tuned ground plane

I have a Velocity - and am planning on using a Comant CI-105
transponder antenna, and place it out on the wing strake area - as I
do not want it close to the engine and near my AHRS setup.

As this airplane is a glass airplane, and this antenna requires a
ground
plane - I have read about a "tuned" ground plane.

The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
5.15229..."

Can someone give me a heads up as to what a tuned ground plane really
is?

Thanks,

John

You want ODD multiples of 1/4 wave for the ground plane like 1/4, 3/4
etc from the center antenna rod to the Ground plane edge. NOT even
like 1/2.

On 17 Nov 2004 19:30:44 -0800, (John Tvedte) wrote:

I have a Velocity - and am planning on using a Comant CI-105
transponder antenna, and place it out on the wing strake area - as I
do not want it close to the engine and near my AHRS setup.

As this airplane is a glass airplane, and this antenna requires a
ground
plane - I have read about a "tuned" ground plane.

The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
5.15229..."

Can someone give me a heads up as to what a tuned ground plane really
is?

Thanks,

John

A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In practicality (with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.

Having said that, you must understand that you cannot make a "perfect" circular
("disk") ground plane for a transponder, as it must operate on two frequencies
simultaneously -- 1030 and 1090 MHz. What is quarter wave for one is not
quarter wave for the other. While the error is slight and relatively
insignificant for transponders, it DOES become significant when you start to
talk about things like the aircraft COM band, where the instantaneous bandwidth
is on the order of 11%.

My chosen way to make a transponder ground plane is to start off with a square
of aluminum using the following calculations:

w = 11810 / f
s = 0.487 w

where "w" is a quarter wave in air at frequency "f"

and "s" is the length of one side of the square of aluminum.

Then punch a hole in the exact center of the groundplane for the antenna.

Then cut the corners of the square to make a regular octagon.

SOMEWHERE along the periphery of that octagon will be an exact quarter wave at
both 1030 and 1090 MHz. IF you select w to be halfway between these two
frequencies (i.e. 1060 MHz.).

So, in the above scenario, w = 11.14" and s = 5.42".


Jim



(John Tvedte)
shared these priceless pearls of wisdom:

-I have a Velocity - and am planning on using a Comant CI-105
-transponder antenna, and place it out on the wing strake area - as I
-do not want it close to the engine and near my AHRS setup.
-
-As this airplane is a glass airplane, and this antenna requires a
-ground
-plane - I have read about a "tuned" ground plane.
-
-The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
-5.15229..."
-
-Can someone give me a heads up as to what a tuned ground plane really
-is?
-
-Thanks,
-
-John

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Ok,

w = c / f; so w= 1 wavelength - use c = speed of light (in air)

s = 0.487 w ; where is the 0.487 from?

I'm also confused why "split the difference" - ie. 1060Mhz, at 1090
Mhz the wave is shorter, and when the sides are cut as you describe,
one ends up with the shortest length = s, and the "tips" of the
octagon across the piece are longer....I would expect that one would
desire a piece that has a section that is as short as the 1090Mhz, and
sections that are as long as 1030Mhz...

I think I am still missing something here....

-------
Jim Weir wrote in message . ..
A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In practicality (with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.

Having said that, you must understand that you cannot make a "perfect" circular
("disk") ground plane for a transponder, as it must operate on two frequencies
simultaneously -- 1030 and 1090 MHz. What is quarter wave for one is not
quarter wave for the other.

John,
I went through this analysis also and elements of understanding also
escape me. I hope that Jim can offer some additional explanation to
clear this up. Anything to take antennas out of the black magic
category will always be appreciated.
Blue skies,
Tom

John Tvedte wrote:

The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
5.15229..."

Can someone give me a heads up as to what a tuned ground plane really
is?

I made similar disc for my antenna, it woks fine with the $20 TED antenna
http://www.aircraftspruce.com/catalo...ransponder.php

French authority required labo test: the emission power level mesured was
150W for 175 nominal for Becker ATC4401-175.

Becker people says a CD audio may do the job (only alumnium ones)

By
--
Philippe Vessaire Ò¿Ó¬

John Tvedte wrote:
Ok,

w = c / f; so w= 1 wavelength - use c = speed of light (in air)

s = 0.487 w ; where is the 0.487 from?

