Tuning Your AntennaGetting the best
performance from your investment.
Coaxial cable, the most common feedline, delivers energy to an antenna in an unequal or "unbalanced" state. RF energy is delivered to the antenna along the center lead. In a perfect system with a 1:1 SWR there will be no current flowing on the coax shield at all. All RF power from your transmitter is radiated away by the antenna. However, antennas are seldom perfect and quite often there is current flowing on the shield of the coax.
The worst of these conditions occurs when feeding a balanced antenna such as a dipole or loop antenna with coax. This is a natural mismatch in feed methods --balanced antenna : unbalanced feedline-- that just begs for problems.
The illustration on the right shows the end of a piece of coax where it connects to a dipole antenna. The arrows represent a moment in time.
The blue arrows represent antenna currents. If the antenna cannot get rid of all of the RF energy current will flow on the inside of the coax shield. This is normal and in this condition the currents are fully contained within the coax.
However, when a balancing mismatch occurs, it is entirely possible for current to flow on the outside of the coax shield, as shown by the red arrow. This undesirable current is not contained inside the coax and can radiate from the coaxial feedline, getting into nearby electronics in very undesirable ways. This is called "common mode" current since it is actually in phase with the center lead of the coax.
This can also happen with unbalanced antennas as well. This most often occurs where the antenna or it's support structure is not grounded or when the antenna's "groundplane" is less than adequate.
If you are having common mode current problems you will notice the SWR of your antenna system changing during a rain storm or when the coax is moved or touched. In severe cases, touching your radio equipment can affect the SWR of your antenna. A very simple way to test for common mode currents is to suspend your coax away from the antenna's support structures, take a reading and then see if the SWR changes when you place it against the support structures.
Fortunately there are relatively easy fixes for this problem...
If you are feeding a balanced antenna such as a dipole or loop you should always use a Balun designed for the range of frequencies in use. A balun is a transformer mechanism that takes the naturally unequal signal from coax and transforms it to a balanced 2 wire signal delivering equal but opposite energies to both sides of the antenna. You are thus feeding a balanced antenna with a balanced signal which should keep both feedline and antenna happy.
If you are feeding an unbalanced antenna such as a mobile whip, groundplane or colinear antenna you can add a common mode choke. This can often be as simple as a few coiled up turns of coax positioned near the antenna. The choke forms an inductor with the outside of the coax shield making it an uninviting place for current to flow. (The internal signals should not be affected) The size of the coil and the number of turns is best determined experimentally; use just enough to eliminate the problem.
An excellent article on the construction of common mode chokes, also called Ugly Baluns , can be found on the Ham Universe website. Despite their larger sizes on HF, the VHF and UHF versions are actually quite compact. As the photo on the left shows, for some of my 2 meter antenna projects, I simply wound 5 turns of the RG-8x coax right around the mast pipe and the antenna settled right in.
It is a good idea to use common mode
chokes or baluns on all your projects. While not absolutely necessary in
all cases, this is a simple precaution that harms nothing if not needed.
Whenever possible you want to adjust the antenna in place on it's mounting structure. In this way you are taking the structure and other unavoidable local objects into consideration.
Because of highly variable conditions, mobile antennas absolutely must be tuned in-place on the vehicle. You should park the vehicle as far from any buildings, light posts or metallic objects as possible. Always take your measurements with all doors or hatches closed.
Portable antennas need to be tuned "in the clear", suspended from a non-conductive cord or standing on a non-conductive mount with as much free space around them as you can get. Those with fold-down stands should be tuned on their mounting structure, simulating real-world conditions.
Omnidirectional base station antennas that can't be tuned in-place, should be mounted on a temporary structure, as far from nearby objects as is convenient.
Directional antennas should be pointed straight up with their reflectors as far above ground as is convenient.
Always keep yourself, your kids, your pets and others well back from antennas while tuning. Beyond the risk of RF burns, there is the matter their body capacity is going to upset your readings. It is best to run a length of feedline to the antenna and set up a testing station, where you take your readings at least 1/4 wavelength away.
Safety first: NEVER activate your
transmitter while anyone or anything is touching the antenna!
The primary goal in tuning an antenna is to make it usable all across the band(s) it is designed for.
Antennas are resonant devices. That is to say they work best at a single frequency. As you move above or below that frequency their efficiency rolls off, producing standing waves. In order to achieve the goal of usability, you will want to tune the antenna for equal SWR readings at each end of the band. Below is a plot of the SWR for a theoretical well tuned antenna.
You want to end up with equal SWR readings at each end of the band you are tuning for. So long as the antenna's design is basically sound, the lowest SWR will naturally occur inside the band, at the antenna's resonant frequency.
