RESONATE FIRST,

MATCH SECOND (OR NOT AT ALL)

By

Eric Nichols, KL7AJ, WD2XSH/27

Whenever a newcomer comes to me with an antenna problem, the first thing I ask is “Where is it resonant?” More often than not, it's also the LAST question I need to ask.

As we will demonstrate, not only is the reactance of any antenna FAR more dependent on frequency than is the radiation resistance, but that the SWR on a transmission line is FAR more dependent on reactance than it is on resistive mismatching. I encourage you to do the following experiments for yourself, thereby, any argument you may have won't be with me, but with the laws of physics themselves!

Although it's nice that affordable antenna analyzers abound nowadays (and are CERTAINLY more convenient than lugging around a GR-916 impedance bridge!), in the wrong hands they can lead to more confusion than enlightenment. A far more useful instrument for ANY new ham...at least until he has some transmission line theory under his belt...is a grid dip oscillator (GDO).

I should mention at this point that I run open wire feedlines for all my antennas...I neither know nor care what the SWR is...ever...since my line losses are negligible. But I realize most new hams will be using coax, which, as we will see, can be EXTREMELY lossy under high reactance load conditions. (For open wire feedline users, the classic QST article “My Feedline Tunes My Antenna” by Byron Goodman, is all the information you'll ever need.)

Now, if you happen to have all three instruments: a GDO, an Antenna Analyzer, and an SWR meter, you can have even more confirmation of some important principles I'll describe. One should ALWAYS be able to obtain the same answer by several means.

The input terminals of an antenna present an impedance in two parts, a reactance (preferably zero), and a real part, which ideally consists of mostly RADIATION RESISTANCE. For nearly any practical antenna, the CHANGE in feedpoint resistance from one end of a ham band to the other, is insignificant. For really short mobile whips, where the radiation resistance is insignificant relative to the LOSS resistances, the change in feedpoint resistance is REALLY insignificant across any ham band.

Reactance is a whole `nother story, though. In fact, for all practical purposes, it is the change in REACTANCE alone which accounts for the change in SWR as you move from one end of a ham band to another. We actually have a “double whammy” working against us in this. REACTANCE changes quickly with frequency for most antennas, and SWR changes rapidly with REACTANCE. The latter factor is easily demonstrated with the Smith Chart, but if that is too daunting for you, any transmission line analysis program like TLA and others, confirms this as well.

But, just in case you have a healthy suspicion of computer programs, you might want to test this out for yourself. GREAT! Let's go!

String up a 40 meter dipole at a height where you can reach the feedpoint with a stepladder. Create a SMALL coupling loop at the feedpoint of the antenna. Just a couple of turns of wire, an inch or so in diameter, clipped across the input gap, should do the job. Now insert your GDO pickup coil into the coupling loop you just made, and sweep the oscillator for a dip. It should be very well defined. Note this frequency. (A GDO won't tell you anything about the RESISTIVE part of the impedance, but with some experience, you can get a crude estimate). Now log this resonant frequency.

If you have an antenna analyzer, use it to confirm this resonant frequency. You should get x=0 very close to the frequency you measured with the GDO. If your antenna analyzer has a built-in SWR function, great...let's do another check. First, look at the RESISTIVE part of the impedance. An antenna at about 1/10 wavelength altitude, over average ground, will have a radiation resistance of about 70 ohms...pretty close to that of free space, oddly enough. Your mileage may vary. Let's call it 70 ohms. The SWR indicated by your analyzer should be 1.4:1. The SWR should always read the ratio of measured resistance to 50 ohms, whichever is larger. Note! This ONLY works at the precise resonant frequency!

Now, hopefully your resonant frequency is somewhere in the 40 meter band. (If it isn't, find out why!) Now, scoot the frequency up by 100 KHz and take another reading. Your radiation resistance probably went up a couple of ohms. But what about the reactance? You probably have about 20 ohms of reactance now, right? It's changed at least TEN TIMES as fast as the resistance.

Now, let's move up 500 kc from the resonant frequency. Take another reading. Well, now the radiation resistance has gone up by 12 ohms or so. But look at that reactance. It's about 105 ohms, plus or minus some spare change! Yikes! Your SWR is up to around 4.65:1. Well, actually, that's just a SMALL yikes, as we will see later. But it could be improved. Let's insert a small series capacitor at the feedpoint, just enough to get rid of the reactance. We still have a radiation resistance of 82 ohms, but the reactance is now zero. But our SWR is now down to 1.64:1, which is good in just about anyone's book!

How good is “good” when it comes to SWR? The best answer to this is to look at something that's really BAD for comparison. Let's look at a worst-case scenario. Well, I guess it can't be an absolute worst-case scenario, because, we know that no matter how bad something is, some ham somewhere has come up with something worse.

So, instead, let's look at a nearly worst-case scenario that a lot of hams actually have! Let's use our 40 meter dipole on 80 meters! I know lots of hams that do that...in fact, I've done it myself. The fact that I actually made contacts with the abomination simply proves that QRP works!

Anyway...scoot your antenna analyzer down to 3.5 MHZ. Take another reading. The R is 27.9 ohms...not great, but not too horrendous. But look at that reactance! -931 ohms! This gives us a whopping SWR of 627:1! That's a GENUINE yikes!

Just how bad is an SWR of 627:1? To answer that, we need to look at line loss. Let's say you're using 100 feet of garden variety RG-8 coax. Calculation with TLA shows us that we have 13.898 dB line loss, or 95.9% line loss! So, if you're pumping 100 watts into your coax, about 4.1 watts actually reaches the antenna. I'd say that qualifies as officially bad. (These figures can be confirmed by using your trusty SWR meter to measure the power at the INPUT of the line and then at the OUTPUT of the line. You don't NEED an expensive calibrated wattmeter to see this. The RELATIVE forward power of your SWR meter will graphically demonstrate all you need to know!

But here's the good news. In fact, it's terrific news! Let's add a pair of decent loading coils right at the feedpoint, changing nothing else. This isn't even the best place to put them; we can get even better improvement by placing them farther out. But let's get this antenna up and running FAST. Guess what? By the mere addition of the loading coils (assuming they're the proper value to precisely achieve resonance, our SWR goes down to 1.79:1! The line loss is down to 9.2%! Can't complain about that at all...especially for a half-size antenna. Just by the mere addition of a proper loading coil! Even better news yet...you don't need an expensive antenna analyzer to determine this proper value...all you need is your trusty old GDO to get a 90% improvement in antenna performance!

Are we saying that resistance has no role in antenna performance? Certainly not. But amateur radio performance is about majoring on the majors and minoring on the minors. That means fixing what you can fix, and not wringing your hands over what you can't! You can't do much to improve the radiation resistance of an antenna...other than making it bigger. (PLEASE read W8JI's dissertations on radiation and feedpoint resistance!) Sometimes making an antenna bigger is not an option, and even if it is, the improvements are usually incremental. But you can make VAST improvements by fixing the reactance...and it's a lot cheaper, too!

I would like to conclude by reiterating that there is ABSOLUTELY NOTHING SACRED ABOUT A SELF-RESONANT ANTENNA. This statement in no way contradicts any of the above discussion. Any method of resonating an antenna will make it resonant (duh!)...either by linear (cutting to length) or lumped elements (coils and capacitors). Remember a 5/8 wave antenna is NOT self-resonant...but it's a better antenna than a wave dipole...with the proper loading. However, if you're using a lossy transmission line, it's crucial that you obtain resonance at the LOAD end of the transmission line. Again, this discussion does not apply to ideal or nearly ideal open wire feeders, where you can perform the resonating ANYWHERE in the system and maintain high efficiency.