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  #21  
Old 03-09-2009, 07:01 AM
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Default Antennas in the the gear legs

I had two antennas in the gear legs in my 6A. I used one for FM radio and the other for my handheld com. I never tested them for performance, but they seemed to work OK. All I did was strip off the shielding from a length of coax and glass it inside the leading edge of the fairing. I used a bulkhead coax connector where it entered the fuse, so that I could unhook them to remove the fairings. If memory serves, I think 21 inches is the correct length for a COM (or FM) antenna.
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  #22  
Old 03-10-2009, 08:21 PM
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Ironflight Ironflight is offline
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Default Updates and Progress!

Well, this has been a fun few days as I wander in and out of the hangar....

Several VAF'ers have responded to me off-line, and offered spare Archer Antennas that they had in their "unused parts boxes" for me to try. I received a Nav antenna the other day, and went in to my wingtip to replace the home-made one I had there. (One thing I noticed is that my Nav light wiring was not secured according to Bob Archer's instructions, and that might very well have caused a large part of the problem.) Someone else is sending me an unused Comm Antenna, and another is shipping me a spare AAE Comm antenna - I intend to try both of those and report back!

In addition, I Pete Howell is building a custom "J-Pole" antenna that should work for VOR, and I am definitely fascinated by the idea of giving that a try. Anything that sounds that cool must be tried! (I have no idea what a "J-Pole" is...)

I also have exchanged emails with Bob Archer himself, as well as the Jerry Chambers at Advance Aircraft electronics, learning interesting things from both. Jerry didn?t hold much hope for the AAE Comm antenna in the RV, but if I am in the tip, I will probably try it. Bob told me that lots of folks who build the copies miss essential trimming that makes it work.

After putting in the new Archer Nav antenna, and dressing the cabling properly, I launched for a test flight. I brought up the list of ?nearest? VOR?s on my 396 screen, and started tuning them in, starting with the closest one. Climbing to 5,000?, I could tell that I had gotten better results already ? but the farthest one I could pull in reliably was at about 60 miles. I then tuned in the ILS at our nearest field, and line dup on final about ten miles out. Localizer was strong (as before), but the glide slope still didn?t come in until right at the FAF. Hmmm?.that got me thinking ? why split the signal? The Garmin 430 has a separate antenna feed for VOR/LOC and GS. I had been feeding them from a single antenna through a splitter, essentially giving away signal strength to each.

I landed and dugout my copy of bob Knuckoll?s book, turning to chapter 13 ? the mystical antenna chapter. He has some interesting designs for home-built VOR and Glide slope antennas (including a copy of an Archer antenna), but he stops just short of explaining how you hook up the Glide slope. Take a look for yourself ? tell me how you hook the Coax up at the antenna end! But then I re-read his text (and several old posts) about how tolerant the Glide slope is to antenna characteristics. OK, I had read about stripping a Coax back 16? and hooking it up ? so I found a chunk of coax, stripped it appropriately, pulled a few avionics, and rerouted my antenna lines. I took out the splitter and ran the wingtip antenna directly to the VOR/LOC portion of the 430. I then took the makeshift antenna I built, velcroed it to the roll-bar of the -8 (that mirror Velcro is good for something!), and hooked it into the GS portion of the 430. Time for a flight check!



Results were great! I now could pull in VOR?s at the 80 mile range from 5,000?. Tuning in the ILS, I had the Glide slope at 10 miles, and proceeded outbound, reaching 18 miles before I had to turn around due to airspace limitations (the glide slope still locked on), and shot the approach ? strong all the way in! So?.splitting the signal gave a noticeable degradation in signal strength, and the wingtip antenna really wasn?t doing a very good job for the glide slope. Here?s a picture of my ?test? GS antenna ? next task will be making it permanent ? I might even put it in the lower cowl.

Next up will be playing with the Comm antennas (I might even try the stripped Coax down the gear leg like Jon Baker suggested) ? and testing that magical J-pole. I?ll update everyone when I have some results!

When it comes to antennas, I remember what an old time engineer taught me ? it?s all ?FM? ? and the EE?s out there will know what I mean?..

Paul
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Paul F. Dye
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RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
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  #23  
Old 03-10-2009, 09:35 PM
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Default

Hmmm.... Similar situation with the GNS430 / splitter, and all. I'm drooling over an 80 mi. vor. Keep writing so I don't have to reproduce your testing . I just spent the whole day under the panel trying to get GRT "GNAV" working to no avail.
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  #24  
Old 03-10-2009, 09:41 PM
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IowaRV9Dreamer IowaRV9Dreamer is offline
 
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Default RF engineer viewpoint

An ideal splitter will cause 3dB of loss... which is half the power. Sounds worse than it is though, but it's pretty noticable at the outer edge of the VOR coverage. Worth 10's of miles for sure.

