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  #1  
Old 03-24-2020, 10:39 AM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 289
Default DIY Comm antenna?

I was planning on making my own comm antenna to be mounted on the turtle deck of my RV-4 below the canopy and following the canopy forward.

I thought a thin wire would do it, but after reading some Kitplane articles it looks like the thinner the antenna, the narrower the bandwidth. So 1/2" or wider copper tape seems indicated, but can't really attach the tape to the canopy and still have the turtle deck as a ground plane?

Mounting it on the side of the canopy, the rollover bar will be close to the antenna tip.

Wing tip mounting does not lend itself to a vertical antenna either (and a longer cable run to boot).

Any ideas (other than spending $120 or more on a Delta Pop antenna hanging out in the breeze)?

Finn
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  #2  
Old 03-24-2020, 10:54 AM
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Mel Mel is online now
 
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Location: Dallas area
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Spend the money and hang the antenna "out in the breeze"! You won't notice the minimal drag and the performance will be sooooo much better.
Home made antennas work OK for receive, but not so much for transmit where VSWR is important.
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  #3  
Old 03-24-2020, 11:14 AM
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gmcjetpilot gmcjetpilot is offline
 
Join Date: Jan 2005
Posts: 4,242
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Quote:
Originally Posted by FinnFlyer View Post
I was planning on making my own comm antenna to be mounted on the turtle deck of my RV-4 below the canopy and following the canopy forward.

I thought a thin wire would do it, but after reading some Kitplane articles it looks like the thinner the antenna, the narrower the bandwidth. So 1/2" or wider copper tape seems indicated, but can't really attach the tape to the canopy and still have the turtle deck as a ground plane?

Mounting it on the side of the canopy, the rollover bar will be close to the antenna tip.

Wing tip mounting does not lend itself to a vertical antenna either (and a longer cable run to boot).

Any ideas (other than spending $120 or more on a Delta Pop antenna hanging out in the breeze)?Finn
Jim Weir is Da man and contributes to Kit plane. He has written about this for decades. He had RST kits and still sells some things. He is a kit for $29 that is enough to make several antennas (VOR, LOC, COM, MB). Check the link below and the link to instructions. He spends a lot of time on VOR "Dipole").

http://www.rstengineering.com/rst/pr...aneantenna.htm

Of course buying an antenna and bolting it on to the belly or turtle deck will be easy, have best performance and the drag I once calculated decades ago. I think we are talking 0.25 mph off top speed. Speed With Economy has numbers. This will be a compromise over a regular VHF mono-pole antenna bolted to the belly or turtle deck with a solid large ground plane which is about 23" inches in all directions. Not discouraging you.

Yep you are right a flat copper tape strip is better than thin wire, so I read like you. I do wounder how much "bandwidth" you are loosing. Voice AM (which is what we are doing) is not Hi-Fi. However since 3M makes sticky back copper tape it is kind of a no brain'er.

The RV-4 (which I flew a bunch but with belly antenna) has a tip over canopy. I can picture the challenge. I'm building a RV-7 slider so putting the antenna in the fixed front windscreen is easier, but the front bow might cause some dead spots. The better canopy is the tip up and using the glare shield as a ground plane. I like a strong radio, so I am going with a VHF antenna on belly.

You will have to terminate the strip and transition to 50 ohm coaxial. It will be an END FED (unbalanced) antenna and requires a ground plane. You will have to bring the feed line to the hinge line, into cockpit and run to your radio, no big deal. You have the cockpit edge longeron to run the coaxial under. The ground plane will be a creative exercise and challenge. You could ground it to the metal skirt around the back of the canopy but really want to tie it to fuselage turtle deck (ground). Also you may be grounding the canopy frame. Not sure what that will do. It could be goodness! It may be possible when you close the canopy you have a ground contact that grounds the skirt to the turtle deck>

Running the antenna over the canopy in the middle near the roll bar may cause some dead directions but you won't know until you try. It should work over 270 degree arc from front to back and off right hand side. I don't think it is worse than the buried antenna in one wing tip idea. Everyone says the wing tip antenna "works". By "work" they mean they can talk to the tower in the pattern, but forget getting out 10-20 miles at low altitude.

