Monthly Archives: July 2020

Bias-Tee Module with DC Switching Logic

The DG8 RXtee Board for 28-432MHz

RXtee board complete

I designed this board because I needed a few compact bias-tee modules with built-in logic for remote switching of preamplifiers, transverters etc. Now that the design is working well, I’ve decided to make the board available in the GM3SEK Odd-Boards series for home constructors.

What’s a Bias Tee?

A “bias tee” is a simple way to feed DC power up the coax to remote devices such as preamplifiers and switching relays. At its very simplest, it consists of a series capacitor to pass the RF, and an RF choke to introduce the DC.

Generic schematic

  • CAUTION: when you complete this project, LABEL THE CONNECTORS! 
    Connecting a bias-tee the wrong way round can cause expensive damage because one of the two RF ports can deliver significant DC voltage and current into whatever equipment is connected.  I have labeled the ports correctly on the PC board – “RF” and “RF+DC”but the rest is up to you.

Features

The DG8 RXtee is several steps smarter than the basic bias tee above! (Although you can also wire it as a simple basic bias tee if that is all you need.)

  • Covers all RF frequencies from 28MHz to 432MHz (at least) with low loss and low VSWR (<1.05)
  • Low-power applications only – not rated for handling high RF or DC power
  • DC supply typically 12-15V, currents up to ~1A with short-circuit protection
  • Versatile TX/RX switching of the DC power (separate logic inputs for ground-to-transmit and TTL)
  • Output for an external indicator LED
  • Designed to fit inside a standard 20x20x37mm tinplate box (optional; user provides box and connectors – see parts list below).
  • Supplied as a bare board only, for home constructors with sufficient experience to source the parts and assemble the board.

BOARDS NOW AVAILABLE – order here

Schematic and Notes

DG8-RX-Tee schematicClick here to download as a printable .jpg file

Construction and connector options

There are more options than you might imagine!

    • This description features SMA female connectors soldered onto a screened tinplate box – a good general purpose option.
    • If you wish, you can solder flying leads of RG174 or RG316 coax directly onto the box (or use a connector on one side, a flying lead on the other).
    • If you don’t need screening, you can leave the box out completely and solder connectors and/or flying leads directly to the board.

Performance

All measurements were made on the prototype shown in the photographs,  using the recommended Epcos 2.2uH RF choke.

DG8 RXtee v096

Click here to download as a printable .jpg file

Marker data from the above traces:
                    Freq        Ins loss    Rtn Loss    VSWR
Marker 1   28MHz     0.05dB       33dB          1.05
Marker 2   50MHz     0.05dB       39dB          1.02
Marker 3   144MHz   0.06dB       42dB          1.02
Marker 4   432MHz   0.14dB       33dB          1.04

Parts List

C1, C5, C7: 100pF C0G dielectric, 0805 SMD (various sources)
C2, C4, C6, C9: 1nF (1000pF) C0G dielectric, 0805 SMD  (various sources)
D1: 1N4148, 0805 SMD  (various sources)
FB: Ferrite bead RF filter, 0805 SMD (various sources)
PTC: ‘Resettable Fuse’ rated to carry 1.0 A (various sources)
Q1: NDT2955, P-channel MOSFET, SMD (eg Farnell 9846271 )
Q2: 2N7002, TO-236AB SMD (eg Farnell 3439577 )
R1, R4: 10K, 0805 SMD (various sources)
R2: 220K, 0805 SMD (various sources)
R3: 1K0 0.25W, 1210 SMD (various sources)
RFC recommended: Epcos B78108S1222K000, 2.2uH 1A (eg Farnell 608440 )

Optional:
RFC1, RFC2, C3 – 0805 pads are provided in case you need a 2-stage filter.
See notes on the Schematic above.
Tinplate box: Schubert type FG1,  20 x 20 x 37mm (un-punched) from G3NYK
External LED.