This is an enjoyable thread... it's got me thinking that I don't have a NAV antenna solution. I may go with the whiskers and sacrifice a bit of speed for good performance.
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  #25  
Old 03-10-2009, 11:14 PM
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az_gila az_gila is offline
 
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Smile Could you have had a poor splitter?

Quote:
Originally Posted by IowaRV9Dreamer View Post
An ideal splitter will cause 3dB of loss... which is half the power. Sounds worse than it is though, but it's pretty noticable at the outer edge of the VOR coverage. Worth 10's of miles for sure.

This is an enjoyable thread... it's got me thinking that I don't have a NAV antenna solution. I may go with the whiskers and sacrifice a bit of speed for good performance.
I'm sure you can find someone nearby at work to test it for you...

As Dave said, whiskers will work for the glide slope.

Did I remember someone on VAF putting a copper tape whiskers equivalent antenna for GS on the inside of the cowling??
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  #26  
Old 03-11-2009, 06:35 AM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by az_gila View Post
I'm sure you can find someone nearby at work to test it for you...

As Dave said, whiskers will work for the glide slope.

Did I remember someone on VAF putting a copper tape whiskers equivalent antenna for GS on the inside of the cowling??
Good question Gil - I actually replaced the splitter with a known good one (that we had taken out of Mikey) a few months back. Now I have TWO surplus, expensive splitters.....

I thought I remember the same thing about a copper foil GS antenna in the cowl - searched and couldn't find it. The stripped Coax helped me confirm that I want the separate antenna - now I'll do a little more research before making my final version - anyone know if there is a better length than 16" (I got that from Knuckoll's dipole design in his book)?

Paul
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RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
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Dayton Valley Airpark (A34)
http://Ironflight.com
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  #27  
Old 03-11-2009, 07:01 AM
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Default It's all FM

Quote:
Originally Posted by Ironflight View Post
When it comes to antennas, I remember what an old time engineer taught me ? it?s all ?FM? ? and the EE?s out there will know what I mean?..
Ah yes, FM. EE's may understand it, but back in my Air Force days, we technicians actually harnessed FM to resolve our most difficult avionics problems -- like those related to a loose nut in the cockpit. The resolution field of those trouble tickets often read "FM adjusted".
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  #28  
Old 03-15-2009, 05:31 AM
wv4i wv4i is offline
 
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Default Antenna facts

A few facts come to mind in this discussion.

Internal wingtip antennas will always have an asymmetrical pattern, whether for transceive or receive. Specifically, they will exhibit multiple deep performance nulls towards the rest of the airplane. Unless the tips are RF transparent, i.e. scraps won't heat in a microwave is a good test, they will also attenuate both the receive and transmit signals.

Obviously then, antennas on the bottom of the fuselage work better with ground stations, on top, with air to air. On the day job big airplane, we sometimes have to switch radios based on bottom/top antenna just to talk to ramp/ground/clearance/etc., not very often, but it happens.

It is difficult to get a completely symmetrical antenna pattern even with fuselage mounted antennas, much more difficult with wing antennas, even when externally mounted.

Comm needs vertical polarity and VHF Nav horizontal. The penalty for cross polarization re to another station is a 20db path loss.

Antenna length in feet is 234/freq(mhz) or 468/freq(mhz), for 1/4 WL whip or 1/2 WL dipole respectively. The length of the coax normally does not present a matching problem, but db loss as a function of coax length does matter. RG-58 is very lossy. Buy the good stuff, including good connectors. We hams usually wrap all connections with Coax Seal but poor connections will still get degraded by moisture over time.

A dipole's impedance is 72 ohms. Bending it to roughly a 120 degree included angle gets it closer to 50 ohms, i.e. closer to 1:1 SWR, what the connected device wants to see, whether xmt or rcv. If you have to change the length of an antenna already cut to the proper length and bend to achieve a lower SWR at a given resonant freq, then something is detuning the antenna, and will degrade its performance. Where a whip antenna (1/4 WL) is concerned, a min SWR not near 1:1 at resonance is indicative of a poor RF ground plane, which is different from a DC ground.

If you want to play with antennas, consider the MFJ-259B antenna analyzer. It generates a low level signal which is used to determine resonant freq of antenna, also resistance and reactance, from 1.8-170 mhz. This unit can also be used as a signal generator and freq counter, easily accurate enough to check Comm or Nav radios, including handhelds.

