Today's Message Index:
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1. 06:27 AM - Re: Need Comm Troubleshooting Advice (Hopperdhh@aol.com)
2. 06:52 AM - Re: Need Comm Troubleshooting Advice (Perry, Phil)
3. 08:03 AM - Re: Need Comm Troubleshooting Advice (Noel Loveys)
4. 11:39 AM - Re: Need Comm Troubleshooting Advice (Noel Loveys)
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Subject: | Re: Need Comm Troubleshooting Advice |
A very good instrument for checking antenna SWR is the MFJ-259B. Many hams
have this instrument. It has a signal generator, frequency counter, and SWR
bridge in one compact battery operated unit.
Dan Hopper K9WEK
RV-7A
************************************** See what's free at http://www.aol.com.
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Subject: | Need Comm Troubleshooting Advice |
You're correct, John. 118 is the bottom and my mistake. No clue where I
got 112 from!
Thanks for pointing that out.
Phil
________________________________
From: John Rippengal [mailto:j.rippengal@cytanet.com.cy]
Sent: Wednesday, July 11, 2007 12:38 AM
Subject: Re: Avionics-List: Need Comm Troubleshooting Advice
Yes but Phil the low edge of the band is 118Mhz NOT 112 Mhz so the
centre frequrency is 127 not 124. Second I would most certainly not
bother tuning a broadband antenna. Assuming it is a professionally made
antenna or something like Jim Weir's tape antenna then there is no point
whatever in tuning it. Just check that the VSWR stays between 2:1 max
over the range - it should be closer to 1:1.5 over most of the band -
but the main idea is to check that there is no fault in the coax or
connectors in which case you would see figures way over the 2:1.
John
From: Perry, Phil <mailto:Phil.Perry@netapp.com>
I agree with John.
The bird meters are great but they're just a company who built
the watt/swr meters. They've been around for years and are not
manufactured any more. and are more of a novelty for hard-core radio
operators to own.
There are certainly many modern meters out there that are less
expensive and more available. The $20 radio shack model is fine.
Here's some a few tips that you can squirrel away for the next
time you need to do some tuning.
First, make sure the antenna is properly grounded to the
aircraft. Several people will ground the antenna to a painted surface,
this is a bad idea! The paint/primer acts as an insulator and the
antenna won't be able to balance the ground plane (aircraft) with the
transmitting element (antenna). If you don't have a good ground, you
will pull your hair out trying to tune the antenna and it won't work
very well anyway. Checking grounding is always a great place to start.
Next, you'll need to tune your antenna to the appropriate
length.
The frequency range determines the length of the antenna. The
lower the frequency; the longer the antenna. The higher the frequency;
the shorter the antenna. This is because low frequencies have longer
wave lengths and higher frequencies have shorter wave lengths. Since
we're dealing with frequencies in the VHF band (108Mhz - 137Mhz), we're
dealing with fairly short antenna lengths.
Keep in mind that an antenna is tuned EXACTLY for only one
frequency. As you deviate (up or down) from that frequency, the antenna
becomes out of tune. The further your deviation, the less in-tune the
antenna will be.
So you will want to tune for a frequency that is in the middle
of your transmit range. Since 108Mhz - 112Mhz are receive-only NAV
frequencies, we aren't too concerned in tuning for those frequencies.
Since 112Mhz - 136Mhz is the range of communication frequencies that we
transmit on, you want to tune for those. So pick a frequency in the
middle and make that the frequency you tune for. In this case, 124Mhz
is exactly in the middle of 108Mhz and 136Mhz. If you tune for 124Mhz,
the antenna will be a perfect match at 124Mhz and only slightly off at
the extreme edges of your transmitting frequency range 112Mhz and
136Mhz.
Let's stop right here for a minute and talk about Standing Wave
Ratios or SWR.
When a transmitter is keyed, a signal is obviously transmitted
down the coax and ultimately destined for the antenna to broadcast. In
a perfect world (or a perfectly tuned antenna) all of the signal power
from the transmitter is radiated by the antenna. So if you have a
transmitter that is radiating 25watts of signal power, all 25 watts exit
the antenna and is radiated out in space for others to hear.
