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Mobile Antenna Notes

Alan Applegate (K0BG) on January 20, 2003
View comments about this article!

The majority of HF mobile antennas are electrically short. For example, an 80 M mobile antenna 8 feet long is electrically 11 in length as opposed to 90 of a full-sized 1/4 wave vertical. Thus, the input impedance is around 15 to 25 ohms depending upon loading coil position and Q factor, and ground and resistive losses. The formula is as follows:

Rt=Rr+Rc+Rg


Where Rt=total or input resistance (more correctly input impedance), Rr the Radiation resistance, Rc the coil resistance, and Rg the ground loss resistance.

We have some control over the radiation resistance, but for our example above, the Rr is under 1 ohm! Since it is a factor of the electrical length of the antenna (loading coil position is also a factor), we must lengthen the antenna to increase it. Obviously, there's a limit here. After all, who could drive around with a full 1/4 wave, 61 foot, 80 meter vertical on their vehicle?

The Rc is the resistive component of our loading coil, which cancels the high capacitive reactance of our short vertical. On 80 meters, the coil will have between 75 to 200 uh of inductance depending upon where in the antenna it is located. The higher up the mast it is located, the higher the radiation resistance, but the larger the coil (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 determines the "Q". The higher Q, the less loss, and the more efficient the antenna will be. On 80 meters it is difficult to obtain Qs much over 200 and even this much requires good construction practices. The ratio between diameter and length to maximize Q is 2:1. Imagine running around with a spare tire-sized loading coil!

The Rg or ground loss resistance typically varies between 2 and 10 ohms, but can be much higher. It's mostly a factor of the size of the vehicle our antenna is mounted on, and how and where it is mounted. Remember our vehicle is not a ground plane for the lower frequencies, but rather a capacitor to ground. Rg can be minimized by proper grounding (bonding) 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 to ground (we want the vehicle coupled to ground, not the antenna). Avoid using bumper mounts, magnet mounts, trunk lip mounts, and similar devices.

In just about every case, an HF mobile antenna will have an input impedance of less than 50 ohms, and typically between 12 and 35 ohms. Obviously if we wish to obtain a low VSWR, we need to match our 50 coax feed to the antenna. There are several ways to do this, but the easiest is to use a 4:1 unun.

Attached to this article is a schematic diagram of a 4:1 unun. Unun stands for Unbalanced to unbalanced, as opposed to Balun, which is Balanced to unbalanced. To the left is our 50 ohm input, and on the right a 12.5 ohm output. By tapping 1/4, 1/2, and 3/4 of the way along the top winding, 1.5:1, 2:1, and 3:1 ratios can be obtained. Or, 33 ohms, 25 ohms, and 16 ohms. In all but the extreme cases, one of these taps will closely match just about any mobile antenna. Using the chart in the ARRL Antenna Book, or by using an antenna bridge like MFJ's 259B, we can guesstimate or measure the input impedance of our antenna and select the best tap.

Construction is easy even for a novice builder, and the ARRL Handbook gives a lot of good tips on how to wind one. In short, the untapped winding should be insulated from the tapped winding with Teflon or high-temperature plastic sleeving or tape. A T200-6 is the best bet, but a T200-2 can be used. The core should be wrapped with glass tape if high power is contemplated. AWG 14 wire is adequate for 1,000 watts of power, and will allow an 11 to 12 turn bifilar winding. The transformer should be mounted close to the base of the antenna, and protected from the weather. Cores, glass tape, and Teflon sleeving may be ordered from Amidon, Palomar Engineering, and many other sources.

Now let's digress for a few moments and take another look at the inherent losses of a short, mobile antenna. Referring to our original formula above, Rt=Rr+Rc+Rg, let's read between the lines. If Rt is say 40 ohms, and affords us a 1.25:1 VSWR, what and where do you think the other losses are? Most generally they are in the coil, and to a lesser degree the ground losses. You might ask then, what does a lossy coil looks like? Well, one thing's for sure, it isn't spare-tire sized!

Many commercial mono band and multi band short mobile antennas will match 50 ohms quite well due to their resistive losses. This author recently measured a 6 foot long, helically wound, bumper mounted antenna with an MFJ 259B, and got 64 ohms at resonance on 40 meters! Its approximate efficiency is under 2%. That is to say, fewer than 2 of those 100 watts inputted were being radiated, and the guy bragged about how well it matched! Well guess what? I have an antenna with a 1:1 VSWR from DC to 1.2 GHz. It's called a dummy load! If your antenna matches as well as this one did, you need a better antenna.

