Author Archives: GM3SEK

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
(To build a high power 1.8-50MHz bias tee, click here .)

  • CAUTION: whenever you build a bias-tee, 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 above, and also on the PC board – “RF” and “RF+DC”but the rest is up to you.


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 ( click here for details of a 1.8-50MHz high power version)
  • 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.


Schematic and Notes

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

Construction and connector options

You have choices! Get used to the idea of building things as you want them – not as someone else decided to sell them.

    • As noted above, you can leave out the DC switching options if you don’t need them.
    • 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.


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 )

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.)

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


Installation in a tinplate box

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)).


2. Accurately centre-punch the correct locations and then accurately drill a 1mm pilot hole in each side. Tip: this thin sheet steel isn’t easy to drill accurately. For the best possible start, use a short 1 x 3.2mm centre drill (eg Farnell 378756 ) to prevent the drill wandering off-centre.Centre drill3. Check that the two holes are exactly opposite each other, square and level across the box, by feeding a 1.0mm drill through both holes. If all is well, use the same centre-drill to open out the holes to 3.2mm.
4. If necessary, line up the two holes using a round needle file. To avoid this horrible job, be careful and accurate when marking and drilling the pilot holes!
Tip: when drilling thin sheet metal, always clamp it down onto a piece of hardwood. (For this particular job, a scrap piece of 19mm floorboard fits nicely between the two sides of the box.)
5. 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.
6. Carefully de-burr the holes, inside and out.

7. 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. More detailed steps:

    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-locating 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.8. Install the RF choke as described above.

9. Find the other part of the box (it must be on the bench somewhere, right?) 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.
10. 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 still remains valid all the way through to 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 always 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 or through 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!
    There is one condition, though: you have to hold onto that idea consistently. Many of the myths in RF engineering arise because people aren’t being logically consistent. They imagine they can 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 to a very high degree of accuracy.
    • Due to the way coax is constructed, 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.

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


  • 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.



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

FAQ1. “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! 



wtKST Download and Setup

wtKST is a clean, simple user interface to the ON4KST online chats – but with some interesting features for Aircraft Scatter prediction.

wtKST was written by DL2ALF, the author of the AS prediction program AirScout and the two programs are linked. If AirScout is running in the background, wtKST will check the list of stations that are logged into KST and warn you about possible AS opportunities. Click on a callsign and the AirScout map will appear with more information.

(This note was first written because the usual download site at was unavailable. It has come back now, but I’ll leave this page here.)

Note: wtKST is not ‘officially’ supported, so please do not ask questions to DL2ALF!


wtKST has one fixed-format screen, divided into four main areas:

  1. All the chat traffic (Left) – just like the official ON4KST chat interface (and just as unreadable when the bands are busy)
  2. A ‘filtered’ chat window (Lower Left) containing only the messages to and from yourself (like the KST2me filtered chat, but no setup required)
  3. Message input line (Top Left).
  4. KST login list (Right). If AirScout is also running, this is where the added info about AS opportunities will appear (see below).

wtKST is less configurable than either the ON4KST interface or KST2me by OZ2M – but the features you most want are already there, and easier to use:

  • Left-click on any other station on the login list, then type in your message at the top right.
  • Your personal message traffic will appear in the lower window (including your own outgoing messages if that option is selected – see below).
  • Right-click on any other station to see info, including distance and beam heading.

wtKST Setup

You should only need to do this once. – just follow these instructions.

  • Before you begin: you must already have signed up with the official ON4KST site to register your username and password.
  • Download: if the site at is unavailable, here is a download link. Unzip the .zip file to a directory of your choice, and wtKST.exe should run.
  • In wtKST: click Options, choose the KST tab, and enter that same login information. Also enter your station info and other selections.

kst tab

  • Next, click the Calls tab and choose which bands and stations you want to display.

calls tab

  • Move along to the Airplane Scatter tab and check the two boxes below (use the default server settings). The bottom line lets you choose the minimum and maximum aircraft ranges you wish to know about.

scatter tab

  • Close the Options dialog and away you go –  wtKST is now your window into the selected KST chat.
  • To get the information from AirScout, open that program, and choose Options from the main screen. Select the Network tab and check Activate main server.

as activate tab

  • wtKST will now begin to highlight Aircraft Scatter opportunities with various stations in the right-hand columns. (The quickest way to populate these columns is to start (or re-start) wtKST after AirScout.)

In Action

The chat function has already been described. The rest of this section is about the extra AS-related features in the Right-hand window of the wtKST display.

