Ruminations on fast TX/RX switching on uBITx

Discussion on the IO Groups BITX20 list has recently turned to how a fast TX/RX switching regime could be achieved for the µBITx.

John K0JD mused that he could probably figure out how the circuit changes to eliminate the relay, but then changing the firmware to let it happen would have to be addressed too.

Madradiomodder suggested that it was easy.  He provided us with this example of the front end of the Elecraft K3, that uses PIN diodes.

You just need to add DC to bias the diodes above the lowest AC cycle plus the diode voltage drop (0.6V in the case of the BAV99’s and the 64-04’s they use here).  The 7T and 7R (7 volts for Transmit or Receive) used for biasing the diode switches… and the PRE_ON and PRE_OFF voltages (again, 7 volts) to pass or block the  preamp Q4.

Argosy II approach to switching using PIN diodes

Jerry, KE7ER  found  a similar circuit for T/R switching (and SWR metering) in a  manual for the Argosy   (see page 51):

The full transmit RF waveform goes through C1, diode D4 ensures the bottom of that waveform is at DC ground, diode D3 then rectifies it to create a DC voltage across C2 that is equal to the peak-to-peak voltage of the transmitted RF.  The notes then say that this relatively high DC voltage available during transmit is used to reverse bias diodes D1 and D2 such that  no transmit signal goes through them to the RCV connector (which then goes into the receiver front end).

Note that the PIN diodes at D1,D2 are called out as 1n4007’s.
And that this transceiver works up through 30MHz.

Another thing to note, the final remains connected to the antenna during receive, shunting some of the receive energy to ground.  Some of the other T/R switches use additional PIN diodes to isolate the final
from the antenna during receive.

Cheap alternative to PIN diodes

Tom, WB6B dug up an old article by  Don Brown KD5NDB that relates to 1N4000 series diodes and their potential for use as a cheap PIN diode.

The 1n4000 series of diodes are general purpose rectifier diodes designed for low frequency use primarily 60 Hz power supplies. The construction is the same for the series except the 1n4007. The lower voltage diodes are basic PN junctions and would not work as a RF switch. The 1n4007 however has an extra intrinsic (undoped) layer forming the P-I-N junction. This is to get the 1Kv PIV rating but it also allows the 1n4007 to work as a RF switch like a true PIN diode.

The RF characteristics are not specified on this diode because it is not designed to do this type of work it just happened to work out that way and they are very cheap. True PIN diodes work better than the 1n4007 but are more expensive and not as readily available. Elecraft has designed the RF switching to work with the 1n4007 and making substations of different diodes may not work. Other 1n4000 series definitely do not have the PIN structure so they will not work.

PIN diodes are selected because they CAN’T switch at RF frequencies

Farhan VU2ESE waded in to clear up some confusion … The PIN diodes that are used for T/R are the ones that CANNOT switch at RF frequencies. That’s precisely why they are used.

An RF charge across the diode travels through precisely because the junction cannot switch on and off so rapidly and hence it remains either on or off. A PIN diode is like that, it can handle large currents, but it cannot change the current fast enough. That’s why for switching 100 watts of RF, you don’t need a few amps of bias current, a few milliamps will do.
There are ways of switching T/R with even 1N4148s (for QRP work). W7ZOI has, as usual, been there and done that. Read this paper to understand the concept :

ElectronicAntenna Switching” by Wes Hayward

This Wes Hayward article  could be useful to those intrepid experimenters looking to use PIN diode switching”


W2AEW video

Jim KH6SKY drew attention to  W2AEW’s video on PIN diode switching for a good intro to the subject.  Using the simple test setup in this video you could check the suitability of the 1n4007s for this purpose.  Interesting that the turn on characteristic of the PIN diodes is so strongly affected by frequency.


Too much RF up the spout

Jonas SM4VEY posted on the IO GROUPS BITX20 list that his µbitx hadn’t survived field day operations After Jonas discovered his transceiver was dead he connected a dummy load via a power meter as an antenna.

The power meter showed 2 w power into the 50 ohm dummy whenever another nearby station transmitted into a nearby antenna.

His poor µBITx was now as deaf as a brick and provided (almost) no power out on transmit.    Jonas mused “What should I suspect first?”

Constructors made a number of suggestions.  Jonas reported the fixes as follows:

“After replacing the 1st mixer diodes the RX came to life again and I could hear -120dBm signals which represented an improvement of around 30 dB.

“I then started measuring the signal levels before and after each amplification block in the TX path and found Q90 having less signal after, then before.  I replaced Q90 with a leaded transistor and the µBITx now works normally again.

