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

"ContestID67" wrote in
ups.com:

As others have pointed out, presuming your antenna is
"correct", its RF feed will have an impedance of 50ohms.
Since the coax is also 50ohms, there will be no
impedance mismatch, therefore no standing wave and VSWR=1.

In practice there will be some non-ideal effects:
- sharp bends in the coax will disturb its impedance
- the antenna will only be "correct" at a single
frequency. The antenna design and nearby objects
determine the frequency band over which it is
"correct enough"
In practice, presuming you have a narrowband signal
(say 5% of the carrier frequency) and the antenna
is for that carrier frequency, I doubt that you will
have much trouble.

If you have trouble, how might it manifest itself?
- the RF amp might blow up: that's your problem
- the RF amp might become unstable and start
transmitting on harmonic frequencies: that's
everybody else's problem, and the regulatory
authoratories can become unpleasantly interested
(for very good reasons)

Safest to:
- get the correct antenna
- measure the VSWR at the tx carrier frequency,
and check it is within the RF amp's limits
- ideally, use a spectrum analyser to check the
level of the 2nd and 3rd harmonics

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?

Unless you are using obsolete logic, you ought to know
about RF effects! Consider that modern logic has
sub-nanosecond rise and fall times, which is equivalent
to frequencies above 1GHz. At such frequencies, PCB
tracks are designed as RF transmission lines. Any more than
0.5" long have to be considered as transmission line
stubs which will set up standing waves which will corrupt
the digital signals. The result would be reduced operating
margins and intermittent pattern-sensitive malfunctions.

One very minor point. As the feedline length increases, the coax loss will
cause the indicated swr to improve. The short length of the coax used makes
this adjustment unimportant in most aircraft.

Colin

"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?


No. To get the most radiated power, the characteristic impedance of the
antenna must match the characteristic impedance of the feedline. If the two
impedances don't match, there will be a reflection of power from the ends of
the feedline, setting up standing waves.
If they're even reasonably close, the feedline length doesn't matter, other
than the slight losses from additional length.
In practice, you adjust the antenna to give a reasonably low SWR -- say,
below 1.5 to 1 or so, and you're done.
Probably more important than obsessing over SWR would be to make sure all
the connections are good, solid connections, keeping the feedline well
supported, and as short as is reasonable.

Tim Ward KD6UTW

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