Antenna and Coax Length

I have a question for this august body.

I found a link on the 1-26 associations web site talking about the
proper length of an antenna for operation at 123.3Mhz. This turns out
to be;

1/4 wave Length: 0.61 m or 23.95 in
3/4 wave Length: 1.82 m or 71.84 in

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

More information. The below was taken from the Microaire M760 manual.

"For certified aircraft the M760 should only be operated with a TSO
DO-160D compliant antenna. The antenna may be ¼ wave whip or ½ wave
dipole, using 50ohm coaxial cable and a BNC connector for connection.
For non-certified aircraft using a non-TSO compliant antenna, the VSWR
must be checked to ensure the ratio does NOT exceed 1:3:1 across
118.000 - 136.975Mhz range. A VSWR of 1:5:1 can be tolerated by the
M760 without injury to the transceiver, but transmission performance
starts to become impaired."

So I guess the next questions are;

1) What is a TSO compliant antenna? - It appears that this is an FAA
"Technical Standard Order" (http://en.wikipedia.org/wiki/
Technical_Standard_Order). Looking at the Aircraft Spruce web site I
find lots of references to TSO transceivers but not to TSO antennas.
I was unable to bring up the http://airweb.faa.gov web site on TSOs.

2) Does my glider have a TSO compliant antenna? Like most glass
ships, my antenna is imbedded in the vertical stabilizer. I guess I
will have to contact the manufacturer.

3) Does a TSO compliant antenna mean that I don't have to worry about
coax length? If so, why not?

Thanks, John

"ContestID67" wrote in message
ups.com...
I have a question for this august body.

I found a link on the 1-26 associations web site talking about the
proper length of an antenna for operation at 123.3Mhz. This turns out
to be;

1/4 wave Length: 0.61 m or 23.95 in
3/4 wave Length: 1.82 m or 71.84 in

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

Where you have an antenna connected to a transmitter or receiver via a
length of coax the important thing is that the characteristic impedance of
the various components match to ensure maximum power transfer and eliminate
reflections in the system. This is achieved by using the correct type of
coax for the equipment and making sure the antenna is the correct length.
You can use a VSWR meter to match the antenna to the rest of the system
(start long and snip bits off).

The length of the coax is not important from the point of view of matching
the impedance. However, because the coax will attenuate the signal, the
shorter it is the better.

These might explain things:
http://en.wikipedia.org/wiki/VSWR (go to the practical implications
section).
http://en.wikipedia.org/wiki/Characteristic_impedance

Stephen

The length of the coax is irrelevant because the coax is a 50 Ohms
transnission line in between a 50 Ohms connection on the transceicver
end and (hopefully) 50 Ohms at the antenna feed point.

So nothing is reflected here (but what is reflected at the Antenna is
transmitted back to the Tx).

If the coax is longer attenuation comes into play but in a glider this
is not an issue.

George

ContestID67 wrote:

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

On 31 Jan 2007 08:00:31 -0800, "ContestID67"
wrote:

I have a question for this august body.

I found a link on the 1-26 associations web site talking about the
proper length of an antenna for operation at 123.3Mhz. This turns out
to be;

1/4 wave Length: 0.61 m or 23.95 in
3/4 wave Length: 1.82 m or 71.84 in

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

If the transmitter output circuit, the coax and the antenna all have
the same characteristic impedance, no reflections occur; the VSWR is
1; radiated power is maximized; and the length of the coax is
essentially immaterial.

If there is an impedance mismatch anywhere in the system, there will
be reflections and the VSWR will be more than 1. This will affect the
signal at least two ways:

(1) The impedance the coax presents to the transmitter will be
something other than its characteristic impedance, creating an
impedance mismatch at the output circuit. That will reduce the amount
of power transferred into the system to begin with, as well as
possibly overloading the output circuit.

The amount of this impedance mismatch will depend on the length of the
coax. The math gets interesting he as an extreme example, a 1/4
wavelength line shorted at one end looks like an open circuit from the
other, and if open at one end it looks like a short from the other!

(2) Every time a wave hits one end of the coax, some part of it gets
transferred and some gets reflected back to the other end, where the
same happens again. Some of the energy goes into the antenna the first
time, and some of it makes multiple trips.

