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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Mon Sep 16, 2019 9:46 am    Post subject: Grounding radio antenna and   transponder antenna | 
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				You said tube structure . . . fabric or metal skin?
 
  If fabric, a 5.6" alum disk .060 or so thick
  is an appropriate ground plane.
 
  [img]cid:.0[/img]
  If your fabric is 'tight' against a structural
  tube, then the ground plane can go on the outside
  surface. It would look cleaner if between skin
  and tube.  One or more clamps should secure the
  ground plane to structure . . . not for electrical
  'bonding' but for mechanical support.
  [img]cid:.1[/img]
 
  
  Whether the ground is inside or outside the
  skin, the antenna should mount in the center
  with good electrical connection between antenna
  base and the ground plane.
 
  If metal skin, simply mount antenna to skin
  but consider double of any practical size
  to re-enforce the mounting surface to preclude
  cracking around mounting hole due to aerodynamic
  buffeting.
 
  VHF comm antennas need good support to the
  ship's tubular structure. If metal airplane,
  ground antenna base to skin. If fabric covered
  airplane, ground a suitable mounting plate
  to the ship's structure then ground antenna
  to the mounting plate. Ship's structure
  becomes the ground plane.
 
  
 
  
    Bob . . .
 
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		Argonaut36
 
 
  Joined: 19 May 2019 Posts: 35
 
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				 Posted: Mon Sep 16, 2019 7:36 pm    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				Bob,
 Thanks for your post and for the sketch.
 Please note that my airplane has a standard certificate.  It is not experimental. The antenna is mounted on a removable panel on the bottom of the airplane; the antenna was installed professionally on the panel and I have not had any structural problems in many years of flying.  The panel is made of metal except for a plexigas window and doubles as the radio antenna ground plane, even if it is kind of small for that purpose.  I have installed some metal tape that goes over the plexigas trying to improve things.  The panel is attached to a vertical metal panel on each side with machine screws/anchor nuts.   At the front, the panel overlaps another metal panel and connects to a little beam with sheet metal screws.  At the back, the panel overlaps a fabric panel and connects to a little beam with sheet metal screws.    
 As mentioned above, this panel is removable and needs to come out of the airplane for maintenance purposes a few times a year.  It would be difficult to make modifications to this panel.  I have noticed that the transponder antenna ground plane is grounded to the plane tubular frame and I am just trying to establish if the radio antenna panel should also be grounded or not and why.  
 Thanks for your help.
 
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		user9253
 
 
  Joined: 28 Mar 2008 Posts: 1944 Location: Riley TWP Michigan
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				 Posted: Tue Sep 17, 2019 12:17 am    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				Since this is a factory built aircraft, and assuming that several of this brand and model have been built without radio problems, then maybe the problem with your radio is not an antenna design problem.  Maybe there is a problem with the coax or its connection at either end.  Would it be difficult to replace the coax?
 What are the dimensions of the metal panel used for the ground plane?  Have you removed the antenna and reinstalled it to be sure that there is a good electrical connection to the ground plane?  We are talking about a com radio antenna, right?
 
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		Argonaut36
 
 
  Joined: 19 May 2019 Posts: 35
 
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				 Posted: Tue Sep 17, 2019 7:24 am    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				The airplane is a Pitts and Pitts are known for poor radio transmissions.
 I know that I cannot get a perfect radio, I am just trying to improve on what I have got.  
 
 The coax was replaced with  a new one, made by a reputable avionics shop.  That made a difference.
 
 The antenna (and yes, we are talking about a com antenna) was replaced with a new one and was installed on the panel by a competent technician, paying particular attention to getting a good electrical connection.
 