One last bit of information... A VSWR bridge cannot measure phase angle, and therefore cannot be used to determine the actual resonate point of any antenna other than a purely resistive one, and that just doesn't happen very often, especially with mobile antennas. What it does measure is the (lowest) voltage, and because an antenna has reactance, the voltage and current are not in-phase. Without a good noise bridge or antenna analyzer, it is mostly guesswork. In short, don't rely on a VSWR bridge to adjust your mobile antenna.


Alan Applegate, KBG

Member Comments:
This article has expired. No more comments may be added.
 
Mobile Antenna Notes  
by K0BG on January 20, 2003 Mail this to a friend!
The schematic for the UNUN didn't get uploaded correctly. Anyone needing it can email me and I'll pass it on.

Alan, KBG
k0bg@aol.com
 
RE: Mobile Antenna Notes  
by K0BG on January 20, 2003 Mail this to a friend!
I've had a few e-mails asking for the schematic of the unun and information on how to wind one. Not everyone has a handbook, so here are the basics. If you have never don't this before, you might ask for help from someone who has.

#14 awg Thermalese enamaled wire is adequate for 1,000 watts, and you'll need two 3 foot lengths. You should cover the core with a layer either plastic electrical tape, or glass tape if you're running high power. Although I suggested insulating the tapped winding, it really isn't needed if one is careful when you wind the turns. Three feet is enough to get 10 or so turns around a T200-6 core. Eleven or 12 turns is OK too. The turns should be side-by-side, should not be twisted, and evenly spaced around the core.

The last four complete turns on one of the windings needs to have a small loop twisted in it for the taps. Each pass through the core is one turn.

Then wire like the schematic shows. Again, I'm sorry the schematic didn't get posted, but I'm happy to pass it along via e-mail.

The connections should be soldered, however, the taps could be selected by using strong alligator clips if you're running low power. For those into building, it can be inclosed in a box if you leave an inch of clearance around the core. You could even use a switch to select the taps, or use separate connectors like the handbook shows. You might want to mark both ends of one of the wires before you start so you can tell which is which. Or use a ohm meter.

By the way, this is just a 4:1 UNUN with taps for the other impedances. The efficiency is on the order of 96% to 98% depending on which tap you use.

Incidentally, you should mount this as close to the base of the antenna as you can, and protect it from the elements.

Good luck with the project.

Alan, KBG
 
RE: Mobile Antenna Notes  
by W4ARM on January 21, 2003 Mail this to a friend!
Please forward the diagram that was not included in the article on eham.net.

Thanks!
W4ARM@arrl.net
 
RE: Mobile Antenna Notes  
by KE1MB on January 21, 2003 Mail this to a friend!
Is there an air core version of this?
 
RE: Mobile Antenna Notes  
by K0BG on January 21, 2003 Mail this to a friend!
Yes, actually there is. Johnson used to make a 4:1 unun that was airwound, and it was about 9" square and 5 " deep.

The real advantage you have with an unun is you nominally don't have to change the tap when you change bands. With either L or C shunt matching, you do.

If you have the parts, it takes all of about an hour to make an unun. Over the years, I've probably make a dozen of them.


Alan, KBG
 
Mobile Antenna Notes  
by KA5TIZ on January 21, 2003 Mail this to a friend!
As you construct mobile antennas it is good to remember that not all losses are equal. Resistive losses produce heat and are a total loss, while inductive losses are resulting from radation in part. Since radiation is the function of the antenna it is reasonable to do everything possible to eleminate resistive losses and compromise on inductive losses. All reduced size antennas are lossy, and we benefit by chosing our design to favor the greatest total radiation. Some antennas are constructed with extreem high Q when a lower Q might radiate more effecently. Good luck with your personal design.
 
Mobile Antenna Notes  
by KA5TIZ on January 21, 2003 Mail this to a friend!
As you construct mobile antennas it is good to remember that not all losses are equal. Resistive losses produce heat and are a total loss, while inductive losses are resulting from radation in part. Since radiation is the function of the antenna it is reasonable to do everything possible to eleminate resistive losses and compromise on inductive losses. All reduced size antennas are lossy, and we benefit by chosing our design to favor the greatest total radiation. Some antennas are constructed with extreem high Q when a lower Q might radiate more effecently. Good luck with your personal design.
 
