Dipole Antenna Question - Attn Jim Weir

wanttaja

wanttaja

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Ron Wanttaja
Gentleman on another forum had a question about using a dipole antenna on his Boredom Fighter. I offered to repost it here, for him:
-------------------------------------------------------------
I have made a dipole antenna, following the instructions found on the web. This looks great and I hope it works!

The problem is that it is, of course, by definition twice as long as a more usual 1/4 wave aerial and my Boredom Fighter fuselage can't accommodate the full length, which has to be vertically orientated.

[Note: The Boredom Fighter is a small biplane, of wood and fabric construction.]

The handiest way I can mount it inside the rear fuselage, just behind the seat bulkhead, results in about 16" of the top element of the aerial sticking out above the turtle deck, into the airflow. The arms of the antenna are made from 3/32"(appx) piano wire and I'm not too confident they will survive the 100mph airflow, without whipping about and thrashing my Boredom Fighter to pieces.

First question is: is this likely?

The second question is: could I safely bend the lower element of the aerial back, at its lower end, such that I could mount the whole thing lower down and keep the entire antenna inside the fuselage? Think I have seen somewhere that these aerials will tolerate a short length being bent back to clear the airframe, without losing much performance. This would also save a little unwanted drag and maybe prevent leaks through the turtle deck.

I plan using this aerial to maximise the efficiency of the small hand held radio I have to use in the open cockpit of the Boredom Fighter.

Look forward to any advice.

Ian
-----------------------------------------------------------------

Ron Wanttaja
 
I'm a retired electrical engineer and a ham radio operator, and my prediction is that degradation in performance would not be noticeable from bending the ends of the antenna at right angles to the main portion of it. (Most of the performance of a dipole comes from the middle portion, where the current is highest.)
 
I'm not sure that vertical orientation is absolutely necessary, though it might be optimal.
Why not use a ground plane and 1/4 wave?
 
I had a tube and fabric aircraft with one 1/4 wave ground plane antenna and one dipole antenna mounted about 30 degrees from vertical. The dipole worked better of the two.
 
jnmeade said:
I'm not sure that vertical orientation is absolutely necessary, though it might be optimal.
Click to expand...

As I told the questioner on the other group, my Fly Baby has a dipole oriented horizontally, and it works fine. Except when the other radio is located in line with the antenna....

Ron Wanttaja
 
On a fabric covered aircraft, the lack of sheet metal to provide a ground plane could reduce the performance of a 1/4 wave antenna.
 
wanttaja said:
As I told the questioner on the other group, my Fly Baby has a dipole oriented horizontally, and it works fine. Except when the other radio is located in line with the antenna....

Ron Wanttaja
Click to expand...

Although having the transmitting and receiving antennas parallel to each other is optimal, the effect is not absolute, because reflections can alter the polarization of the received signal.
 
Palmpilot said:
On a fabric covered aircraft, the lack of sheet metal to provide a ground plane could reduce the performance of a 1/4 wave antenna.
Click to expand...

Why can't one use a metal ground plane disk as one does on a plastic airplane? It would take up about half the space of a dipole. One would use a ground plane disk if one were using a transponder, for example.
 
Palmpilot said:
Although having the transmitting and receiving antennas parallel to each other is optimal, the effect is not absolute, because reflections can alter the polarization of the received signal.
Click to expand...

Wouldn't directionality be as important as polarity? If your antenna is end-on to the the other, signals might be in a null spot, wouldn't they? I'd think that would be generally less likely with a vertical antenna [absent trying to talk while practicing 60° banks, of course:) ]
 
jnmeade said:
Why can't one use a metal ground plane disk as one does on a plastic airplane? It would take up about half the space of a dipole. One would use a ground plane disk if one were using a transponder, for example.
Click to expand...