Find a HAM operator near you who has an antenna analyzer to test your antenna for SWR and Impedance. You understand about antenna length vs Freq. It is fun project, can be done cheap, you will learn a lot, even if you revert to a stick in the breeze. Again drag from one com antenna can't be measured it's so small but it is well under 1/2 mph. BTW I had a VOR dipole on the belly under the horizontal stab out in the breeze. When I did a few cross country races I took it off. If you are a speed demon you can take a com antenna off to race and use a handheld.
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Last edited by gmcjetpilot : 03-24-2020 at 11:29 AM.
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  #4  
Old 03-24-2020, 11:45 AM
Ralph Inkster Ralph Inkster is offline
 
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Clear communications is worth the price of a good antenna!
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  #5  
Old 03-24-2020, 02:29 PM
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9GT 9GT is offline
 
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Location: Southern Michigan
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Quote:
Originally Posted by Ralph Inkster View Post
Clear communications is worth the price of a good antenna!
Amen to that. Nothing aggravates me more than trying to decipher garbled, or poor quality radio transmissions from other aircraft.
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  #6  
Old 03-24-2020, 03:02 PM
Northernliving Northernliving is offline
 
Join Date: May 2005
Location: Boston, MA
Posts: 374
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Quote:
Originally Posted by gmcjetpilot View Post
Jim Weir is Da man and contributes to Kit plane. He has written about this for decades. He had RST kits and still sells some things. He is a kit for $29 that is enough to make several antennas (VOR, LOC, COM, MB). Check the link below and the link to instructions. He spends a lot of time on VOR "Dipole").

http://www.rstengineering.com/rst/pr...aneantenna.htm

Of course buying an antenna and bolting it on to the belly or turtle deck will be easy, have best performance and the drag I once calculated decades ago. I think we are talking 0.25 mph off top speed. Speed With Economy has numbers. This will be a compromise over a regular VHF mono-pole antenna bolted to the belly or turtle deck with a solid large ground plane which is about 23" inches in all directions. Not discouraging you.

Yep you are right a flat copper tape strip is better than thin wire, so I read like you. I do wounder how much "bandwidth" you are loosing. Voice AM (which is what we are doing) is not Hi-Fi. However since 3M makes sticky back copper tape it is kind of a no brain'er.

The RV-4 (which I flew a bunch but with belly antenna) has a tip over canopy. I can picture the challenge. I'm building a RV-7 slider so putting the antenna in the fixed front windscreen is easier, but the front bow might cause some dead spots. The better canopy is the tip up and using the glare shield as a ground plane. I like a strong radio, so I am going with a VHF antenna on belly.

You will have to terminate the strip and transition to 50 ohm coaxial. It will be an END FED (unbalanced) antenna and requires a ground plane. You will have to bring the feed line to the hinge line, into cockpit and run to your radio, no big deal. You have the cockpit edge longeron to run the coaxial under. The ground plane will be a creative exercise and challenge. You could ground it to the metal skirt around the back of the canopy but really want to tie it to fuselage turtle deck (ground). Also you may be grounding the canopy frame. Not sure what that will do. It could be goodness! It may be possible when you close the canopy you have a ground contact that grounds the skirt to the turtle deck>

Running the antenna over the canopy in the middle near the roll bar may cause some dead directions but you won't know until you try. It should work over 270 degree arc from front to back and off right hand side. I don't think it is worse than the buried antenna in one wing tip idea. Everyone says the wing tip antenna "works". By "work" they mean they can talk to the tower in the pattern, but forget getting out 10-20 miles at low altitude.