I know there are some strong opinions out there about where/how to mount antennas, especially from some vendors of wing tip antenna kits, but the facts are what they are.
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  #29  
Old 03-15-2009, 01:39 PM
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RVjim RVjim is offline
 
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Default REAL Bob Archer VOR antenna's

Quote:
Originally Posted by wv4i View Post
A few facts come to mind in this discussion.

Internal wingtip antennas will always have an asymmetrical pattern, whether for transceive or receive. Specifically, they will exhibit multiple deep performance nulls towards the rest of the airplane. Unless the tips are RF transparent, i.e. scraps won't heat in a microwave is a good test, they will also attenuate both the receive and transmit signals.

Obviously then, antennas on the bottom of the fuselage work better with ground stations, on top, with air to air. On the day job big airplane, we sometimes have to switch radios based on bottom/top antenna just to talk to ramp/ground/clearance/etc., not very often, but it happens.

It is difficult to get a completely symmetrical antenna pattern even with fuselage mounted antennas, much more difficult with wing antennas, even when externally mounted.

Comm needs vertical polarity and VHF Nav horizontal. The penalty for cross polarization re to another station is a 20db path loss.

Antenna length in feet is 234/freq(mhz) or 468/freq(mhz), for 1/4 WL whip or 1/2 WL dipole respectively. The length of the coax normally does not present a matching problem, but db loss as a function of coax length does matter. RG-58 is very lossy. Buy the good stuff, including good connectors. We hams usually wrap all connections with Coax Seal but poor connections will still get degraded by moisture over time.

A dipole's impedance is 72 ohms. Bending it to roughly a 120 degree included angle gets it closer to 50 ohms, i.e. closer to 1:1 SWR, what the connected device wants to see, whether xmt or rcv. If you have to change the length of an antenna already cut to the proper length and bend to achieve a lower SWR at a given resonant freq, then something is detuning the antenna, and will degrade its performance. Where a whip antenna (1/4 WL) is concerned, a min SWR not near 1:1 at resonance is indicative of a poor RF ground plane, which is different from a DC ground.

If you want to play with antennas, consider the MFJ-259B antenna analyzer. It generates a low level signal which is used to determine resonant freq of antenna, also resistance and reactance, from 1.8-170 mhz. This unit can also be used as a signal generator and freq counter, easily accurate enough to check Comm or Nav radios, including handhelds.

I know there are some strong opinions out there about where/how to mount antennas, especially from some vendors of wing tip antenna kits, but the facts are what they are.
Bio - Bob Archer designed antenna's for TRW. He has lived the facts for the design of the "GAMA" wingtip antenna design. No guesswork, no opinion's. Just cold hard test data taken on an aircraft signal range.

My Opinion - If you want a solid wingtip VOR antenna installation, buy the Sportcraft Antenna (Bob Archer's) AND READ THE INSTALLATION INSTRUCTIONS.
A "real" Bob Archer antenna mounted in accordance with the instructions is the best possible installation for a VOR antenna. Any other mounting location will have poorer overall performance.

Information from Bob Archer - The "GAMA" antenna receives the VOR signal from the ground plane. The airframe itself. There will be two very sharp null points in the signal. One directly across from the antenna location. And one at a 45 degree angle behind the aircraft on the side the antenna is mounted. These null points are supposed to be so sharp that they can not be held to loose the signal.
The capacitance between the GAMA antenna element and the GAMA antenna ground plane is CRITICAL. A physical copy of Bob Archer's antenna design probably won't even be close on this.
(My interpretation of Bob's comments.)
A friend of mine tried to copy Bob Archer's VOR antenna with very limited success. I finally gave him a real Bob Archer wingtip VOR antenna.

I've had a Bob Archer wingtip antenna in my RV-3 for the last 20 years. It has at least 10 miles more range than a stock external VOR antenna on a Cessna 210.

Regards,
Jim Ayers
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  #30  
Old 03-15-2009, 03:13 PM
alcladrv alcladrv is offline
 
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Quote:
Originally Posted by RVjim View Post
My Opinion - If you want a solid wingtip VOR antenna installation, buy the Sportcraft Antenna (Bob Archer's) AND READ THE INSTALLATION INSTRUCTIONS.
A "real" Bob Archer antenna mounted in accordance with the instructions is the best possible installation for a VOR antenna. Any other mounting location will have poorer overall performance.

Regards,
Jim Ayers
I agree with Jim. In the right wingtip of my -7A, an 80nm reception range with the "real" Archer antenna is the norm.

Mike
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