Post by Gremlin on Mar 27, 2008 1:35:17 GMT -4
Although almost every C.B. radio antenna is now made to frequency at the factory, I, nor anyone else, can simulate all installation possibilities.
Therefore most antennas may need slight adjustments.
Also, the addition of springs and or quick disconnects will make your antenna
electrically longer and tuning may become necessary.
Take all SWR measurements with the antenna in it's final mounting position including all accessories, then adjust as necessary.
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 (Roof Mount, Trunk Mount, Magnet Mount, etc) but 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 should always 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. 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. (Antenna Feed-Line Only )
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 proper contact. BNC's are known for their short circuits as they can be more difficult to assemble.
The Installation
When installing any Base & Lead Assembly (Coax & Mount) 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 area 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 it should always be a multiple of 1/4 wave length of the frequency i.e.: for 27MHz the length should be either 9' or 1.8m. Jumpers of other lengths will work but may cause false readings. In the case of VHF & UHF it is not important make it as short as possible.
SWR Readings
INITIAL TEST
It is of the utmost importance that the antenna have a cap on it during all SWR measurements.
Tests made without antenna cap in place will produce false information. Your goal is to achieve the lowest REF SWR reading possible but NOTE SWR readings lower than 1.3 to 1 are quite acceptable and 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.
If this is the case see below.
DIFFERENT ANTENNAS DIFFERENT READINGS
Note: We are dealing with two different Antenna lengths (1) Physical or actual length, (2) Electrical length.
At this point, depending on the type and most important the Physical or actual 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 and if the antenna is more than 40" long, the antenna's SWR after tuning should fall between Ch 1 ~ Ch 40 under 1.3 to 1. However the shorter the antenna the more narrow band 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 1.3 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
A: If SWR readings are 1.3 to 1 or below on all frequencies throughout the frequency spectrum of your transceiver, then no adjustments are required.
B: If the SWR FAVORS 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
C: If SWR FAVORS 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.
D: If the antenna is longer than 40" long and the SWR is worse than 2 to 1 on Ch 1 then
check for open circuit or short circuit in coax.
E: If the antenna is shorter than 40" long and the SWR is worse than 2 to 1 or even in the red don't be concerned but check coax then proceed to tune.
F: Small antennas work best on small ground plane areas i.e.: Rubber Ducky's work best on gutter mounts and 1.8mtr antennas work best in the center of the roof of the Vehicle.
G: Most importantly, always test antennas away from any metal obstacles such as garage doors, fences or metal tanks. A good distance to use is 36' or 12mtrs.
Tuning The Antenna
Set your transceiver to the lowest operating frequency or Channel 1.
Remove cap from top of the antenna, remembering that the cap makes a difference as it lowers the frequency slightly, so remember to replace the cap 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 1.3 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 cap on to prevent water, moisture or loosing the cap.
PROBLEMS TO LOOK OUT FOR
Broken, shorted, pinched or kinked coax. Low grade coax cable, there is a lot of it about.
Poor ground connections or shorted mounts. Improper coax. All CB systems use 50 ohm coax. Excess coax wound into coil less than 10" in diameter to use up slack
(this forms an induction coil which can hinder tuning and performance).
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 minum good distance should be 1 wavelength (27MHz = 36' or 12metres).
Insufficient ground plane area under antenna. Antenna cap not in place.
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.
Faulty test meters or static charges on meters with plastic face covers.
Amplifiers installed out of phase with respect to the 1/2 wave coax rule,1/4 wave in,1/4 wave out, (minor coax reaction will be greatly increased if signal enters amp out of phase).
ANTENNA TOO SHORT ?
27MHz CB Antennas More Than 40" long
Replace cap with a longer one will lower the frequency but only very slightly.
Cut heatshrink off the top and solder wire to top and rewind, then re-heatshrink.
27MHz CB Antennas Less Than 40" long
Replace tip with a longer one will lower the frequency.
A spring or a quick release mount will lower the frequency.
You may need to add some wire on the top of the antenna so cut 20mm heatshrink off the top release wire then solder some wire to the end, wind a loose wound coil on top then re-heatshrink, note heatshrink is available from your local Radio Shack store.
