Relay board from VK4PP given thumbs up in testing for isolation

Nick VK4PP sent one of his LPFx4  test boards to Warren WA8TOD for blow-by testing.   It looks like the bare PCB has excellent isolation between its input and output, of around 90db.

Matched with RF rated and separate in/out relays this board will perform very well.   The updated pic above shows a few tweaks and more ground stitching.

The only downside is the US$1.50 each relays (x8).    Relay K3 can potentially be reused, or you can source AXICOM relays for a bit more.

Nick is interested in feedback from constructors on the level of demand for these boards.


Variable bandwidth filter mod

Broadening the crystal filter

Michael N2ZDB notes that the BITX40 he is building receives somewhat OK but the transmit audio is terrible with distortion and/ narrow bandwidth.

He figured he may have one or two bad 12.0 MHz crystals, as the bandpass is only around 1,700 Hz at best (11.998800 to 11.997100)!  The crystals are from MEC and the letters / numbers on them are: 12.000 G   MEC AH6L1 .  He asked whether anybody knew of where to obtain suitable replacements.

Well, one supplier was disclosed by Lawrence W8LM:


between YOU and I ONLY!!!!!

12Mhz Crystals 5 for a $1.00  or 15 cents each in lot 100 quanities…

Right! 5 for a DOLLAR.!!!…. over $75 free shipping, under $75 flat $7 shipping.. They have 1/8″ uBITX volume knobs too..

Also read my review on 50ft. RG58/u cables with BNC’s both ends (GO KIT SPECIAL) … and adjustable 6.8uH inductors..
I usually get my order Calif. to Kansas in 2 days…

Variable bandwidth filter design

Meanwhile, Jim AB7VF, posted a circuit for a variable bandwidth filter for the BITx transceivers.  This is based on a circuit patented by TenTec (the Jones Filter design).  Some constructors may want to try this one out, as it involves only a few additional parts and replacement of C217 -C221.


The last challenge: reducing IMD

One of the last challenges to be conquered with the µBITx is the unacceptably high intermodulation distortion produced in the IF stages on transmit.

The IMD seems to be sourced in the Termination Insensitive Amplifiers used as bi-directional IF amps at both 45 and 12 MHz.  

Warren WA8TOD is experimenting with a prototype MMIC amplifier utilising the ERA-3SM+ (80 cents each on eBay) mounted on an RF prototyping board ($2.50 each from SV1AFN) that is the proper size and form factor to replace both transmit TIAs. The MMIC/board measure flat response from 300 Hz to well over 200 MHz, and will provide around 22dB of gain.

This test quantifies the IMD performance of the combo. The reference point on the SA is shifted +6 dB so that the readings reflect the power level that would be achieved with a single tone for convenience of interpretation.

Measurements were taken at Vcc of 3.3 volts and current draw was the recommended 35 ma.

The blue trace shows performance at 0 dBm out and is an acceptable -42 dBc (-36 dB minus 6 dB for a single tone carrier). Performance at – 10 dBm out, the level needed to drive the uBitx driver/PA chain, is an excellent -51 dBc (-45 dB minus 6 dB for a single tone carrier).

Yet to be determined is how to add the LM1117-3.3 regulator to hold the Vcc at the required level.


Replacing relays with Axicom brand relays


Jim Sheldon W0EB has now replaced all stock relays in one of his µBITx transceivers with Axicom D2n 12v relays.

All five relays have been replaced with Axicom D2n 12v parts.  Photo courtesy Jim W0EB.

This improved filtering by a wide margin on all bands except 40m.   The results:

80 Meters 3.500 MHz
7.000 (2nd harmonic) – 65.7 dBC
14.000 (3rd harmonic) -76.4 dBC

40 Meters 7.000 MHz
14.000 (2nd) -58.0 dBC
21.000 (3rd) – 47.2 dBC

30 meters 10.000 MHz
20.000 (2nd) -76.9 dBC
30.000 (3rd) -75.8 dBC

20 meters 14.000 MHz
28.000 (2nd) -65.2 dBC
42.000 (3rd) -64.8 dBC

Jim didn’t test 17, 12 and 10 meters, but he suspects their harmonic suppression is in the same boat as the rest.

Closeup of the four Axicom relays in the output stage. Photo courtesy Jim W0EB.

Graph showing results

The graph above provided by Gary AG5TX shows the resultant effects of using Axicom relays (using Jim’s data points).

Gary says:

“Your shared data maybe useful as you have shown the same board and relay with different measured results. For that particular board and the measurement data you gathered, I would say the results ARE encouraging from this viewpoint:
You have 4.5 dBc more margin to FCC spec on your worst case data point (40m 3rd harmonic) with the supplied data set.”



Sourcing Axicom relays

These relays can be obtained from a number of sources.  If you live in the US they can be found at Arrow, Digikey and Mouser for less than US$3 each.  They are also available from Aliexpress for under US$2 each in a set of 10 and from Component HK in Hong Kong for under US$1 each.

More details on using Axicom relays for RF switching

For those interested in the benefits of Axicom relays see this webpage for a switched Bandpass filter bank:

There are wealth of tips on this page for those building switched filter banks.


It appears that the Axicom relays are the answer, and Ashhar Farhan has already committed to replacing these relays in production µBITx, presumably when the current µBITx stocks have been depleted.  Hopefully Raj’s mod for removing spurs will also feature in a v4 board upgrade shortly.

