from the real ground. These antennas are used in the tops of skyscrapers, effectively, though.  

   

Other notes for those with enquiry minds (like me):  

   

-The bigger the diameter of the antenna conductor, the broader the bandwidth of response (and a slight decrease in gain, most likely).  A small diameter wire will tend to be highly tuned to one frequency. I think that the copper tape will simulate a bigger diameter, and that is good, I suppose.  

   

-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?  

   

-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.   

Hence at the lower end of the band (118Mhz), the antenna will look short to the signal, and the inductance in the tank circuit will compensate somewhat for the VSWR issue, since its reactance lowers, while capacitive reactance increases.  

At the high end of the band (136Mhz), the physical antenna will look long to the electrical signal, and the capacitance in the tank circuit will compensate for the frequencies higher than the center, since its reactance will lower, while the inductive reactance increases.  

I would try setting Xc (capacitive reactance) equal to Xl (inductive reactance) at both ends of the band, when calculating the tank circuit.  

   

I should probably leave the above topic out, since for all practical purposes, the high frequency ferrite cores placed on the end of the coax (antenna side) will reduce the VSWR, since it presents a lossy environment for energy lost in the bouncing signals in the coax, and then there is no need for impedance compensation. This is how the Europa style Antenna is, and is perfectly adequate, especially for line of site signals, and is the easiest to implement.  

   

-From my reading, it appears that the dipole antenna does not HAVE to be straight! The tips can be bend slightly with very little effect, since most of the radiated energy is closer to the feedline.  

This knowledge could be handy for tight spaces. To use this info to advantage, I am planning on installing a glideslope made out of copper tape, following the curvature of the Europa fuselage top (inside the fuselage, of course), and located just behind the front main window. This should take out some of the ‘parasitics’, such as ME as an antenna parasite (hmmm…).  Mostly it is to keep the antennas away from each other.  

   

Hopefully this information will be useful for someone. It is the type of info I have been craving of late.  

   

   

Remember, I have never taken antenna theory before now, and have never built an aircraft antenna yet, so I am only just learning (work in progress), so let me know of any additions or better explanations, or if I have made a mistake here. From reading the responses on the server, I know there are other technical types out there, so If there is a frown, let us know J Also, Although the copper tape antenna is omnidirectional, I assume it has slightly more radiant power either orthogonal to the plane of the tape, or does it have slightly more radiant energy parallel to the plane of the tape? (if anyone knows)  

   

   

Greg  

   

     

   

  

     

________       

  

From: owner-europa-list-server(at)matronics.com [mailto:owner-europa-list-server(at)matronics.com] On Behalf Of Carl Pattinson

 Sent: Monday, August 31, 2009 3:17 PM  

    

My Physics is a bit rusty but i am sure there are those that would know the formula for aerial length. v = f  X wavelength (I think) though im not sure what v should equal ?  

    

   

    

Hope that helps.  

  [quote]    

---

frans(at)paardnatuurlijk.
Guest

Posted: Tue Sep 01, 2009 5:53 am Post subject: VOR Aerials

William Harrison wrote:



 	  Quote: 	
		   Can I ask a simpler question re VOR aerials? Is there an optimum

 location and orientation for the "rubber duck" antennae you get with

 handheld Navcoms, eg ICOM?


	


You need to hold it horizontally, with its side towards the VOR you want

to receive.



 	  Quote: 	
		   These are a bit rubbish for VOR range when


	


They are indeed. You'd better make small dipole just behind the

windscreen. Would cost down to nothing, but give far better reception

than a rubber duck antenna will do.



Someone on the forum already calculated the approximate dimensions, for

reception I wouldn't bother to tune the antenna perfectly.



Frans

frans(at)paardnatuurlijk.
Guest

Posted: Tue Sep 01, 2009 5:53 am Post subject: VOR Aerials

craig bastin wrote:

 	  Quote: 	
		   Is a full wave loop installed in the tail an option here, feed point on the

 floor or roof to create the horizontal polarization


	


If you would have the room for it, yes. But I'm afraid the tailplane is

much too small for that.



Frans

craigb(at)onthenet.com.au
Guest

Posted: Tue Sep 01, 2009 12:45 pm Post subject: VOR Aerials

atcually i was thinking the tail itself, not a tailplane, about 12 inches

behind the baggage bay

give you a nice run of 93 inches around the tail cone



craig



--

gregoryf.flyboy(at)comcas
Guest

Posted: Wed Sep 02, 2009 8:08 am Post subject: 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)

___________________________________________________



 	  Quote: 	
		  > -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
	


 	  Quote: 	
		  > of the radiated energy is closer to the feedline.


	


 	  Quote: 	
		  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.



___________________________________________________

 	  Quote: 	
		  > -Lowest VSWR should be at 127Mhz, however it will increase at both ends

of
	


 	  Quote: 	
		  > 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?


	


 	  Quote: 	
		  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 



___________________________________________________



 	  Quote: 	
		  > -Antenna can be impedance compensated, ie a resonant tank circuit between

> the two antenna elements, set to resonate at the center frequency of

127Mhz.
	


 	  Quote: 	
		  > At 127Mhz, the signal will see the added components as an open circuit.


	


 	  Quote: 	
		  > 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
	


 	  Quote: 	
		  > long, requiring a capacitive reactance to compensate. 


	


 	  Quote: 	
		  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...

___________________________________________________

 	  Quote: 	
		  >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

gregoryf.flyboy(at)comcas
Guest

Posted: Wed Sep 02, 2009 8:43 am Post subject: 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  

   

       [quote][b]

gregoryf.flyboy(at)comcas
Guest

Posted: Wed Sep 02, 2009 8:43 am Post subject: 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  

    

   

    

[quote]craig  

   	  Quote: 	
		    
	


    [b]

Rowland_Carson

Joined: 04 Jul 2008
Posts: 155
Location: Cheltenham, England

Posted: Wed Sep 02, 2009 12:22 pm Post subject: VOR Aerials

At 2009-09-01 21:23 +1000 craig bastin wrote:



 	  Quote: 	
		  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(at)clara.net>

frans(at)paardnatuurlijk.
Guest

Posted: Wed Sep 02, 2009 2:56 pm Post subject: VOR Aerials

Greg Fuchs wrote:



 	  Quote: 	
		   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.



 	  Quote: 	
		   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.



 	  Quote: 	
		   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.



 	  Quote: 	
		   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

gregoryf.flyboy(at)comcas
Guest

Posted: Wed Sep 02, 2009 11:17 pm Post subject: VOR Aerials

Frans-



....Looks like this thread is slowed (for now).



Its just a matter of looking for the best way to do things.

I think we all strive for that, while building our Europa.



Regards,

Greg Fuchs XS TRI



 	  Quote: 	
		  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.


	


 	  Quote: 	
		  Frans

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