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[Articles Home]  [Add Article]  

80-Meter Base-Loaded Mobile Antenna

Cecil A. Moore (W5DXP) on December 8, 2010
View comments about this article!

Analysis of an 80m Base-Loaded Mobile Antenna

Cecil Moore, W5DXP, Rev. 1.2, 11/28/2010

Introduction

The previous article, Degrees of Antenna Occupied by a Loading Coil, showed how The Hamwaves Inductance Calculator can be used to determine the parameters of an 80m mobile loading coil. Note that the accuracy of the inductance calculator is thought to be 10%. The specifications for the 80m mobile loading coil are:

2 inches in diameter (50.8 mm)
100 Total Turns, 10 inches long (254 mm)
#18 wire (1.024 mm in diameter)
Design Frequency = 3.5 MHz

The above graphic shows how the parameters were entered into the inductance calculator and the following outputs were obtained.

Axial Propagation Factor = 1.8118 radians/meter
Characteristic Impedance: Z0 = 4747 ohms
Effective Series Inductance at 3.5 MHz = 98.6 uH
Effective Series Reactance at 3.5 MHz = 2168 ohms
Effective Series AC Resistance at 3.5 MHz = 3.86 ohms
Effective Unloaded Q of Coil at 3.5 MHz = 562

In the previous article, we converted the Axial Propagation Factor to degrees/inch by multiplying by 1.4553 and then multiplied by the 10 inch length of the coil to obtain the number of electrical degrees occupied by the loading coil at 3.5 MHz. We also calculated the Velocity Factor of the coil at 4% of the speed of light in free space. Knowing the VF of the 10 inch coil allowed us to calculate the RF propagation time through the coil.

Degrees Occupied by the Loading Coil = 26.37 degrees
Velocity Factor (VF) of the Coil = 0.04
RF Propagation Time through the Coil = 21.2 ns

3.5 MHz Base-Loaded Mobile Antenna Analysis

EZNEC was used to estimate the length of the whip above the coil in order to resonate the mobile antenna on 3.5 MHz. That length is 8.83 feet. So the 3.5 MHz base-loaded mobile antenna consists of the specified 10 inch coil at the base with a whip length of 8.83 feet. It is interesting to note that the reactance of the coil is j2168 ohms while EZNEC says the impedance looking into the 8.83 foot whip is 0.4-j2187 ohms which is in reasonable agreement.

One wavelength at 3.5 MHz is 281 feet so we can estimate that the whip occupies 8.83/281 = 0.0314 wavelength. We can calculate that the loading coil occupies 26.37/360 = 0.0733 wavelength.

The coil occupies 0.0733 wavelengths (26.4 degrees). The whip occupies 0.0314 wavelength (11.3 degrees). That adds up to 0.1047 wavelengths = 37.7 degrees. We know that the resonant mobile antenna is 0.25 wavelength (90 degrees) long. Where are the missing 52.3 degrees?

Smith Chart Representation

When we represent the mobile antenna on one half of a Smith Chart, the "missing" degrees are revealed. The 0.0733 wavelength occupied by the coil is plotted "toward the load" with the low impedance feedpoint on the left. The 0.0314 wavelength occupied by the whip is plotted "toward the source" with the high impedance open end on the right. We can now calculate where those "missing" 52.3 degrees come from.
The above image can be viewed in a larger format at: Smith Chart: 3.5 MHz Mobile Antenna The impedance at the coil to whip junction can be obtained by reading the approximately -j0.5 value from the Smith Chart at the top of the coil and multiplying by the characteristic impedance of the coil where Z0 = 4747 ohms.

Approximate impedance at coil/whip junction =
4747(-j0.5) = -j2374 ohms 10%

Note: There is a very small (less than one ohm) radiation resistance of the whip which can be ignored for the purpose of finding the "missing" degrees in this mobile antenna.

Knowing the impedance at the coil/whip junction allows us to calculate the characteristic impedance (Z0) of the whip. The normalized value from the Smith Chart on the whip end is approximately -j5.0 which we know from the above calculation is -j2374 ohms. Therefore, the Z0 of the whip at the coil/whip junction is 2374/5 = ~475 ohms which is a reasonable value.