That should work.
 
jnmeade said:
Wouldn't directionality be as important as polarity? If your antenna is end-on to the the other, signals might be in a null spot, wouldn't they? I'd think that would be generally less likely with a vertical antenna [absent trying to talk while practicing 60° banks, of course:) ]
Click to expand...

Good point. And as with polarization, reflections could mitigate the effect.
 
jnmeade said:
Why can't one use a metal ground plane disk as one does on a plastic airplane? It would take up about half the space of a dipole.
Click to expand...
I built a replacement antenna for my Fly Baby using a Radio Shack antenna and plumbing parts from Lowes. For the ground plane, I used aluminum foil.
ground_plane.JPG

The antenna was mounted under the shelf with the aluminum foil.

It worked fine for ten years, then I noticed some problems receiving signals. Not sure what went wrong, probably corrosion in the innards of the antenna. Switched back to the 30-year-old dipole that had been built into the aircraft.....

Ron Wanttaja
 
Several years ago I faced a similar dilemma, also with a Fly Baby, and ended up defying conventional advice by building/installing a Dipole, not in the fuselage but in the wings, tip to tip. It ended up being a healthy half wave and worked ... especially when I was right over the transmitting antennas.

About the only null area seemed to be when the base station was directly off the nose. Some electrical interference from the magnetos. Inside a 10 to 12 mile range it worked very well.

I used very light weight television coax RG-59 with what is called bare (or solid) copper in a silicone sleeve, and wrapped with an aluminum braid shield.

I stripped off the jacket, foil wrap and aluminum shield, compressed on a BNC connector (standard fitting on all radios) and pulled one cable into each wing. Note* These are BNC connectors, NOT the "F-Type" or SMA coax connectors used for tv.

"Ear Tested" it two ways; 1. bare copper wires 2. leaving it in the silicone jacket. Without a meter there was no noticeable difference.

If you catch a Dish Network or DirecTV installer working in your neighborhood he'll probably give you all the 10 or 15 foot scraps of what they use, RG-6, you can carry to your car. It'll save him having to schlep it to the trash dumpster. You can get the fittings at Radio Shack or on eBay for less than a buck a piece. Of course on eBay, the shipping will be 9 dollars.
 
Many thanks 1wise1 - very interesting and your advice is much appreciated.

Ian
 
I am 100% a novice on aviation RF but am fairly versed in professional audio RF.

We do not use anything but 50 ohm cable for antennas on wireless mics...my question is will the 75 ohm cable the Direct TV folks use attenuate the signal to a noticeable degree?

1wise1 said:
Several years ago I faced a similar dilemma, also with a Fly Baby, and ended up defying conventional advice by building/installing a Dipole, not in the fuselage but in the wings, tip to tip. It ended up being a healthy half wave and worked ... especially when I was right over the transmitting antennas.

About the only null area seemed to be when the base station was directly off the nose. Some electrical interference from the magnetos. Inside a 10 to 12 mile range it worked very well.

I used very light weight television coax RG-59 with what is called bare (or solid) copper in a silicone sleeve, and wrapped with an aluminum braid shield.

I stripped off the jacket, foil wrap and aluminum shield, compressed on a BNC connector (standard fitting on all radios) and pulled one cable into each wing. Note* These are BNC connectors, NOT the "F-Type" or SMA coax connectors used for tv.

"Ear Tested" it two ways; 1. bare copper wires 2. leaving it in the silicone jacket. Without a meter there was no noticeable difference.

If you catch a Dish Network or DirecTV installer working in your neighborhood he'll probably give you all the 10 or 15 foot scraps of what they use, RG-6, you can carry to your car. It'll save him having to schlep it to the trash dumpster. You can get the fittings at Radio Shack or on eBay for less than a buck a piece. Of course on eBay, the shipping will be 9 dollars.
Click to expand...
 
Kelvin said:
I am 100% a novice on aviation RF but am fairly versed in professional audio RF.