 The dimensions of the ground plane are length 7"  x  width 14".  The 14" width is made up follow: central section: 6":,   side sections: 4" with an angle of 15-20 degrees in relation to the central section.
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Tue Sep 17, 2019 8:38 am    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 10:24 AM 9/17/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "Argonaut36" <fmlibrino(at)msn.com>
 
  The airplane is a Pitts and Pitts are known for poor radio transmissions.
  I know that I cannot get a perfect radio, I am just trying to improve on what I have got. | 	  
 
     Do you still have the old coax?
     I'd like to put my hands on it . . .
 
  The antenna (and yes, we are talking about a com antenna) was replaced with a new one and was installed on the panel by a competent technician, paying particular attention to getting a good electrical connection.
 
     Hmmm . . . vhf comm is line-of-sight
     bounded with very low path losses.
     Except for 'shielding' effects of surrounding
     structure, satisfactory communications can
     be expected with rather low power and
     less-than-perfect antennas.
 
  The dimensions of the ground plane are length 7"  x  width 14".  The 14" width is made up follows: central section: 6":,   side sections: 4" with an angle of 15-20 degrees in relation to the central section.
 
     . . . but attached to metal airframe
     components at various places around
     the edges?  While not an 'ideal'
     ground plane it should certainly
     be adequate for all but the occasional
     extreme range situation while x-country.   
 
  I am just trying to establish if the radio antenna panel should also be grounded or not and why. 
 
     If there are numerous fasteners connecting
     the removable panel to metallic components
     of the airframe, it's unlikely that
     any additional 'grounding' will produce
     observable improvement.
 
     You have a new antenna and coax . . . are
     you still experiencing unsatisfactory
     performance? Have you ever had occasion
     to use a hand-held radio in this 
     airplane?
 
     Sorry for the run-off into the weeds.
 
  
 
  
    Bob . . .
 
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		user9253
 
 
  Joined: 28 Mar 2008 Posts: 1944 Location: Riley TWP Michigan
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				 Posted: Tue Sep 17, 2019 10:06 am    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				7 x 14 inches is way too small for a ground plane if it is not attached to the airframe on all 4 sides.  So make your own ground plane.  Here is a quote from chapter 13 of Bob's book:
 "VHF ground-planes can be fabricated from radial strips of copper foil, soldered to a communing disk at the base of the antenna.  Make these strips 1” wide and trim them off 22” from the base of the vertical.  4 to 10 strips are recommended.  These may be cemented to the underside of the skin and structure.  By fabricating a commoning disk from copper, the entire assembly can be soldered."
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Tue Sep 17, 2019 1:17 pm    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 01:06 PM 9/17/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
 
  7 x 14 inches is way too small for a ground plane if it is not attached to the airframe on all 4 sides. | 	  
   While a long way from the idealized ground
   plane, it's not insignificant. After all,
   how much ground plane does a hand-held
   transceiver have?
 
   We're told that these aircraft have a
   history of poor radio performance . . .
   but they've been built in various types
   and quantities for over 50 years. Given
   that this is a t/c aircraft, I'm inclined
   believe that the problem(s) with this
   airplane may be more selective than a
   poorly designed antenna installation.
 
   We're advised that replacing the coax
   made an improvement . . . and that
   the antenna has been replaced with
   extra attention to process. We're
   not sure yet if this fixed things.
 
  
  
    Bob . . .
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Tue Sep 17, 2019 1:26 pm    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 01:06 PM 9/17/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
 
  7 x 14 inches is way too small for a ground plane if it is not attached to the airframe on all 4 sides. | 	  
   While a long way from the idealized ground
   plane, it's not insignificant. After all,
   how much ground plane area does a hand-held
   transceiver have?
 
   We're told that these aircraft have a
   history of poor radio performance . . .
   but they've been built in various types
   and quantities for over 50 years. Given
   that this is a t/c aircraft, I'm inclined
   to think that the problem(s) with this
   airplane may be more selective than a
   poorly designed antenna installation.
 
   We're advised that replacing the coax
   made an improvement . . . and that
   the antenna has been replaced with
   extra attention to process. We're
   not sure yet if this fixed things.
 