“I’ll look at adding 2 reversed (one each way) diodes to ground from the RX line before the 1st mixer. I’m not sure these will survive 2W though…”


Ignition noise tips for your mobile uBITx

Robert N4FBZ has been experimenting with locating his µBITx in his vehicle, but says, “I’ve given up on trying to use my Bitx40 as mobile. Nothing I’ve done eradicates the ignition noise (and the alternator whine came back).”

Robert’s comments, and those of other constructors provide pointers to addressing ignition noise that may be helpful to others seeking to take their µBITx mobile.

Bob shares a couple of tips from this…

1) the radio is awesome, but very sensitive to impulse-type noise. The more barriers you can throw up between the noise source and the radio, the better! That means chokes in the power line, filter caps, and so on – and if possible, shielded wires! The noise may actually be getting in somehow via a path besides the antenna and knowing the spectrum could give a clue as to how it gets in.

2) if you’re trying to go mobile, grounding grounding grounding!!! While Bob’s best attempt  wasn’t good enough, it might be good enough in another situation. Ground everything you can… and don’t just trust to bolted-together sheet metal (frame). Also, usually drawing the power from a very low impedance source (direct to the battery) with good shielding and filters is the usual fix. In the case of Bob’s µBITx, it wasn’t enough.

Other suggestions:

  1. Ensure your µBITx is in a metal enclosure without large penetrations (like Nextion screens)
  2. Is everything inside the radio grounded to a common ground? (Power, antenna, board, raduino, mic and key jacks, etc.)
  3. How and where your antenna is mounted is important (i.e. far away from ignition sources, and with a good RF bond to the chassis)
  4. Use a common mode choke using ferrite material and with a number of turns of coax through it.
  5. Check for alternator whine.

Vic WA4THR observed on his first BitX40 trip that he didn’t have ignition noise, but the alternator whine was horrible.  He found it was coming through the power line, and not the antenna.   The noise didn’t change when the antenna was disconnected.

He used a Radio Shack “Heavy Duty Automotive Electrical Noise Filter” (cat. no. 270-051B) in series with the power lead and was amazed that it actually quieted everything.  This was subsequently installed inside the BitX40 plastic case.

On the road, the receiver now sounds as good as when at home on a power supply. Vic suspects that these filters were designed for CB radios, and are the right size (rated 10 amp) for QRP.   You can often find them quite cheap, too.

A good resource:

  1. K0BG’s website contains information on bonding surfaces, dealing with ignition noise, spark plug wires, etc.


Two tone generator for testing SSB

Mike K5ESS referred to posts by Raj VU2ZAP for an online generator that can provide two tones for testing your SSB output:

Another alternative is to download the application f-Generator for loading on to your cellphone.  The free version will give you a single tone, but you will need to pay a dollar or two in app to get two tones.

Mike also alerted us to an article in ARRL’s March/April QEX by Phil Salas AD5X describing a two tone generator fabricated from two Wien bridge oscillators that are available from Ebay. I’ve put together a PDF that describes the generator for those that might want to build one but don’t have access to QEX.


Q90 blown by strong RF input on antenna line

Further to an earlier post about the vulnerability of Q90 in the µBITx, Gary
W6RAG wrote to today noting that the potential danger to Q90 from nearby strong RF signals is well founded.

Recently Gary was attempting an FT8 contact on 80 meters with his son about 400 miles distant.  Being unsuccessful with the µBITx, he turned off the rig and turned on his TS430 and made contact at about 60 watts output power. The ubitx remained connected to an end fed wire about 10 ft from his dipole that was in use with the TS430.

A few minutes later he tried the ubitx again and it had no output. He traced the failure to Q90. The emitter was shorted to the base just as reported by others.
The diodes in the balanced mixer will not save Q90.

Gary recommends that µBITx users disconnect the antenna if they are going to operate other transmitters with nearby antennas.

Toroid winding hints

Mark N7EKU/VE3 has  found the calculator on to be very accurate — so much that he rarely measure after winding anymore. quotes a quite generous length of wire to use (even considering the 1″ leads) so he usually subtracts an inch.

No need to re-wind if you loose count. As long as you have a cell phone or other digital camera, just take a picture and then zoom in on it. It becomes very easy to count if you do that.

How to wind: first put the wire through the center, gather the ends, and pull them taut. Now wind one half of the windings, and then the other. This way you only have to deal with one half the length of wire at a time (instead of threading the entire length through for the beginning turns).


Microphony in your uBITx

Peter G8FWY had a problem with his µBITx.  It picked up sound from touching the case and pushed this through his headphones.

If you have a similar problem it is most likely to be ceramic capacitors in the audio stages of your rig.

In this case it turned out to be C50 in the audio pre amp.  Peter replaced it with a leaded electrolytic capacitor and the problem went away.