Antenna Length Optimization Recommendations

Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas and
SWR meters can help me out. I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them. I
want to cut them for 123.4 MHz so they will work well at glider frequencies.
However, theoretically calculated antenna lengths don't usually agree with
ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4 MHz
and measure and record the SWR value of the antenna. Then I'll need to cut
a bit of the antenna off and see if the SWR value improves. I am completely
ignorant so I don't know if low or high SWR value is better. At some point
in the process the SWR value should level off. I'd hate to continue on
until the SWR value goes the other way - because that would be a waste of a
perfectly good antenna.

Any suggestions? Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

On Sep 24, 12:07*pm, "Paul Remde" wrote:
Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas and
SWR meters can help me out. *I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them. *I
want to cut them for 123.4 MHz so they will work well at glider frequencies.
However, theoretically calculated antenna lengths don't usually agree with
ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4 MHz
and measure and record the SWR value of the antenna. *Then I'll need to cut
a bit of the antenna off and see if the SWR value improves. *I am completely
ignorant so I don't know if low or high SWR value is better. *At some point
in the process the SWR value should level off. *I'd hate to continue on
until the SWR value goes the other way - because that would be a waste of a
perfectly good antenna.

Any suggestions? *Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.http://www.cumulus-soaring.com

A 5/8 wave antenna has slight gain over a quarter wave for vertically-
polarized signals, which is their attraction. They are pretty close
to 5/8 of a wavelength (about 1.5 meters or 5 feet long for 123.4
MHz). In order to match the impedance of the antenna to a 50-ohm
feedline, a small inductor is needed, usually at the base of the
antenna. Like a 1/4 wave antenna, a 5/8 also needs either a ground
plane or several radials (1/4 wave are normally used).

Standing Wave Ratio indicates the impedance mismatch between the
antenna, feedline and transmitter. It's not critical, but most
transmitters are happiest if this is less than 2. An SWR of 1.5 or
less is generally considered very good.

The length of the antenna for best match depends on the loading
inductor and ground plane. If your antenna's close, but a bit long,
set the antenna up where it is going to be used, transmit on the
desired frequency and snip gently. I built a bunch of these in my
amateur radio days.

Mike

On Sep 24, 12:07*pm, "Paul Remde" wrote:

...Any suggestions? *Is there an easier way to do this?

Just for the sake of posting something that Jim Weir can say is
wrongedy wrong because he's a EE and I'm not:

If you're talking about relatively simple whip antennas of relatively
small wire size, you can probably do fine using the theoretical value,
but breaking out the SWR is a good plan.

One thing you can do is SWR tune the antenna by trimming it to a
longer wavelength frequency, and use that to figure out a correction
factor that you can apply to the theoretical length for 123.4 MHz.
That way you can achieve a pretty high degree of confidence in your
tuning without ending up with a shorty. The easiest thing to do would
be to use the numerically lowest frequency your radio transmits on.

Getting back to fundamentals, for the theoretical wavelength, it's
sometimes reassuring to go back to first principles:

wavelength=speed of light/frequency

in our case, it's:

w=186000 miles per second / 123,400,000 cycles per second
= 0.0015072933549432739059967585089141 miles

Ok, so not many people have measuring tapes marked out in miles.
Multiply by 5280 for feet:

= 7.9585089141004862236628849270665

And then multiply by 12 for inches:

= 95.502106969205834683954619124797

That agrees pretty well with the 11800/f value that Weir suggests in
RST-802. Jim's number results in a slightly longer antenna, so it's
probably a better place to start since as you observe it's easier to
shorten than lengthen.

11800/123.4=95.62

Multiply that by the 5/8 wave of your whip:

95.62 * 5/8 = 59.76"

So consider maybe tuning for 118.0 Mhz:

11800/118 * 5/8 = 62.5"

And once you get the ideal length for 118 MHz apply that correction
factor to find the corresponding ideal length for 123.4 MHz.

Over to you, Jim!

Thanks, Bob K.