   Given the ease with which VHF communication
   propagates between aviation facilities,
   I'm inclined to believe that root cause
   for unsatisfactory performance is more
   profound than a soggy ground plane.
   In the words of the immortal, slightly
   short-circuited Number 5, "Input! Input!
   I need more input!"
 
  
  
    Bob . . .
 
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		Argonaut36
 
 
  Joined: 19 May 2019 Posts: 35
 
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				 Posted: Tue Sep 17, 2019 6:39 pm    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				Thanks to both for the additional comments. 
 I reply to the questions of Bob as follows:
 •	I do not have the old coax any more
 •	I have never used a hand-held radio in this airplane
 •	The radio performance is ok for Class D operations, but not so good for Class B and Class C, when you need to communicate from further out and clarity of communications is even more important
 •	As far as additional input, I can tell you that the radio harness was replaced without appreciable changes
 I am posting a file that includes 2 pictures of my panel/antenna.  The copper strips are just taped (not soldered) and there is no communing disk.  The strips do not extend laterally, because of the limited width of the panel.  I assumed that, as we normally fly towards the radio station we are talking to, that would be kind of acceptable.
 Could you please make comments on my copper foil strips and elaborate a little further on how the set up described by Bob is in his book could be implemented on my panel (keeping the panel removable from the airplane)?
 Note: in my previous post with the dimensions of the ground plane I have just disregarded the copper foil strips and I have assumed that the section of panel behind the plexigas window and the double plate work together as a ground plane.
 Thanks
 
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		user9253
 
 
  Joined: 28 Mar 2008 Posts: 1944 Location: Riley TWP Michigan
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				 Posted: Wed Sep 18, 2019 7:25 am    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				That copper tape does not do much good because it all goes in one direction.  And it is questionable if it is making good contact with the antenna mounting plate.
 Here is a suggestion: Cut 4 pieces of coax each 24 inches long.  Attach a ring terminal to the shield of each coax (not the center conductor).  Connect one length of coax to each of the 4 antenna mounting screws using the ring terminals.  When the panel is mounted to the aircraft, arrange the coax ground plane so that it extends outward in 4 different directions.  Elastic cord attached to the coax ends could help to position it.
 This experiment is worth a try and will not cost much except your time.
 
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 _________________ Joe Gores
  Last edited by user9253 on Thu Sep 19, 2019 5:33 am; edited 1 time in total | 
			 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Wed Sep 18, 2019 12:09 pm    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 10:57 PM 9/17/2019, you wrote:
   	  | Quote: | 	 		  If the copper tape isn't soldered or otherwise electrically bonded to the rest of the metal ground plane (panel), then it's not doing anything productive. You could get what you were hoping for by using strips of aluminum; even something really thin like strips cut from aluminum flashing material. They could be riveted on one end to the aluminum doubler shown in the pics, and glued with clear 'sensor safe' rtv to the plexi, and screwed at the other end using the plexi mounting screws. 
 
  Having said that, the antenna still won't be centered in the ground plane. You could add strips going the other direction using similar techniques. The 'ideal' ground plane extends out from the base as far as the antenna height. As few as 4 equally spaced radial arms can get the job done pretty effectively. | 	  
    Actually, there's a relatively simple experiment
    that can be conducted. 4 radials ~1" wide, extending
    from the antenna base and simply taped to the outside
    surface of the a/c will emulate an excellent ground
    plane. It does not need to be electrically bonded to
    the airframe. Where it is in close proximity to the
    airframe, there will be significant electro-static
    coupling to the airframe. I'd make them 24" long or
    so. Length not critical when so closely coupled to
    the a/c skin. Here's a tape suited to the task of
    temporary attachment to the airplane.
 
   https://tinyurl.com/y3wrc7kx
 
    I've used this stuff to run ribbon cable through
    the baggage door seal, down the side of the fuselage
    and past the entry door seal to bring investigatory
    signals from the hell-hole of a Beechjet into the
    cabin.
 
