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.