Raj’s second spurs mod … simpler again

Raj VU2ZAP has found good reduction of spurs with the following mods:

1. Replace L5 with an SMD inductor 680nH or 681nH.  Mount at right angles to old toroid

2.  Replace L7 as above.  Note that C211 is bypassed with this mod.

Don’t be distracted by the missing C216 and C210.  These parts were never populated.

An additional 45MHz filter is not required and, therefore, you should get almost the same power out as stock boards.  If you ALSO use the second 45MHz filter the spurs get further reduced, but with some loss of output power.  Raj used Murata inductors 1210 size. 1206 size will do.

With Mikes W0MNE relay fix and this mod, Raj has the uBitxv3 and v4 boards now operating within normal emission standards.

Raj thinks that a combination of high Q, small size and possibly internal shielding contribute to the reason why.   Another hypothesis is that the original inductors caused ringing.  Raj has also replaced L1-4 with 330nH SMD and that helped a little on one test board.    His interest was to fix the  problem of spurs with the simplest solution and without major surgery.


W0MNE simply replaces the existing relays to fix the LPF blow by!

Mike W0MNE has come up with a fix for the harmonics problem with the µBITX that is simpler than other fixes …

He has two Ver 3 and one Ver 4 uBiTX rigs. He simply replaced K3, KT1, KT2, and KT3 relays with Axicom relays.

This has knocked down the 3rd, 5th, and 7th harmonic levels from -35dB to below -50dB on all 3 rigs.   No board modifications are required, and no changes to the LPF circuitry is required.   This may not be reproducible on all rigs,  but with 3 rigs showing the same results it may be something constructors should consider.

Mike purchased the relays from Mouser – part # 655-V23105A5403A201. Cost was US$2.89 per relay plus shipping.

Raj VU2ZAP found a picture of the “innards” of the Axicom relay, and suspects the improvement in isolation is the result of the contacts being closer to the ground plane (as the relay coil is on top in this relay, whereas the standard relays supplied with the main board have the relay coil underneath the contacts).

Before the fix on 40m …

And after the fix on his Ver 4 rig …


10w linear amp from QRP-Labs now available to purchase

Hans G0UPL who operates QRP-Labs has released his 10W HF Linear Power Amplifier.  This kit comfortably produces 10W from a 12V supply.  It is a compact design with huge heatsink included, which will not overheat even on continuous 100% duty-cycle operation.   The amp provides 26dB gain with +/- 1dB gain flatness from 2 to 30MHz.

This 10W HF Linear Power Amplifier kit has no Surface Mount Components (SMD) to solder. There are a number of small transformers to be wound, and assembly requires care and patience.

The push-pull driver stage uses two BS170 transistors in the amplifier design used in the SoftRock transmitter stage. The final uses two IRF510 transistors in push-pull. Yes, this humble low-cost MOSFET really is capable of excellent performance all the way up to 10m band and beyond! Short lead-lengths and PCB layout are extremely important, they are the key to success.


  • 10W output from 2 to 30MHz, using 12V Supply
  • Generously-sized heatsink, will not overheat even on continuous 100% duty-cycle modes
  • 2-stage amplifier provides 26dB of gain
  • Push-pull driver and push-pull finals, for high linearity and low harmonic content
  • +/- 1dB gain flatness from 2 to 30MHz
  • 4dB down at 6m (50MHz) and 8dB down on 4m (70MHz)
  • Standard 50-ohm input and output
  • Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
  • Standard inexpensive components throughout
  • Tested for 1 hour at full-power 10W, 100% continuous duty-cycle with no forced air cooling
  • Tested for 15 minutes at 20W, 100% continuous duty-cycle with no forced air cooling
  • Tested at 20V supply
  • Tested into open load, shorted load and various mismatches without instability (oscillation)

As Hans advises on the BITX20 I/O Groups list:

I am not suggesting that this amplifier will solve all problems and without wishing to be offensive, the “garbage in, garbage out” rule always applies – in particular, if you feed it a signal containing spurs, harmonics, or other problems… then you will get the same thing at the other end, just amplified 26dB. It also still applies that all stages leading up to the final amplifier must have flat gain. If the pre-drivers cannot provide a big enough signal then you won’t get full power output.

But, it could be useful or interesting, in the BITX context, to look at the details of this amplifier kit and see the short lead lengths, symmetric PCB and extensively “stitched” groundplanes.

For further information see the product on Hans website.


Teensy audio board schematic and simplified audio “board”

Dr Flywheel (aka Ron N7FTZ) has provided some details of the Teensy audio add-on board.  This is used in the upcoming JackAl add-on board expected to be available shortly, but can almost certainly be home-brewed by savvy constructors using the Teensy 3.6 and audio add-on board.

Here is the circuit diagram for the PJRC audio module:

The CODEC part is very much the reference design provided by the part manufacturer. The support for the EEPROM and the SD card is optional.

The libraries will be compatible with any SGTL5000 or for that matter a long list of other I2S compatible CODECs (look at the source code). If you are homebrewing an audio processor on the Teensy you should consider integration of the CODEC. The libraries will support the internal A/D-D/A of the Freescale ASIC with very few components (resistors, capacitors) added.

Sample resolution of 12-bits is sufficient for the type of audio DSP that we need, though the higher bit-rate CODECs are much better. It is a matter of cost effectiveness and board space.

An alternative simpler approach


Below shows mono circuits for a lower grade Teensy, however the circuit is the same for the Teensy V3.6 with the corresponding pins.

The input circuit (mono):

Here is the output circuit (mono):