NB: you will only see info about those stations that are logged into KST – don’t forget there are others too!

Same as with AirScout, purple blobs in the RH columns mean “AS possible now”, red means “possibly soon”, and brown/orange means “aircraft currently too low”. Bigger blobs indicate better opportunities (probably more planes).

Depending on your minimum and maximum distance settings (see above),  you may also see < which means “too close”, or > which means “too far away for AS”.

If you have further info about this area of the wtKST display, please post a comment here so I can update the blog.

Clean Up Your Shack – 2019

The online follow-up to my talk for the 2019 RSGB Convention.

3 chokes

What’s new since 2015

  • Choke Chart and RadCom Plus article by Steve Hunt G3TXQ (SK)see separate blog entry below
  • Revised Choke Cookbook by Jim Brown K9YC
  • Large Fair-Rite #31 core
    These are a game changer!  
    See ordering details below.


  • Constant changes in the prices of ferrite cores (but still the same Best Buy supplier)
  • Updated Whole-Shack Mains Filter



  • Extended slide pack (PowerPoint) includes more slides showing how to build the updated Whole-Shack Mains Filter.
  • View ‘Clean Up Your Shack 2015’ (YouTube) for more detailed background.
  • Slides from my 2010 talk about chokes and baluns  to drill deeper into that subject.
  • The two mains earth (green/yellow) wires are connected together at the earth tag on the filter. (Sorry, I can’t add a better photograph yet – the filter is away at the radio club, in a building under lockdown.)

Fair-Rite 0431177081 snap-on core

Not cheap, but well worth it. If RF noise threatens to take your hobby away, then surely it’s worth something to get it back?

  • Manufacturer’s data  Ignore the photograph – this biggest bead doesn’t look like that.
  • Current Best Buy supplier – and always near the top of the list –  is
    Again, ignore the photograph, trust the part number.
    Notice the big price break at quantity 10. This makes an ideal club purchase.
    UK prices are in GB£ but do not include VAT. However, that is all you have to pay. Three-day FedEx shipping from the USA is streamlined and free for orders over £50+VAT.

Parts list for the Whole-Shack Mains Filter

  • Packaged 15A or 16A mains filter, single phase, 250VAC rated.
    Within 2 days of publication, the Roxburgh RES5-F15 filter from Farnell was sold out! The specific type of mains filter is not critical, so instead let’s try the Schaffner FN2030-16-06 (also from Farnell, but many more in stock).
  • Large Fair-Rite core 0431177081: Mouser (see above) or Farnell
  • Qty 2, Fair-Rite oval core 2643167851: Mouser or Farnell
  • Plastic box CE-TEK GR17012 to fit the above parts: CPC  EN84544
  • Qty 2, plastic cable glands, 5-10mm size: CPC CBBR7352
  • At least 3m of 3-core 2.5mm² mains flex, 90ºC rated: eBay, eg here
  • 13A socket strips to meet your requirements
  • 13A plugThe maximum total current supplied to all sockets is 13A, limited by this plug.

Steve Hunt, G3TXQ – In Memoriam

New Year’s Morning, 2019

G3TXQ was well known and respected throughout the technical world of Amateur Radio, through his website and countless postings to a wide range of groups and mailing lists. Always meticulous and deep-thinking, Steve was a fount of good information – and also a very nice guy.

Steve passed away peacefully on 30th December 2018 after a long battle with cancer, which he also recorded in that same meticulous manner.

G3TXQ is known particularly for his optimized versions of the Cobweb and Hexbeam antennas, and for his work on common-mode chokes. His website is already archived at
and if you haven’t already visited, I strongly recommend it. Take the tour, and you will meet the man he was.

Common Mode RF Chokes


In addition to the material on his website about common-mode chokes, in 2015 Steve wrote a stand-alone article for RSGB’s online technical magazine RadCom Plus.  That article is in danger of falling into obscurity, taking with it some new information that has not been published anywhere else, so I am archiving it here in memory of Steve:

Common Mode Chokes: G3TXQ 2015

The article included a new version of Steve’s famous graphic to help in selecting broadband ferrite chokes to cover various HF amateur bands. The best chokes are the ones that cover the required band(s) with both the dark green shading and the black underline. Orange and red shading are to be avoided – the choke may do some good, but may also have problems. (For clarity, I have added a large ‘X‘ across the three chokes at the bottom. As Steve explains, those were included as particularly BAD examples, NOT to be copied.)

Writing this is not the way I had hoped to begin a New Year.
RIP, Steve.