Paul,

The SWR should be as low as possible. Ideally 1:1 over the entire
aero-band. Which is pretty impossible for a 19 MHz span around 125
MHz.
If the SWR is high, an increasing part of the RF power sent to the
antenna is reflected back to the transmitter, and back again.
Depending on the quality and the length of the coaxial cable, more or
less power is lost in it each time the RF energy runs through the
cable, with less RF energy radiated by the antenna as a result.
Practically, considering the relative good quality of RG-58 coax at
these frequencies, and the short length of the cable between the
transceiver and the antenna, an SWR of, say 2:1 at some frequencies is
far from dramatic.

You should first plot the curve of SWR versus frequency for the
antenna as is. Put the antenna on some insulated pole, with no
metallic parts near the antenna. Connect the SWR bridge between the
antenna and the RF generator (an aero-handheld XCVR will do the job).
Measure the SWR for each Mhz. You will get some kind of a 'U'-shaped
curve, with the SWR increasing below and above the antenna's resonant
frequency.
If the lowest SWR-frequency is higher than 123.4, your antenna is too
short.
If the dip is on a lower frequency, you antenna must be clipped.
Cut no more than 10 mm at once, to start.
If you cut too much, getting a resonant frequency, say at 120 MHZ,
your antenna can still be used over the entire band, and nobody will
notice...

I used to do some curveplotting and there is a difference between a
glider on the ground and in flight, obviously due to ground proximity.
SWR stands for 'Standing Wave Ratio'. Broadly speaking, if the
caracteristic impedance of the output stage of the transmitter, the
coaxial cable, the connectors and the antenna are all the same (52 ohm
in most cases), you get a 'travelling wave' from the transmitter to
the antenna, and no part of the wave is reflected.
If on the other hand there is a impedance mismatch, the vectorial sum
of the incoming and reflected travelling waves results in a 'Standing
Wave'. The ratio between them is what your SWR-bridge measures and
calculates.

Does this answer your question?

Willy VINKEN - Cirrus OO-ZNY - Ham callsign: ON5WV
--------------------------------------------------------------------

On Wed, 24 Sep 2008 19:07:15 GMT, "Paul Remde" wrote:

Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas and
SWR meters can help me out. I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them. I
want to cut them for 123.4 MHz so they will work well at glider frequencies.
However, theoretically calculated antenna lengths don't usually agree with
ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4 MHz
and measure and record the SWR value of the antenna. Then I'll need to cut
a bit of the antenna off and see if the SWR value improves. I am completely
ignorant so I don't know if low or high SWR value is better. At some point
in the process the SWR value should level off. I'd hate to continue on
until the SWR value goes the other way - because that would be a waste of a
perfectly good antenna.

Any suggestions? Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

On Wed, 24 Sep 2008 23:32:08 +0200, Willy VINKEN
wrote:


Paul,

The SWR should be as low as possible. Ideally 1:1 over the entire
aero-band. Which is pretty impossible for a 19 MHz span around 125
MHz.

Make that WAY impossible...;-)

When my partners and I bought our LS-4, the radio output was
miserable, so I hauled it home and dived in. Turned out the seller,
while he was an A&P, must have learned about radio from a CB magazine.
He had set out to get the best possible signal the same way a CB'er
might do it, by adding an antenna tuner. Of course, there weren't any
antenna tuners in the Wings & Wheels catalog, so he used one meant for
the AMATEUR TWO-METER BAND!

Now that unit had a tuning capacitor to adjust it to the operating
freq in a band 4 MHz wide, and it was hooked up to a signal 20 MHz
below its lower end...you can guess the results. Other operators had
to fiddle with their squelch to hear it in the pattern. And this guy
had flown with it for several years.

Ralph NY0F

I think that what Paul is asking is..."I have a bunch of antennas
ready to sell. I want to pre-tune them to 123.4 so that my customers
can install them right away for optimal glider performance."
Hopefully this is correct and it is an admirable goal.