We do not use anything but 50 ohm cable for antennas on wireless mics...my question is will the 75 ohm cable the Direct TV folks use attenuate the signal to a noticeable degree?
Click to expand...

I think that's difficult to predict. One reason is that when the impedance is mismatched, results become more dependent on the length of the cable. However, a 75 ohm cable with a 50 ohm load gives a standing wave ratio (SWR) of only 1.5, which is not all that bad as mismatches go.

You're probably already familiar with the concept of SWR, but for those who aren't:

http://en.wikipedia.org/wiki/Standing_wave_ratio

This article helps put SWR in perspective;

http://www.qsl.net/4/4z4tl//pub/swr_obsession.pdf
 
Last edited:
Thanks for the thread! It is not about my question, but provided enough info for me to find my answer. (I was trying to figure out what kind of cable to get to go from my Yaesu hand held to one of my plane's installed VHF antennas. Now I need to find a 6 ft, 50 ohm cable with female BNC at both ends.)

-Jim
 
Kiddo's Driver said:
Thanks for the thread! It is not about my question, but provided enough info for me to find my answer. (I was trying to figure out what kind of cable to get to go from my Yaesu hand held to one of my plane's installed VHF antennas. Now I need to find a 6 ft, 50 ohm cable with female BNC at both ends.)

-Jim
Click to expand...
I am pretty sure I have one sitting on the shelf...PM me your mailing address...and I'll check in the morning to see if I do...
 
Kelvin said:
I am 100% a novice on aviation RF but am fairly versed in professional audio RF.

We do not use anything but 50 ohm cable for antennas on wireless mics...my question is will the 75 ohm cable the Direct TV folks use attenuate the signal to a noticeable degree?
Click to expand...

I forgot to ask: Are you connecting to a dipole or a quarter wave ground plane? The former are close to 75 ohms, the latter to 50 ohms. And with a dipole, it's best to connect the coax to the antenna with a balun.
 
Palmpilot said:
I forgot to ask: Are you connecting to a dipole or a quarter wave ground plane? The former are close to 75 ohms, the latter to 50 ohms. And with a dipole, it's best to connect the coax to the antenna with a balun.
Click to expand...

Actually we use this helical antenna for many of our applications.

The 'standard' is a dipole antenna similar to this unit.

We uses BNC connectors on 50 ohm cable exclusively to connect to the receivers or transmitters.
 
Kelvin said:
Actually we use this helical antenna for many of our applications.

The 'standard' is a dipole antenna similar to this unit.

We uses BNC connectors on 50 ohm cable exclusively to connect to the receivers or transmitters.
Click to expand...

It's hard to tell from their description, but the dipole array at your second link could have a built-in balun, and some type of provision for matching it to a 50 ohm line.
 
Somewhat off topic (okay, way off), but does anyone have a suggestion for a good SW antenna (better than random wire) for a Tecsun PL-660?
 
The best solution is to have an antenna resonant for the frequency you are interested in. There are some tricks to making a given antenna broadbanded across a wider range of frequency, typically either thickening up the elements or doing something intersting with the design like a Discone.
 
Thanks. I'm experimenting with a few things, but I'm new to it and have a lot to learn. Seems like it's all going internet and computer-based nowadays. Old school Tesla and Marconi systems seem to be going extinct.
 
wanttaja said:
Gentleman on another forum had a question about using a dipole antenna on his Boredom Fighter. I offered to repost it here, for him:
-------------------------------------------------------------
I have made a dipole antenna, following the instructions found on the web. This looks great and I hope it works!

The problem is that it is, of course, by definition twice as long as a more usual 1/4 wave aerial and my Boredom Fighter fuselage can't accommodate the full length, which has to be vertically orientated.

[Note: The Boredom Fighter is a small biplane, of wood and fabric construction.]

