Contents: Basics; How Efficient Is It?; Noise Level; Q Factors; Bandwidth; Cheating; Odds & ends;
Antenna efficiency is a balancing act between minimizing ground losses, minimizing stray capacitance losses, maximizing length, and maximizing radiation resistance. The latter includes keeping the Q high, but once the Q exceeds 300 (about the practical limit), gains diminish.
What follows is an expansion of these factors. It's important to keep in mind, minimizing ground and stray capacitance losses go hand-in-hand. In fact, proper mounting is so important to efficiency, I've covered the details in another article.
Suffice to say, mounting your antenna low, atop long stalks (as shown), and/or far removed from its image plane (the vehicle in this case), is a prescription for poor performance. The worst part is, very few amateurs realize just how much difference in efficiency (performance) proper mounting can achieve. In fact, the difference can be as much as 25 dB, perhaps more. So before you decide to buy an amplifier, perhaps you should look at your antenna mounting methodology first.
In the following are several references to ground (image) planes. If you want a little more technical look at why an adequate ground plane is so important, you should read this article.
Most amateurs know HF mobile antennas are inefficient when compared to a beam or simply dipole, but how much worse is a misunderstood concept. The simple reason is, efficiency is relative. It requires a defined baseline and without one it is just another word. The fact that you worked a DX station only proves one point; just how easy it is to work around the globe regardless of the ERP (effective radiated power).
Before we set a baseline for efficiency, we need to know a little more about HF mobile antennas. The majority of HF mobile antennas are electrically short. For example, an 80 meter, unloaded mobile antenna 8 feet long is electrically 11° in length as opposed to 90° of a full-sized 1/4 wave vertical.
The input resistive component is about .5 ohms, and its capacitive component is about 1,800 ohms (.5r-j1800). In order to cancel the capacitive reactance, a loading coil is inserted in the antenna to bring it into resonance. Depending on the requisite loading coil's position (bottom loaded or center loaded) the coil's Q factor, ground and stray losses, and resistive losses, the input impedance is closer to 25 ohms.
The formula is: Rt=Rr+Rc+Rg, where Rt=total or input resistance (more correctly input impedance), Rr the Radiation resistance, Rc the coil resistance (at the operating frequency, not DC resistance), and Rg the ground (Image Plane) loss resistance. There are some other losses too. Shunt capacitance losses, conductor losses, matching losses, and even losses caused by our mounting method. For the most part we can ignore these as they’re small (usually, but not always) compared to the other losses were talking about.
We have some control over the radiation resistance, but for our 80 meter example above, the Rr is .5 ohms! Since it is a factor of the electrical length of the antenna we must lengthen the antenna to increase it. Doubling the length raises the radiation resistance by a factor of four. Obviously there is a limit to how long it can be, with 11 to 14 feet about the maximum (suburban, versus rural). If you live east of the Appalachians, perhaps just 8 feet (all assuming bumper mounting).
We can use cap hats to raise the Rr, but there is more to cap hats than meets the eye. Aside from adding complexity and wind loading, they have to be designed correctly, and not be in close proximity to the coil. Correctly implemented, it is possible to raise the radiation resistance by a factor of 4, on the lower bands. If they're incorrectly implemented, they will have the opposite effect. If you need more information on cap hats, Tom Rauch, W8JI, has an article on his web site which explains the facts. There is also a downloadable PowerPoint presentation Tom did at Dayton in 2004 that may be easier to follow.
The Rc is the resistive loss component of our loading coil which cancels the high capacitive reactance of our short vertical as measured at the operating frequency. This is not the same as its DC resistance. On 80 meters, the typical antenna loading coil will have between 75 to 200 uH of inductance depending on where in the antenna it is located. The higher up the mast, the higher the radiation resistance, but the larger the coil (more inductance) needs to be. There is a trade off limit, however, because the larger the inductance, the greater the resistive losses of the coil. The reactive resistance versus the resistive losses (at the operating frequency) determines the Q factor. The higher the Q factor, the less loss, and the more efficient the antenna will be. In a mobile environment it is difficult to obtain Qs much over 300 and even this requires good construction practices (I revisit Q factors later on in this article).
By far the greatest loss is Rg or ground (Image) plane loss. It typically varies between 5 and 20 ohms, but can be much higher. It’s dependent on frequency, the size and type of the vehicle our antenna is mounted on, and how and where the antenna is mounted. Keep in mind, any vehicle (even a semi) is an inadequate Image Plane at HF frequencies. Since the body of the vehicle capacitively couples to the surface under it, Rg losses can be minimized by proper Bonding of all bolted-on parts including doors, hoods, trunks, tail and exhaust pipes, bumpers, etc. Mounting the antenna as high as possible on the vehicle also helps as this reduces the coupling between the antenna and the surface under the vehicle. After all, we want the vehicle coupled to surface, not the antenna.