Paul - You may not be able to get there from here. Remember that
tuning for VSWR is from the connector at the radio end to the antenna
which includes the coax. I looked at your catalog and see that most
of your antennas (antennae?) are sold without coax. Thus if you tune/
trim the antenna with a piece of coax you had lying around, odds are
that it won't be right for your customer.

For those antenna that come with coax, you would assume that the
manufacturer cut the coax for the "right" length for the middle of the
aircraft band (you'd need to ask them). The middle appears to be
122.5Mhz (108-137mhz) which seems pretty darn close to what you want
to tune for. Then you have to worry that your customers will cut the
factory length of coax to make a nice neat installation.

So if you want to experiment, using a transmission source (your radio
presumably), a VSWR meter, and the manufactured antenna/coax combo,
you should be able to trim the antenna for maximum performance (as
close to 1:1 as possible). Let's say you were able to go from a VSWR
of 1.7:1 to 1.6:1. How much further can you now transmit? I have no
clue. I'm the type of EE called a "bit pusher", not an RF jock. Your
goal is making your tranmission go further, but It is nearly
impossible to predict how much further you will get by retuning short
of renting some very expensive facilities).

So you have to ask yourself, is your re-tuning in your office really
worth it to your customers? My answer would be no. My $0.02.

- John DeRosa

Hi John,

Good points. Thanks for the note. Thank you to everyone who sent the many
helpful notes.

The antennas I'm trying to pre-tune for my customers are magnet base
antennas (for use on a car) and a base station antenna for mounting to an RV
or truck. The base station antenna has 4 small antennas that spread out at
the base of the main antenna which probably act as the ground plane - so I
won't worry about a ground plane for that one. They will all ship with
coaxial cable and a BNC connector for connection to a handheld radio or
Dittel base station radio. I'm selling a quarter wave antenna and a 5/8
wave antenna.

I agree that 122.5 is directly in the middle of the aircraft radio frequency
band, but I think that most customers would prefer that the antenna be tuned
for 123.4 (between 123.3 and 123.5) - which would still give excellent
response at the nearby 122.8, 122.9 and 123.0 common airport frequencies -
and all other aircraft band frequencies. I would think that most car to
glider communication would be at 123.3 or 123.5.

The testing is necessary because the antennas are nowhere near the correct
length. The manufacturer of the 1/4 antenna gives a very nice length table
that I will use. But for the 5/8 wave antenna they don't have any
recommendations. I have talked to the manufacturer directly.

There have been many very helpful responses to my query. I appreciate them
all very much. A common note was that a coil is required on the 5/8 wave
antenna. I have that under control. I have all the right parts.

I think that if I can make the magnet base antennas work well on my car they
will work fine on any car. I understand that every car will be a little
different, but I believe they will be close enough.

I think I am now armed with enough knowledge to tune the antennas.

This newsgroup is a fantastic connection to a community of very
knowledgeable and helpful glider pilots!

Thank you,

Paul Remde


"ContestID67" wrote in message
...
I think that what Paul is asking is..."I have a bunch of antennas
ready to sell. I want to pre-tune them to 123.4 so that my customers
can install them right away for optimal glider performance."
Hopefully this is correct and it is an admirable goal.

Paul - You may not be able to get there from here. Remember that
tuning for VSWR is from the connector at the radio end to the antenna
which includes the coax. I looked at your catalog and see that most
of your antennas (antennae?) are sold without coax. Thus if you tune/
trim the antenna with a piece of coax you had lying around, odds are
that it won't be right for your customer.

For those antenna that come with coax, you would assume that the
manufacturer cut the coax for the "right" length for the middle of the
aircraft band (you'd need to ask them). The middle appears to be
122.5Mhz (108-137mhz) which seems pretty darn close to what you want
to tune for. Then you have to worry that your customers will cut the
factory length of coax to make a nice neat installation.

So if you want to experiment, using a transmission source (your radio
presumably), a VSWR meter, and the manufactured antenna/coax combo,
you should be able to trim the antenna for maximum performance (as
close to 1:1 as possible). Let's say you were able to go from a VSWR
of 1.7:1 to 1.6:1. How much further can you now transmit? I have no
clue. I'm the type of EE called a "bit pusher", not an RF jock. Your
goal is making your tranmission go further, but It is nearly
impossible to predict how much further you will get by retuning short
of renting some very expensive facilities).

