# Bias Tee for HF: noise and losses?

I have a homebrew remote antenna switch. It uses relays and it has a nRF24L01-based wireless link to the band selector (Arduino, taps into CAT and switches antennas accordingly).

I would like to power this device without running a separate power cable. I thought of using a bias tee with about 100 to 470nF capacitors, KV rated, several in parallel, and big inductors for DC injection and extraction. Some sources online recommended using a ferrite rod to make the coil, which should provide an inductance into the millihenries.

The questions are:

1. Is this a good idea?
2. Will this introduce significant losses into the system?
3. Will this introduce significant noise into the system?
4. Is there anything else I should be concerned about?

I've seen at least one supplier of this sort of "bias tee" circutry for HF, but at a price exceeding $250 (which I'm not willing to spend), so I believe it "should work". ## 1 Answer You might be guided by the circuit for the Ameritron RCS-4 Remote Antenna Switch. I have used this unit on HF for decades without any noticeably deleterious effects, though I have never performed any measurements. The manual shows L201 and C203 filtering the supply to the relay coils. L201, p/n 409-2150, is a 1A 90uH choke. At 1.8-MHz, this provides about 1K$$\Omega$$ of inductive reactance, which will effectively isolate the RF and DC/low-frequency parts of the system. Of course, this isolation increases with operating frequency. Off-the-shelf components with these or similar specs (e.g., 1A 100uH) are available in unit quantities for less than US$1 from the usual suppliers (DigiKey, Mouser, Newark, etc.). It's difficult to imagine doing better by winding your own inductor.

• That's very interesting, I have several inductors that can do the job in my junkbox, including 150uH and 330uH coils. I did some math and thought I'd need much bigger coils to keep the impedance high enough but I suppose if the antenna is matched appropriately, practically all of the power will be sinked by the antenna and nothing by the inductors. – hjf May 7 at 13:45