# Ferrite Rod Antenna Array?

The ferrite rod is a reasonably effective receiving antenna for LW, MW, and lower HF spectrum. It is well known that a ferrite rod is much more effective than an air cored coil of the same dimensions. An array of ferrite rods would receive more power if tuned and phased properly. If all the rods were bundled together would they act as one large diameter rod? If the rods were spaced many diameters apart they would be independent. How far apart is reasonable? Would a spaced array be better than bundled rods due to a bigger capture area?

• I've not tried this, but it's an interesting idea. The spacing between the antennas would need to be worked out. Ferrites bend magnetic flux lines into themselves, so I suspect if the antennas are too close to each other they will "steal" signal. Dec 16 '19 at 14:49
• I did some modeling in Ansys of ferrites at 100's of kHz, and observed what looks like the magnetic analog of the "skin effect". I suspect that if you took a tube of some diameter and placed ferrite rods like a fence around the perimiter, the bundle would act similarly to a single large ferrite. Probably not quite as good, but a lot lighter and less expensive. Dec 16 '19 at 14:52
• Bundle? A bundle could be many rods combined to make a composite fat rod. Or a bundle could be many rods combined end-to-end to make a composite long rod. I think the long-rod version is better. End-to-end rod connection may suffer an air-gap: this should be done carefully. Have ferrites prepared to try end-to-end, but have no RF measurements. Apr 28 '20 at 18:25

## 1 Answer

Ferrite rods are smaller than loop antennas, but since the $$Q_u$$ (unloaded Q) is lower (200 is already very good as maximum) a copper or aluminium loop with same effective diameter ($$\mu_r$$ of ferrox material is about 130) and $$Q_u$$ of 500 for the loop is better. Dimension is indeed a factor larger; diameter goes factor $$\sqrt {130}$$ = factor 11,4 up for same aperture of H-field.

And to go back to your question: it makes a difference wheter the antenna is tuned or not. When tuned loops are within the range where coupling comes in the region of $$k*\sqrt {Q_l}$$ = 1 then the antennas have unwanted mutual effects. ($$Q_l$$ is loaded Q.)

When more loopsticks antennas are coupled with single tuning capacitor then you created a larger ferroceptor antenna: higher effective height and better reception, better sensitivity and lower noise floor. Noise floor on medium wave 1 MHz is seldom lower than 3$$\mu$$V/m in RBW = 3 kHz ("radio bandwidth"). Finally, when you make separate wideband antennas (no resonance, wideband noise matching) loop and ferroceptor loopstick antennas can be placed closer together.

Spaced array: coherence of signal is high within a wavelength separation. Local interference can vary. Last week I did a test with RSP-duo and 2 antennas and found only decorrelation for separation beyond 0,7$$\lambda$$; probably only beam forming from an array. PA0FSB

• Hello, Frans, and thanks for joining the Ham StackExchange. Apr 28 '20 at 17:22
• Frans, I've added your callsign to your profile, if you don't mind. You can delete that, and optionally add anything that you would like there. You could even add your callsign after your name, if you want to. :-) Apr 28 '20 at 18:18