But what happens if the antenna isn't tuned perfectly? Actually
the signal will traverse the coax and reach the antenna. But since the
antenna isn't the appropriate length only a portion of the wave (signal)
will be radiated. The portion that isn't radiated is reflected back
down the coax and toward the transmitter. (YIKES!) Transmitters are
made to transmit 25 watt signals, not have them blasted back down the
coax like a fire hose.
The ratio of signal that is radiated by the transmitter vs the
amount reflected by the antenna is referred to as the Standing Wave
Ratio (SWR). In the perfect situation your standing wave ratio should
be 1 to 1 (presented as 1:1). This means that all of your power is
being radiated and there is no reflective wave coming back to the
transmitter.
So it's important to tune for the best SWR because: 1) You want
as much of your signal to leave the antenna and be radiated for the rest
of the world to receive and 2) Excessive reflected waves WILL RUIN your
transmitter.
So lets tune our antenna. Install it like the manufacture
suggests and put a SWR/Watt meter inline with the coax. (The closer to
the transmitter, the better)
If you have a low-power option on your radio, select it. The
least amount of power you can use, the better. This will keep the
reflected signals to a minimum, while the antenna still isn't correctly
tuned. Not to mention you'll be less likely to interrupt a frequency
that's in use.
Dial up 124Mhz and make sure the frequency isn't in use! If it
is, just change to another unused frequency nearby.
Now it's time to check the SWR. Key the transmitter and check
your reading on the meter. (You'll need to follow the directions that
came with your meter on how to set it up. I can't go through the setup
procedure for your meter, but I promise. It's not complicated!)
Measure your SWR. If you see 1:1, you're lucky! More than
likely, you won't. You might see something like 1.7:1 or 2:1 or worse.
So how do you figure out if you have to lengthen or shorten your
antenna? It's pretty simple, trial and error.
Most antennas have a set screw that you can loose and adjust
then antenna length. First try lengthening the antenna by 1/4 inch and
re-checking your SWR. Did it improve or did it get worse? If it got
worse, try the other direction and shorten the antenna by 1/4 inch. If
it got better, try lengthening it a hair more.
It's a good idea to keep track of each adjustment on a piece of
paper, so you can get an idea of how each change is affecting your SWR.
This will help you zero in on that perfect length. As you get closer
and closer to 1:1, you're adjustments need to be refined with smaller
increments.
Don't be surprised is you can't get exactly 1:1. The
transmitted waves can be reflected back toward the antenna by metal
(such as your empennage). So when you tune your antenna, get it away
from metal buildings or other structures and tune for the best available
SWR at 124Mhz. You want the signal to be as unaffected as possible.
If you can get it down to 1.1:1 or 1.2:1, you're doing great! I
wouldn't be happy with anything over 1.5:1. 1.5 is my personal minimum
and it's easy to obtain.
Once you think you've tuned your antenna for 124 Mhz, you need
to check the extreme edges of your transmit range.
Dial down to 112 Mhz. Once again, make sure you won't be
interrupting anyone. Check your SWR.
It will be higher than it was at 124 Mhz, because at 112Mhz the
antenna is slightly out of tune. But if your inside 1.5:1, you're good.
I'd be okay with 1.7:1 on the extreme edges.
Next dial up 136Mhz and give it a shot. Once again, make sure
no one is using the frequency. I'm okay with 1.7ish:1 up there too.
Antenna problems really aren't that complex or difficult to
tune. A little practice and you'll have it in no time.
A couple of other things.
1) Make sure you use the correct coax. Usually 50 Ohm and don't
buy the cheap stuff!! Radio shack and other retailers have cheap coax.
I am a fan of Belden RG58-A/U coax. It has relatively low loss at the
cable lengths required for an airplane, the center conductor is braided
(important for high vibration installations), it has a very good braided
external conductor (Most cheap cables will skimp here), and it has a
solid inner-core insulator. Nearly any Amateur Radio (Ham) store will
carry Belden Cable. If you can't find it, I know aesham.com carries it.
The coax is the only thing between your transmitter and your
antenna, make sure you use good cable! It's worth a couple of extra
dollars.
