HF Antenna Splitter

Most SWL's can use a handy accessory such as this. If you have more than one HF receiver and room for only one antenna, this HF antenna splitter will accomodate connecting two receivers to the same antenna. It provides the necessary isolation to prevent the receivers' local oscillators from interfering with each other, a problem that is common when using a simple T connector. This device was designed out of necessity, since I have two HF receivers but didn't want to put up another antenna. The antenna splitter/preamp is basically a preamplifier which takes the input from the antenna and feeds it to a 1:1 transformer with the winding from the preamp being primary and all other windings being secondary.  This was done by winding five equal lengths of magnet wire around a toroid. I first built this device in an Altoids tin, using just three antenna connectors & windings to split the antenna into two outputs. It was built as a simple passive device, but I decided to add an RF preamplifier to overcome the losses.  The preamp also prevents spurious signals from one receiver to enter the front end of the other receiver.   The preamplifier design is a Norton low noise, broadband RF preamplifier design I found here.  It is the third circuit down on the page, under the heading "A Low-Noise, High Dynamic Range Broadband RF Amp."

Since I was adding some circuitry, I needed a larger enclosure than the Altoids tin I used for the first, passive version of the splitter.  I had an A/B data switchbox which was about the right size.   I constructed the preamp semi-Manhattan style buy cutting traces on a piece of PC board.  After all the components were installed and the circuit was tested, I secured it in the case with double-sided foam tape.  I used a flat piece of metal to cover the oblong holes left by the DB-25 connectors in the rear of the switchbox.  The SO-239 jacks hold it in place inside the rear.  I also installed a 2.1 mm power jack to feed the preamp with 12VDC.  The splitter will not work without the preamp being powered.  A wall wart may be used, however, you will want to make sure that the one you use is well filtered.  If the power supply is noisy, it may introduce noise or hum into the preamp which will most likely induce hum on the received signal.  I feed this antenna splitter from my 50 Amp power supply that also powers some other radios.  If a wall wart is used, it would be a good idea to install a large electrolytic capacitor between the +12V feed and ground.

The toroid I used was of unknown composition with an outer diameter of 1" an inner diameter of 3/8", and thickness of 1/2". It was salvaged from an old piece of electronics equipment which I believe may have been a power supply. I took five lengths of magnet wire and wound them around the toroid until I filled the entire diameter with wire. The wire diameter is not critical, and small-diameter insulated wire can be substituted for magnet wire. The above illustration shows the original two output splitter transformer with three lengths of wire.  Any number of outputs can be used, just be sure to use one piece of wire for the primary and one for each output. The wires should all be wound the same direction, and is easily done by wrapping all  wires at the same time.  The wires don't need to be labeled, however, you don't want to mix up the leads for each side of the transformer since one end of each wire will be connected together at a chassis ground. The above schematic shows how the circuit comes together.  in this schematic only two outputs are used, but more could be added. However, the more outputs you add the more signal loss you will have in a passive device, so if more than two outputs are required it is recommended to install a preamp.  If a toroid ring cannot be found to construct the splitter, a ferrite rod from an old AM transistor radio could be substituted. Simply start wrapping the wires around one end and work your way down to the other. As with the audio isolation transformer I built, I figured I would have to do some experimenting to get good results with the toroid, but I was happy to find that the initial construction provided excellent results. I can now connect two to four receivers to one HF antenna, and I have absolutely no interference between receivers. I tuned the receivers 455 kHz apart and had no interaction. The best part is that the preamp eliminates any signal loss.  However, the preamp design I used is designed for use from 1.8 MHZ to 150 MHZ.  It does work OK on the MF and LF band, but with reduced results.   A splitter could also be made for VHF/UHF for scanners, but a smaller toroid should probably be used.  A different preamp covering low VHF to UHF would definitely need to be used, since as I stated above, the one I constructed was designed for up to 150 MHZ. The label for the front of the box was designed & printed by computer.  I attached it to the front panel with a thin layer of Elmer's glue, then oversprayed it with clear spray paint.  The power switch I used is a lighted switch which originally contained a 120V neon bulb, but I replaced it with a superbright LED and 1K resistor to operate on 12V.  Important note: Do not even think about trying to use this device with an transceiver. If you transmit into this device even with low power you will most likely burn up the front ends of the other radios connected to it and the splitter's preamp.  This device will only provide isolation from each receiver's LO, it will not isolate Watts of RF. I would recommend not even connecting it to an HF transceiver just to listen, since there's always the chance you may later forget that the HF rig is connected to the splitter and not straight through to the antenna, and then key the rig. It's always better to be safe than sorry. This device is designed and intended for receivers only.  Also: Before you connect this device to an older hollow state (tubed) receiver, be sure to check to make sure there is no voltage present on the antenna connection.  My Hallicrafters S-38B has 120VAC across the antenna terminal and ground.  If I were to connect this radio to the splitter along with my modern, solid state receivers, I'd be putting 120VAC into their antenna jacks with obviously disasterous results.  It would probably also burn up the preamp.  A transformer will not block AC voltage, and this transformer is no exception.

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