    This tape would work fine for the experimental ground
    plane installation as well.
 
   	  | Quote: | 	 		   
 
  Having said *that*, are you sure your problems are purely transmission range? Tube/rag a/c are notoriously noisy in the cockpit. You may have as much a problem with *audio* signal to noise ratio as with transmission distance. Ability to accurately describe comm deficiencies is pretty rare, even for controllers. If you think that could be a possibility, we can expand on that.  | 	     
    Excellent point! There was a list-thread on
    this very topic way back when . Turns out
    that transmission intelligibility was
    completely dependent on the noise cancelling
    quality of microphone . . . I'm embarrassed
    for not to have recalled this.
 
    Before hammering on the antenna installation,
    do try another mic/headset combination. They
    are not all the same . . . particularly
    with respect to cancelling low frequency
    'buffeting' kinds of noise common to airplanes
    where creature comfort is rather far down on the
    list of design priorities.
 
    Is it just your transmitted signal that's
    deficient . . . or both transmit and receive?
 
    This brings up the point that few users
    of two-way radios have the experience and vocabulary
    to describe poor signal quality. For example,
    you can have a strong radio frequency signal
    that is dead quiet when not talking but the
    audio is weak/distorted. This is ALWAYS an
    audio/microphone problem. You can have
    a marginal radio frequency signal (just
    beginning to present 'popcorn' noises
    when not talking and uncharacteristically
    poor audio when adding voice modulation.
    Then there's the truly weak-signal which
    can be a combination of radio/coax/antenna
    issued. Years ago, I asked a reader to
    send me a recording of his received signal
    as heard on the ground . . . his problem
    turned out to be in the audio system causing
    a badly under-modulated transmitter.
 
    Charlie's memory jog suggests an
    important avenue of investigation
    that supercedes fiddling with the antenna.
 
  
 
  
    Bob . . .
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Wed Sep 18, 2019 12:09 pm    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 10:25 AM 9/18/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
 
  That copper tape does not do much good because it all goes in one direction.  And it is questionable if it is making good contact with the antenna mounting plate.
  Here is a suggestion: Cut 4 pieces of coax each 22 or 23 inches long.  Attach a ring terminal to the shield of each coax (not the center conductor).  Connect one length of coax to each of the 4 antenna mounting screws using the ring terminals.  When the panel is mounted to the aircraft, arrange the coax ground plane so that it extends outward in 4 different directions.  Elastic cord attached to the coax ends could help to position it.
  This experiment is worth a try and will not cost much except your time. | 	  
    That would work too . . . the same tape could
    be used to hold it against the skin.
 
  
  
    Bob . . .
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Thu Sep 19, 2019 4:55 am    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 10:57 PM 9/17/2019, you wrote:
   	  | Quote: | 	 		  If the copper tape isn't soldered or otherwise electrically bonded to the rest of the metal ground plane (panel), then it's not doing anything productive. You could get what you were hoping for by using strips of aluminum; even something really thin like strips cut from aluminum flashing material. They could be riveted on one end to the aluminum doubler shown in the pics, and glued with clear 'sensor safe' rtv to the plexi, and screwed at the other end using the plexi mounting screws. 
 
  Having said that, the antenna still won't be centered in the ground plane. You could add strips going the other direction using similar techniques. The 'ideal' ground plane extends out from the base as far as the antenna height. As few as 4 equally spaced radial arms can get the job done pretty effectively. | 	  
    Actually, there's a relatively simple experiment
    that can be conducted. 4 radials ~1" wide, extending
    from the antenna base and simply taped to the outside
    surface of the a/c will emulate an excellent ground
    plane. It does not need to be electrically bonded to
    the airframe. Where it is in close proximity to the
    airframe, there will be significant electro-static
    coupling to the airframe. I'd make them 24" long or
    so. Length not critical when so closely coupled to
    the a/c skin. Here's a tape suited to the task of
    temporary attachment to the airplane.
 
   https://tinyurl.com/y3wrc7kx
 
    I've used this stuff to run ribbon cable through
    the baggage door seal, down the side of the fuselage
    and past the entry door seal to bring investigatory
    signals from the hell-hole of a Beechjet into the
    cabin.
 