VHF-UHF Baluns

This is the talk that I presented at the RSGB Convention in October 2018.

Update: RSGB have now released the video at (member login required; it will probably be several more months before the video is transferred to YouTube).

I always cringe to hear myself talking, but the sound track does add some context to the previously released slide pack ( still available here ).

Even without the voiceover, the important points are clear:

  • Today, “baluns” are about controlling interference, reducing noise
  • Choke baluns are best for doing this.

So we should aim to:

  • Minimize common-mode RF currents on the outside of the coax
    and along the boom.
  • Don’t upset the good feedpoint balance that VHF-UHF Yagis already have.

Comments and discussion, please!  This is new information for VHF-UHF, and certainly not the last words to be said about it. It’s still very much ‘work in progress’.
Leave a reply


Technical slide content © 2018 Ian White, GM3SEK
Slide template © 2018 RSGB
Images as attributed (except where that would be embarrassing).

Interdigital Filter Design

Here is a scanned pdf of my 1984 Radcom article called A Simple Way to Design Narrowband Interdigital Filters .

Doug McArthur VK3UM (SK) wrote a program to implement this design method. Since Doug’s legacy website has now closed, the program is now hosted here. Click to download, open the zip file and run the install program.

Many people have used this method – design your filter, build it well, and it will work.

A key requirement for good performance is an excellent electrical connection all the way around the base of each resonator element. Use a lathe to countersink the base and make a sharp, level edge all around (there is really no way to do this freehand). Drill and tap a deep hole into the base of each element, and assemble the filter using strong steel screws to pull each element tightly into the supporting bar (you’re actually hoping to make a cold weld).


Switching 24V Relays from a 12V DC Supply

How to operate a 24V relay with no 24V DC supply

It only needs this simple circuit.

Relay pulser topside


Most amateur equipment operates from 12-13.8V DC; but many of the best RF coaxial relays are military surplus and designed to operate from 24-28V DC. Obviously you can generate 24-28V using a switch-mode power supply operating from 12V, and circuit boards are very cheap on eBay. But that isn’t the only way to do it… and not always the best way, either.

“Relay Speed-up” Circuit

This little circuit was originally published in my Radcom ‘In Practice’ column for April 2002 as a way of speeding up the slow open-frame antenna changeover relays that are used in many older power amplifiers. It works by providing a switching pulse at twice the normal voltage, which will roughly halve the switching time. For more details, download the Radcom article.

The idea appears to have originated with K1KP and K6XX. To make it compatible with the switching outputs of most modern transceivers, I added the low voltage / low current switching interface (TR1) so the final circuit looked like this:


Corrections: TR1 should be ZVN3306A (or 2N7002) and TR2 should be ZVN3306A (or similar)

Fig.1: Relay speed-up using a pulsed low-side switch.

Notice that the relay(s) are connected to the positive supply rail, and the switching circuit TR2-TR3 is connected at the ‘low side’ between the relays and ground. For that reason, this type of circuit is often called a “low-side switch”.

Alternative Use: Operate 24V Relays from 12-13.8V

This same circuit also has another use: it will allow the large majority of 24V relays to function directly from a 12-13.8V DC supply, with no 24V supply at all! This is because most so-called “24V” relays only need the full voltage while actually switching over. Once the relay has pulled in, most “24V” relays will hold in quite satisfactorily on 12V or even less. So there’s actually no need for a continuous 24V supply – all you really need is that first short pulse.

If a low-side switch is what you need, simply connect Fig,1 to a 12-13.8V DC supply rail and most 24V DC relays will function almost as normal. However, we can simplify it a little more…

Now Even Simpler

Fig.2 is even simpler than Fig.1 was. We have now converted the whole circuit into a ‘high side switch’ which no longer needs a separate control interface. The active switch is connected between the supply rail and the relay; and the ‘low’ side of the relay is connected directly to the ground return. We can now control the relay directly from the gate of Q2, so Fig.2 is simplified to just the two transistors and a handful of other components.

Fig.2: High-side pulsed switch.

Parts List for Fig.2

C1: 220uF 35V, 10mm dia, lead spacing 5mm (eg Farnell 9451293 )
C2: 1000uF 35V, 13mm dia, lead spacing 5mm (eg Farnell  9451323 )
D1, D2: S1M (SMD version of 1N4007, eg Farnell 2675069 )
Q1: TIP32C PNP bipolar, TO220 (eg Farnell 9804161 )
Q2: NDT2955 P-channel MOSFET, SMD (eg Farnell 9846271 )
R1: 1K0  0805 SMD (various sources)
R2, R3: 10K  0805 SMD (various sources)
R4: 100K 0805 SMD (various sources)

CIrcuit Notes for Fig.2

About C1 and C2: C1 is the capacitor that adds an extra 12V pulse in series with the permanent 12-13.8V supply. The value of C1 usually isn’t at all critical, and in most cases 220uF will give a switching time almost identical to what you’d see with a conventional 24V supply. You can experiment with different values for C1 while using an oscilloscope to monitor the pulse shape and voltage. 