The speed of light in air isn't the number to be using. EM waves propagate
at slower speeds in different media. Normally this speed is expressed as
a "velocity factor" which indicates what fraction of the speed in vaccuo

John Tvedte wrote:


The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
5.15229..."

Can someone give me a heads up as to what a tuned ground plane really
is?

Just another thing: don't applys any reduction factor. The 0.95 reduction
factor is only for radian lenth, not ground plate.

By
--
Philippe Vessaire Ò¿Ó¬

"Jim Weir" wrote in message
...
A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In practicality
(with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.

Having said that, you must understand that you cannot make a "perfect"
circular
("disk") ground plane for a transponder,

Jim, I once built such an antenna (a 1/4 wave radiator with a 1/4 wave disk
ground plane) for a specific 800 MHz frequency and it was a disaster, with
almost infinite SWR at the cut frequency. I cut 4 large V's out of the disk
leaving 4- 1" 1/4 wave tabs, and I now had perhaps a 3 to 1 SWR. I finally
bent the tabs down 45 degrees into the usual "ground plane antenna"
configuration, and suddenly the thing had a near-perfect SWR with a wide
bandwidth.

I learned two things from that experience, 1) there is a lot I still don't
know about antennas, and 2) be cautious of tuned disk ground planes.

Perhaps you can tell me what went wrong.

Vaughn

Jim Weir wrote in message
...

(John Tvedte)
shared these priceless pearls of wisdom:

-I have a Velocity - and am planning on using a Comant CI-105
-transponder antenna, and place it out on the wing strake area - as I
-do not want it close to the engine and near my AHRS setup.
-
-As this airplane is a glass airplane, and this antenna requires a
-ground
-plane - I have read about a "tuned" ground plane.
-
-The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
-5.15229..."
-
-Can someone give me a heads up as to what a tuned ground plane really
-is?
-
-Thanks,
-
-John
A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In
practicality (with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.

Having said that, you must understand that you cannot make a "perfect" circular
("disk") ground plane for a transponder, as it must operate on two frequencies
simultaneously -- 1030 and 1090 MHz. What is quarter wave for one is not
quarter wave for the other. While the error is slight and relatively
insignificant for transponders, it DOES become significant when you start to
talk about things like the aircraft COM band, where the instantaneous bandwidth
is on the order of 11%.

My chosen way to make a transponder ground plane is to start off with a square
of aluminum using the following calculations:

w = 11810 / f
s = 0.487 w

where "w" is a quarter wave in air at frequency "f"

(A) 'w' is mis-identified. This is a _full_ wavelength in air, not a quarter.
Assumes frequency in mHz, gives result in inches.
only accurate to 4 sig figs -- good enough for most 'practical' work.
Use 11811.02 [ (300 million meter/sec) * (inches/meter) / one million ]
for 'high precision' "in vacuum" and then correct for propagation in air
vs. in vacuum (299702547/299792458), giving 11807.45775338017 grin

(B) the 'mysterious' 0.487 is a combination of two things.
1) we want a 1/4 wavelength _radius_, so the "diameter" will be 1/2 wave.
2) we want the 'minimum' dimension of the octagon to be resonant at
1090 mHz, while we did the 'w' calc at 1060 mHz.
3) guess what 1060/1090, _divided_by_2_ is? grin
(0.48623853211009+, for those without a calculator handy)

Thus, from the center, to the _middle_ of any side is a 1/4 wave at 1090 mHz.

And, the distance from the center of the octagon, to a 'corner' is enough
longer than the distance from the center to the 'middle' of a side (a factor
of 1.0823922002+ [sqrt(1**2+(sqrt(2)-1)**2), if anybody cares) that the 1/4
wave resonant frequency on the 'diagonal/ is 1007 mHz.

Thus, 'somewhere' between the middle of a side, and the corner, the length
will be 'right' for a 1030mHz 1/4 wave.


and "s" is the length of one side of the square of aluminum.

Then punch a hole in the exact center of the groundplane for the antenna.

Then cut the corners of the square to make a regular octagon.

SOMEWHERE along the periphery of that octagon will be an exact quarter wave at
both 1030 and 1090 MHz. IF you select w to be halfway between these two
frequencies (i.e. 1060 MHz.).

So, in the above scenario, w = 11.14" and s = 5.42".


Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com