Therefore most antennas may need slight adjustments.
Also, the addition of springs and or quick disconnects will make your antenna
electrically longer and tuning may become necessary.
Take all SWR measurements with the antenna in it's final mounting position including all accessories, then adjust as necessary.
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 (Roof Mount, Trunk Mount, Magnet Mount, etc) but 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 should always 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. 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. (Antenna Feed-Line Only )
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 proper contact. BNC's are known for their short circuits as they can be more difficult to assemble.
The Installation
When installing any Base & Lead Assembly (Coax & Mount) 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 area 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 it should always be a multiple of 1/4 wave length of the frequency i.e.: for 27MHz the length should be either 9' or 1.8m. Jumpers of other lengths will work but may cause false readings. In the case of VHF & UHF it is not important make it as short as possible.
SWR Readings
INITIAL TEST
It is of the utmost importance that the antenna have a cap on it during all SWR measurements.
Tests made without antenna cap in place will produce false information. Your goal is to achieve the lowest REF SWR reading possible but NOTE SWR readings lower than 1.3 to 1 are quite acceptable and 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.
If this is the case see below.
DIFFERENT ANTENNAS DIFFERENT READINGS
Note: We are dealing with two different Antenna lengths (1) Physical or actual length, (2) Electrical length.
At this point, depending on the type and most important the Physical or actual 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 and if the antenna is more than 40" long, the antenna's SWR after tuning should fall between Ch 1 ~ Ch 40 under 1.3 to 1. However the shorter the antenna the more narrow band 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 1.3 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
A: If SWR readings are 1.3 to 1 or below on all frequencies throughout the frequency spectrum of your transceiver, then no adjustments are required.
B: If the SWR FAVORS 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
C: If SWR FAVORS 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.
D: If the antenna is longer than 40" long and the SWR is worse than 2 to 1 on Ch 1 then
check for open circuit or short circuit in coax.
E: If the antenna is shorter than 40" long and the SWR is worse than 2 to 1 or even in the red don't be concerned but check coax then proceed to tune.
F: Small antennas work best on small ground plane areas i.e.: Rubber Ducky's work best on gutter mounts and 1.8mtr antennas work best in the center of the roof of the Vehicle.
G: Most importantly, always test antennas away from any metal obstacles such as garage doors, fences or metal tanks. A good distance to use is 36' or 12mtrs.
Tuning The Antenna
Set your transceiver to the lowest operating frequency or Channel 1.
Remove cap from top of the antenna, remembering that the cap makes a difference as it lowers the frequency slightly, so remember to replace the cap 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 1.3 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 cap on to prevent water, moisture or loosing the cap.
PROBLEMS TO LOOK OUT FOR
Broken, shorted, pinched or kinked coax. Low grade coax cable, there is a lot of it about.
Poor ground connections or shorted mounts. Improper coax. All CB systems use 50 ohm coax. Excess coax wound into coil less than 10" in diameter to use up slack
(this forms an induction coil which can hinder tuning and performance).
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 minum good distance should be 1 wavelength (27MHz = 36' or 12metres).
Insufficient ground plane area under antenna. Antenna cap not in place.
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.
Faulty test meters or static charges on meters with plastic face covers.
Amplifiers installed out of phase with respect to the 1/2 wave coax rule,1/4 wave in,1/4 wave out, (minor coax reaction will be greatly increased if signal enters amp out of phase).
ANTENNA TOO SHORT ?
27MHz CB Antennas More Than 40" long
Replace cap with a longer one will lower the frequency but only very slightly.
Cut heatshrink off the top and solder wire to top and rewind, then re-heatshrink.
27MHz CB Antennas Less Than 40" long
Replace tip with a longer one will lower the frequency.
A spring or a quick release mount will lower the frequency.
You may need to add some wire on the top of the antenna so cut 20mm heatshrink off the top release wire then solder some wire to the end, wind a loose wound coil on top then re-heatshrink, note heatshrink is available from your local Radio Shack store.