The coupling I'm referring to is more correctly called stray capacitance. These stray capacitances include, but are not limited to, the capacitance in the mount (2 to 4 pF for the average ball), that caused by the close proximity of the mast to surrounding metal (difficult to measure, but could be as much as the antenna itself; 20 to 45 pF effectively shunting a large portion of the input power to ground), and that caused by the coil being too close to metal (again, difficult to measure, but this type of coupling has the greatest effect on efficiency and bandwidth). Tom Rauch, W8JI, covers this subject very well in the aforementioned article. When you read Tom's article, pay particular attention to his comments at the end of the article.
Efficiency can be calculated (not exactly, but close enough) if we know Rr, Rc, and Rg. All we have to do is add these factors together to get Rt, and divide Rr by Rt. For an average 8 foot antenna mounted on an average vehicle, and using an estimated Image Plane loss, the efficiency ranges between .2% on 80 meters to maybe 80% on 10 meters. These figures are based on data taken from the ARRL Antenna handbook.
It should be noted that the so-called ground loss figures used by the ARRL (approximately 10 ohms on 80 meters to about 2 ohm on 10 meters which relates to about .004 uF of coupling) are low because stray losses are not included, but they should be. Remember, a mobile antenna and the vehicle it is mounted on should be viewed as a system! As a result, 20 ohms to about 5 ohms respectfully is more realistic for the average installation, and perhaps a lot more depending on the antenna mounting location and mounting style.
While coil and other resistive losses are important, Image Plane loss has the largest effect on efficiency. As the frequency goes down the losses go up. Remember, we are coupled to the surface, and capacitive reactance goes up as the frequency goes down. This is why it is so important to minimize Image Plane losses with proper bonding. It's also why radiation resistance becomes paramount especially on the lower bands. It further explains the popularity of cap hats on 160 and 80 meters where ground loss is highest and radiation resistance is lowest.
An antenna's gain (or lack of it as is the case of an HF mobile antenna) is the same with respect to both transmitting and receiving. However, they are not reciprocal in transmitting and receive performance. It is important to understand why, especially in a mobile environment.
Short, poorly-mounted, and inefficient antennas do not provide as good of a signal to noise ratio (S+N/N) as longer, well-mounted, and more efficient ones do. The often used terms to describe this fact are capture area, and hearing ability (CBers call it ears). None of these terms are definitive.
Digressing for a moment. Very narrow bandwidth antennas (ones with a high system Qs, not necessarily ones with high loading coil Qs) receive less band noise than wide bandwidth antennas. While this point-of-fact could be applied to mobile HF antennas, it would be hard to quantify. The implication here is, although less lossy HF mobile antennas exhibit a better S+N/N ratio than their counterparts, it isn't necessarily caused by a reduction in the amount of received band noise.
You often see receiver sensitivity expressed as 10 dB S + N/N (a 10 dB ratio of signal + noise / noise) at a given microvolt level. However, in a mobile scenario, it's important to consider distortion as well as the noise. Remember, a vehicle is not an ideal environment because of the RFI and Image Plane issues involved.
Thus, we need a measurement method which includes these issues, and that method is SINAD. It is an abbreviation for Signal-Including-Noise-And-Distortion, hence the acronym SINAD. Expressed in dB, it is 20 * log of the signal + noise + distortion / noise + distortion, all expressed in volts. It is measured at the audio output, so it includes distortion and noise generated anywhere in the receive signal path.
It's important to remember that distortion and noise can mask weaker incoming signals, while your transmitted signal may be easily heard by another station with a better SINAD. If the incoming signal is great enough to overcome your poor SINAD, it may very well be the other station won't be able to hear you due in part to your low transmitting efficiency. While this scenario can be true in any installation, it's more prevalent in a mobile one.
Using terms like I can work anything I can hear, to describe your mobile installation, isn't being realistic. For example, in a mobile scenario it is not uncommon to have the background noise level 40 dB or more above the receiver's noise floor. This means any station you work has to be about 50 dB above the noise floor, or you won't hear them.
Although somewhat off the subject, it is important to note that some transceivers handle incoming band noise (however it is generated) better than others. Probably the most important attribute to look for in a mobile transceiver is one with the least front end bandwidth. As I write this, there are no commercial transceivers, designed primarily for mobile operation, which have roofing filters. This situation will certainly change in the near future, but for now, narrow IF filters (1.9 kHz SSB), and IF based DSP are our only options.
In simple terms, the Q of a loading coil is the ratio of the inductance and the resistance of the coil measured at the resonant frequency. Under laboratory conditions, it is possible to obtain coil Qs in the 800 to 1,000 range. However, in the real world of mobile loading coils, it is difficult to obtain Qs over 300 when mounted in (on) an antenna. Even this takes good construction practices.
Regardless of what hype you read or hear, most commercial antennas have Qs in the 100 to 200 range, and some antennas are as low as 50 (even less on 80 meters). Most spirally wound antennas fall into this latter category.