Construction Notes

  • For a high-resolution view of the completed DG8 RXtee, click here or click on the lead photo.
  • For most applications you can use a single wire-ended RF choke, as shown in the prototype. If you really do need two-stage RF filtering, 0805 SMD pads are provided for RFC1, C3 and RFC2.
  • If you are intending to mount the board in a tinplate box, see the detailed notes below.  Your order of assembly must be as follows:
    • file the board to be a snug fit inside the box with no gaps (board is supplied slightly over-width to allow this)
    • check that the RF striplines end about 0.5-1.0mm away from the walls of the box.
    • assemble the board and test the DC switching first (the underside of the board will be inaccessible when soldered into the box)
    • install the RF choke last of all, after the board has been soldered inside the box.
  • Here are close-ups of the top and bottom of the board with parts assembled ready for DC testing.  (Later batches of the boards will have printed part numbers as shown on the right.)

Topside
C4 is soldered on top of C5; C6 is soldered on top of C7.

Underside

 

Installation in a tinplate box

“Listen very carefully, I will say thees only once.”

Many small RF construction projects involve
a PC board inside a tinplate shielding box,
with accurately drilled holes for connectors.

The detailed instructions below will be
a useful checklist
for other projects too.

1. Mark the box very carefully for drilling a pair of holes on opposite sides, as shown below (left, dimensions in mm)).

Drilling

2. Accurately centre-punch the correct locations and then accurately drill a 1mm pilot hole in each side, preferably using a 1 x 3.2mm centre drill (eg Farnell 378756 ).Centre drill3. Feed a 1.0mm drill through both holes and check that the two holes are exactly opposite each other, square and level across the box. Use the same centre-drill to open out the holes to 3.2mm, and if necessary adjust their positions with a needle file. (To avoid this horrible job, be careful and accurate when marking and drilling the pilot holes.)
4. If you are installing SMA connectors as shown above, open out the holes further to 4.0mm – again being careful not to go off-centre.
If you are installing flying leads of RG174 or RG316 coax, leave the holes at 3.2mm diameter.
5. Carefully de-burr the holes, inside and out.

6. The next big step is to position the board accurately inside the box, and solder it into place. This is much easier If you are using SMA connectors that have a flat rear face.

    1. Trim the PTFE insulation off the rear face and trim the remaining centre pin down to about 2.5mm. You should now find that the connector is self-positioning inside its 4.0mm hole. Solder each  connector into place, making sure the solder covers the entire rear of the flange (not easy to do neatly – see above).
    2. With the connectors in place, feed the pre-assembled and tested PC board into the box, underneath the connectors. Position the board straight and level within the box, with the two connector pins accurately centred along the RF striplines on the PC board.
      Take your time to get this right.
      Check that the ends of the RF striplines are not touching the side wall of the box! If in doubt, remove the board and use a sharp knife to chamfer the ends of the striplines (but don’t remove more than is really necessary).
    3. Tack-solder the connector pins into place on both striplines. Check again that the board is level, and then tack-solder the far end of the board to the side wall of the box to hold it.
    4. Now seam-solder the PCB ground-plane to the wall of the box, at each side of each connector.
    5. Underneath the board, also seam-solder the accessible ends of the ground-plane as well.

7. Install the RF choke as described above.
8. Find the other part of the box and trim one end to 8.0mm as shown above. This will leave room to connect wires to the row of pads on the board.
9. Last of all, assemble the whole box and test for correct operation.

As I said above, these “European tinplate box” techniques
will prove useful for a wide variety of small RF projects.

 

Page, circuit design and PC board,
all © 2020 IFWtech (Ian White, GM3SEK
)

The Private Life of Coaxial Cable

This is a common theme from several of my talks over the years about Ferrite Chokes and Baluns. By expanding the topic into a blog format, I hope to be able to explain it in more detail, and add some FAQs as time goes on.

This well-known image originated with Walt Maxwell W2DU, the author of many articles about transmission lines. It brings out several important points.

  • The inside of a coaxial cable truly is “private” – completely screened from the outside world. This is one of the rare statements that can be handed to absolute beginners as a simple fact, and yet remains valid all the way through into graduate-level EM physics.
  • The private interior of coax is due to the skin effect which means that, at RF, current flows only on the surfaces of conductors.