Find a HAM operator near you who has an antenna analyzer to test your antenna for SWR and Impedance. You understand about antenna length vs Freq. It is fun project, can be done cheap, you will learn a lot, even if you revert to a stick in the breeze. Again drag from one com antenna can't be measured it's so small but it is well under 1/2 mph. BTW I had a VOR dipole on the belly under the horizontal stab out in the breeze. When I did a few cross country races I took it off. If you are a speed demon you can take a com antenna off to race and use a handheld.
I think George is right. If you find a HAM radio guy to help you tune an antenna (like a Bob Archer wing tip antenna), they can work extreamily well. The key is tuning it and getting it just right. I have an "out in the breeze" belly antenna that I'm underwelmed with. It works okay with just about every place but my class D home airport when on the ground. It's very frustrating. I wish I had one mounted elsewhere. I'll be working my my brother who is oild time vaccum tube, di-pole HAM guy to tune a wingtip antenna for my second radio.
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  #7  
Old 03-24-2020, 03:26 PM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,802
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Quote:
Originally Posted by FinnFlyer View Post
I was planning on making my own comm antenna to be mounted on the turtle deck of my RV-4 below the canopy and following the canopy forward.

Any ideas (other than spending $120 or more on a Delta Pop antenna hanging out in the breeze)?

Finn
Mate, if you’re worried about spending $120 on a comm antenna...well, you really need to ask yourself if you’re in the right sport.
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  #8  
Old 03-24-2020, 04:06 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 289
Default Handheld

"If you are a speed demon you can take a com antenna off to race and use a handheld."

Funny you should mention that, George.

I flew my old RV-3 for 575 hours with nothing but a handheld Icom mounted front/left on the canopy frame, rubber duck antenna sticking up clear of the canopy skirt. (I removed its battery case and made a mount to slide it on that also fed it 12 volts). Of course adapters and wiring to jacks for a headset. I wish I could find an old picture to scan.

Never had an issue.

"Speed With Economy has numbers"
5 mph! Page 149. Well, that's Comm, VOR and Loran His guess was 2mph for the comm antenna.

"Mate, if you’re worried about spending $120 on a comm antenna...well, you really need to ask yourself if you’re in the right sport. "

$120 here, $1,400 for ADSB-Out there, ... it all adds up

I'll re-read Jim's instructions. May even build my own SWR meter

Finn
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  #9  
Old 03-24-2020, 04:28 PM
Jpm757 Jpm757 is offline
 
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Location: Sherman, CT
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Seriously, here's one from Spruce for $85 bucks! If that's not in your budget then maybe owning an RV is over your head.
.jpg](https://postimg.cc/Cn5MdJdM)
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Last edited by Jpm757 : 03-24-2020 at 04:30 PM. Reason: Ad
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  #10  
Old 03-24-2020, 04:34 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,423
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Quote:
Originally Posted by gmcjetpilot View Post

Yep you are right a flat copper tape strip is better than thin wire, so I read like you. I do wounder how much "bandwidth" you are loosing. Voice AM (which is what we are doing) is not Hi-Fi. .
‘Bandwidth’, as used wrt antenna performance, means how well it performs at 118 and 135 MHz, when cut to spec for 126 MHz. Up to a point, using wide conductors, as opposed to thin wire, can improve this somewhat. ‘Bandwidth’, as used wrt the audio frequencies passed by our com receivers, is a function of the filters used within the radio. With aviation coms the bandwidth is deliberately limited to improve signal to noise ratios.
For poster #6. I also have a belly whip and an Archer-clone wingtip antenna, for com 1 and com 2, respectively. The external whip is generally better, especially beyond 25 nm or so. I have also encountered, but rarely, trouble using the belly whip when on the ground. Rotating the airplane 30 deg has always fixed the problem. (Might also be how much rebar was below the antenna!). If you install an Archer com, make certain you bend the antenna, to get as much vertical throw as possible in the leading edge (where the current is the highest) of the radiating element. In my -10, I mounted the ground element against the end rib as high as possible, then bent the radiating elements down until they were against the bottom of the wingtip. Getting as much vertical polarization as possible is important.
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