Large (6 to 8 inches diameter) bug catcher coils are often advertised as having Qs in the 1,000 range. Fact is, they're closer to 300. These big coils also have low self-resonant points negating their use on the higher bands. While bigger is better in most cases, there is a limit, and in mobile loading coils that is about 4 or 5 inches depending on construction.
Coil Q is important because it directly relates to the efficiency of the antenna as a system. It's difficult to measure coil Q without sophisticated lab equipment, but you can use a few thumb rules as a relative measure. For example, long skinny coils have low Qs. Short fat ones typically have high Qs. The best ratio is about 1:1. In other words, as long as they are in diameter. However, there are other deciding factors. Coils with a large amount of inductance, like those used for 160 and 80 meters, need to have a lower ratio. In this case, about 3:1 (3 times longer than the diameter) seems to be optimum. What's more, the size of the wire, the spacing of the individual turns, and the dielectric constant of the coil form all have major effects on Q.
When the coil in mounted in (on) the antenna, the Q goes down! After all, the whip and the mast are in close proximity, and these masses of metal take their toll. An even bigger toll is taken by large metal end caps you see on some commercial antennas like the Hustler® high power (?) coils. Screwdriver antennas are another example. The large metal masts these antennas require greatly effects the Q.
Keep in mind that the position of the coil in the antenna is important too. The higher up it is mounted, more inductance is required to resonant the antenna. This increases losses because the coil Q goes down (higher resistive component). Depending on the operating frequency, the ground losses involved, and whether or not a cap hat is used, the most advantageous position maybe anywhere from the base to nearly the top of the antenna. The reasons for this are beyond the scope of this article. In general terms, the middle of the antenna is the best overall position.
There is another Q factor we have to contend with, and that is the Q of the antenna as a system. The Q of a short HF mobile antenna is directly related to the coil's Q, the overall length of the antenna, the ground losses, and the other resistive losses including radiation resistance. You might notice these are the same losses we deal with in maximizing efficiency. In short, while we strive to increase efficiency, we also increase the antenna system's Q which reduces the effective bandwidth of the antenna. In other words, to increase efficiency we have to lower the resistive losses, or increase radiation resistance, or both. You can do both to a point, but there are diminishing returns with respect to cost, complexity, and of course physical size.
It is common to relate bandwidth edges to the points above and below resonance where the SWR reaches 2:1. This isn't much of a concern on 20 meters or above, but below 20 meters it is. Put another way, the bandwidth of an 20 meter antenna of reasonable quality will be about 150 kHz. The bandwidth of a similar quality antenna on 80 meters may be just 10 to 15 kHz. In other words, when we increase efficiency, we reduce bandwidth, all else being equal (see Cheating below).
This fact causes some amateurs to assume that inexpensive, low Q antennas are superior to some higher priced ones. The false assumption is, they don't have to retune their antenna as often so it's got to be better. Adding insult, a few misguided manufacturers tout their products extended bandwidths as an advantage. Both of these premises are false.
Unlike the song about horses and carriages going together, efficiency and bandwidth are inverse qualities. You can cheat them some by using a coax-fed (or internal) auto-coupler or small mobile tuner to extend (to a point) bandwidth, however, the fact remains it is a trade off we can't ignore.
The words when we increase efficiency, we reduce bandwidth, all else being equal, need a little more explanation. You can have a very narrow bandwidth antenna, that is very lossy. And, you can have a wide bandwidth antenna that is very efficient. However, we're discussing HF mobile antennas, so the statement is generally true.
Besides using a tuner, another way to extend the bandwidth is to use a shorted coax stub across the antenna terminals. Selecting the correct length will not only match the antenna's input impedance to the feed line, its reactance is exactly opposite the antenna's reactance with any given change in frequency. Thus the 2:1 bandwidth increases (typically 30% to 50%). While the trick works well for a single band antenna (a different stub is required for each band), it's not a good solution for a remotely tuned antenna.
Let's address short tapping once more. One of the basic design premises for all screwdriver antennas, is the fact the coil is not short tapped. Short tapping refers to shorting out some of the coil's turns to resonant the antenna. Contrary to popular opinion, this has very little effect on coil Q until a large portion (>70%) of the coil is shorted out. Even then the effect is minimal, and measuring the difference requires good laboratory equipment.
In the Technical Correspondence section of the September 2006 issue of QST (page 57), are a few paragraphs written by Dr. Jack Belrose, VE2CV. Jack explains how to use an antenna analyzer and EZNEC to calculate the efficiency of a mobile antenna. The basic premise is to compare the measured input impedance of your mobile antenna, compare it to the modeled impedance given by EZNEC, and then adjusting the ground losses until the two impedances equal. The scenario given works well, but it's very important to know the true Q of the coil. EZNEC calculations can come close, but other factors (stray capacitance for example) can skew the results.
Incidentally, Jack wrote a series of articles for QST (circa 1953) on the various aspects of mobile antenna efficiency. My introduction was via the 1960 issue of the ARRL Mobile Manual which contains a compilation of the articles. I still use the manual for reference, as it is the basis for all later treatises on the subject.