    This tape would work fine for the experimental ground
    plane installation as well.
 
   	  | Quote: | 	 		   
 
  Having said *that*, are you sure your problems are purely transmission range? Tube/rag a/c are notoriously noisy in the cockpit. You may have as much a problem with *audio* signal to noise ratio as with transmission distance. Ability to accurately describe comm deficiencies is pretty rare, even for controllers. If you think that could be a possibility, we can expand on that.  | 	     
    Excellent point! There was a list-thread on
    this very topic way back when . Turns out
    that transmission intelligibility was
    completely dependent on the noise cancelling
    quality of microphone . . . I'm embarrassed
    for not to have recalled this.
 
    Before hammering on the antenna installation,
    do try another mic/headset combination. They
    are not all the same . . . particularly
    with respect to cancelling low frequency
    'buffeting' kinds of noise common to airplanes
    where creature comfort is rather far down on the
    list of design priorities.
 
    Is it just your transmitted signal that's
    deficient . . . or both transmit and receive?
 
    This brings up the point that few users
    of two-way radios have the experience and vocabulary
    to describe poor signal quality. For example,
    you can have a strong radio frequency signal
    that is dead quiet when not talking but the
    audio is weak/distorted. This is ALWAYS an
    audio/microphone problem. You can have
    a marginal radio frequency signal (just
    beginning to present 'popcorn' noises
    when not talking and uncharacteristically
    poor audio when adding voice modulation.
    Then there's the truly weak-signal which
    can be a combination of radio/coax/antenna
    issued. Years ago, I asked a reader to
    send me a recording of his received signal
    as heard on the ground . . . his problem
    turned out to be in the audio system causing
    a badly under-modulated transmitter.
 
    Charlie's memory jog suggests an
    important avenue of investigation
    that supercedes fiddling with the antenna.
 
  
 
  
    Bob . . .
 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Thu Sep 19, 2019 4:56 am    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 10:25 AM 9/18/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
 
  That copper tape does not do much good because it all goes in one direction.  And it is questionable if it is making good contact with the antenna mounting plate.
  Here is a suggestion: Cut 4 pieces of coax each 22 or 23 inches long.  Attach a ring terminal to the shield of each coax (not the center conductor).  Connect one length of coax to each of the 4 antenna mounting screws using the ring terminals.  When the panel is mounted to the aircraft, arrange the coax ground plane so that it extends outward in 4 different directions.  Elastic cord attached to the coax ends could help to position it.
  This experiment is worth a try and will not cost much except your time. | 	  
    That would work too . . . the same tape could
    be used to hold it against the skin.
 
  
  
    Bob . . .
 
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		user9253
 
 
  Joined: 28 Mar 2008 Posts: 1944 Location: Riley TWP Michigan
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				 Posted: Thu Sep 19, 2019 5:37 am    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				I changed the length of the ground plane radials to 24 inches in my post above to be the same as Bob's suggestion.
 
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 _________________ Joe Gores | 
			 
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		nuckolls.bob(at)aeroelect Guest
 
 
 
 
 
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				 Posted: Thu Sep 19, 2019 7:27 am    Post subject: Grounding radio antenna and   transponder antenna | 
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				At 08:37 AM 9/19/2019, you wrote:
   	  | Quote: | 	 		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
 
  I changed the length of the ground plane radials to 24 inches in my post above to be the same as Bob's suggestion. | 	  
   The length won't be 'critical' in this case.
   When crafting a free space ground plane for
   a vertical antenna, the idea is to approximate
   the 'perfect' ground plane . . . a solid conductor
   radiating from the base of the antenna for a
   long ways. Since an perfect (infinite) 
   plane isn't practical, the common approximations
   take on the form of radial 'whiskers'
 