So you have to ask yourself, is your re-tuning in your office really
worth it to your customers? My answer would be no. My $0.02.

- John DeRosa

At 19:07 24 September 2008, Paul Remde wrote:
Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas
and
SWR meters can help me out. I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them.
I
want to cut them for 123.4 MHz so they will work well at glider
frequencies.
However, theoretically calculated antenna lengths don't usually agree
with

ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4
MHz
and measure and record the SWR value of the antenna. Then I'll need to
cut
a bit of the antenna off and see if the SWR value improves. I am
completely
ignorant so I don't know if low or high SWR value is better. At some
point
in the process the SWR value should level off. I'd hate to continue on

until the SWR value goes the other way - because that would be a waste
of
a
perfectly good antenna.

Any suggestions? Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized
lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

Hi Paul,

The method you suggest is basically correct.

However, for many years now, I have used an "MFJ SWR Analyzer" and would
not want to go back to using a simple SWR meter (the link is:
http://www.mfjenterprises.com/Produc...uctid=MFJ-259B

For the manual, the link is:
http://www.mfjenterprises.com/man/pdf/MFJ-259B.pdf

With this you can find the frequency for which the antenna has the best
SWR. This tells you how far you are from the required frequency and if the
antenna is too long (optimum frequency is lower) or too short (optimum
frequency is higher).
--

Tim Newport-Peace >
Skype: specialist_systems
http://www.spsys.demon.co.uk/icom.htm

Hi Tim,

That testing device looks like an awesome recommendation. It will eliminate
the need to transmit on a variety of frequencies to find the frequency with
the lowest SWR. That will make it much easier to find the perfect length
for a desired frequency. Very nice! I have just ordered one.

Thanks!

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com


"Tim Newport-Peace" wrote in message
...
At 19:07 24 September 2008, Paul Remde wrote:
Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas
and
SWR meters can help me out. I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them.
I
want to cut them for 123.4 MHz so they will work well at glider
frequencies.
However, theoretically calculated antenna lengths don't usually agree
with

ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4
MHz
and measure and record the SWR value of the antenna. Then I'll need to
cut
a bit of the antenna off and see if the SWR value improves. I am
completely
ignorant so I don't know if low or high SWR value is better. At some
point
in the process the SWR value should level off. I'd hate to continue on

until the SWR value goes the other way - because that would be a waste
of
a
perfectly good antenna.

Any suggestions? Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized
lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

Hi Paul,

The method you suggest is basically correct.

However, for many years now, I have used an "MFJ SWR Analyzer" and would
not want to go back to using a simple SWR meter (the link is:
http://www.mfjenterprises.com/Produc...uctid=MFJ-259B

For the manual, the link is:
http://www.mfjenterprises.com/man/pdf/MFJ-259B.pdf

With this you can find the frequency for which the antenna has the best
SWR. This tells you how far you are from the required frequency and if the
antenna is too long (optimum frequency is lower) or too short (optimum
frequency is higher).
--

Tim Newport-Peace >
Skype: specialist_systems
http://www.spsys.demon.co.uk/icom.htm

On Sep 24, 12:07*pm, "Paul Remde" wrote:
Any suggestions? *Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized lengths.


It is best if the antenna is tuned when installed in the position it
will be used. I would suggest you sell the antennas long and let the
purchaser optimize them for their particular installation. Perhaps
include a suggested length for those that do not wish to tune them.

This is probably particularly important for magnetic base antennas as
the ground plane characteristics may be very different between
installations.

When I used a magnetic mount 5/8 base loaded antenna on my old Chevy
Malibu it was tuned for a specific location on the roof and it was
always mounted in that exact position.

Some good quality 1/4 and 5/8 base loaded antennas allow the length to
be adjusted and set without any cutting.

Andy