C2 is a reservoir capacitor connected across the 12-13.8V supply rail, to help meet the pulsed current demand. If the relay is mounted remotely (eg at the masthead) then the whole circuit needs to be located close to the relay itself, but the presence of C2 at the load end of the supply cable makes it unnecessary to use heavy wiring.

C1 and C2 see no higher voltage than the 12-13.8V supply rail, so a typical voltage rating would be 16V. (Higher voltage ratings are perfectly OK, of course; but not actually needed.)

Do not connect a ‘protection’ diode across the relay coil – it isn’t needed. The switch-off surge is absorbed by recharging C1 and C2.

The whole circuit can be built on a scrap of Matrix Padboard (search or Veroboard. To fill some free space on a PC panel that was mainly for another project, I made a very compact semi-SMD version as shown in the Lead Photo and Fig. 3.

Relay pulser underside

Fig.3: Compact version of the pulsed high-side switch.


Page © 2017-2020 IFWtech (Ian White, GM3SEK)
Fig.1 and related links © RSGB

“Play Nicely Together”: Integrating the K3, SDRplay RSP1, HDSDR and N1MMlogger+


These notes describe how to configure the HDSDR software to allow your SDR to act as a 2nd receiver, fully co-ordinated with your existing transceiver and logging software.

The procedure has been tested with the Elecraft K3 but should also work with any OmniRig compatible transceiver.

1. Create a Virtual Port Splitter

The COM port numbers used below are typical examples only, and could be changed. However, it is good practice to keep virtual port numbers well separated from physical port numbers, so I begin my virtual port numbering at COM11.

[ Physical and Virtual COM ports:
COM ports are used for serial data communication. Physical COM ports are associated with real hardware connectors. In older PCs these were 9-pin ports for RS232 communication, but in modern PCs these are typically USB sockets.
Physical port numbers are allocated by the operating system and are usually single-digit numbers starting from COM1.
Virtual ports exist only in software, and their numbering is normally user-configurable.]

The transceiver is connected to the PC using the USB port (in my example, COM6). But a physical COM port can only support one program at a time – any other programs trying to connect to that port will produce error messages about port conflicts.

We get around this problem by using Virtual Serial Port Emulator from  The 32-bit version is freeware, but the 64-bit version requires a contribution to help pay for software licences.


VSPE has a “virtual port splitter” facility which can connect to a physical COM port (COM6 in this example) and then creates multiple instances of a virtual COM port with a different number (which I configured as COM11). These virtual COM11 ports are all identical, but are hidden from each other by the VSPE program. This allows several different programs to connect to the same virtual “COM11” port.

VSPE’s description of a virtual Splitter ‘device’.

VSPE must be started first, to create those virtual COM11 ports. When you start up each program, its own port configuration menu will now include COM11 so that is the one you should select (COM6 will still be listed, but make sure you don’t connect to that any more).

These menu selections show how to configure the port splitter:


2. Start your logging software

Now you can start up your existing logging program, reconfigure it for the new virtual port COM11 and make sure that it talks to your transceiver exactly as it did with the old hardware COM port connection. The logging program should be able to control the transceiver, and the logging software should also follow any frequency/mode changes that you make on the transceiver.

3. Connect your SDR

Now is also the time to connect your SDR and arrange for its RX feed.  Usually this will come from the transceiver, either a dedicated IF output or a signal-frequency feed which is shared with the transceiver itself (typically a rear panel RX OUT connector).

For HF you can probably connect the SDR to the HF transceiver’s own dedicated IF output (at 8.215MHz in the case of the K3). However, if the transceiver is connected to a VHF/UHF transverter, a better signal/noise ratio will probably be available at the output of the transverter (typically 28MHz). Use a 3dB hybrid splitter to isolate the HF transceiver from the input to the SDR, reducing the possibility of birdies.

CAUTION: make sure that your SDR cannot be overloaded and damaged by your transmitter!

Next you can start up HDSDR. When you configure HDSDR for COM11 (details below) there should be no error messages about port conflicts. When everything is working OK, go back into VSPE and save your port-splitter configuration as a .vspe file that can be loaded automatically at startup.

