Antenna Ground Plane help

[warbugdriver]

I'm thinking about adding an external antenna to my L-2. I'm currently using a handheld with the stock 'rubber-ducky' antenna which has performance I would describe as minimal/marginal.

Textbook describes the ground plane needed for aviation VHF frequencies as 4x4 feet. Can the airframe be used as the ground plane or does it need to be a literal square of metal? My current thoughts are mounting a belly antenna on the bottom of the boot cowl -- my concern here is that the boot cowl is only about 28-30 inches wide, max. How much effect will the narrow width have on radio performance?

[Guest]

Re: Antenna Ground Plane help

Mine is mounted on a sheet of aluminum that is about 6" by 9" at the cabin roof at the root of the RH wing.

It has worked fine and I flew for many years from a controlled field so it got a lot of use.

Dave

[Nefj40]

Re: Antenna Ground Plane help

I've got a 9X9, so it's just a little bigger than Dave's mounted to the X brace on top of my cabin. With my Icon A6 I can pick up traffic 300 miles away at times.

[M Towsley]

Re: Antenna Ground Plane help

My former L-2B had the antenna mounted to the bottom of the boot cowl. I could talk to people in the next state with it and I am centrally located in Illinois. My current BC-12D has it mounted on the bottom of the boot cowl and I get the same results from an Icom handheld. My L-2 radio was a Microair 760 in-dash that fit like an instrument. You will have to have a reinforcing plate in there. I think I posted pictures on here somewhere from my BC-12D install. I will look.

[M Towsley]

Re: Antenna Ground Plane help

Look here: http://vb.taylorcraft.org/showthread...a-Installation

[mike lutz]

Re: Antenna Ground Plane help

My 1940 bl65 has it mounted on the wing root fairing just behind the rear spar, and works great. A friend of mine has a 46 bc12d and his is on the boot cowl on the bottom and it works great.

[Scott]

Re: Antenna Ground Plane help

it's a bit of a crap shoot. I used to connect my portable VHF to my ELT antenna which was mounted on the right wing root faring. I flew the Tcraft in and out of Montreal International many many times (before transponders were mandatory) with no problems. However I have no idea what would have happened trying to transmit and receive on all other frequencies. So there may have been and probably were, frequencies where performance was marginal or non-existent.

I was going to install a ground plane during my restoration but then I came across this: http://advancedaircraft.com/#ourproducts

I cannot yet attest to its performance but I have installed one by gluing it to a piece of spruce and attaching it with adel clamps between the top and bottom fuselage cross-frames aft of the baggage compartment. I'll add photos later

[PA1195]

Re: Antenna Ground Plane help

There's many answers and solutions to aircraft antennas. They can range from "fair>best".

The basic rubber ducky is typically an electrically and physically shortened antenna that depends upon the user's body (via the hand contact) to act as the ground plane, or other half of the antenna system. Adding a single 22-24" electrical wire (insulated wire with metal to metal contact at the radio end) to the radio's BNC antenna base or chassis screw can improve performance via it becoming a single counterpoise and turns the hand-held radio's antenna into a compromised vertical dipole described below. It's a fair setup that is portable. Mounting the hand-held radio to the metal of the airframe or panel can also improve performance.

Adding two or more wires of similar length (typically four, fine tuned via length trimming to the desired radio frequency using an external SWR meter) to a fixed external antenna's base mount can provide better performance. Same for a solid metal base or plate. The 4x4' base approximates an infinite number of tuned wires. A smaller base like the plates mentioned or root fairings also work, and the results vary from good to best depending upon available room and metal. Adding wires to the four corners of a small plate can improve performance. That's what Bellanca used to do to the small base for com antennas on Citabrias

The link above is for dipole antennas. One half of the assembly is the antenna, the other a single counterpoise or ground plane similar to the handheld rubber ducky/single wire setup. They are best installed vertically in or on the airframe away from metal. Performance in a direction off the two ends is poorest. The coax feed should leave the antenna @ 90 degrees or the signal can couple to the coax shield and cause issues.

Most radios will hear ok even with a poor antenna providing the local noise in the aircraft is minimized. Ignition, charging, and other devices can create signals that can mask and limit desired reception. Reducing local interference is often more helpful improving reception than a better antenna. And where a good ground plane helps most is in transmission...being heard is often improved.

Edit: Brief antenna theory>

https://en.wikipedia.org/wiki/Whip_antenna

https://en.wikipedia.org/wiki/Monopole_antenna

https://en.wikipedia.org/wiki/Dipole_antenna

https://en.wikipedia.org/wiki/Ground_plane

https://en.wikipedia.org/wiki/Counterpoise_(ground_system)

Gary

[warbugdriver]

Re: Antenna Ground Plane help

Thanks to everyone for your replies! Sounds like some folks have had success with attachment to the boot cowl so I'm leaning in that direction.

Gary: my chief complaint is exactly as you identified -- transmission. I run a Sporty's SP-200 in a cradle and have it tied into my intercom. With the stock rubber antenna, transmission power leaves much to be desired although it does seem to work pretty well on the ground -- likely your explanation of the user becoming the ground plane is the reason.

Marty: Thanks for the photo link! I have the same antenna.

[PA1195]

Re: Antenna Ground Plane help

The simplest permanent setup would be to add an antenna to the wing root fairing or cowl (suitably supported with adequate metal thickness and fastening to the nearby frame) and a run of coax between it and the radio. That way the antenna has the required two components (vertical mast and base metal) to function. The antenna and metal base material have to make electrical contact...bare metal...to work well.

Alternatively in the meantime just attach a 22-24" piece of wire to the bare metal base of the rubber ducky and let it hang down.

Gary

[M Towsley]

Re: Antenna Ground Plane help

Yes, this has been re-hashed over and over and everyone has their 'druthers'. After much thought, research on-line and a lot of experience and training in making and using antennas for lots of various transmitters, (body wires, covert transmitters, etc.), I chose the boot cowl. It offers a good-sized ground plane and excellent transmission/reception. The ONLY thing I have found is on the ground the landing gear legs MAY blank out a transmission to the control tower. I just cock the plane a bit and that takes care of it. It usually only happens for me when I am talking to ground control at a very particular point on the airport.

Be extremely careful in using antennas not tuned to the transmitter's frequency range. Transmitting with the improper antenna, no antenna or a length of wire is a good way to let the smoke out of your handheld. And you can never put the smoke back in, (old electronics joke). Handhelds are not cheap and there is no reason to experiment.

[PA1195]

Re: Antenna Ground Plane help

It's been surprising to me how few builders, and even some techs, end up measuring and optimizing the final radio<>antenna system match. Reasonably priced in-line VHF SWR, output watt, and field strength meters are available, as are more expensive self contained and portable antenna system analyzers that provide info on SWR and complex impedance. Google source products.

That step can involve more effort and expense than most care about doing, but in the end when using low power AM radios we still expect to hear and be heard.

The simplest method is to use a good antenna system, and make sure the plane is as electrically quiet as possible...add magneto and charging system filters plus insure good electrical bonding exists in the frame and between the frame and engine.

Gary

[astjp2]

Re: Antenna Ground Plane help

You cant go wrong with ac43.13-2a. it basically says the ground plane must be the size of the antenna is long or something along those lines. Tim