2) If you suspect a cable issue or short in the cable,
disconnect the cable from both the transmitter and the antenna. Use a
continuity meter and check to make sure you have continuity through the
center conductor. Same thing for the external braided conductor, check
it.
If you've got continuity all the way through, you've eliminated
the possibility of a broken wire.
Next check and see if you have continuity from the center
conductor to the external conductor. If you do, there's a problem.
Either your cable has a short or you've got a connector that isn't
soldered on properly. Check the connectors first be re-soldering them.
If that doesn't fix it, you need a new cable.
Finally, if you're still having problems, check your connectors.
Make sure the solder joints are good. Your continuity can be fine, but
if you've got a cold solder joint, that can cause problems.
Please don't use the cheap retail connectors, they're junk. Get
good quality connectors. If you can find silver connectors, they are
the best and are the easiest for solder to stick to. I use the ones
made by Amphenol. If you're gonna get the good coax, make sure you get
the good connectors too! Most places that sell Belden also sells
Amphenol connectors. I know aesham.com has them too, but it's been
several years since I've had to place and order with them. I'm sure
they still carry both brands.
Anyway, that's all I can think of. Hopefully you'll tuck this
stuff away somewhere and use it one day. Otherwise,the delete button
looks like a red "X" at the top of the screen. :)
Take care,
Phil
________________________________
From: John Rippengal [mailto:j.rippengal@cytanet.com.cy]
Sent: Tuesday, July 03, 2007 2:00 PM
To: avionics-list@matronics.com
Subject: Re: Avionics-List: Need Comm Troubleshooting Advice
Glad it seems to be ok now Roger. An aircraft standing around
for quite a time while a new avionics pack is being fitted is a good
clue to there being a flat battery. However the other advice given to
you to check with a VSWR meter is a good one. The Bird Thru' Line meter
is a sort of Rolls Royce professional type and you really don't need
that for a good check on your antenna/coax/connector setup. Try Radio
Shack or similar for a cheap VSWR meter that covers the 2 metre amateur
band. Should only cost about $20 or less and every home installer of
avionics should have one. It will give you confidence in the antenna
installation together with the feeders and connectors.
John
From: Roger Roy <mailto:Savannah174@msn.com>
Hi John,
How's the avionics guru? I did as you suggested and put
my battery charger on the battery and all seem to workload and clear. I
was unable to had another person TX with the portable I will check this
out tomorrow, again thank you, Cheers
RJ
From: John Rippengal
<mailto:j.rippengal@cytanet.com.cy>
Roger,
What are the battery volts when you switch the
equipment on?? The fault is common to both comms so suspect something
common like the battery. You should check the equipment with the battery
on charge and a battery voltage of about 14v WITH the equipment switched
on.
John
href="http://www.matronics.com/Navigator?Avionics-List">http://www.matr
o
nics.com/Navigator?Avionics-List
href="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/Navigator?Avionics-List">http://www.matr
o
nics.com/Navigator?Avionics-List
href="http://forums.matronics.com">http://forums.matronics.com
Message 3
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Subject: | Need Comm Troubleshooting Advice |
Well written Phil.
Two additions for the totally uninitiated.
1 The grounding Phil was talking about is not the centre radiator of the
antenna but the ground from the coaxial braid. Even a small bit of
corrosion under that connection can drive your SWR into the
stratosphere.
2 Under an antenna mounted on a cloth plane ( on one of the frame tubes
) it
is a good idea to install a nice foil ground plane. I like to see a
circle
of foil where the radius is equal to the height of the radiator ( centre
radiating post )
I don't have a Bird but if any one is going to junk one...... They are
a
lot lighter than lugging around a Cushman ;-)
Noel
-----Original Message-----
From: owner-avionics-list-server@matronics.com
[mailto:owner-avionics-list-server@matronics.com] On Behalf Of Perry,
Phil
Sent: Tuesday, July 10, 2007 4:04 PM
Subject: RE: Avionics-List: Need Comm Troubleshooting Advice
I agree with John.
The bird meters are great but they're just a company who built the
watt/swr
meters. They've been around for years and are not manufactured any more.
and are more of a novelty for hard-core radio operators to own.
