Practical Antenna Theory - Part 2
One of the most often discussed topics in Ham Radio (and the most misunderstood) is SWR. In this series of articles we will learn some basics, some theory, and dispel a few myths that are generally perpetuated in the Ham Radio community.
In our first installment we learned some terms used in discussing antennas and feed lines. We also dispelled the myth that a high SWR sends RF back to your finals and can cause damage from the RF. If that is what you still think, reread the first article and think about it some more!
This would be a good time to discuss the nature and purpose of antenna tuners, often called a matchbox. This device is quite simple in application even though much engineering often goes into the design.
Let’s look at a common hookup for the tuner. The coax from the rig goes to the “transmitter” terminal on the matchbox. The antenna coax goes to the “antenna" terminal(s) on the tuner.
Adjusting the “Antenna”, “Transmitter” and “Inductor” controls on the front panel in various combinations will seem to indicate that you can lower the SWR of an antenna. Sorry to bust your bubble again, but this too is a myth. Myth #2 comes from the false assumption that the SWR meters indicate that you have “tuned” your antenna. What has actually happened is that the tuner has compensated for the mismatched impedances between your rig and the coax. You have, in effect, “tuned your coax to the rig”. The impedance seen by the tuner is a result of the entire antenna system from antenna feed point to feedline connector in the shack. Adjusting the tuner (manual or automatic – internal or external) supplies reactance and resistive characteristics to the radio that it expects. This does nothing for the other end of the coax in terms of “tuning” the antenna. The low SWR readings make your rig happy, because it is operating with good output efficiency. The final tubes or transistors will be able to generate RF without having to work very hard (heat up).
What if we move the tuner to the other end of the coax? Granted not every situation would make this a practical situation, but for our purposes – let’s say we could do it (some automatic tuners are made to be mounted at the antenna as they come or could be mounted in a suitable box for protection). What happens then?
Normally speaking, the “Transmitter” port of the tuner is connected to the coax from the rig in the shack to the antenna. Moving the tuner to the antenna, we must insert the tuner in the feed line before it reaches the antenna. The “Antenna” port of the tuner connects, either directly or with a short piece of appropriate feedline, to the antenna feed point (e.g. some antennas, like HF verticals, inverted “L”, etc. need only a single wire from the tuner to the antenna and one from the tuner to ground). Tuning the controls will now match the antenna to the feedline at the frequency used to tune it to the lowest VSWR.
Ultimately, this is the best of all situations when available.
The question then becomes - Why?
We discovered earlier in our “tuner in the shack” scenario, that a tuner in the shack will not change the SWR presented to the feedline by the antenna. That will remain the same. In that scenario, the tuner only matches the shack end of the coax to the rig. This scenario could represent an operational problem.
RF returning from the antenna to the rig tends to flow on the outside of the coax cable. This unshielded RF flow can then radiate just like your antenna. Twin lead users are not immune either (although the effect is much less noticeable). Because SWR is an additive component (reference or discussion from the first article) the same radiation occurs in twin wire transmission line as coaxial cable because the delicate balance between the two conductors will not be equal. This is an unintended consequence of uncontrolled SWR. It is often manifested in incidental RF sparking or burns in the ham shack when transmitting. Also, the radiation pattern of your antenna may be altered due to the additional radiation from the feedline – adding a more or less omni directional component rather than the usual directional pattern expected. Sometimes you don’t know about it until the neighbors complain you are getting into their telephone or stereo.
One unenlightened, but well meaning, ham decided to eliminate the RF on the coax by grounding the coax shield at his vertical antenna (no it was not me HI HI). When he tried to transmit, he discovered nothing would tune up properly. If you think about it, what he had done was to essentially reduce the feedline impedance by half and made it into a giant tuned circuit (remember coax has both series inductance and parallel capacitive characteristics). Depending on the length of the cable, it could become a very broadband tuned circuit. All sorts of problems could arise from this including but not limited to radiation on a different frequency than your transmitter is tuned to.
So, if we cannot put the tuner at the antenna, how do we avoid the antenna system SWR problem and should we?
Ideally, the antenna would exhibit exactly (or almost nearly) the same characteristic impedance as the feedline at any frequency we wish to transmit on. Practically, however, this is rarely (if ever) the case. The closest we could practically come is to use a multi-band log periodic antenna that covers all the frequencies we normally transmit on. Even then, the impedance characteristics are not perfect on every band within its operating range. There will be VSWR changes as we change frequency.
The best situation for most hams is to construct multiple single, two, or three band antennas that have a very low SWR over the normal operating range of the individual bands served. The same principal applies to multi-band beam antennae.
This is a compromise at best, but one most hams can live with. When possible, placing the tuner remotely at the antenna will always provide the highest available matching possibilities and most beneficial results for broad frequency / band coverage. For operating conditions that do not allow this, use some practical guidelines to avoid the undesirable effects mentioned earlier.
Rule #1: Don’t try to match SWR conditions that exceed 3.5:1 with your matchbox. Remember the SWR on the feedline and antenna will still be the same no matter how low the tuner is able to indicate the SWR is in the shack.
Rule #2: Construct well-designed, low SWR antennas for a broader frequency range than you will operate. In other words; if your antenna will stay under 3:1 from 14.000 to 14.100, trying to use it in the 14.200-14300 range of the band would not be recommended even with a tuner unless the tuner is remotely located at the antenna.
Rule #3: Know the characteristic impedance of your antenna at the frequency you will be transmitting. Use only good quality transmission line that is the closest match for the antenna. If the antenna feedpoint impedance is 72 +j75 as measured by an analyzer, RG216 or RG6X is a better choice than even the most expensive RG8 or RG214. Let your tuner provide the rig-to-coax matching of impedances.
In the past, experimentation and general knowledge of antenna and transmission line characteristics was difficult to obtain. The high cost and low availability of measuring instruments were prohibitive for most hams. Today, very sophisticated antenna and network analyzing instrumentation is available for the ham shack in the under $1000 range. The most notable of these is the MFJ SWR analyzer in various models. This valuable tool (or several others that serve the same purpose) should be one of the first purchases for the serious antenna experimenter (worry about an SWR meter later – when you fully understand SWR). If you like to homebrew, the 1980 ARRL Handbook has a project to build an antenna impedance bridge. This device is not as accurate, but will go a long way toward presenting appropriate data to the experimenter.
Next time:
To Balun or not to Balun, is Unun the answer?