How to Tune an antenna !

The following, is advice that is applicable to all kinds of antennas

For more information, ideas and advice please see our  Frequently Asked Questions
Celebrating more than 40 Years of Manufacturing Antennas in Australia.

 

 

Mobile One Antennas

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 adapters 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

(CD470's, S474's, M474's, RF470's, FDW470,s, BNCHW,s, CP474,s, etc)

All Mobile One UHF Antennas do not need to be tuned, they are very broad-band and will have a SWR below 1.5 to 1 as long as they are installed correctly.

Mobile One 27MHz Pre tuned Antenna Range

(DX125CD to DX160CD & DX136FW to DX172FW, DX745CD, etc

The DX125CD to DX160CD antennas have been made with 1/4" constant diameter fiberglass 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)
The cable length should be a multiple of 1/2 wave length of the frequency i.e.: for 27MHz the length should be either 3.6m, 7.2m, 10.8m or 14.4m etc, but this is not at all essential for performance but rather for accuracy in testing the antenna!.
In the case of UHF CB the length of cable 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 center has fallen in and is not making a proper contact, BNC connectors are known for their short circuits as they can be more difficult to assemble and can become faulty over time.

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. If possible you should check your SWR after installation.

The SWR Readings

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.

Checking SWR

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 meters). 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.

The Initial Testing

It is of the utmost importance that the antenna have a tip on it during all SWR measurements. Tests made without the 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 1 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 importantly, the length of the antenna, you will see different results.
The longer the antenna, the more broad-band the antenna is, i.e.: in the case of 27MHz CB, if the antenna is more than 40" long, then antenna's SWR after tuning should fall between Ch 1 ~ Ch 40 under 1.5:1, however, the shorter the antenna, the more narrow banded it is, i.e.: 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 1.5:1 SWR. For example, a short CB antenna such as a Rubber Ducky will only have a SWR reading of better than 1.5:1 only over a few channels. 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.

Readings in a Nutshell

If the SWR readings are 1.5:1 or below on all channels of your transceiver, then no adjustments are required.

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 the 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 e.g.: 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. Use a razor blade, knife or side cutter (don't crush the fiberglass, cut it!) Carefully cut 3mm 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 may need to cut as much as 30mm.

You need to achieve a SWR of better than 1.5:1 on Ch 1 to Ch 40 with the best reading on Ch 20 (you decide)

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)  If you encounter problems see below or contact the service dept. at Mobile One during working hours on 02-46556677

Some problems to look out for

  1. Broken, shorted, pinched or kinked coax.
  2. Low grade coax cable, there is a lot of it about.
  3. Poor ground connections or shorted mounts.
  4. Improper coax, all CB systems use 50 ohm coax.
  5. Excess coax, do not coil, run as direct as possible.
  6. Insufficient ground plane area under the antenna.
  7. Antenna tip not in place.

Is the Antenna too short or tuned too far?

If you replace the plastic tip with a longer one will lower the frequency but only very slightly. A spring or a quick release mount will slightly lower the frequency (Not for UHF Antennas). You could cut heatshrink off the top and solder wire to top and rewind, then re-heat-shrink
NOTE heatshrink is available from some Dick Smith Electronic stores or from Mobile One.


    Call Peter at Mobile One for help on 02-46556677
     tech@mobileone.com.au
All Mobile One Antennas are covered with the Mobile One No Worries 5 Year Warrant