There are certainly many modern meters out there that are less expensive
and
more available. The $20 radio shack model is fine.
Here's some a few tips that you can squirrel away for the next time you
need
to do some tuning.
First, make sure the antenna is properly grounded to the aircraft.
Several
people will ground the antenna to a painted surface, this is a bad idea!
The paint/primer acts as an insulator and the antenna won't be able to
balance the ground plane (aircraft) with the transmitting element
(antenna).
If you don't have a good ground, you will pull your hair out trying to
tune
the antenna and it won't work very well anyway. Checking grounding is
always a great place to start.
Next, you'll need to tune your antenna to the appropriate length.
The frequency range determines the length of the antenna. The lower the
frequency; the longer the antenna. The higher the frequency; the
shorter
the antenna. This is because low frequencies have longer wave lengths
and
higher frequencies have shorter wave lengths. Since we're dealing with
frequencies in the VHF band (108Mhz - 137Mhz), we're dealing with fairly
short antenna lengths.
Keep in mind that an antenna is tuned EXACTLY for only one frequency.
As
you deviate (up or down) from that frequency, the antenna becomes out of
tune. The further your deviation, the less in-tune the antenna will be.
So you will want to tune for a frequency that is in the middle of your
transmit range. Since 108Mhz - 112Mhz are receive-only NAV frequencies,
we
aren't too concerned in tuning for those frequencies. Since 112Mhz -
136Mhz
is the range of communication frequencies that we transmit on, you want
to
tune for those. So pick a frequency in the middle and make that the
frequency you tune for. In this case, 124Mhz is exactly in the middle
of
108Mhz and 136Mhz. If you tune for 124Mhz, the antenna will be a
perfect
match at 124Mhz and only slightly off at the extreme edges of your
transmitting frequency range 112Mhz and 136Mhz.
Let's stop right here for a minute and talk about Standing Wave Ratios
or
SWR.
When a transmitter is keyed, a signal is obviously transmitted down the
coax
and ultimately destined for the antenna to broadcast. In a perfect
world
(or a perfectly tuned antenna) all of the signal power from the
transmitter
is radiated by the antenna. So if you have a transmitter that is
radiating
25watts of signal power, all 25 watts exit the antenna and is radiated
out
in space for others to hear.
But what happens if the antenna isn't tuned perfectly? Actually the
signal
will traverse the coax and reach the antenna. But since the antenna
isn't
the appropriate length only a portion of the wave (signal) will be
radiated.
The portion that isn't radiated is reflected back down the coax and
toward
the transmitter. (YIKES!) Transmitters are made to transmit 25 watt
signals, not have them blasted back down the coax like a fire hose.
The ratio of signal that is radiated by the transmitter vs the amount
reflected by the antenna is referred to as the Standing Wave Ratio
(SWR).
In the perfect situation your standing wave ratio should be 1 to 1
(presented as 1:1). This means that all of your power is being radiated
and
there is no reflective wave coming back to the transmitter.
So it's important to tune for the best SWR because: 1) You want as much
of
your signal to leave the antenna and be radiated for the rest of the
world
to receive and 2) Excessive reflected waves WILL RUIN your transmitter.
So lets tune our antenna. Install it like the manufacture suggests and
put
a SWR/Watt meter inline with the coax. (The closer to the transmitter,
the
better)
If you have a low-power option on your radio, select it. The least
amount
of power you can use, the better. This will keep the reflected signals
to a
minimum, while the antenna still isn't correctly tuned. Not to mention
you'll be less likely to interrupt a frequency that's in use.
Dial up 124Mhz and make sure the frequency isn't in use! If it is, just
change to another unused frequency nearby.
Now it's time to check the SWR. Key the transmitter and check your
reading
on the meter. (You'll need to follow the directions that came with your
meter on how to set it up. I can't go through the setup procedure for
your
meter, but I promise. It's not complicated!)
Measure your SWR. If you see 1:1, you're lucky! More than likely, you
won't. You might see something like 1.7:1 or 2:1 or worse.
So how do you figure out if you have to lengthen or shorten your
antenna?
It's pretty simple, trial and error.