We can now conclude that the "missing" 52.3 degrees is caused by the impedance discontinuity at the coil/whip junction, i.e. where the Z0 changes from 4747 ohms to 475 ohms. The same thing happens when two transmission lines are connected together if they have different characteristic impedances, i.e. different Z0s.

Conclusion

In a base-loaded mobile antenna, there are three components that contribute to the 90 electrical degrees required for resonance.

1. The coil contributes a certain number of degrees, 26.4 degrees in the above example.

2. The impedance discontinuity at the coil to whip junction point contributes a certain number of degrees, 52.3 degrees in the above example.

3. The whip contributes a certain number of degrees, 11.3 degrees in the above example.

Thus a base-loaded mobile antenna can be analyzed in the same manner that Dual-Z0 Shortened Stubs can be analyzed. The following dual-Z0 1/4WL stub analysis is virtually identical to the above base-loaded mobile antenna analysis. Note that the phase shift at the impedance discontinuity is proportional to the ratio Z0H/Z0L and 4747/475 = 500/50.

Other Configurations

Having a bottom section under the loading coil (center-loading) complicates the analysis by introducing an additional negative phase shift at the bottom section to coil impedance discontinuity where the Z0 changes from the low Z0 value of the bottom section to the high Z0 value of the coil. Because the phase shift at the bottom of a center loading coil is negative, the inductance of the coil must be increased to add to the degrees occupied by the coil, in order to compensate for the number of degrees lost by the negative phase shift. That's why, for the same overall antenna length, the center loading coil must have more inductance, i.e. must occupy more degrees of antenna, than a base loading coil configuration.

W5DXP will follow up on center-loaded mobile antennas with a short future article. The primary concepts concerning degrees occupied by a loading coil are covered in this present article.

Member Comments:
This article has expired. No more comments may be added.
 
80-Meter Base-Loaded Mobile Antenna  
by W5DXP on December 8, 2010 Mail this to a friend!
While re-formatting this article for the eHam.net web page, I left out a paragraph under "Other Configurations". Here it is:

There are two reasons why a center-loading coil must have more reactance, i.e. occupy more degrees of the antenna. The primary reason is that the whip on a base-loaded antenna is longer than the whip on a center-loaded antenna of the same length. A four foot whip exhibits much more capacitive reactance than an 8 foot whip so more inductive reactance in the loading coil is required to neutralize that added capacitive reactance.
--
73, Cecil, www.w5dxp.com
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K0CBA on December 8, 2010 Mail this to a friend!
To quote W.C.Fields.."Godfrey Danials!".
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by G3LBS on December 8, 2010 Mail this to a friend!
Just goes to show a picture is worth a thousand words
W2/G3LBS
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by WI7B on December 8, 2010 Mail this to a friend!

Thanks for the photos, especially the Smith Chart readings.

73,

---* Ken
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K0BG on December 8, 2010 Mail this to a friend!
One thing you forgot to mention Cecil. A standard 102 inch whip has some fairly high resistive losses at 3.5 MHz.

Alan, KBG
www.k0bg.com
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by WA2TNO on December 8, 2010 Mail this to a friend!
Alan,

Does the resistive loss of the whip matter much in a short base loaded antenna, once you get past the loss in the loading coil? I always wondered about this because the HF whip on a boat I used to work on was made from a pretty thin wire inside of a fiberglass tube. There was no capacitive hat at the top of the whip, so the amount of current in the whip itself must have been pretty limited in the low MHz region. I never tried to model it back then, but it always bugged me. It always seemed to me that the autotuner was working awful hard. The ships HF rig was a Kenwood marine product, very compact and simple to operate. I think the tuner, mounted directly under the whip, was bigger than the radio. Connection from the tuner to the whip was about one foot of insulated wire, with the insulation looking like the insulation on an ignition wire. The hull had an aluminum buffer rail that ran the length of the hull, that was ground for us, along with connection to a plate under the fiberglass hull.

The whip was around twenty feet. It worked pretty well, the marine bands in those days were pretty good for discipline, a little signal can go a long way when there is no QRM.