RE: Mobile Antenna Notes  
by K0BG on January 22, 2003 Mail this to a friend!
Gene, you are only partially correct. While there is some radiation from the coil, it is so small it typically is not included in any calculation. I refer you to J. Belrose's book "VLF, LF, adn MF Antennas" for a better description of this phenomena.

As for a lower Q radiating more efficiently, this is incorrect. Some amateurs believe that an antenna which matches well will radiate well. Certainly a 50 ohm dummy load matches well, but it sure doesn't radiate well.

Further, Q is something every mobile antenna maker touts by saying his is better than anyone else's, when in fact, they are universally poor compared to what you can build yourself.

But make no mistake about it, the higher the Q of the coil, the less the over all losses are, and the more efficient the antenna will be.

Alan, KBG
 
RE: Mobile Antenna Notes  
by WB2WIK on January 22, 2003 Mail this to a friend!
Absolutely. Q is a ratio of reactance to resistance, and the "perfect" reactor (such as a coil) has no resistance at all.

Only the resistance of an inductor can dissipate power and have loss.

And even some quite large inductors have plenty! If you refer to the latest issue of QST (Feb '03) and read the product reviews on "high power" antenna tuners, you'll see how much some of them have!

WB2WIK/6
 
Mobile Antenna Notes  
by KA5TIZ on January 25, 2003 Mail this to a friend!
My comments are not intended to argue the mathamatics of antennas, but are based on personal experance and results from mobile antenna contests. In general antennas that show a high Q will radiate more effeciently than those with lower Q, however there are exceptions. For example screwdriver antennas often outpreform larger coils and heavier wire used in bugcatcher designs. Sometimes antennas that have the coils more near the base than in the center give the strongest preformance. Perhaps the challange is that antennas are much more complex than is allowed by the math used to describe them. Experience however often demonstrates that an antenna may preform better than the mathematicians would allow or suggest. Also, it seems apperant to me that many are not understanding the information that they are publishing. One of the best examples that I know is QST and their articles describing increadible losses in various antenna tunners, while they claim others are very effecient. I have extensive experience building and using my own antenna tunners, so a few years ago when they came out with what I believe was their first series of articles, I set up various tests with simple units that I have built. These tunners were equlivent to the ones they tested and I found that when they were tunned correctly, nearly no losses could be found in the tunners, but I could adjust them so that nearly all the power was lost in the tunner. With proper use the tunner they rated as worst were as good as the best that they tested. The Arrl has lost my subscription and a lot of respect because they are misrepresenting the facts about antenna tunners. I am sure that the authors have the math to prove their position, but that does not make it true. Construction is a lot of fun. Do your best and enjoy your project, and don't give up if your antenna is as lossy as the math says that it is. Just build a better mousetrap.
 
RE: Mobile Antenna Notes  
by W8JI on January 31, 2003 Mail this to a friend!
Q is used many ways. There is loaded and unloaded Q. There is the Q of a component, and the Q of a system containing a component or components.

EM radiation from a linear-wound coil is actually just about perfectly proportional to its physical length, just as if it were a section of the actual antenna.

With that in mind, it's doubtful the coil has much "good" resistance.

The UNloaded Q of the coil need to be as high as possible, and the reactance as low as possible, to minize what you are really trying to get rid of....and that is ESR. ESR is the equivalent series resistance.

You want both minimum ESR and minimum current, so even if loss resistance (ESR) increases by moving an inductor higher in the antenna, losses can be reduced if the current has a chance to change. This is NOT the case in a very short antenna, because current is essentially uniform in the area below the antenna.

Loaded or system operating Q is a function of the amount of reactance required in the coil, capacitance shunting the coil (that is a very bad capacitance since it increases ESR drastically while reducing bandwidth), and loss resistances.

You want the lowest possible operating Q by minimizing coil reactance WITHOUT shunting the coil with capacitance, and without adding series resistance or reducing radiation resistance.

On lower bands, there is virtually no difference at all in ground resistance by antenna position, as long as you are getting a short connection to the vehicle chassis and sheetmetal.

My full size Super Duty extended cab truck with long bed has about 25 ohms of ground resistance on 160 meters, and over 10 ohms on 75 meters. I can move what might APPEAR to be that resistance all over the place by moving the antenna around, but what I am really doing is forming a closed loop and increasing circulating currents between the antenna capaciatnce and chassis and making myself "think" I made ground losses change.

Factually the idea you can measure ground loss by subtracting out coil and the tiny radiation resistance virtuually never works. FS measurements will prove that, and will prove that the lowest resistance might not be the best configuration. I can build a configuration that makes the ground loss look like 2 ohms, and actually has less efficiency than one that appears to have 10 ohms of ground loss.