No, I didn't miss the middle when
drawing the line. Most antennas behave a little differently below
resonance than above and it is rare that you will get the lowest reading
exactly in the center of the band. The important goal is equality at the
band edges. This ensures the antenna is usable all across the entire band.
Hooking Up The Meter
Your SWR meter needs to be connected into the coax between your radio and antenna. For this you will need your meter and a short jumper of coax with the correct connectors on it (usually PL-259s).
Connect one end of the jumper to the back of your radio. Now connect the other end to the "Transmitter" socket on the SWR meter. The antenna's coax now connects to the "Antenna" socket on the meter.
Make sure all connectors are well seated and done up snugly.
Most SWR meters will not be damaged if you get them backwards (I've done it more often than I care to admit) but they will not give you accurate readings as the forward and reflected functions will be reversed.
The operation of SWR meters varies a bit
from model to model, so be sure to read the instructions for your meter
carefully before proceeding.
Testing and Adjusting
To reduce the risk of interference with other hams or nearby equipment you should always use your transmitter's minimum power setting when adjusting SWR.
The actual adjustments you will make depend entirely on the type of antenna you are tuning. Those with impedance matching devices are more complex than those with simple top whips. Multi-band antennas introduce a whole new level of complexity. But it's all doable.
The general measuring procedure is always the same...
Set your radio to CW or FM mode (not SSB).
Tune to the low edge of the band you are adjusting for.
Transmit and calibrate your SWR meter1
Transmit and take an SWR reading and write it down.
Tune to the high edge of the band you are adjusting for.
Transmit and calibrate your SWR meter1
Transmit and take an SWR reading and write
(1 single needle meters only)
These readings will tell you if the antenna is too long or too short:
If the low edge has the lower SWR the antenna is too long.
If the high edge
reads lower the antenna is too short.
The adjustments you make based on this information will depend on the type of antenna you are adjusting:
Whips, Mobiles and Groundplanes
These antenna types are adjusted by changing the length of the radiating element(s). There is usually a provision to slide the element(s) in and out for tuning. If the antenna reads too long, adjust the element shorter. If it reads too short, make it longer.
Wire Antennas, Dipoles and Loops
Wire antennas should always be deliberately cut too long at the start. The only adjustment you have here is to clip a little bit off the end. Be careful to keep the sides of dipoles the same length and make sure the feedpoints of loops stay centered. Cut carefully and in small increments. If you get too short, making them longer is going to be a huge undertaking that might well result in antenna failure once the weather gets at your splices.
Antennas with Gamma, Y or T Matches
These antennas require multiple adjustments. The best plan here is to set everything on the manufacturer's recommended starting points then move the matching device's rods a little bit, test again and see if you went the right way. Once you know the correct direction, adjust the matching device for the lowest SWR. Then, once you find the lowest SWR by this means, try adjusting the length of the driven element to get the SWR equal at the band edges. This might send you back to re-adjusting the matching device. With patience, each cycle will result in smaller movements as you zero in on the perfect settings.
J-Pole Style Antennas
These are also antennas with matching devices. Fortunately they are a lot easier to adjust than a beam. Here you move the coax up and down on the matching stub to find the lowest SWR. If the antenna reads too long or too short after adjusting the stub, you can try adjusting the length of the long radiating element a little bit.
Ring Matched Antennas
These antennas use a sliding contact on a ring device. The general procedure is to set the radiating element at the factory recommended length then adjust the sliding contact for the lowest SWR. If the antenna reads too long or too short you can generally adjust the length of the radiating element a little bit to even out the ends of the band.
Dual/Multi Band Antennas
These antennas are a fair bit more complex to adjust. Here you adjust the length of the shortest section for the highest frequency band. Then move to the next section along the antenna for the next lower band and so on, moving to longer sections for lower frequencies, until you have all sections matched. You may need to go back and forth a few times to get everything working right. Also note that multiband antennas use coils and chokes along their length so these adjustments can be quite sensitive.
Once you have your antenna all tuned up, don't forget weatherproofing. Any place water can get in will cause problems. Use tape on your connectors, put weather caps on tubes, apply liquid rubber to exposed RF connections, etc. Once mounted at the top of your brand new 30 metre tower, you won't have much chance to get up there and dry things off or melt the ice out.
An SWR meter is an essential tool for every ham. While tuning antennas is sometimes a tedious task, the good news is that after properly tuning and weatherproofing an antenna, the SWR seldom changes on it's own.
A carefully tuned antenna will always outperform a poorly tuned one.
73, L.D.Blake, VE3VDC
LEARN MORE ABOUT SWR - Making Sense of those readings!