Most antennas have a set screw that you can loose and adjust then
antenna
length. First try lengthening the antenna by 1/4 inch and re-checking
your
SWR. Did it improve or did it get worse? If it got worse, try the other
direction and shorten the antenna by 1/4 inch. If it got better, try
lengthening it a hair more.
It's a good idea to keep track of each adjustment on a piece of paper,
so
you can get an idea of how each change is affecting your SWR. This will
help you zero in on that perfect length. As you get closer and closer
to
1:1, you're adjustments need to be refined with smaller increments.
Don't be surprised is you can't get exactly 1:1. The transmitted waves
can
be reflected back toward the antenna by metal (such as your empennage).
So
when you tune your antenna, get it away from metal buildings or other
structures and tune for the best available SWR at 124Mhz. You want the
signal to be as unaffected as possible.
If you can get it down to 1.1:1 or 1.2:1, you're doing great! I
wouldn't be
happy with anything over 1.5:1. 1.5 is my personal minimum and it's
easy to
obtain.
Once you think you've tuned your antenna for 124 Mhz, you need to check
the
extreme edges of your transmit range.
Dial down to 112 Mhz. Once again, make sure you won't be interrupting
anyone. Check your SWR.
It will be higher than it was at 124 Mhz, because at 112Mhz the antenna
is
slightly out of tune. But if your inside 1.5:1, you're good. I'd be
okay
with 1.7:1 on the extreme edges.
Next dial up 136Mhz and give it a shot. Once again, make sure no one is
using the frequency. I'm okay with 1.7ish:1 up there too.
Antenna problems really aren't that complex or difficult to tune. A
little
practice and you'll have it in no time.
A couple of other things.
1) Make sure you use the correct coax. Usually 50 Ohm and don't buy the
cheap stuff!! Radio shack and other retailers have cheap coax. I am a
fan
of Belden RG58-A/U coax. It has relatively low loss at the cable
lengths
required for an airplane, the center conductor is braided (important for
high vibration installations), it has a very good braided external
conductor
(Most cheap cables will skimp here), and it has a solid inner-core
insulator. Nearly any Amateur Radio (Ham) store will carry Belden
Cable.
If you can't find it, I know aesham.com carries it.
The coax is the only thing between your transmitter and your antenna,
make
sure you use good cable! It's worth a couple of extra dollars.
2) If you suspect a cable issue or short in the cable, disconnect the
cable
from both the transmitter and the antenna. Use a continuity meter and
check
to make sure you have continuity through the center conductor. Same
thing
for the external braided conductor, check it.
If you've got continuity all the way through, you've eliminated the
possibility of a broken wire.
Next check and see if you have continuity from the center conductor to
the
external conductor. If you do, there's a problem. Either your cable
has a
short or you've got a connector that isn't soldered on properly. Check
the
connectors first be re-soldering them. If that doesn't fix it, you need
a
new cable.
Finally, if you're still having problems, check your connectors. Make
sure
the solder joints are good. Your continuity can be fine, but if you've
got
a cold solder joint, that can cause problems.
Please don't use the cheap retail connectors, they're junk. Get good
quality connectors. If you can find silver connectors, they are the
best
and are the easiest for solder to stick to. I use the ones made by
Amphenol. If you're gonna get the good coax, make sure you get the good
connectors too! Most places that sell Belden also sells Amphenol
connectors. I know aesham.com has them too, but it's been several years
since I've had to place and order with them. I'm sure they still carry
both
brands.
Anyway, that's all I can think of. Hopefully you'll tuck this stuff
away
somewhere and use it one day. Otherwise,the delete button looks like a
red
"X" at the top of the screen. :)
Take care,
Phil
_____
From: John Rippengal [mailto:j.rippengal@cytanet.com.cy]
Sent: Tuesday, July 03, 2007 2:00 PM
Subject: Re: Avionics-List: Need Comm Troubleshooting Advice
Glad it seems to be ok now Roger. An aircraft standing around for quite
a
time while a new avionics pack is being fitted is a good clue to there
being
a flat battery. However the other advice given to you to check with a
VSWR
meter is a good one. The Bird Thru' Line meter is a sort of Rolls Royce
professional type and you really don't need that for a good check on
your
antenna/coax/connector setup. Try Radio Shack or similar for a cheap
VSWR
meter that covers the 2 metre amateur band. Should only cost about $20
or
less and every home installer of avionics should have one. It will give
you
confidence in the antenna installation together with the feeders and
connectors.