    There are many confusing illustrations in textbooks that make some people imagine the skin effect only applies to electrical conductors of some particular shape and size, or in some particular type of circuit. That isn’t true! The best and most generalized derivation of the skin effect I have been able to find is reproduced here. All the high-level EM physics and maths leads to a simple and very powerful conclusion:
    Whenever and wherever an RF current is flowing (regardless of the circuit configuration, regardless of the reason) the skin effect will be present. RF current will be forced to flow very close to the outer surface of the conductor, and there is no net current flow into the thickness of the material. 
    Because we know the skin effect will always be present, we can use that as a starting point when tracing RF current pathways on complex shapes such as coaxial cables and shielded loops. Suddenly, the whole topic of shielding (and what you need to do to make it work correctly) will begin to make sense!
    That is, provided you hold onto the idea consistently. A great many myths in RF engineering are because people aren’t being logically consistent. They switch these basic ideas on-and-off to suit their own preconceived notions… and physics just ain’t like that.  
  • The skin effect in coaxial cables means that the outside surface of the shield is a completely separate conductor from the inside surface.
    The outside surface of the shield is part of the outside world. This is where the so-called “common-mode” currents flow.
  • Let’s look again at that diagram. It shows the open end of a coaxial cable carrying three separate RF currents (I1, I2 and I3). The cable is connected to two wires coming from the antenna and carrying currents I4 and I5.

Private Life of Coax

  • Inside the coax, current I1 flows on the outside surface of the centre conductor, and current I2 flows on the inside surface of the shield. Because the centre conductor is entirely surrounded by the shield, electromagnetic coupling is very tight and this forces I1 and I2 to be exactly equal and opposite. Even a very short length of coax is sufficient to enforce this. So:
    • The inside of the coax will only support energy transfer in the differential, transmission-line mode.
    • Inside the coax, the common-mode current is zero.
  • Now look at the currents I4 and I5 close to the feedpoint of the antenna. In the real world, there is no such thing as a perfectly balanced antenna. Here are a couple of slides to make that point:

Because a perfectly balanced antenna is such an UN-natural phenomenon, I4 and I5 will almost always be UNequal. 

  • When we connect the antenna to the coaxial feedline, the current I4 becomes re-labeled as I1 flowing on the centre conductor, but it’s still the same current.

    Now if I4 and I5 happened to be exactly equal, I5 would flow entirely into the inside surface of the shield to become I2, and everything would be fine. But I4 and I5 never are the same, so what happens to the difference?
  • Point X on the diagram is where I5 from the antenna can divide into two parts, shared between I2 and I3. I2 is the part the flows into the inside surface of the shield, and as we have already seen, this must be numerically equal to I1.

    The difference between I4 and I5 becomes I3, flowing on the outside surface of the shield. Note that this happens only at the open end of the coax, where the inside and outside of the shield meet together. Everywhere else along the coax, I2 and I3 each create their own skin effects on opposite surfaces of the shield, that keep those two currents physically separated.

That was a lot of observations to keep in mind, but they all lead to one simple conclusion.

Summary

  • Because the currents I4 and I5 in the antenna are never quite equal, they can be mathematically resolved into a differential-mode component and a separate common-mode component.

  • However, a coaxial feedline will physically do the same thing! At RF, where the skin effect creates a third conductor on the outside of the shield, the pure differential-mode component of the incoming two-wire connection will automatically be routed into the private, shielded interior of the coax to become I1 and I2. Meanwhile the common-mode component is routed separately onto the third conductor, the outside of the shield, to become I3.

And that’s the private life of coax.

 

FAQ

Ooh, we’ve got lots of those… if you follow this blog, I will add to and answer these from time to time.

1. “I don’t feel comfortable about using the term ‘common-mode’ in relation to coax.”

The classical concepts of common mode and differential mode were based on twin-wire transmission lines, which can support the two different modes on the same two conductors. However, this was long before coax was invented, and coax doesn’t work like that.

The difference is that, at RF frequencies, the skin effect creates a third conductor on the outside of the shield, so the coax becomes a three-conductor transmission line which will carries its common-mode current separately on the outside of the shield. Seen from that viewpoint, it is perfectly justifiable that engineers call I3 “the common-mode current”… because that’s exactly what it is!