In order to remotely power my SG-230 Smartuner which will (temporarily) live in my attic to “tune” a large vertical loop antenna, I opted to use a bias-T to inject DC power onto the coax. Bias tees are passive components (subject to losses) which allow coupling and de-coupling of DC on an RF transmission line. When used properly (in the correct orientation), the device (theoretically) never sees DC on the TX/RX port but there exists a DC “offset” or bias to the RF signal.

In lieu of sourcing the components myself, I opted to purchase 1 x MFJ-4117 (on/off switch) and 1x MFJ-4116. These bias tees are relatively simple devices (see schematic) that can be invaluable in powering remote equipment without the need to run additional antennas (I mean wires). The bias tees are specified to handle 1 amp DC at up to 50 volts and are said to handle up to 1500 W CW (I have my doubts).

The question is: what are the trade-offs?

With the addition of anything in the signal path, one should expect some bi-directional loss. I wanted to know how much loss the two bias tees would add to my particular setup, so it was time to fire up my trusty Rigol DS815-TG.

I started by connecting two N-to-PL235 cables with a SO-237 coupler in order to “zero” the tracking generator output with the spectrum analyzer. This is done to account for any cable losses, impedance discontinuities and to give the spectrum analyzer a reference point for what its tracking generator is outputting along the frequency sweep. This normalization process is done each time the setup changes. I even repeat normalization if I vary the frequency range or receive bandwidth (RBW). The result after normalization was a 0.03dB loss (to be subtracted from the test results later). I could have accounted for this system loss automatically but I prefer to see all the variables and work with them as I go.

The next step was to replace the SO-237 coupler with the MFJ-4117 in its place. As I primarily operate on 20M (14MHz) I wanted to know the insertion loss at that frequency in particular.

With the spectrum analyzer set for a frequency range of 0-60 MHz with RBW of 1 kHz, I measured a mere 0.06dB of loss at 14 MHz (accounting for systematic 0.03dB loss).

Interestingly, it was noted that 3.63dB loss was found to be at 37.5 MHz which is not an issue for amateur radio use, but interesting none the less. It may be possible to make some assumptions as to the component values and/or circuit design knowing this frequency dependent loss figure.

Sweeping from 1 MHz to 30 MHz with an RBW of 1 kHz, the frequency response curve shows a little more detail as to what the loss / frequency relationship will be for each bias tee in the system.

The loss only becomes 0.3dB once you reach approximately 24 MHz (12M WARC @ 24.890–24.990 MHz and 10M @ 28-29.7 MHz in Canada affected).

The bias tees that I will be using are only specified to work from 1-60 MHz but I thought it would be an interesting exercise to test the devices outside that range. I swept 0-1 MHz and not surprisingly, there is a dip in the frequency response graph. There is 18.2dB of loss at 206 kHz. There is only 0.3dB (or so) loss starting at 300 kHz, improving until the roll-off as previously seen.

All said and done, I am very happy with the minimal loss that I expect to experience. Testing shows that I am looking at 0.12dB total loss for the addition of the bias tees. This does not however account for the connector and cable losses required by the addition of these units. Perhaps I should re-test with the actual jumper cables being used.

Nice measurement, thank you!

Hi,

I have now tried a number of values for the “loss resistors” in my bias-T design and have achieved a gain of -6.085dB +/-0.003dB over the range 1MHz to 100MHz with three parallel tuned circuits. The total resistance in the DC path through the chokes is 0.72 ohms and the maximum current is 0.66 A. This is OK for me but I would really like to find a range of larger inductors.

Of course, this is all a mathematical model. I must now buy some components and make some real measurements.

73, Richard G0REL.

Hi again,

When two parallel tuned circuits are connected in series there will be a frequency at which the magnitude of the capacitive skirt of one has the same magnitude (but opposite phase) as the inductive skirt of the other. At this frequency the reactive components cancel out and one is just left with the resistive losses. In the bias-T this is from the line to ground – hence the dip in the response.

It seems that the lossy dips could be reduced by increasing the loss in the tuned circuits ie lowering their Q. Since there must be a clear path for the DC, resistors might be added in parallel to the coils.

Returning to the analysis of the circuit that I am planning to build, I have three Murata PS 22RxxxC inductors (where xxx is the inductance in uH), 10uH, 33uH & 100uH in series where MFJ have two. The data sheet specifies the effective series resistance and self resonant frequency from which the stray capacitance can be calculated.

If there were no loss, a 50 ohm source driving a 50 ohm load through the bias-T would have a gain of -6dB. By picking suitable loss resistors, it appears that a gain that varies between -6.04dB and -6.11dB is possible.

I look forward to hearing from you.

73, Richard G0REL

Hi,

I have just bought an LDG RT100 automatic tuner which receives its 12V DC supply along the coaxial cable. Looking around for a bias-T, I found the MFJ4117. While trying to find out why it cost US$50 and what opinions were about it, I found your detailed measurements. While analysing the bias-T circuit in the RT100 manual, I stumbled across the answer to your comment about a dip at 37.5MHz.

Having no data on the MFJ inductors than their inductance, I used the data for Murata PS 22R104C and PS 22R105C even though their current rating is too low. As you can imagine the parallel resonant tuned circuit formed by each inductor and its self capacitance give zero loss at their self-resonant frequencies. However, at a frequency midway between them, the impedance to ground is not infinite and so it forms a voltage divider with the source impedance.

This is all much clearer if one has the frequency response diagrams which I will be glad to send you if you contact me in a way that permits such attachments.

Incidentally, the MFJ inductance values seem very high and the coils must have very low self-capacitance to give the dip at 37.5MHz.

73, Richard G0REL.