John
From: Roger Roy <mailto:Savannah174@msn.com>
Hi John,
How's the avionics guru? I did as you suggested and put my battery
charger
on the battery and all seem to workload and clear. I was unable to had
another person TX with the portable I will check this out tomorrow,
again
thank you, Cheers
RJ
From: John Rippengal <mailto:j.rippengal@cytanet.com.cy>
Roger,
What are the battery volts when you switch the equipment on?? The fault
is
common to both comms so suspect something common like the battery. You
should check the equipment with the battery on charge and a battery
voltage
of about 14v WITH the equipment switched on.
John
href="http://www.matronics.com/Navigator?Avionics-List">http://www.matr
onics
.com/Navigator?Avionics-List
href="http://forums.matronics.com">http://forums.matronics.com
Message 4
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Subject: | Need Comm Troubleshooting Advice |
you also check the VSWR for corrosion at the ground plane. All things
being
equal a new antenna should be almost flat across the band. Generally
aircraft coming in with TX problems are well corroded at the antenna and
radio ends of the Coax. Depending on the condition of the coax ( its
wet
and salt here) the connectors are cleaned and the coax may be replaced.
The
VSWR meter will tell all.
BTW I never tune for the centre of the band ( older wire antennas ) I
always
tune for .39 the band (.61 top down). = 125.5mHz. Some antennas are
used
for the full VHF band (e.g. small nav/com) there fore the antenna should
cover the whole band, nav and com, 108 to 137 MHz. for that I tune for
119.5mHz. centre.
Here in Canada the amateurs are allowed to run a full Kw on all bands,
Exception is some 160Meters in maritime locations, One learns very fast
that
line of site 5W is more than enough power for aircraft..... in many
instances 1W is more than enough.
Still waiting for somebody to donate a nice Bird to the cause. :-)
Noel
-----Original Message-----
From: owner-avionics-list-server@matronics.com
[mailto:owner-avionics-list-server@matronics.com] On Behalf Of John
Rippengal
Sent: Wednesday, July 11, 2007 3:08 AM
Subject: Re: Avionics-List: Need Comm Troubleshooting Advice
Yes but Phil the low edge of the band is 118Mhz NOT 112 Mhz so the
centre
frequrency is 127 not 124. Second I would most certainly not bother
tuning a
broadband antenna. Assuming it is a professionally made antenna or
something
like Jim Weir's tape antenna then there is no point whatever in tuning
it.
Just check that the VSWR stays between 2:1 max over the range - it
should be
closer to 1:1.5 over most of the band - but the main idea is to check
that
there is no fault in the coax or connectors in which case you would see
figures way over the 2:1.
John
From: Perry, <mailto:Phil.Perry@netapp.com> Phil
I agree with John.
The bird meters are great but they're just a company who built the
watt/swr
meters. They've been around for years and are not manufactured any more.
and are more of a novelty for hard-core radio operators to own.
There are certainly many modern meters out there that are less expensive
and
more available. The $20 radio shack model is fine.
Here's some a few tips that you can squirrel away for the next time you
need
to do some tuning.
First, make sure the antenna is properly grounded to the aircraft.
Several
people will ground the antenna to a painted surface, this is a bad idea!
The paint/primer acts as an insulator and the antenna won't be able to
balance the ground plane (aircraft) with the transmitting element
(antenna).
If you don't have a good ground, you will pull your hair out trying to
tune
the antenna and it won't work very well anyway. Checking grounding is
always a great place to start.
Next, you'll need to tune your antenna to the appropriate length.
The frequency range determines the length of the antenna. The lower the
frequency; the longer the antenna. The higher the frequency; the
shorter
the antenna. This is because low frequencies have longer wave lengths
and
higher frequencies have shorter wave lengths. Since we're dealing with
frequencies in the VHF band (108Mhz - 137Mhz), we're dealing with fairly
short antenna lengths.
