ADS-B DIY Antenna

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ab cd

Senior Member
Again I refer to this diagram. Can someone help me understand what length L refers to? As in, L = 234/1090 Mhz = .2146 (presumably meters, or 214.6 mm). On one side of the diagram they show λ/4. On the opposite side L=234/f(MHz). I could take that to mean that L = λ/4, but that makes no sense when we know that λ/4 of 1090 Mhz is 69 mm. :confused: Without L=234/f(MHz) the diagram makes sense to me.

View attachment 1292
L = 234/1090 Mhz = .2146 feet = 0.2146 x 12 inches = 0.2146 x 12 x 25.4 mm = 65.4 mm
Confusion caused by ARRL diagram not mentioning units. They take for granted that every one in the world uses FPS (foot, pound, seconds) system.
 

jepolch

Active Member
@giacomo1989: I think I'll make one with a 69 mm balun. Then I can always shorten it if I need to. By the way, in your post #2131 above, did you notice the two diagrams show different techniques for attaching the baluns?
 

jepolch

Active Member
L = 234/1090 Mhz = .2146 feet = 0.2146 x 12 inches = 0.2146 x 12 x 25.4 mm = 65.4 mm
Confusion caused by ARRL diagram not mentioning units. They take for granted that every one in the world uses FPS (foot, pound, seconds) system.
Hi ab cd. After Giacomo wrote that I remember that ARRL uses feet and inches, right? Why do you think they compute 65.4 mm instead of 69 mm for 1/4 wavelength?

ETA: Maybe ARRL uses the Imperial speed of light vs. Metric speed of light. :D
 
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ab cd

Senior Member
I think you're right. 234/1090 = 0.21467 X 12 (inches) = 2.576 inches = 65.43 mm. Close to 69 mm, but not exact. :)
Wavelength (meters) = Speed of Electromagnetic waves in meters per sec/frequency in Hz
Wavelength (feet) = Speed of Electromagnetic waves in feet per sec/frequency in Hz

Speed of electromagnetic waves is same as speed of light, as light is also electromagnetic wave.
The speed of light (MKS or SI or Metric units) = 299,792,458 meters/sec
The speed of light (FPS or Imperial units) = 186,282 miles/sec = 983,571,056 ft/sec.

For practical purposes, these figures can be rounded to:
300,000,000 meters/sec
984,000,000 ft/sec

Hence:
Wavelength in Metric (MKS/SI) units = 300,000,000/ MHz x 1,000,000 = 300/MHz (meters)
Wavelength in Imperial (FPS) units =984,000,000/ MHz x 1,000,000 = 984/MHz (feet)

1/4 wavelength in meters = 1/4 x 300/f (Mhz) = 75/f (Mhz)
1/4 wavelength in feet = 1/4 x 984/f (Mhz) = 246/f (Mhz)

The ARRL Book uses 234/ f (Mhz) , which gives about 5% less length than 246/f (Mhz). Why? no idea.

***
Measurement Systems:
CGS = Centimeter Gram Seconds - old scientific system, no more in use.
MKS = Meter Kilogram Seconds.
SI = International Standard Units = MKS (almost).
FPS = Imperial = Foot, Pound, Seconds.
 
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DougJohnson

New Member
@giacomo1989: I think I'll make one with a 69 mm balun. Then I can always shorten it if I need to. By the way, in your post #2131 above, did you notice the two diagrams show different techniques for attaching the baluns?
Don't forget to adjust for the velocity factor of the coax. But I am looking forward to your results. Maybe I can save my poor Franklin.

The problem I have with tweaking tap locations is the same as yours. The only instrumentation I have is very crude --- stick the thing up for a few days and count messages, range, and dBFS.
-- Doug
 

jepolch

Active Member
Wavelength (meters) = Speed of Electromagnetic waves in meters per sec/frequency in Hz
Wavelength (feet) = Speed of Electromagnetic waves in feet per sec/frequency in Hz

Speed of electromagnetic waves is same as speed of light, as light is also electromagnetic wave.
The speed of light (metric units) = 299,792,458 meters/sec
The speed of light (FPS or Imperial units) = 186,282 miles/sec = 983,571,056 ft/sec.

For practical purposes, these figures can be rounded to:
300,000,000 meters/sec
984,000,000 ft/sec

Hence:
Wavelength in metric units = 300,000,000/ MHz x 1,000,000 = 300/MHz meters
Wavelength in metric units =984,000,000/ MHz x 1,000,000 = 984/MHz meters

1/4 wavelength in meters = 1/4 x 300/f (Mhz) = 75/f (Mhz)
1/4 wavelength in feet = 1/4 x 984/f (Mhz) = 246/f (Mhz)

The ARRL Book uses 234/ f (Mhz) , which gives about 5% less length. Why? no idea.
Great answer. I think I have all the information I need now to start cutting. I'll start with 69 mm and do a test, then maybe cut back to 65 mm and do more testing.
 

jepolch

Active Member
Don't forget to adjust for the velocity factor of the coax. But I am looking forward to your results. Maybe I can save my poor Franklin.