If the coax were made of perfect conductors with perfect insulators,
this back-and-forth travel would be immaterial and all the power that
gets into the line (after being reduced by the impedance mismatch at
the transmitter) would eventually get radiated by the antenna. But
they don't sell perfect conductors and insulators at Radio Shack, so
every foot of coax represents measurable resistive losses.

rj

This brings to mind a clever circumvention of the FCC regulations on Family
Radio Service (FRS)tranceivers. The rules say that a FRS transmitter and
antenna must be integrated - i.e. handhelds are OK but not roof antennas.
Radio Shack brought out a transceiver/antenna unit with a very long mike
cord and all transceiver controls in the mike which meant that the
antenna/transceiver with a magnetic base could be on the roof of a car. It
met the rules and worked really well but the FCC frowned on the effort and
it was withdrawn from the market.

So, why not extend the idea to built-in units? Put a small tranceiver unit
in the antenna base and a separate control head in the panel. This
eliminates all the RF coax problems.

Not without problems of course. I'm sure it't more important in the GHz
bands than in the 2-meter bands. Mounting a tranceiver in the fin with the
antenna creates access and W&B issues but it looks solvable.

Bill Daniels


"Ralph Jones" wrote in message
...
On 31 Jan 2007 08:00:31 -0800, "ContestID67"
wrote:

I have a question for this august body.

I found a link on the 1-26 associations web site talking about the
proper length of an antenna for operation at 123.3Mhz. This turns out
to be;

1/4 wave Length: 0.61 m or 23.95 in
3/4 wave Length: 1.82 m or 71.84 in

I was curious about the length of the coax. I was under the
impression that to get the maximum power out of the antenna, that the
combined total length of the coax *AND* the antenna needs to be taken
into consideration and needs to be an even number of wavelengths.
That prevents power from reflecting at the tip of the antenna and then
back into the trasceiver. This can not only rob radiated power but
might also damage the transceiver. A VSWR meter is used to tune this
for maximum radiated power typically by adjusting the length of the
antenna.

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

If the transmitter output circuit, the coax and the antenna all have
the same characteristic impedance, no reflections occur; the VSWR is
1; radiated power is maximized; and the length of the coax is
essentially immaterial.

If there is an impedance mismatch anywhere in the system, there will
be reflections and the VSWR will be more than 1. This will affect the
signal at least two ways:

(1) The impedance the coax presents to the transmitter will be
something other than its characteristic impedance, creating an
impedance mismatch at the output circuit. That will reduce the amount
of power transferred into the system to begin with, as well as
possibly overloading the output circuit.

The amount of this impedance mismatch will depend on the length of the
coax. The math gets interesting he as an extreme example, a 1/4
wavelength line shorted at one end looks like an open circuit from the
other, and if open at one end it looks like a short from the other!

(2) Every time a wave hits one end of the coax, some part of it gets
transferred and some gets reflected back to the other end, where the
same happens again. Some of the energy goes into the antenna the first
time, and some of it makes multiple trips.

If the coax were made of perfect conductors with perfect insulators,
this back-and-forth travel would be immaterial and all the power that
gets into the line (after being reduced by the impedance mismatch at
the transmitter) would eventually get radiated by the antenna. But
they don't sell perfect conductors and insulators at Radio Shack, so
every foot of coax represents measurable resistive losses.

rj

My glider has a dipole antenna built into the rear edge of the rudder. How
do I tune the VSWR for this antenna. Cutting a chunk off is not possible.

John

John Scott wrote:
My glider has a dipole antenna built into the rear edge of the rudder. How
do I tune the VSWR for this antenna. Cutting a chunk off is not possible.

John

Rear edge? Odd, most gliders have them in the leading edge of the
vertical stabilizer.

Anyway, the bottom line is that you cannot change the length of the
antenna. We are all [retty much in the same boat. It would be great
if we could retune the antenna (change the length) for 123.3 (or
whatever frequency you use). This would optimize our power out.

However, this is not practical so we live with antennas that are
probably optimized only for the middle of the 118.000 - 136.975Mhz
range. What is the middle? 127.4875Mhz. This is at least pretty
close to where we usually transmit.

John Scott wrote:
My glider has a dipole antenna built into the rear edge of the rudder. How
do I tune the VSWR for this antenna. Cutting a chunk off is not possible.

What glider? Some of the old ones did have a tuning adjustment. First,
measure the VSWR at the output from the radio: less than 2 - you're
golden; over 3, it's worth some effort to discover the problem.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

It is an APIS. The antenna is in the trailing edge of the rudder due to the
fuselage, vertical stab, and most of the rudder being carbon fiber.

I do have an inexpensive VSWR meter. I also have a 50 ohm dummy load (left
over from thin wire ethernet days....). I'll check my meter with that.
I haven't installed my Microair radio in the glider yet. I did a check
after installing the wiring and connector using my handheld ICOM and the
VSWR was a little less than 2 then. Looks like I should be okay.

John