Re: size not a major consideration in wireline phone sets [Telecom]

So that is what is under that cap on the back of the phone. The price for that adapter is fine, now if it would work with a standard external antenna, I can't see spending that kind of money, maybe if I was in the car a lot and did my work there.

WB6VHI

Use the shortest possible available coax between the adapter and the antenna. At those frequencies an large amount of the cell phone power goes into warming up the coax.

Tony

That's always the way for any RF line to a new antenna position, you have to calculate if the line losses are outweighed by the extra signal by the newer/better/higher antenna, otherwise there isn't much point.

It's almost waveguide rather than coax territory for these frequencies now?

In article ,

In a test-lab environment, something like that is trivial to do. :)

For an end-user, in the real world, it's somewhat more complicated. If you have some sort of signal-strength indicator on the phone -- i.e. a " how many bars" thing -- *AND* you can get far enough away from the 'nearest' tower so that it shows a 'less than max' strength on the built-in antenna, then you can switch to the external one, and, maybe, see a visible improvement. Or, go out in the boonies -- just to the point you get a 'no service' when using the external antenna -- and see how much closer you have to get to civilization before service re-establishes itself.

I know from personal experience with an old analog "bag phone" (a Motorola), mag-mount whip on the middle of the vehicle roof gave me a _lot_ more range than the rubber-duckie on the back of the brick. e.g. I had service out in the "middle of nowhere" in th Rockies, some 17+ miles off the nearest paved road, in BLM wastelands.

That same antenna significantly outperformed the rubber-duckie, used *inside* an AMTRAK "Superliner" train -- antenna stuck to the inside of the roof of the rail car.

Depends *greatly* on the path. Coax has significantly higher losses per linear foot. Waveguides have higher losses at turns/bends.

Um, OK, but I wasn't looking for something that involved: the question should be "Is there a kind of waveguide that I can put in my car to get better cell coverage than I can get with coax?".

Bill Horne Temporary Moderator

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For what it's worth, [my] emegency CB kit said the mag mount antenna-- placed on the metal surface of the car--was designed to work with the metal, that is, it made the metal part of the antenna. You weren't supposed to just hand hold the antenna or put it on the ground. I don't know the physics, but presumably the metal surface acted as some sort of radio wave resonator for the signal. (Maybe the radio folks here could explain this more accurately).

My present digital cell phone handset has a pull out antenna a few inches long. I don't get to fringe areas, but I had hoped the antenna would save on battery consumption, [however] this handset only gives about 75 minutes of talk time since it was new. (Now it dies at 55 minutes, but after it shuts down I can turn it on the next day and get another 20 minutes out of it when it dies for good and I have to recharge it.) I don't use the phone that much, but if I do go away for the day, I do need more talk time between charges in a single day's usage.

Bill Horne asked:

This is a complex issue. Coaxial cable almost always has considerably higher attenuation per unit length than waveguide. But waveguide requires specialized bends since flexible waveguide has its limits on how much it can be bent and still work properly. But the biggest issue is size.

Standard rectangular waveguide for the S-band (2.60 to 3.95 GHz) is WR-284 in the EIA designation and also as RG48 or WG10. This is 1.34 x

So waveguide would be an ideal transmission line at a tall cellular tower having far lower losses than coaxial cable. But it would not be practical to connect a hand-held cell phone to an auto's rooftop antenna.

Probably the largest waveguide I have ever read about was at the Arecibo Radio Telescope in Puerto Rico. Its lower cutoff frequency was below 400 MHz. Today, the receivers are mounted at the "feed horn" eliminating the need for such immense waveguide.

I once visited the laboratory originally used by Charles Townes at Duke University. They were doing research into millimeter wavelength frequencies at the time. Instead of using waveguides, they were using quasi-optical techniques to transfer the millimeter waves around the room. What I found especially interesting was the use of lenses machined from PTFE (Teflon) to focus the beams. While Townes is mainly known for his work in the maser and the laser, he is also considered the father of microwave spectroscopy.

I was thinking that it may be possible to include a waveguide as part of the car's body: your S band dimensions would work for some of the support pilars (and I reserve the rights if anyone does it), but the question is how precisely a waveguide must be machined.

I'm surprised at the difference in loss of waveguide vs. coax: all the cell sites I've ever seen appear to use coax, so either they're using the flexible type or the cellular engineers are employing the coax loss to contribute a large part of their loss budget for the antenna arrays being used.

Bill Horne Temporary Moderator

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Waveguide is moderately forgiving as long as you do not push the frequency close to cutoff. [Waveguide functions similarly to a high-pass filter.] With some simple adjusting screws, mismatch could be tuned out. But at each end of the waveguide you would likely need coaxial to waveguide transitions. You would also need to electroplate the inside of the waveguide to increase its conductivity. However, I really like your idea. Thinking out of the box...

