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#1
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Aircraft antennas
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Charlie |
#2
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Aircraft antennas
ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Please excuse my "piggybacking" your question, can anyone with an informed opinion weigh-in on this: http://www.aircraftspruce.com/catalo...nnasystems.php You put it inside a tube and fabric structure. Seems like a good idea at first glance... |
#3
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Aircraft antennas
In article ,
"ccwillwerth" wrote: Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Charlie The ground plane should extend an antenna length around the antenna base. Copper or aluminum tape will serve the purpose very well. This is outlined in CAM 18. Make sure that your ground has a good electrical connection with the elements of the ground plane. |
#4
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Aircraft antennas
Charlie: I recently installed a VHF com antenna on the tailboom frame of my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the length of the antenna. I ran an antenna check using an MFJ 250 antenna analyzer and the highest VSWR I got was 2.2. This was so much better than the bent antenna that had been on the belly of the ship which did have a large ground plane but had VSWR greater than 3 and in some cases as high as 5. One thing you can do is try out a prototype ground plane and then have a Ham or someone with the right equipment do a check. -- Stuart Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 ph (760) 408-9747 publication cell "ccwillwerth" wrote in message ... Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Charlie |
#6
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Aircraft antennas
Orval Fairbairn wrote: In article , "ccwillwerth" wrote: Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Charlie, you will get several opinions, but here is mine. If at all possible, get a "broadband" VHF Com antenna, not a wire whip. The "broadband" fiberglass antennas have a VSWR of less than 2:1 across the range of 118 to 136MHz, while the metallic whip will have an bandwidth of only about 5Mhz where the VSWR is below 2:1. At the band edges, the VSWR will be high enough to cause the VSWR protection circuitry in transistorized transmitter to shut the output power down to nearly zero. Although the wire-whip can be cut&tuned to just Unicom frequencies (122.7 to 123.6 Mhz), it could be marginal for transmission at some ATC frequencies. The whip will work ok for receiving even at the band-edges, because the receiver doesn't care about the VSWR. If your fuselage is anything like my Piper PA20, there are enough metal tubes to act as a ground plane without adding any additional conductive material, other than a mounting plate. I would put the antenna base on a metal plate which is just below the plane formed by the fabric. Radius the edges of the plate so that the fabric doesn't ride on a sharp edge. The plate could be long enough to bridge between two fuselage cross-brace tubes, but only about 4 to 6" wide. It must be electrically "bonded" to the cross-braces, so to avoid drilling holes in the cross-braces, your idea of welding some attach "ears" to the cross-braces to mount the plate is good. |
#7
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Aircraft antennas
Jim Carriere wrote: Please excuse my "piggybacking" your question, can anyone with an informed opinion weigh-in on this: http://www.aircraftspruce.com/catalo...nnasystems.php You put it inside a tube and fabric structure. Seems like a good idea at first glance... VHF COM, NAV, and ELT antennas placed inside a welded-tube fuselage aren't worth crap! What is the biggest complaint of anybody trying to use a Air-band VHF hand-held transceiver inside the cabin of a metal aircraft? They cant be heard! They all end up putting an external antenna on their aircraft. And no, it is not just the inefficiency of the rubber-ducky that causes this. The fundemantal problem is that the wavelength at 120Mhz is 2.5 meters. In order for a radio wave to pass through an opening in a metallic structure, the dimensions of the opening need to approach a half wavelength in diameter, and even then the wave is greatly attenuated. Just to make the point, a half-wave at 120Mhz is 49". How many of the openings whose edges are defined by the fuselage longerons, the cross-brace and diagonal brace tubes are 49" across? Answer, none of them. Even the windscreen opening is usually not that large. A welded tube fuselage makes a real good "screen room"... Do yourself a favor, and put the antennas on the outside where they belong... |
#8
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Aircraft antennas
ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Charlie Our experience is that anything inside a structure especially metal is NOT going to work. As one other said... put the aerial on the outside where it's meant to go. Also we have just added a table on VSWR effects and power drop off with a bad aerial, check http://www.mcp.com.au/xcom760/faq/faq.html for details... right down the bottom of the page (All the way down) Interesting if you have a bad aerial Regards Michael from XCOM |
#9
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Aircraft antennas
"Jim Carriere" wrote in message ... ccwillwerth wrote: Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Two comments. One is that the steel tube fuselage will be an adequate ground plane if the attach plate is electrically connected to the fuselage tubes. Two, that wire whip antenna was just fine for the day in which we had 90 channel radios and didn't go much above 124 MHz.. Do you and your radio a favor and get one of those fiberglass whips that have the word "broadband" in their descriptor. You will be a much happier camper. Please excuse my "piggybacking" your question, can anyone with an informed opinion weigh-in on this: http://www.aircraftspruce.com/catalo...nnasystems.php You put it inside a tube and fabric structure. Seems like a good idea at first glance... Read it again. The antenna is meant for composite aircraft or metal tube aircraft IN A FIBERGLASS WINGTIP. No VHF antenna will work worth a darn inside of a steel tube fuselage. Google on "Faraday Cage" or "Screen Room" to get a picture of what is happening. Jim |
#10
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Aircraft antennas
"XCOM" wrote Our experience is that anything inside a structure especially metal is NOT going to work. As one other said... put the aerial on the outside where it's meant to go. With careful reading, I take that he is asking about putting JUST the ground plate under the fabric, with the aerial on the outside, as you suggest. -- Jim in NC |
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