Keep in mind that an antenna is tuned EXACTLY for only one frequency.
As
you deviate (up or down) from that frequency, the antenna becomes out of
tune. The further your deviation, the less in-tune the antenna will be.
So you will want to tune for a frequency that is in the middle of your
transmit range. Since 108Mhz - 112Mhz are receive-only NAV frequencies,
we
aren't too concerned in tuning for those frequencies. Since 112Mhz -
136Mhz
is the range of communication frequencies that we transmit on, you want
to
tune for those. So pick a frequency in the middle and make that the
frequency you tune for. In this case, 124Mhz is exactly in the middle
of
108Mhz and 136Mhz. If you tune for 124Mhz, the antenna will be a
perfect
match at 124Mhz and only slightly off at the extreme edges of your
transmitting frequency range 112Mhz and 136Mhz.
Let's stop right here for a minute and talk about Standing Wave Ratios
or
SWR.
When a transmitter is keyed, a signal is obviously transmitted down the
coax
and ultimately destined for the antenna to broadcast. In a perfect
world
(or a perfectly tuned antenna) all of the signal power from the
transmitter
is radiated by the antenna. So if you have a transmitter that is
radiating
25watts of signal power, all 25 watts exit the antenna and is radiated
out
in space for others to hear.
But what happens if the antenna isn't tuned perfectly? Actually the
signal
will traverse the coax and reach the antenna. But since the antenna
isn't
the appropriate length only a portion of the wave (signal) will be
radiated.
The portion that isn't radiated is reflected back down the coax and
toward
the transmitter. (YIKES!) Transmitters are made to transmit 25 watt
signals, not have them blasted back down the coax like a fire hose.
The ratio of signal that is radiated by the transmitter vs the amount
reflected by the antenna is referred to as the Standing Wave Ratio
(SWR).
In the perfect situation your standing wave ratio should be 1 to 1
(presented as 1:1). This means that all of your power is being radiated
and
there is no reflective wave coming back to the transmitter.
So it's important to tune for the best SWR because: 1) You want as much
of
your signal to leave the antenna and be radiated for the rest of the
world
to receive and 2) Excessive reflected waves WILL RUIN your transmitter.
So lets tune our antenna. Install it like the manufacture suggests and
put
a SWR/Watt meter inline with the coax. (The closer to the transmitter,
the
better)
If you have a low-power option on your radio, select it. The least
amount
of power you can use, the better. This will keep the reflected signals
to a
minimum, while the antenna still isn't correctly tuned. Not to mention
you'll be less likely to interrupt a frequency that's in use.
Dial up 124Mhz and make sure the frequency isn't in use! If it is, just
change to another unused frequency nearby.
Now it's time to check the SWR. Key the transmitter and check your
reading
on the meter. (You'll need to follow the directions that came with your
meter on how to set it up. I can't go through the setup procedure for
your
meter, but I promise. It's not complicated!)
Measure your SWR. If you see 1:1, you're lucky! More than likely, you
won't. You might see something like 1.7:1 or 2:1 or worse.
So how do you figure out if you have to lengthen or shorten your
antenna?
It's pretty simple, trial and error.
Most antennas have a set screw that you can loose and adjust then
antenna
length. First try lengthening the antenna by 1/4 inch and re-checking
your
SWR. Did it improve or did it get worse? If it got worse, try the other
direction and shorten the antenna by 1/4 inch. If it got better, try
lengthening it a hair more.
It's a good idea to keep track of each adjustment on a piece of paper,
so
you can get an idea of how each change is affecting your SWR. This will
help you zero in on that perfect length. As you get closer and closer
to
1:1, you're adjustments need to be refined with smaller increments.
Don't be surprised is you can't get exactly 1:1. The transmitted waves
can
be reflected back toward the antenna by metal (such as your empennage).
So
when you tune your antenna, get it away from metal buildings or other
structures and tune for the best available SWR at 124Mhz. You want the
signal to be as unaffected as possible.
If you can get it down to 1.1:1 or 1.2:1, you're doing great! I
wouldn't be
happy with anything over 1.5:1. 1.5 is my personal minimum and it's
easy to
obtain.
