Shortening PowerFLARM Brick Antenna and GPS cables

A properly terminated transmission line is infinitely long from the
perspective of the transmitter. The reason the unterminated spool of coax
had nothing coming out the end was due to nearly 100% reflections in the
line.


"Mike the Strike" wrote in message
...
Yes, but all feed lines are lossy and at the frequencies we are using here
the lengths of coax are dumb-ass. We don't have an awful lot of signal to
begin with, so throwing half of it away is plain stupid. I remember in my
ham radio days a friend used an unterminated roll of coax as a dummy load
when experimenting up in the GHz range. The signal barely made it to the
other end, making a terminating resistor unnecessary!

Mike

On 9/15/2012 6:56 PM, Dan Marotta wrote:
A properly terminated transmission line is infinitely long from the
perspective of the transmitter. The reason the unterminated spool of
coax had nothing coming out the end was due to nearly 100% reflections
in the line.


"Mike the Strike" wrote in message
...
Yes, but all feed lines are lossy and at the frequencies we are using
here the lengths of coax are dumb-ass. We don't have an awful lot of
signal to begin with, so throwing half of it away is plain stupid. I
remember in my ham radio days a friend used an unterminated roll of coax
as a dummy load when experimenting up in the GHz range. The signal
barely made it to the other end, making a terminating resistor unnecessary!

There should not be any reflections _in_ the line, unless it is
improperly made or damaged. More likely, as Mike suggests, most of the
power was absorbed along the way by the coax itself, so there was
precious little power to be reflected. The coax is effectively
"terminated" by the distributed losses that occur in the center and
outer conductors, and the dielectric.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Nope.

As a lab experiment in my college days, I located a break in a very long
coax by timing a signal's reflection using an oscilloscope. An undamaged
line, terminated in its characteristic impedance will deliver most of its
power to the load. Reflections will be minimal. I'm describing an ideal
situation which, I know, is not possible. You're describing a poory
designed, built, installed, or maintained system which, considering us
glider pilots, is probably the norm.



"Eric Greenwell" wrote in message
...
On 9/15/2012 6:56 PM, Dan Marotta wrote:
A properly terminated transmission line is infinitely long from the
perspective of the transmitter. The reason the unterminated spool of
coax had nothing coming out the end was due to nearly 100% reflections
in the line.


"Mike the Strike" wrote in message
...
Yes, but all feed lines are lossy and at the frequencies we are using
here the lengths of coax are dumb-ass. We don't have an awful lot of
signal to begin with, so throwing half of it away is plain stupid. I
remember in my ham radio days a friend used an unterminated roll of coax
as a dummy load when experimenting up in the GHz range. The signal
barely made it to the other end, making a terminating resistor
unnecessary!

There should not be any reflections _in_ the line, unless it is improperly
made or damaged. More likely, as Mike suggests, most of the power was
absorbed along the way by the coax itself, so there was precious little
power to be reflected. The coax is effectively "terminated" by the
distributed losses that occur in the center and outer conductors, and the
dielectric.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email
me)

On 9/16/2012 8:17 AM, Dan Marotta wrote:
Nope.

As a lab experiment in my college days, I located a break in a very long
coax by timing a signal's reflection using an oscilloscope. An
undamaged line, terminated in its characteristic impedance will deliver
most of its power to the load. Reflections will be minimal. I'm
describing an ideal situation which, I know, is not possible. You're
describing a poory designed, built, installed, or maintained system
which, considering us glider pilots, is probably the norm.

"Mike the Strike" wrote in message
...
Yes, but all feed lines are lossy and at the frequencies we are using
here the lengths of coax are dumb-ass. We don't have an awful lot of
signal to begin with, so throwing half of it away is plain stupid. I
remember in my ham radio days a friend used an unterminated roll of coax
as a dummy load when experimenting up in the GHz range. The signal
barely made it to the other end, making a terminating resistor unnecessary!


Hi Dan - You are indeed describing an ideal situation; Mike was
describing a non-ideal situation. If you make the line long enough, it
will absorb all the power before it reaches the end of the cable; for
high frequencies (transponder and higher), "long enough" might be only
20 or 30 feet with low-cost cable.

Without knowing the specific cable used in the PF antennas, I can't say
they are significantly degrading the signal, but shorter is better.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Hi, Eric,

It appears we're talking the same story after all. When I installed my
transponder (Trig TT22) I used some RG-58 that was laying around the
airport. It didn't work too well. So I went to an avionics shop and got
the right stuff. It worked much better.


"Eric Greenwell" wrote in message
...
On 9/16/2012 8:17 AM, Dan Marotta wrote:
Nope.

As a lab experiment in my college days, I located a break in a very long
coax by timing a signal's reflection using an oscilloscope. An
undamaged line, terminated in its characteristic impedance will deliver
most of its power to the load. Reflections will be minimal. I'm
describing an ideal situation which, I know, is not possible. You're
describing a poory designed, built, installed, or maintained system
which, considering us glider pilots, is probably the norm.

