Aviation Forum / General / Soaring / September 2008

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

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
--------------------------------------------------------------------

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/Product.php?productid=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).

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