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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.
  11. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Assembled a simulated "real-life" situation in the following maner:

    Tracking generator output is connected to "transceiver" end of bias tee. 50Ohm dummy load - to the "antenna" end. The "power supply" end is connected as it should be - to 12V 500ma linear power supply. Analyzer is connected to the power supply also via DC Blocking adapter. The above mentioned setup (4x6800pf & 200uh choke) has minimum level of rejection of -20db @ 1mhz, it quickly goes to about -40db around 4mhz and stays there for up to 24mhz, where it has quite wide peak at -20db and then fades again.
  12. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Ok finally I've come up to solution which provides rejection level around -65dB across all HF range (consists of two coils and two capacitors). Regarding the bypass capacitor, what if I use mylar/teflon one? like ones used as chokes in PC power supply? (0.68uf, 250V)
  13. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Or even larger ones, 1uf @ 275V ?

    I've tried to measure their insertion loss with analyzer. Curve is perfectly flat till 70mhz, where it starts to drop, and reaches -8db @ 100mhz, I believe, for HF operation this capacitor looks to be OK ?
  14. WA7PRC

    WA7PRC Ham Member QRZ Page

    The bypass capacitors on the cold end of the inductors need to have low ESR at the frequency of interest. For wideband bypassing, you use multiple bypass capacitors of different value. For 1.8 to 30 MHz, use a 0.1μF ceramic capacitor in parallel with a 0.01μF ceramic capacitor.
  15. KO6WB

    KO6WB Premium Subscriber QRZ Page

    The voltage present in the feed line for the RF is determined by values that don't change such as resistance. The formula Watts = [Volts^2]/Resistance indicates that at 250V and the resistance is 50 ohms then the wattage is 1250W. You will notice that the resistance cannot change but in an antenna it will present a different resistance when the SWR does not equal 1:1. This doesn't take into account the reactance and the effect that will have on the the voltages present. You may want to use highly overrated components for best results.
    Last edited: Apr 21, 2012
  16. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Ok, here it is. Tested it with dummy load on all HF bands for continious 10 minute key down in CW mode - no warm-up was detected. SWR is also flat in 1.8-50mhz range.

  17. GM3SEK

    GM3SEK Ham Member QRZ Page

    Here is another one. With the recommended components and construction, this bias tee will handle 1kW+ from 1.8 to 50MHz with low VSWR.

    The web link leads to a full-length article and a wrap-around drilling template for the Hammond diecast box.


    73 from Ian GM3SEK
  18. 4L4AGU

    4L4AGU Ham Member QRZ Page

    I've re-used diecast case of 20dB 50ohm pad. I don't have power excess 100W to test on high powers :)
  19. G3TXQ

    G3TXQ Ham Member QRZ Page

    I've no idea what your toroids are, or what sort of DC current they will be passing, but have you considered the risk of core saturation?

    Steve G3TXQ
  20. WA7PRC

    WA7PRC Ham Member QRZ Page

    He said the toroids have 6 turns. If they're 200uH as he says, they must be ferrites with high permeability. I wouldn't have tried using the film capacitors anywhere near 30MHz. I will be using disc ceramics in mine, used to switch an 80m inv vee to operate as a T antenna on 160m (with a radial field).
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