"Mike the Strike" wrote in message
...
Yes, but all feed lines are lossy and at the frequencies we are using
here the lengths of coax are dumb-ass. We don't have an awful lot of
signal to begin with, so throwing half of it away is plain stupid. I
remember in my ham radio days a friend used an unterminated roll of coax
as a dummy load when experimenting up in the GHz range. The signal
barely made it to the other end, making a terminating resistor
unnecessary!


Hi Dan - You are indeed describing an ideal situation; Mike was describing
a non-ideal situation. If you make the line long enough, it will absorb
all the power before it reaches the end of the cable; for high frequencies
(transponder and higher), "long enough" might be only 20 or 30 feet with
low-cost cable.

Without knowing the specific cable used in the PF antennas, I can't say
they are significantly degrading the signal, but shorter is better.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email
me)

I took one of the little dipole antennas apart today. Easy, once you know that there are two screws hidden under the adhesive pad. Remove the pad to get to the screws. Each element of the dipole has a brass base with coax center conductor soldered to one, shield to the other. Easy to change length to remove the "bundled" excess to minimize line loss.

But, that does bring up a question. I remember years ago that at least for lower frequencies the length of the coax wanted to be multiples of 1/4 or 1/2 wave adjusted for velocity or some such. Is that a factor or worth messing with at 900 MHz?

bumper

I took apart the GPS antenna which has by far the LONGEST excess of cable in the brick PowerFlarm setup. It's a wee bit trickier soldering than on the PF antennas, but if you're inclined it's not a huge task. I removed probably 6' of cable. I left the butterfly display back at the gliderport so I haven't been able to test it, but I'm confident it will work. The shield is soldered to one spot and the center conductor is soldered to a small pad on the PC board. The only trick is the careful trimming of the shield and conductor insulation to match what they did. It's also fairly easy to remove and re-use the strain relief.


Mark

On 9/16/2012 7:18 PM, wrote:
I took apart the GPS antenna which has by far the LONGEST excess of
cable in the brick PowerFlarm setup. It's a wee bit trickier
soldering than on the PF antennas, but if you're inclined it's not a
huge task. I removed probably 6' of cable. I left the butterfly
display back at the gliderport so I haven't been able to test it, but
I'm confident it will work. The shield is soldered to one spot and
the center conductor is soldered to a small pad on the PC board. The
only trick is the careful trimming of the shield and conductor
insulation to match what they did. It's also fairly easy to remove
and re-use the strain relief.

The GPS antenna cable doesn't carry high frequency RF, so tidiness is
the only value in reducing the length.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On Sunday, September 16, 2012 10:27:47 PM UTC-4, Eric Greenwell wrote:
On 9/16/2012 7:18 PM, wrote:

I took apart the GPS antenna which has by far the LONGEST excess of

cable in the brick PowerFlarm setup. It's a wee bit trickier

soldering than on the PF antennas, but if you're inclined it's not a

huge task. I removed probably 6' of cable. I left the butterfly

display back at the gliderport so I haven't been able to test it, but

I'm confident it will work. The shield is soldered to one spot and

the center conductor is soldered to a small pad on the PC board. The

only trick is the careful trimming of the shield and conductor

insulation to match what they did. It's also fairly easy to remove

and re-use the strain relief.



The GPS antenna cable doesn't carry high frequency RF, so tidiness is

the only value in reducing the length.



--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)

Eric, so why do we need a GPS antenna wire if the signal from the satellite magically gets to the receiver without the RF signal first going through the antenna wire :=P

On Sunday, September 16, 2012 10:27:47 PM UTC-4, Eric Greenwell wrote:
On 9/16/2012 7:18 PM, wrote:

I took apart the GPS antenna which has by far the LONGEST excess of

cable in the brick PowerFlarm setup. It's a wee bit trickier

soldering than on the PF antennas, but if you're inclined it's not a

huge task. I removed probably 6' of cable. I left the butterfly

display back at the gliderport so I haven't been able to test it, but

I'm confident it will work. The shield is soldered to one spot and

the center conductor is soldered to a small pad on the PC board. The

only trick is the careful trimming of the shield and conductor

insulation to match what they did. It's also fairly easy to remove

and re-use the strain relief.



The GPS antenna cable doesn't carry high frequency RF, so tidiness is

the only value in reducing the length.

--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)
Eric, so why do we need a GPS antenna wire if the signal from the satellite magically gets to the receiver without the RF signal first going through the antenna wire :=P I'm fairly sure that the GPS antennas are "active" antennas getting phantom dc power from the receiver but I thought they only had an amplifier; are you sure they also have a local oscillator & mixer to convert the RF to an IF (I guess like an LNB)? Amazing for such a cheap device.