Around here in rural South Carolina, large coaxial cables are used too. I believe they are Andrews Heliax cables or their equivalent. Depending on the size of the cable, they may use closed-cell foam dielectrics or even a spiral of insulation between the center conductor and the outer conductor (air dielectrics). Air or nitrogen has much lower dielectric losses than does plastic insulation. These are designed to be moderately flexed during installation only as the conductors are usually corrugated pipe. These cables have considerably less loss than the small, flexible cables usually used with Ham and CB installations.

At one time circular waveguide was considered by the telephone system for replacing long lines. However keeping the polarization maintained was a problem, Then fiber came along and the waveguide idea was dropped. [Well not exactly, the fiber is an optical waveguide!]

And cost.... Waveguide was/is solid copper, and recently many siteowners have had it stolen.

The AT&T 2Ghz KS-5759 antennas used plastics as delay lines to focus the output.

Note the coax types he cited would NEVER be used for microwave; the larger Heliax [?sp] coax style line is often air-insulated, not foam, and has much less loss.

Waveguide is expensive to buy, and expensive to install, and needs dry air and ....

You sometimes see long runs of waveguide & then transitions to coax for the final connection. There is also flexible [and I use the term loosely; "not rigid" might be better....] waveguide.

________________________________

The metal roof was supposed to act as a ground plane. But it was an equal voltage plane near 0 volts. It acts as a mirror and gives the whip an apparent length twice it's actual length. For each frequency there is an ideal whip length. For FM it's ~30 inches. The new cars often cheat by wrapping the antenna spirally around the rod to make the whip shorter. Rabbit ears are an example of an antenna that doesn't need a ground plane. It's differential between the two ears.

Mark L. Smith

Waveguide today is generally aluminum with the flanges and mitre bends hot-dip brazed. It has lower loss than brass which is sometimes used, but slightly more loss than copper. Large Andrews Heliax cable has lots of copper too.

The sandwitching of plastic with thin sheet metal creates an artificial dielectric as noted in the article. In the shorter millimeter wave (terahertz) region, solid PTFE is practical. Because of the differences in wavelength, the PTFE does not have to be polished to a high degree like a lens for visible light.

When we think of lenses, we think of the refractive index of the material the lens is made from. But refractive index is the square root of the dielectric constant.

But these small cables are used with computer networking antennas. I chose those cables to demonstrate that the added range obtained by elevating an antenna is often lost due to the increased cable losses. Andrews Heliax transmission lines still have more loss than waveguides. The gas insulated lines with spiral insulation begin to have problems at higher frequencies with propagation modes other than TEM being possible. And these cables require pressurization systems in the same way that waveguide does.

Yes, flexible waveguide is very much akin to liquid tight flexible metal conduit - only stiffer. It is also much lossier than regular waveguide. Short sections of flexible waveguide are sometimes used at the ends of long runs of regular waveguide because the thermal coefficient of expansion of copper or aluminum is quite different than that of the steel used to make the towers that support the antennas.

I wish I could answer more of Bill Horne's questions about microwave links and licensing. Unfortunately my specialty was online chemical instrumentation. Hence my knowledge about microwave spectroscopy and dielectrics. I do hold a FCC General Radiotelephone license (grandfathered from 1st Class) with Ship Radar Endorsement* and the Amateur Extra Class ham license. but these are not applicable to Bill's questions.

(i.e.trans-continental) microwave routes used horn reflector antennas

One drawback of this waveguide was that it supported more than one mode: 2 or 3 modes in the 3.7 to 4.2 GHz band (depending on frequency), and 6 modes in the 5.925 to 6.425 GHz band. Any non-uniformity, and also the transitions from square guide (used by the frequency and polarization combining networks) at the base of the tower to the circular guide, and from the circular guide to the antenna, would convert some of the energy to and from the dominant mode to the higher-order modes. Each mode traveled at a different velocity, so when energy from a higher-order mode reconverted to dominant, an echo resulted. The towers ranged up to 250 feet high, and thus the echos had appreciable time delays.

One of the worst offenders was an 8-foot length of flexible circular waveguide which connected the top of the vertical waveguide run to the feedhorn of the antenna, to allow the antenna to be pointed in elevation. Eventually, we had to replace each pieces of flex guide with a piece of rigid guide especially bent to shape, on site, to fit between the antenna and the vertical waveguide run. First, the crew measured the relative positions of the two flanges. Then an 8-foot piece of rigid guide was packed with sand to keep it from buckling, and put onto a machine which put a gradual bend in the guide.

You bring back a memory: my father was a pluber, and he used to bend pipe that way when he had to have a non-standard angle. I used to hold the torch, and he'd heat a copper pipe to orange, then use a lalley column to form the bend: every so often, there'd be some moisture in the sand, which would flash-burn any human flesh within a foot of the ends.

BTW, please explain what "Mode" means in this context. TIA.

Bill Horne Temporary Moderator

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