Antenna and Coax Length

Hi John:

Remember that your dummy load may not have the power rating of your
Microair. I think your Microair is rated at 3 watts carrier and your dummy
load may only be 1 watt. That will still work - but keep the test
transmissions short - enough to get a useful reading. 10 second or so
transmissions should be fine.

Colin

Generally, close is good enough. As Eric says, under 2:1 is probably fine.
A number of ham operators have impedance bridges or you can use a reflected
power meter. However, before cussing at your antenna, be sure the meter is
designed for the frequency range and is of good quality. The cb meters will
not give you reliable readings here.

If your swr is bad, or you are having communication problems, the connector
is more likely the problem than the antenna. I have helped out on a number
of installations and in almost every case the antenna failure is in one of
the connectors.

Remember, also, that if you place a metal object or wire in the close
proximity of the antenna, it may affect the resonance.

Colin

COLIN LAMB wrote:
Generally, close is good enough. As Eric says, under 2:1 is probably fine.
A number of ham operators have impedance bridges or you can use a reflected
power meter. However, before cussing at your antenna, be sure the meter is
designed for the frequency range and is of good quality. The cb meters will
not give you reliable readings here.

It's measuring a ratio, not an absolute quantity, so they work just
fine. Sure, the response is lower, but you calibrate the meter each time
you use it, and that compensates for the response difference as well as
the output power of the transmitter. That was the conclusion of a long
thread on this a year or two ago. Radio Shack has them cheap enough, as
do other places, so every pilot can own one, or know a pilot that does.

--
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

Although inexpensive bridges calibrated for one frequency or band can be
useful at a different frequency for comparative standing wave ratio
measurement, I have found enough unreliable instruments to not trust them.
If you compare them in the aviation band with a known meter, that is one
thing - but trusting them off the shelf may give you a false reading. Many
of the meters must zero the reading into a 50 ohm load at the frequency in
question. That zero can vary with frequency.

You can generally check the usefulness of the meter by doing two things.
First, test the meter with a known 50 ohm dummy load at the frequency in
question. It should read zero reflected power. Note the forward power or
ratio. Then, reverse the leads. The same readings should be observed with
the meter readings reversed. If not, the meter is not going to tell you the
truth.

Some excellent meters, like the Bird wattmeter, will work fine as an swr
indicator far outside the design frequency, even though the power indication
is off.

Colin

"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