The problem I have with tweaking tap locations is the same as yours. The only instrumentation I have is very crude --- stick the thing up for a few days and count messages, range, and dBFS.
-- Doug
It occurred to me that we're an interesting hybrid of non-HAM radio hobbyists. Of course, there are some exceptions, like ab cd and a couple others here. But I think most of us ADS-B hobbyists are plug and pray types. :D

ETA: Thank goodness there are a few HAMs here to help us with the calculations!
 

ab cd

Senior Member
It occurred to me that we're an interesting hybrid of non-HAM radio hobbyists. Of course, there are some exceptions, like ab cd and a couple others here. But I think most of us ADS-B hobbyists are plug and pray types. :D

ETA: Thank goodness there are a few HAMs here to help us with the calculations!
I am NOT a HAM, nor ever have been, but I do have a keen interest in Telecommunications, Electronics, & Computers, and like DIY & Experimenting.

Plug & Pray .... me too :D
 

jepolch

Active Member
I am NOT a HAM, nor ever have been, but I do have a keen interest in Telecommunications, Electronics, & Computers, and like DIY & Experimenting.

Plug & Pray .... me too :D
Aha. I thought you were a HAM from something you posted once - Morse Code?
 

giacomo1989

Member
@giacomo1989: I think I'll make one with a 69 mm balun. Then I can always shorten it if I need to. By the way, in your post #2131 above, did you notice the two diagrams show different techniques for attaching the baluns?
I had noticed it :) It's just a wire. Use the second as it has less resistance and just easier to do :D Let me know
 

trigger

Member
@jepolch:
@DougJohnson:

Before you try decoupling sleeves & balun, try to adjust cable tap of Franklin. This can best be done with Franklin on a temporary support (pvc pipe, wooden plank etc) in your attic or open yard, and at height of arms reach. Weather now is good for working in open yard :)
Don't give up on the Franklin.

I did pretty much what ab cd suggests. I sat under my patio umbrella with the antenna hanging off one of the spokes. I ran adsb Scope and got a feel for where the edge of detection was. I watched a plane which was just at the edge of the detection but coming towards me. Move the tap a couple of mm and if the plane disappears put the tap back to where it was (the plane should re-appear). Move the tap a couple of mm in the opposite direction. Repeat for the next plane. It helps if you have a beer in your spare hand ;)

I cut a metal connectors out of a connector block, cut it in half then slipped them onto the wire before bending to shape. I soldered the ends of the coax which made connection to the cut down connector easier. Moving the tap was just a matter of unscrewing the screw and pushing the connector along the wire. I also put a bit of insulation tape between the 2 connectors in case they touched.
upload_2015-3-13_18-52-39.png
 
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ab cd

Senior Member
Don't give up on the Franklin.

I did pretty much what ab cd suggests. I sat under my patio umbrella with the antenna hanging off one of the spokes. I ran adsb Scope and got a feel for where the edge of detection was. I watched a plane which was just at the edge of the detection but coming towards me. Move the tap a couple of mm and if the plane disappears put the tap back to where it was (the plane should re-appear). Move the tap a couple of mm in the opposite direction. Repeat for the next plane. It helps if you have a beer in your spare hand ;)

I cut a metal connectors out of a connector block, cut it in half then slipped them onto the wire before bending to shape. I soldered the ends of the coax which made connection to the cut down connector easier. Moving the tap was just a matter of unscrewing the screw and pushing the connector along the wire. I also put a bit of insulation tape between the 2 connectors in case they touched.
View attachment 1296
THANK YOU! :cool:
 
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DougJohnson

New Member
Don't give up on the Franklin.

I did pretty much what ab cd suggests. I sat under my patio umbrella with the antenna hanging off one of the spokes. I ran adsb Scope and got a feel for where the edge of detection was.
Do you have a feel for how much difference you were able to get? It is too late for me to try the terminal block idea for this one, but I am sure there are other things I could try. -- Doug
 

jepolch

Active Member
Don't give up on the Franklin.

I did pretty much what ab cd suggests. I sat under my patio umbrella with the antenna hanging off one of the spokes. I ran adsb Scope and got a feel for where the edge of detection was. I watched a plane which was just at the edge of the detection but coming towards me. Move the tap a couple of mm and if the plane disappears put the tap back to where it was (the plane should re-appear). Move the tap a couple of mm in the opposite direction. Repeat for the next plane. It helps if you have a beer in your spare hand ;)

I cut a metal connectors out of a connector block, cut it in half then slipped them onto the wire before bending to shape. I soldered the ends of the coax which made connection to the cut down connector easier. Moving the tap was just a matter of unscrewing the screw and pushing the connector along the wire. I also put a bit of insulation tape between the 2So connectors in case they touched.
View attachment 1296
Thanks for the tips. So you got yours working good? How many elements? Will Diet Coke work OK? :D
 

jepolch

Active Member
Do you have a feel for how much difference you were able to get? It is too late for me to try the terminal block idea for this one, but I am sure there are other things I could try. -- Doug
I'll bet you could wiggle the single connector around the bends to get it where you need to.
 
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