  [img]https://www.bing.com/th?id=OIP.frzhTHSL4nWMxO4KqsPwpAHaDU&w=300&h=134&c=7&o=5&dpr=1.25&pid=1.7[/img]  
 
   In this case, they are just more antennas
   brought together at the base such that their
   impedances are paralleled. The more radials,
   the lower the ground impedance, the better
   the plane. These are resonant ground planes.
   I.e. 1/4 wave in free space elements. 
 
   This technique is closely approximated
   in composite airplanes with copper or
   aluminum strips bonded to the inside
   skin of the aircraft.
 
   The DIY transponder antenna for composite
   applications suggeseted a solid disk
   ground plane with a radius equal to height
   of the antenna.
 
  [img]cid:.0[/img]
 
   This 'plane' looks like an infinite number
   of radials joined at the center. But
   unless the plane is in pretty much free
   space with respect to surrounding conductors,
   it's near-ideal characteristics are
   degraded. 
 
   For the purposed of our 'experiment',
   the proposed 1/4 wave elements will be in
   closest practical proximity to a metal
   airframe; no longer in free air and
   certainly not resonant at the frequency
   of interest.
 
   So the exact length is no longer significant.
   Instead we're looking for some electro-static
   or capacitive coupling to the airframe to
   combine with planar effects of the now-random
   lengths of conductor. In this case, the
   wider strips taped to the skin are more
   desirable to increase the capacitive coupling
   effects . . . but the suggested wire elements
   may well produce the desired effect of compensating
   for a suspected inadequate ground plane.
 
   While the microphone performance issue is
   worth examination, one of the 'grains of
   sand' in this study says that performance
   is degraded with DISTANCE from the other
   station which discounts audio problems
   (consistent irrespective of range) and
   re-enforces the notion that there is a
   signal strength issue.
 
  
    Bob . . .
 
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		Argonaut36
 
 
  Joined: 19 May 2019 Posts: 35
 
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				 Posted: Thu Sep 19, 2019 5:50 pm    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				Reference is made to the following comment of Bob in a recent post:
 Quote
 Years ago, I asked a reader to send me a recording of his received signal as heard on the ground . . . his problem turned out to be in the audio system causing a badly under-modulated transmitter. 
 Unquote
 When the radio was almost new, it failed and I flew for a while with a loaned radio (identical) and then I got the replacement radio (also identical) that I have now.  The three radios worked exactly the same and for this reason I did not consider the radio itself being the source of the troubles.  At the time I flew with the other radios, however, I had different antenna, coax and harness that could have masked issues with the radio.
 Going back to the alternative antenna configurations that have been proposed, could Bob please answer the following questions:
 What is the recommended 1” copper foil tape and where I can buy it?
 How is the tape terminated at the antenna? Does it stop at a certain distance from the antenna or right at the antenna?
 Does the recommended 2" white vinyl tape leave adhesive residue on the paint? Considering that testing will probably require only a single short flight, could perhaps a less strong tape that does not leave residue be used? 
 Thanks
 
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		user9253
 
 
  Joined: 28 Mar 2008 Posts: 1944 Location: Riley TWP Michigan
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		Argonaut36
 
 
  Joined: 19 May 2019 Posts: 35
 
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				 Posted: Fri Sep 20, 2019 1:34 pm    Post subject: Re: Grounding radio antenna and   transponder antenna | 
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				Thanks for the links for the copper foil tape and the installation suggestions.  I am not too sure about how easy would be to remove my existing antenna.  I remember that an adhesive that would provide a very strong bond between the antenna and the panel was used during  installation.
 The other antenna option that uses the coax cable that was discussed in this thread seems easier and I plan on testing it the next time I have the panel out of the airplane.
 
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