The handiest way I can mount it inside the rear fuselage, just behind the seat bulkhead, results in about 16" of the top element of the aerial sticking out above the turtle deck, into the airflow. The arms of the antenna are made from 3/32"(appx) piano wire and I'm not too confident they will survive the 100mph airflow, without whipping about and thrashing my Boredom Fighter to pieces.

First question is: is this likely?

The second question is: could I safely bend the lower element of the aerial back, at its lower end, such that I could mount the whole thing lower down and keep the entire antenna inside the fuselage? Think I have seen somewhere that these aerials will tolerate a short length being bent back to clear the airframe, without losing much performance. This would also save a little unwanted drag and maybe prevent leaks through the turtle deck.

I plan using this aerial to maximise the efficiency of the small hand held radio I have to use in the open cockpit of the Boredom Fighter.

Look forward to any advice.

Ian
-----------------------------------------------------------------

Ron Wanttaja
Click to expand...

Sorry I missed this one, gang. I don't get over to this forum very often (obviously).

If the antenna is truly a dipole (2 lengths of wire ABOUT 22" long each) then bending the ends really isn't going to be a problem. As a matter of fact, to eliminate the "hole in the donut" effect, dipoles should NOT be truly axial from element to element. Shouldn't be a problem.
 
Palmpilot said:
I'm a retired electrical engineer and a ham radio operator, and my prediction is that degradation in performance would not be noticeable from bending the ends of the antenna at right angles to the main portion of it. (Most of the performance of a dipole comes from the middle portion, where the current is highest.)
Click to expand...

Well, shucks, the folks I worked with in the Teledyne Antenna Lab for five or six years always said that most of the performance of a dipole comes from the ENDS of the dipole where the VOLTAGE is the highest.

Jim
 
jnmeade said:
I'm not sure that vertical orientation is absolutely necessary, though it might be optimal.
Why not use a ground plane and 1/4 wave?
Click to expand...

Ground stations are mostly vertical, so any angle from vertical at the aircraft end is throwing away some power or range. However, up to about 45 degrees of cross-polarization isn't going to do much to degrade performance.

The reason you don't use a ground plane and a vertical quarter-wave spike is that the ground plane needs to be quarter wave in RADIUS from the center of the vertical antenna. That is a ground plane about 4 feet in diameter.

Jim
 
jnmeade said:
Why can't one use a metal ground plane disk as one does on a plastic airplane? It would take up about half the space of a dipole. One would use a ground plane disk if one were using a transponder, for example.
Click to expand...

Nope, twice the space of a dipole.

Jim
 
1wise1 said:
Several years ago I faced a similar dilemma, also with a Fly Baby, and ended up defying conventional advice by building/installing a Dipole, not in the fuselage but in the wings, tip to tip. It ended up being a healthy half wave and worked ... especially when I was right over the transmitting antennas.

About the only null area seemed to be when the base station was directly off the nose. Some electrical interference from the magnetos. Inside a 10 to 12 mile range it worked very well.

I used very light weight television coax RG-59 with what is called bare (or solid) copper in a silicone sleeve, and wrapped with an aluminum braid shield.

I stripped off the jacket, foil wrap and aluminum shield, compressed on a BNC connector (standard fitting on all radios) and pulled one cable into each wing. Note* These are BNC connectors, NOT the "F-Type" or SMA coax connectors used for tv.

"Ear Tested" it two ways; 1. bare copper wires 2. leaving it in the silicone jacket. Without a meter there was no noticeable difference.

If you catch a Dish Network or DirecTV installer working in your neighborhood he'll probably give you all the 10 or 15 foot scraps of what they use, RG-6, you can carry to your car. It'll save him having to schlep it to the trash dumpster. You can get the fittings at Radio Shack or on eBay for less than a buck a piece. Of course on eBay, the shipping will be 9 dollars.
Click to expand...

TV and cable use 75 ohm cable. Aircraft nav and com radios use 50 ohm cable. While the mismatch probably won't kill you, I'd be more than happy to pay you for all the good it will do you.

Jim
 
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