Bruce
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by N2RRA on December 8, 2010 Mail this to a friend!
Cecil,

Great job on this article and is the type that would make the "EHam Classics" in their article section. Worth reading and well layed out to understand. Bravo!



Alan,

Someone beat me to it ,but I will ask. Is there really any significant difference in signal, gain, or any other between stainless whip or fiber glass?

Would that highly resistive capacitance in loss would be different if it were center load some how?

73!



 
RE: 80-Meter Base-Loaded Mobile Antenna  
by N2RRA on December 8, 2010 Mail this to a friend!
Alan,

Ment to say....Would that highly resistive loss would be different if it were center load some how?
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by W5DXP on December 9, 2010 Mail this to a friend!
> K0BG wrote: One thing you forgot to mention Cecil. A standard 102 inch whip has some fairly high resistive losses at 3.5 MHz. <

Alan, I'll correct that oversight on my web page. However, for the purpose of determining the approximate number of degrees occupied by the loading coil, those "high resistive losses" can be ignored. Here's why:

The feedpoint impedance of a 102 inch whip used on 3.5 MHz might be in the ballpark of 15-j2200. Of course, that feedpoint resistance is of primary importance in calculating efficiency but my article is not about efficiency and vars >> watts on the whip.

15-j2200 ohms = 2200 ohms at 89.6 degrees. For the purposes of determining the approximate degrees occupied by the coil, we can round that off to -j2200. After all, the accuracy of the Hamwaves inductance calculator is thought to be around plus or minus 10 percent and 15 ohms is 0.68% of 15-j2200 ohms. In fact, we can assume a completely lossless system and the degrees occupied by the whip (or coil) will not change appreciably.

However, if we were discussing efficiency, Rloss > Rradiation so Rloss is of *primary importance*. Thanks for pointing out that potential point of confusion.
--
73, Cecil, www.w5dxp.com

 
RE: 80-Meter Base-Loaded Mobile Antenna  
by W5DXP on December 9, 2010 Mail this to a friend!
> N2RRA wrote: ...Would that highly resistive loss would be different if it were center load some how? <

In my "ARRL Antenna Book", 20th edition, page 16-5, it compares a base-loaded antenna to a center-loaded antenna. The approximate efficiency can be calculated by dividing the radiation resistance by the feedpoint impedance.

Base-loaded on 3.8 MHz: 0.35/16 = 2.2%

Center-loaded on 3.8 MHz: 0.8/22 = 3.6%

That's a 2+ dB difference. Actual measured difference at a CA 75m mobile shootout was 3 dB.
--
73, Cecil, www.w5dxp.com
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K0BG on December 9, 2010 Mail this to a friend!
I wish it were so simple as to just compare a stainless steel whip to a fiberglass one with a wire up the center. For example, we don't know the exact dimensions of the wire; whether it is round, or flat; and we don't know if it is wound around, or straight the FB form? While most whips are 17-7 stainless, some of the latest ones are made from 17-4. While similar, they do have different permeabilities. Here's some test data worth reading: http://vk1od.net/antenna/conductors/loss.htm

I recently spend about $150 getting a stainless whip coated first with nickel (you can't copper plate one directly), and then copper plated. When compared to an identical whip without plating, there is a slight difference in unmatched input impedance (&#8776;2 &#937;). There is an increase in field strength measurements too (&#8776;2 dB). While the change isn't all that much, there are times when even 1 dB determines a contact. Whether that is worth $150 to your average Joe Ham, remains to be seen.

Incidentally, you can do almost as good by covering the whip with silver plated, copper braid. That cost almost nothing, other than a few pieces of left-over RG228.

Alan, KBG
www.k0bg.com
 
80-Meter Base-Loaded Mobile Antenna  
by K1XT on December 9, 2010 Mail this to a friend!
K0BG wrote :

"For example, we don't know the exact dimensions of the wire; whether it is round, or flat; and we don't know if it is wound around, or straight the FB form?"

If yo refer to fiberglass whips, all fiberglass whips I have opened up have #18 straight up the center.

