Measure Tape Antenna efficiency

After spending some time searching for a DIY friendly portable antenna for my Baofeng, I come across a measure tape antenna who has a priori all the benefits I was searching for: foldable (particular importance at VHF), cheap, DIY friendly, good reviews, etc. But some thing made me some noise in my head that impede to me to start intermediately making one: Skin-effect.

Skin effect is the interaction between the variable magnetic field generated by the current circulating in a conductor and its same current. the effect is detrimental because magnetic field tend to interact negatively with charge carriers generating a net force who push them outwards, reducing on this way the effective conducting area of the conductor. this phenomena was studied extensively on round conductors (one of the best cases) and because of the geometry of this particular problem the charge distribute uniformly over the perimeter of the conductor.

Some conductors don’t have the same luck, for example a rectangular bar carry most of the charges on a really small part of his cross-section, on this way the electrical resistance at high frequency will be much higher than his DC counterpart.

Source: https://www.edn.com/power-tip-26-current-distribution-in-high-frequency-conductors/

Tape measure antenna suffer from this phenomena the same way as the last example but with a added disadvantage: is made from a carbon-iron instead of copper. This gives two new problems to the table: First DC resistance will be an order of magnitude higher and second the magnetic permittivity of iron worsen the magnetic field inside the conductor in respect to skin effect.

After taking into consideration all those drawbacks one must say that an tape measure antenna is more like a tape measure dummy load with leaks. The fact that this antenna has widespread small satellite use implies some success after all, but some improvements are needed.

Simulations (F.E.M.M.) shows how bad is the problem at UHF frequencies. Current density on the edge of a measure tape antenna D:10mm T=100um, F=433MHz I=1A.

But a simple fix to this problem was always in known of the RF industry: copper plating conductors. Almost all 75ohms coaxial cables installed by ISP’s have a center conductor made of an iron base with copper plating, this technique can be used on this antenna as well but the plating must be thin to retain the springy feel of the iron tape but thick enough to carry almost 90% of the feeding current.

When using electrolytic plating method, plating tends to accumulate over sharp edges an this phenomena is beneficial because en edges of the tape is where most of the current flows. Because of this I run a second simulation with a 20um plating and rounded edges to se how is the new current distribution over the new, improved configuration.

As seen on the image the main current is carried over the copper plating and is distributed in a much better way than the previous simulation.

Post processing the simulation results gives 87% less AC resistance on the plated version, so scrape the yellow paint and made a copper bath is a smart move if high radiation performance is desired.

Simulation Files (FEMM 4.2):
copper_plated
bare_iron

10 responses to “Measure Tape Antenna efficiency”

  1. […] pratiques s’avèrent en fait étonnamment efficaces, mais comme nous pouvons le voir sur cette analyse approfondie des caractéristiques des antennes à rubancela tient probablement en grande partie à la […]

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  2. […] of these expedient antennas actually turn out to be surprisingly effective, but as we can see from this in-depth analysis of the characteristics of tape measure antennas, a lot of that is probably down to […]

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  3. […] of these expedient antennas actually turn out to be surprisingly effective, but as we can see from this in-depth analysis of the characteristics of tape measure antennas, a lot of that is probably down to […]

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  4. […] of these expedient antennas actually turn out to be surprisingly effective, but as we can see from this in-depth analysis of the characteristics of tape measure antennas, a lot of that is probably down to […]

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  5. […] of these expedient antennas in fact change out to be surprisingly effective, but as we can see from this in-depth evaluation of the features of tape measure antennas, a lot of that is in all probability down to […]

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  6. Saw this featured on Hackaday. While most the technical stuff goes over my head, maybe taking a look into the Sony Tick-Talk talking portable clock radio would be worth it. Those use a thin strip of metal that is the same mechanism as a tape measure (though doesn’t auto retract) for an Antenna. Only device I’ve seen with it.

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  7. […] of these expedient antennas actually turn out to be surprisingly effective, but as we can see from this in-depth analysis of the characteristics of tape measure antennas, a lot of that is probably down to […]

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  8. […] of these expedient antennas actually turn out to be surprisingly effective, but as we can see from this in-depth analysis of the characteristics of tape measure antennas, a lot of that is probably down to […]

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  9. […] Tape measure antenna efficiency Scrape the yellow paint and make a copper bath if high radiation performance is desired. LU2ARZ […]

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