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I'm thinking about building a 3-element beam using EMT for some parts and then steel tubing at each end to get the length of element I need. EMT will give the antenna more back bone but still give me the overall length I need.

For example, to get a 20 ft element I might use 10 ft of EMT and 5 ft of smaller steel tubing at each end of the EMT to total 20 ft. Can I combine conductive materials in this way when building an antenna?

user3486184
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Zane Lile
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  • What is this "smaller steel tubing" that you speak of? Is it plated or bare steel? – Mike Waters Jul 15 '23 at 02:57
  • Smaller steel tubing 1/2" outside diameter. - bought from an antenna builder very ridged tubing. 1/2" EMT with the other tubing inside as a connection. Thanks. – Zane Lile Jul 16 '23 at 12:13

2 Answers2

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Yes any metal tubing will work for a yagi. If you join tubes make sure there is a good electrical connection, and one that will withstand some rain and movement without coming loose or rusting.

Usually antennas are made of aluminium, because it is inexpensive, light weight and corrosion-resistant, but I've made Yagis out of all sorts of things, steel and brass and copper. If electrical conduit is the best price per length / weight / stiffness then go for it!

I suggest you take a look at an ARRL handbook (almost free at used book stores etc), or this this comprehensive free ebook about antennas, which has my standard go-to table table of lengths for yagis (in wavelengths):
yagi table

For an 11 metre wavelength, 36 feet, you would get the following dimensions:

  • Reflector: 208.7" or 17.4'
  • Spacing: 86.6" or 7.2'
  • Driven element (see below)
  • Spacing: 86.6"
  • Director: 191.4" or 16'

So for example, you could cut up and use six 10' lengths of metal tubing, bolted somehow onto a 14 foot 2x4. Reflector and Director are continuous metal connected in the middle.

The driven element of this compact yagi will have a very low impedance, the table says 8.6 ohms, so to feed it you'll need to use a "Gamma Match". All fairly easy with scraps of tube and standard electrical connection hardware.

Some things to consider though:

  1. In some countries you're not allowed to use a high gain antenna for CB radio. In the US the height of the antenna is restricted which may reduce the benefit of a beam antenna.

  2. This would be quite a heavy beam antenna - consider instead a cubical quad which would be a cube about 9 feet on a side mostly made of some 8' pieces bamboo or other lightwight materials, and thin wire, quite easy to build. There are many variations using triangles, single masts, guy ropes etc.

tomnexus
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  • Last time I knew (1980s) Yagis were legal on CB in the USA. – Mike Waters Jul 15 '23 at 15:20
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    Yes I remembered wrong, perhaps thinking of UHF CB/FRS/443. In the US it's just an antenna height limit, which actually favours the horizontal yagi, for DX at least. – tomnexus Jul 16 '23 at 00:06
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Different metals have different conductivity, so changing the metal can affect antenna performance. The higher resistance of non-copper will decrease the efficiency of the antenna. (Although aluminum is also highly conductive, but the diameter is larger for the same conductivity.) Also, the different conductivity changes the effective diameter of the elements and some antenna models are more sensitive to element diameter than others, although this can be compensated for in the antenna design if it is an issue.

Overall, both of these effects are small and generally won't prevent the antenna from working, but if the antenna is very tightly designed, it may matter for the intended performance level of the antenna.

A bigger issue is mechanical. It is not unusual to use steel for structural support, and then plate with copper for greater efficiency since skin effect causes only the outside of the elements to matter. Copper clad steel wire is very commonly used to make antennas. However, for large tubes, bare steel is probably fine as long as the high resistance isn't a problem.

Corrosion is also a big deal. If you mix metals in the antenna, you need to understand which metals will become sacrificial anodes and make sure that corrosion of those metals won't be a problem, otherwise the antenna will not last long. This is most critical for selecting screws that hold the antenna together. (If a screw becomes a sacrificial anode, it will be gone soon!)

For numerical comparison (by volume), Gold is about 45% less conductive than copper, and aluminum is 57% less conductive. Iron is 477% less conductive, and steel is worse.

user10489
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  • My experience has been that it makes negligible difference which metal is used in this instance, especially at HF, and the impact on antenna performance can be ignored. You would consider material conductivity for ultra low efficiency antennas such as a DF loop, or when very high power > 2 kW is involved. – Andrew Jul 16 '23 at 00:08
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    Yes, it is largely negligible. Structural integrity is a bigger concern than the conductivity of the metal. – user10489 Jul 16 '23 at 00:40
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    Also, steel has eddy current losses on top of the lousy electrical conductivity. :-) – Mike Waters Jul 16 '23 at 02:48
  • https://ham.stackexchange.com/a/1390/8717 – Mike Waters Jul 16 '23 at 02:56
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    This is a good answer; antennas are primarily mechanical animals. An interesting fact is that skin effect is a much bigger deal on an HF wire dipole than a 2.4 GHz dipole. It really hurts at low frequencies because the conductors are typically thin and the dipole elements are long. – tomnexus Jul 16 '23 at 03:23
  • I assume you mean thin skin hurts HF more while the skin effect helps higher frequencies rather than hurting? – user10489 Jul 16 '23 at 04:42