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Practical Bias T examples for HF usage?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 4L4AGU, Apr 20, 2012.

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  1. 4L4AGU

    4L4AGU Ham Member QRZ Page


    Recently needed a DC Injection Bias T, and cheapest offer was from MFJ, around $25, which is way too much as I believe for an inductor. I decided to do one by myself, since I have all the hardware needed. Unfortunately, I found no practical schematics online. Wikipedia article gives some formulas, in which I'm not so strong unfortunately. I'm planning to build Bias T using 4 pieces of 6800pf, 3kVAR rated capacitors in parralel, and ferrite core wound inductor with 200uh of inductance, the core is relatively large, there are only 6 windings on it, and they are quite separated from each other, to reduce cross-capacitance. Will this work in 1.8-30mhz range? btw, I also have spectrum analyzer with sweep generator, so I can take a sweep of the band to see if there's any attenuation, but how to check for RF leakage at DC supply point?
  2. K7JEM

    K7JEM Ham Member QRZ Page

    I would use more capacitance, the inductor seems large enough. I'm not sure you need 3KV rated caps, either. Should work OK. I built one years ago that used 3 .01 caps, 1000v disks. And a 180uH inductor. Never used it at more than 100 watts, but it worked OK. Wasn't using it at 160 meters, either. Mainly 40, 20, 15, 10.

  3. 4L4AGU

    4L4AGU Ham Member QRZ Page

    These are just smallest voltage, RF usable caps that I have at hands, others are 5, 10, 20, 50kVAR. And this is kVAR - reactive power, not voltage.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

  5. K7JEM

    K7JEM Ham Member QRZ Page

    That looks a lot like what I built. I left out most of the bypass caps, fed DC through a switch from a 12v wall wart. The other end was a relay. Voltage applied, antenna two. Voltage absent, antenna one. We used these to switch between 40M and triband antennas at FD, worked very well. Used a 20A automotive type relay for RF. Works OK at HF.

  6. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Thanks, looks like caps will be my limiting factor, and considering the price of such new ones, factory made bias t does not looks so expensive any more :D
  7. W0BTU

    W0BTU Ham Member QRZ Page

    Exactly what kind of antenna are you using your bias-T with? When I think of a bias-T, I think receive antennas, but it sounds like you're planning on running a kW through it.
  8. 4L4AGU

    4L4AGU Ham Member QRZ Page

    I'm planning an experimental vertical for HF bands with elevated radials, located at distant tower (cable length will about 55 feet of LMR-240) MFJ-925 Tuner will be located at feedpoint, so I need a power injector to feed it. Power will be only 100W at current stage.
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    I think you're on the right track. Keeping interwinding capacitance to a minimum should keep the SRF (Self-Resonant Frequency) high. A sweep of the inductors will tell you if any resonances fall in any of the ham bands of interest. You want shunt X[SUB]L[/SUB] to be several times the system impedance at the lowest frequency of interest. 200µH = 2.262KΩ @ 1.8MHz. It's much higher at 30MHz (37.7KΩ). You want series X[SUB]C[/SUB] to be a fraction of the system impedance at the lowest frequency. 0.027uF = 3.275Ω @ 1.8 MHz. Bypassing the cold end of the inductors with 0.01µF and 0.1µF capacitors will ensure no RF gets past the inductors.

    I'm also building a bias T. Mine will drive a relay to switch my 80m cage inverted vee to operate as a T antenna (agains a radial field) on 160m.
  10. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Tried these 4 x 6800pf caps between tracking generator output and analyzer input. Set sweep to 1-33mhz. No signs of signal dropping so far. For sure, I can use 10 x 6800pf caps.
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