4. HDSDR configuration


HDSDR control panel. Lower right is a magnified spectrum/waterfall display of the current frequency (144.388MHz). Ctrl+Shift toggles the whole control panel on/off. For menu options, click Options or press F7.

Assuming that you already have HDSDR set up and working with your SDR as a standalone program, most of the additional changes can be found in the following HDSDR sub-menus (F7 or click Options on the control panel).

4.1 Options > CAT to Radio (OmniRig) > OmniRig Settings

Configure Port to the number of your shared virtual port (eg COM11). The rest of your port settings should be pretty much the same as you were already using. However, you should set RTS and DTR to be permanently Low so that HDSDR cannot accidentally key the transmitter.

4.2 Other settings in Options > CAT to Radio (OmniRig)

  • Sync Rig 1 = ON
  • Sync To OmniRig = ON
  • Sync From OmniRig = ON

Those three settings should ensure that your transceiver, logging program and HDSDR can all communicate with each other. However, I noticed that the VFO B knob on the K3 has considerable delays in tuning the SDR, so I normally tune HDSDR directly with the mouse. Click on the waterfall for large frequency changes and then roll the mouse wheel for fine tuning.

Also set Sync Tune Frequency = ON. (Do not select Sync LO because the LO frequency needs to remain fixed.)

4.3 Options > CAT to HDSDR

Not needed. Just make sure that Activated = OFF (unchecked).

4.4 Options > RF Front-end Config

Full sync in both directions = ON

With my VHF and UHF transverters, I use Transverter mode to give a direct signal-frequency display. For direct HF operation you will need one of the other options (which I haven’t explored).

4.5 LO settings

I always tune the band with a fixed frequency span, covering the whole range of interest and remaining static on the screen. This is important to maintain the straight vertical waterfall display (see the header photo above).

Setting the frequency span is not easy in HDSDR. You have to juggle the Spectrum Zoom slider on the control panel to set the desired width of coverage (very delicate) and each time you do that, the spectrum frequency scale ribbon will move as well. Drag the spectrum scale ribbon sideways to make it cover the exact frequency span required. This may require a few iterations between the two controls – and then be very careful not to move it again!

With the 2m transverter I select 144.150-144.400 which is the normal contest sub-band. The LO frequency then needs to be approximately centered within the display span, which in this example is 144.275.

You should now be able to QSY by clicking on either the HDSDR waterfall or the cursor in the spectrum segment. Watch to see that the transceiver follows these changes.

4.6 Choose the correct VFO

A small problem: at the moment you’re tuning the wrong VFO in the K3! The default in HDSDR is VFO A, but if you want to use the SDR as a second receiver you’ll normally need to link it to the K3’s VFO B. Fortunately it’s easy to change. On the HDSDR  frequency control panel, click on [A] (the small square box to the right of LO) and that label will change to [B]. You should now be able to tune VFO B correctly instead.

Notice that the LO frequency should not be changing (it will only change if you move completely outside of the visible frequency span.) If the LO frequency has changed for some reason, reset it to the centre of your desired display span.

Finally, right-click on the word LO to lock the LO.

At switch-on, HDSDR will restart with your previous LO and Tune frequencies, but it always defaults back to VFO A with the frequencies unlocked. You simply need to repeat the little routine above.

CAUTION:  When in use, VFO control of the K3 can occasionally jump back to VFO A.  This seems to happen if the VFO A and B frequencies are too close (eg if you have pressed A=B on the transceiver). VFO tracking can usually be reset to follow VFO B by splitting the two frequencies more widely apart, and then repeating the routine above.

4.7 HDSDR screen

I prefer the configuration in the header photo above, with a very small spectrum on top and the largest possible waterfall below. This maximizes the time history that is visible on-screen.

Setting the desired frequency span is described in 4.5 above.

The HDSDR control menu and the small passband spectrum appear in the lower part of the HDSDR window, but once your setup is OK you can press Shift-C to close those displays, and then the waterfall can flow further downward.

CAUTION: Your logging software probably has a very large number of keyboard shortcuts… but so too does HDSDR, and many of them use the same shortcut keys. Take great care that the Windows focus is on the correct program before you start typing!


Those settings should get you there… or at least somewhat closer to SO2V using HDSDR as the second receiver.

If this works for you, there are also ways to automate the setup and shutdown.

Special thanks for all the help and support from David, GM4JJJ.


Ian White GM3SEK
v2, 2017-09-30