I have been using a long transmission line. I connect to the antenna and my transceiver. I then take the line that is tangled everywhere and make a nice coil. I have just been assuming the ground shield on the coax prevents significant effects. Thoughts please. This is a 50 ft cable, and I only need 20 feet to go from the antenna to the transceiver.
- KC3LUE asked 3 weeks ago
- last edited 3 weeks ago
Ham radio operators can be right, wrong, or practical. Many things in ham radio are a compromise. This is a hobby, not a profession. While I went to engineering school, I found out I was NOT good enough to be a “real” engineer, so I kept the hobby and changed careers. There is perfect enough. Just do the best you can and have fun. Keep the coil rather large compared to the diameter of the cable. You’ll be fine. However, in general, the shorter the cable, the better.
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Hi Howard. Coiling a coax cable can actually (read usually) have some unintended effects, even with the ground shield. When you coil the cable, it can act like an inductor, potentially creating an RF choke that could impact your signal, leading to things like impedance mismatches or signal loss, especially at higher frequencies. This might also cause some electromagnetic coupling, which could lead to interference, either with your own equipment or with other nearby electronics.
Also, coiled cables do not dissipate heat as effectively as straight cables, so if you’re running high power, the cable could heat up more than you’d like. While the ground shield does help prevent interference, it’s not always perfect, and coiling the cable could still lead to unwanted radiation or reception of RF signals, especially if there are any unbalanced currents.
Instead of coiling, you might want to consider cutting the cable down to the length you actually need if possible. If you’re not comfortable doing it, you could go to RF Connection in Rockville. If cutting isn’t an option, try laying out the excess in a loose zig-zag or serpentine pattern to minimize any inductive effects. And definitely check your SWR to ensure you’re not running into any impedance issues. Hope this helps, and good luck with your setup!
Sorry, one last thing. I do want to emphasize that RF chokes can be useful. I use them often. They are typically used when you need to suppress common-mode currents to improve your antenna system’s performance and reduce unwanted interference. These common-mode currents can occur when RF energy travels back along the outer surface of the coax shield rather than being radiated by the antenna. This can lead to various issues, such as causing interference with nearby electronic devices or creating RF feedback that might affect your equipment’s operation or even result in hot spots on metal surfaces in your shack. Using an RF choke, often referred to as a “choke balun,” helps prevent these currents, ensuring that the RF energy is correctly radiated by the antenna. It’s especially useful when you want to maintain a clean radiation pattern and reduce the chance of unwanted signal radiation or pickup, making it a valuable tool in maintaining an efficient and interference-free station setup. But again, frankly, if you can cut, you’ll be better off.
73
Jim
- Jim answered 3 weeks ago
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I mostly agree with Jim’s comments, but whether the possible ill effects matter depends on some details.
- Cable type, parameters. For example, RG8x vs LMR400 vs RG-174
- Highest frequency of operation. For example 10 m versus 2 m
- Power level. A transceiver may put out 5 W max or up to 200 W max. Think ahead. If one day you purchase an amplifier, don’t forget any calculations that assumed only 100 W.
- Number of turns, diameter of how you coil up the cable.
- Match between the antenna and coax cable. If you use a multiband antenna and rely on a tuner at (or in) the transceiver to bring down a high SWR, then coiling the cable could have more consequences.
The importances of the effects Jim describes depend upon the above parameters. In many cases, coiling a coax cable has no adverse consequences. If I was using a well-tuned single band antenna at a power level well withing the ratings of the coax cable, I would not hesitate to coil the coax and see the results. However, if I was using a very small size coax at relatively high frequency (RG-174 at 10 m), I would shorten the coax to reduce the attenuation of the coax cable. 50 ft to 20 ft would matter.
Finally, cutting the cable seems innocuous enough, but it can have its own consequences. Cables spliced using UHF connectors will have impedance bumps (discontinuities) at very high frequencies. Only BNC and N connectors should be used above 2 m. If the splice is outdoors, a splice is less waterproof than many cables. Proper sealing is essential. Finally, if the antenna has a severe mismatch at one of the frequencies you use, changing the length of a cable could bring the high current or high voltage point along the cable into the shack and into the tuner or rig. An antenna that once seemed to work fine would suddenly and unexpectedly fill the shack with unwanted RF on certain bands.
- wa3ltj answered 3 weeks ago
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I’d like to enlarge on something Jim wrote in his first paragraph. Yes, coiling a cable (or any wire) creates inductance, but in coax, the inductance is seen only by current flowing on the outside of the cable. It has no impact on current flowing inside the coax. To explain why, you need to know about “skin effect.”
At RF frequencies, virtually all the current flows very close to the surface of a conductor. This is called “skin effect.” On 80 m, virtually all of the current in a copper conductor is in the 35 µm closest to the surface. At 6 m, the skin depth is less than 10 µm. What that means, in practical terms, is that the outside of the coax shield behaves as if it is an entirely separate conductor from the inside of the shield, at least for RF currents. (Caveat: this is not true in the case of shielded audio cables, but we’re talking about RF here.)
That result is that coiling the cable has no effect on the RF current that you intend to flow through the cable between your antenna and the radio. Any current flowing through the center conductor of the coax is returned along the inner surface of the coax shield. For that desired RF signal, the main thing you have to be concerned with is the loss of the cable at the highest frequency of interest. Cable manufacturers (and most cable vendors) publish a chart of loss (in dB per 100′) versus frequency for every type of coax they sell. For most of us, a good rule of thumb is keep the feedline loss under 1 dB at your highest operating frequency. If you care about performance, you can buy better (and more expensive) coax to reduce your loss to a fraction of a dB.
As Jim mentioned, coax feedlines are often formed into a coil, or choke, near the antenna feedpoint. In the absence of such a choke, the antenna might induce undesirable RF current onto the outer surface of the shield when transmitting. There are several problems with that. One is that this makes the outside of the coax effectively a part of the antenna. Part of the energy from the transmitter will be radiated from the outside of the coax, distorting the antenna pattern and affecting its performance in other ways. This undesirable current may also introduce RF into the shack when you are transmitting. In addition, given that the feedline may pass close to RF interference sources, any noise current picked up on the outside of the coax will be conducted to the antenna feedpoint and then pass down the inside of the coax to the receiver. (However, a good balun does the same thing as an RF choke at the antenna feedpoint, so depending on your antenna configuration, you might or might not need the RF choke.)
If the coil is at the shack end of the feedline instead of the antenna end, it is still going to reduce RF current on the outside of the coax, but it doesn’t provide any of the other benefits mentioned above.
73, Al
- kn3u answered 2 weeks ago
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