It's virtually impossible to explain everything in one short article, and the guy did a good job. Sorting oout the mixing of models and folklore would take several pages.

The only thing I would add is get the hat up near the very top, or don't use it. Keep the coil above the sheetmetal, and use the best component you can.

As for matching, I never waste time with transformers. Ive measured shunt capacitors, transformers, and simple shunt inductors and could not measure the diffference between them on my commercial FSM, which has accurate resolution to 0.1 dB.

I use a simple shunt coil because it moves the correct direction with frequency change, is easy to tweak, handles any amount of power, and has exactly the same FS as an UN UN or any other efficienct matching method.

My best DX is a few VK's on 160 meters, and Europe on SSB 160 while actually DRIVING down the highway (and not on or near the coast, either). I've worked VK longpath on 40 with no sweat. So an eight foot tall antenna can do very well, if you pay attention to details.

73 Tom


 
RE: Mobile Antenna Notes  
by K0BG on January 31, 2003 Mail this to a friend!
Gene and Tom, let's try and not mix apples and oranges. Gene is correct; Both UNloaded and loaded Q are important. but, we're talking two different things here.

Antenna Q is VERY important, and all other considerations aside, the higher the unloaded Q of the coil, regardless of where it is placed in the antenna, the higher the better the performance. And regardless of what you read, the average Q of most antennas, with few exceptions, is in the neighborhood of 150 to 200 best case. One of the best known screwdriver units out there is less than 50 over all but its highest bands. And short tapping (shunting turns) will in most cases halve the Q. There is a difference of opinion about this even between the various publications by the ARRL and their authors.

Antenna tuners are more esoteric in nature because there are two Q factors to consider; Loaded and unloaded.

The actual components need to have a high Q to minimize I squared R losses, and loaded Q (under conjuate match conditions) must be low for the same reason. And Tom, you may have cancelled your ARRL membership for the wrong reason.

The ARRL recent "shootout" between antenna tuners is as fair and reasonable as any I have ever read. And the facts are dead on. The reason "T" type tuners are lossy is because their loaded Q is typically high?>15 and as high as 50! The TenTec unit on the other hand has a loaded Q of about 10.

The TenTec is an LC and the main shortcoming is that the values of the shunt C must be rather high, where as the values of the series C in a "T" match are relatively low. Incidentally, this is its only attribute. Running legal limits through a "T" match into other than a 4:1 or lower VSWR produces a shack heater to rival any modern amp.

As for ground loss, here again this is a debatable issue. Jack Kuecken says it runs an average of 15 ohms. I disagree. If you mount a good quality mobile antenna over a groundplane of 120 radials of 100 feet in length, and the input of the antenna is say 15 ohms, and you mount it on your car and it measures 20 ohms, then the ground loss must be very near 5 ohms.

It has been my experience that ground loss can be as high as 25 ohms, and as low as 5 ohms from 160 through 10 meters. Mounting height matters, bonding the various parts of the vehicle matters, and mounting method matters.

One quick test is to mount your antenna and measure the input impedance. Then bond the various parts and measure the input again. Don't take my word for it, try it. And don't be surprised if it goes down by 20 to 30% after proper bonding.

Alan, KBG
 
RE: Mobile Antenna Notes  
by WB4PGT on April 16, 2003 Mail this to a friend!
Could you send me the missing diagram on ur Antenna matching unum

73& TNX Russ WB4PGT
 
Mobile Antenna Notes  
by AC6DN on July 22, 2003 Mail this to a friend!
I have a Don Johnson DK3 antenna. I had one of his original Screwdriver antenna too, it had a UnUn toroidal balum, later I had the one with 10t (turn coil) at the base.
What are the differences between the toroid UnUn Vs. an 10t coil at the base of the antenna? Loss, Q, etc???
 
Mobile Antenna Notes  
by N1KK on October 2, 2003 Mail this to a friend!
Could I get the article of the 4:1 unun sent
to me.

thanks,

Ken
N1KK
 
RE: Mobile Antenna Notes  
by N6GBW on May 11, 2004 Mail this to a friend!
Please pass on the schematic/drawing for the unun transformer.

Thanks & 73s, Bob
 
RE: Mobile Antenna Notes  
by KI4CRS on February 18, 2005 Mail this to a friend!
can you please send me a copy of schematic for the UNUN.
 
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