Practical Bias T examples for HF usage?

[4L4AGU]

Hello,

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?

[K7JEM]

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.

Joe

[4L4AGU]

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.

[WB2WIK]

This might be helpful:

http://ad5x.com/images/Articles/RemoteDC%20RevA.pdf

[K7JEM]

This might be helpful:

http://ad5x.com/images/Articles/RemoteDC%20RevA.pdf

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.

Joe

[4L4AGU]

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

[W0BTU]

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.

[4L4AGU]

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.

[WA7PRC]

Hello,

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?

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_L 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_C 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.

[4L4AGU]

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.