 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K0BG on December 10, 2010 Mail this to a friend!
I have one, assumingly made by Shakespeare, and it uses a copper ribbon with a very slow twist around the core.

Alan, KBG
www.k0bg.com
 
80-Meter Base-Loaded Mobile Antenna  
by K1XT on December 10, 2010 Mail this to a friend!
K0BG:
"I have one, assumingly made by Shakespeare, and it uses a copper ribbon with a very slow twist around the core."

Interesting. Is it somewhat coiled then? Is it near resonant at a 1/4 wave as to its physical length, or is it lower in frequency? The ones I have used have been CB whips at 102". I have shortened a few according to what coil/mast combination I use.

 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K0BG on December 11, 2010 Mail this to a friend!
It is in the attic, but as I recall it is 98 inches long. It does have a marine fitting on the bottom.

Alan, KBG
www.k0bg.com
 
80-Meter Base-Loaded Mobile Antenna  
by WA1RNE on December 12, 2010 Mail this to a friend!

Cecil,

Nice job. In the "Other Configurations" category, have you tried a capacitance hat, and even a step further, center loading with a capacitance hat?

It's been done by others many years ago, but for modeling purposes and as a comparison to the very high feed impedance of the base loaded configuration,
it would be very interesting to see graphically as you've shown quite well here, possibly as Part 2 of this article.


....WA1RNE
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by W5DXP on December 12, 2010 Mail this to a friend!
> WA1RNE wrote: In the "Other Configurations" category, have you tried a capacitance hat, and even a step further, center loading with a capacitance hat? <

I have figured out how to estimate the number of degrees occupied by a wire and/or a loading coil, but I haven't given it enough thought to figure out how many degrees are occupied by a top hat. At the moment, my mind is occupied with the problem of easily measuring the actual delay through a loading coil and I have had an epiphany. It is a trivial matter to measure the actual delay through a loading coil if one understands the basics. There is a key requirement in the test setup which W8JI didn't meet in his test setup at:

http://www.w8ji.com/inductor_current_time_delay.htm

Therefore, his reported results are not valid. The key requirement is to adjust the load resistor value until the current magnitudes are equal at each end of the loading coil - then measure the phase shift. This is such a simple concept, I can't believe it has taken me five years to realize it. So I'm busy writing yet another article.
--
73, Cecil, w5dxp.com
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by K5LXP on December 13, 2010 Mail this to a friend!
Wouldn't the known/fixed source and load Z on a network analyzer net you the same current magnitude on both ends?


Mark K5LXP
Albuquerque, NM
 
RE: 80-Meter Base-Loaded Mobile Antenna  
by W5DXP on December 14, 2010 Mail this to a friend!
> K5LXP wrote: Wouldn't the known/fixed source and load Z on a network analyzer net you the same current magnitude on both ends? <

No Mark, it won't. Just as the source impedance has no effect on SWR, it also has no effect on the ratio of the currents at each end of a coil. That ratio is determined by the characteristic impedance of the loading coil and the load impedance (both limited to resistive values for this discussion).

Steve, G3TXQ, sent me some current bench measurements that he had made on a large air-core coil which prompted me to recognize the pattern. I then used EZNEC to confirm what I thought I was seeing. EZNEC has the advantage that the source current can be set to a constant 1.0 amps at 0 degrees. With the load resistor set to a value below the Z0 of the coil, the load current is greater than 1.0 amp. With the load resistor set to a value above the Z0 of the coil, the load current is less than 1.0 amp. With the load resistor set to the Z0 of the coil, the load current is 1.0 amps and the reflection coefficient at the load is zero, i.e. no reflections, i.e. *that 1.0 amp of current is a pure traveling wave*.

The point is that to validly determine the actual delay through a loading coil using W8JI's procedure, the reflection coefficient at the load must be close to zero. When the current magnitude at each end of the coil is equal, the reflection coefficient is close to zero. (The Rloss of the coil will cause a negligible error because Rload >> Rloss.)

I have no idea how accurate EZNEC is for this purpose so consider this a qualitative discussion rather than a quantitative one.
--
73, Cecil, w5dxp.com
 
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