Once you think you've tuned your antenna for 124 Mhz, you need to check
the
extreme edges of your transmit range.
Dial down to 112 Mhz. Once again, make sure you won't be interrupting
anyone. Check your SWR.
It will be higher than it was at 124 Mhz, because at 112Mhz the antenna
is
slightly out of tune. But if your inside 1.5:1, you're good. I'd be
okay
with 1.7:1 on the extreme edges.
Next dial up 136Mhz and give it a shot. Once again, make sure no one is
using the frequency. I'm okay with 1.7ish:1 up there too.
Antenna problems really aren't that complex or difficult to tune. A
little
practice and you'll have it in no time.
A couple of other things.
1) Make sure you use the correct coax. Usually 50 Ohm and don't buy the
cheap stuff!! Radio shack and other retailers have cheap coax. I am a
fan
of Belden RG58-A/U coax. It has relatively low loss at the cable
lengths
required for an airplane, the center conductor is braided (important for
high vibration installations), it has a very good braided external
conductor
(Most cheap cables will skimp here), and it has a solid inner-core
insulator. Nearly any Amateur Radio (Ham) store will carry Belden
Cable.
If you can't find it, I know aesham.com carries it.
The coax is the only thing between your transmitter and your antenna,
make
sure you use good cable! It's worth a couple of extra dollars.
2) If you suspect a cable issue or short in the cable, disconnect the
cable
from both the transmitter and the antenna. Use a continuity meter and
check
to make sure you have continuity through the center conductor. Same
thing
for the external braided conductor, check it.
If you've got continuity all the way through, you've eliminated the
possibility of a broken wire.
Next check and see if you have continuity from the center conductor to
the
external conductor. If you do, there's a problem. Either your cable
has a
short or you've got a connector that isn't soldered on properly. Check
the
connectors first be re-soldering them. If that doesn't fix it, you need
a
new cable.
Finally, if you're still having problems, check your connectors. Make
sure
the solder joints are good. Your continuity can be fine, but if you've
got
a cold solder joint, that can cause problems.
Please don't use the cheap retail connectors, they're junk. Get good
quality connectors. If you can find silver connectors, they are the
best
and are the easiest for solder to stick to. I use the ones made by
Amphenol. If you're gonna get the good coax, make sure you get the good
connectors too! Most places that sell Belden also sells Amphenol
connectors. I know aesham.com has them too, but it's been several years
since I've had to place and order with them. I'm sure they still carry
both
brands.
Anyway, that's all I can think of. Hopefully you'll tuck this stuff
away
somewhere and use it one day. Otherwise,the delete button looks like a
red
"X" at the top of the screen. :)
Take care,
Phil
_____
From: John Rippengal [mailto:j.rippengal@cytanet.com.cy]
Sent: Tuesday, July 03, 2007 2:00 PM
Subject: Re: Avionics-List: Need Comm Troubleshooting Advice
Glad it seems to be ok now Roger. An aircraft standing around for quite
a
time while a new avionics pack is being fitted is a good clue to there
being
a flat battery. However the other advice given to you to check with a
VSWR
meter is a good one. The Bird Thru' Line meter is a sort of Rolls Royce
professional type and you really don't need that for a good check on
your
antenna/coax/connector setup. Try Radio Shack or similar for a cheap
VSWR
meter that covers the 2 metre amateur band. Should only cost about $20
or
less and every home installer of avionics should have one. It will give
you
confidence in the antenna installation together with the feeders and
connectors.
John
From: Roger <mailto:Savannah174@msn.com> Roy
Hi John,
How's the avionics guru? I did as you suggested and put my battery
charger
on the battery and all seem to workload and clear. I was unable to had
another person TX with the portable I will check this out tomorrow,
again
thank you, Cheers
RJ
From: John Rippengal <mailto:j.rippengal@cytanet.com.cy>
Roger,
What are the battery volts when you switch the equipment on?? The fault
is
common to both comms so suspect something common like the battery. You
should check the equipment with the battery on charge and a battery
voltage
of about 14v WITH the equipment switched on.
John
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