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Tuning Mobile One Antennas

Only Mobile One, gives the after sales support you need! so why buy any other brand?
Although each Mobile One Antenna is made to frequency at the factory, we, nor anyone else, can simulate
all installation possibilities. Therefore most  antennas unless stated  otherwise may need slight adjustments.
The addition of springs  or adaptors may make your antenna  electrically longer  so tuning may become
necessary. Take all SWR measurements with the antenna in it's final mounting position include all accessories
then adjust if necessary.

Mobile One UHF Antenna Range
(S474, S474B, M474, RF470, FDW470, BNCHW, COL6M, ETC)
All Mobile One UHF Antennas do not need to be tuned they are extremely broad band and will have a SWR
below 1.5 to 1 if they are installed correctly.

Mobile One Pretuned Antenna Range
(DX125CD, DX140CD, DX148CD, DX160CD, DX170CD)
These antennas have been made with 1/4" constant diameter fibreglass rods that are identical in diameter,
and have been wound using a computer operated winding machine that ensures each one has been made
identically to the previous one. The helical wind in these antennas is designed to be as broad band as
possible, the end result is these antennas will work with a SWR below 1.5 to 1 on almost any position on
the vehicle as long as they have been installed properly with sufficient ground plane area.

The Base, Connector & Cable Assembly
The Base should always be suitable for the frequency it is to operate on, for example, with the frequency
range 0~100MHz any good base is suitable (Base A, Base UL,Base BSS, Dipole Base,Sam Base, etc)
However the frequency range of 100~1200MHz is sensitive to loss so it's very important that you use
a base that is suitable such as (Base UL, Dipole Base etc) If in doubt shout
     
 The Cable should always be a multiple of 1/2 wave length of the frequency ie: for 27MHz the length
should be either 3.6m, 7.2m, 10.8m or 14.4m etc. In the case of UHF CB it is not important as the
vast majority of UHF antennas are broad band and do not need to be tuned.

The Connector is usually a PL259 or a BNC. In either case it is important that you ensure that there is
proper connectivity and no shorts in the connector. PL259's are known to give problems if the coax is loose
in the barrel. With BNC connectors it is common that the pin in the centre has fallen in and is not making
a proper contact. BNC's are known for their short circuits as they can be more difficult to assemble.

If possible you should check your SWR after installation.

The Installation
When installing any Base & Lead Assembly it is important that the Base is properly earthed to the vehicle.
The earth is the braid or the outer part of the coax and is connected to the base plate of the base.
This base plate is usually the part that earths to the underside of the vehicle body. It is important that
there is no short circuit between the base plate and the base bolt that the antenna is connected to.
It is a good idea that if you are installing a Base & Lead Assembly through the inside of a vehicles motor
you keep the cable as far away from the electrical system such as ignition coils as possible as this will help
to reduce noise. After installing the Base & Lead Assembly, using a multimeter check for continuity between
the collar of the connector and any metal part of the vehicle, also check for short circuit on the collar end.
If you have excess coax and have no other choice but to coil it then try to run it up and down some way.
If you coil coax you may create inductance and this may lead to problems.

The SWR Meter
It is important that the SWR Meter should be of good quality and cover the frequency that you wish to test
and that the lead that goes from the SWR meter to the Transceiver be as short as possible.

The SWR Readings
INITIAL TEST
It is of the utmost importance that the antenna have a tip on it during all SWR measurements.
Tests made without antenna tip in place will produce false information. Your goal is to achieve the lowest
REF SWR reading possible but NOTE SWR readings lower than 1.5 to 1 are quite acceptable, if your antenna
is within that range it is not necessary to tune it. After installing the SWR meter the first thing to do is
measure the SWR reading on channel on then measure the SWR reading on channel 40, if the SWR is better
on Ch 1 than on Ch 40 then the antenna is too long and will need to be tuned, however if the SWR is better
on Ch 40 than on Ch 1 the antenna is to short and cannot easily be adjusted any better.  see below.

DIFFERENT ANTENNAS DIFFERENT READINGS
At this point, depending on the type and most important the length of the antenna, you will see different
results. The longer the antenna the more broad band the antenna is, ie: In the case of 27MHz CB if the antenna
is more than 40" long, the antenna's SWR after tuning should fall between Ch 1 ~ Ch 40 under 1.5 to 1.
However the shorter the antenna the more narrow band it is, ie: In the case for 27MHz CB antennas that
are shorter than 40" long the antenna's SWR after tuning will not cover all the 40 channels below 1.5 to 1 SWR.
For example, a short CB antenna such as a Rubber Ducky will only have a SWR reading of better than 1.5 to 1
on a few channels, so it may be necessary in this case to decide which particular channels you wish to
operate on. So, the shorter the antenna, the more quickly it reacts to adjustments. Extreme care should be
taken when tuning antennas less than 25" long as even a cut of 1/8" may make a big difference.

THE READINGS IN A NUTSHELL
If SWR readings are 1.5 to 1 or below on all frequencies throughout the frequency spectrum of your transceiver,
then no adjustments are required. (Dont fix it it aint broke) If the SWR FAVOURS the lowest frequency and
SWR on the highest frequency of your transceiver is above 1.5 to 1, then your antenna appears LONG on
your system, and may be CUT TO TUNE If SWR FAVOURS the highest frequency on your transceiver and the
SWR readings on the lowest frequency are above 1.5 to 1, then your antenna cannot easily be adjusted.
See below
If the SWR is worse than 3 to 1 then check for open circuit or short circuit in coax. Small antennas work
best on small ground plane areas eg: Rubber Ducky's work best on gutter mounts and 1.5mtr antennas work
best in the center of the roof of the Vehicle. Most importantly, always test antennas away from any metal
obstacles such as garage doors, fences or metal tanks. A good distance to use is 10mtrs.

Tuning The Antenna
Set your transceiver to the lowest operating frequency or Channel 1. Remove TIP from top of the antenna remembering that the TIP makes a difference as it lowers the frequency slightly, so remember to replace the
tip when measuring SWR. Using a razor blade, knife or side cutter, carefully cut 1/4" off the top, then replace
tip, then measure SWR reading on Ch 1 and Ch 40, The SWR will improve on Ch1 quicker than on Ch 40.
Continue to cut the antenna. As the antenna lowers in SWR you need to achieve a SWR of better than
1.5 to 1 on Ch 1 to Ch 40 with the best reading on Ch 20.CAUTION: As SWR reaches it's lowest point,
cut only small amounts of wire off antennas to avoid over trimming. Once you have tuned the antenna it is
a good idea to glue the tip on to prevent water, moisture or loosing the Tip
(Note that spare tips are available on request)

When checking SWR, do so in an open area.
SWR readings that are inside a building or near other objects or antennas can cause reflection
which may give a false reading. A good distance should be 1/2 wavelength  (27MHz = 5 metres).


If you encounter problems see "Problems" or "Antenna Too Short" below
or contact Rochelle at Mobile One during working hours on 02-46556677 sales@mobileone.com.au

Some problems to look out for

Side mounts installed too far below the top edge of the vehicle, a minimum of 2/3 the length
of the antenna should be above the roof line. Portions of the antenna which are below roof line
and close to metal will absorb reflected signals that bounce off of the vehicle this will make it
hard to tune the antenna and the performance will suffer.