Will putting a ferrite bead or beads near (but not at) the end(s) of a wire antenna electrically lengthen or shorten it? (lower or raise its resonant frequency)?
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$\begingroup$ Are you planning on transmitting or receiving? $\endgroup$– Mike WatersSep 6, 2018 at 17:58
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2 Answers
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Lengthen. The bead increases the inductance, similarly to a loading coil. Near the ends the current is low, so the effect is diminished compared to the same coil near the feedpoint.
I wouldn't recommend it for a transmitting antenna. Ferrites have significant loss, and may saturate. Non-linear components are definitely not something you want in a transmitting antenna.
answered Sep 6, 2018 at 16:57
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$\begingroup$ Agreed, never on a transmitting antenna. I once placed an FB-73-801 ferrite bead in series with a receiver tuned to about 2 MHz, and the signal level dropped by over 20 dB. Keep that in mind. $\endgroup$ Sep 6, 2018 at 17:57
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$\begingroup$ Not as a pure inductor loading coil in a resonant antenna. But for travelling wave HF broadband antennas, ferrites are better than discrete RL losds. With the right bead you can have an impedance that changes with frequency, they are light and cheap, don't require the wire to be cut and joined, and can dissipate plenty of power as it's spread between so many beads. RL loads are heavier for the same power. $\endgroup$– tomnexusSep 7, 2018 at 20:14
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$\begingroup$ Transmit at QRP levels to prevent ferrite saturation? sites.google.com/site/g3xbmqrp3/antennas/ferrite_tx $\endgroup$– hotpaw2Dec 24, 2019 at 5:41
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Actually, the answer is "it depends".
Polydoroff did some work on this in the 60's and documented it in his book:
https://books.google.com/books/about/High_frequency_Magnetic_Materials.html?id=GC4jAAAAMAAJ
I'd quote the relevant part but my copy isn't with me at the moment. He showed significant shortening of the antenna with the ferrites he tested. He used ferrites of very low permeability, something like 8 or 10 if I remember correctly.
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$\begingroup$ Could you offer at least a brief description of why that happens? $\endgroup$ Dec 20, 2019 at 21:00