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Tool tips – making a rectangular hole

John KZ1G notes a recent post by Fred (K3TXW):

“…I have problems making a rectangular hole in a piece of aluminium of exactly the right size, with the edges exactly parallel to the edges of the case. I inevitably make the opening too big or slightly wavy. The result screams “homebrew carelessness” though I’m not careless. Maybe there is a rectangular punch or something to do this right, but I don’t have such a tool; I use a set of files.”

In addition to files for work on front panel fabrication, I recommend an electronics shop include:

1.  A drill press with a chuck that will accept 0.50-in.-diameter bits.  Harbor Freight sells a couple of bench-top units for under $100.  Put a magnet on the base as a place to keep the chuck wrench.  (It’s usually best to clamp work to the drill press table.)  Once you have a drill press you’ll find many other jobs for it.

2.  A good set of sharp drill bits.  Bits with a titanium nitride coating remain sharp for a long time.

3.  A step drill bit.  I use an Irwin Tools Unibit 3/16-Inch to 7/8-Inch Step-Drill Bit with a 3/8-Inch shank.  Great tool when you need to make larger holes for controls or to start a rectangular cutout.

4.  An Adel-brand metal nibbler.  They show up on Ebay.  Or buy a new one at https://www.adelnibbler.com.  I’ve used one since I was a teenager and couldn’t work on chassis or panels without it.

Lay out your hole with masking tape around the outside.  To make a rectangular hole (see attached image) I use a step bit and smaller bits to make round holes that remove a lot of metal.  Just don’t get too close to the rectangle’s edges.  Next I use the nibbler to remove remaining metal close to, but not at, the rectangle edges.  Finally I clamp the panel or chassis in a bench vise so an edge of the hole aligns with the top of the vice jaws.  File away any remaining metal until the edge is parallel with the vice jaws.

Kevin KU8H hides imperfections around a rectangular hole in the front panel with a bezel. A bezel will hide a lot of sins. They are also easier to fabricate with straight, clean edges. Rough edges that might show a little on internal panels of chassis – who cares.

Your use of the bench vice to keep the edge straight during filing is good but will eventually damage the vice. I use a sacrificial piece of steel like a piece of angle iron clamped up along with the workpiece. In woodworking we call that a “fence”.

Kevin VK3DAP / ZL2DAP clamps a short length of angle iron along the line of the opening, and files to the edge of angle iron. This gives a nice straight line. Rectangular chassis punches are available but are costly, and the larger ones require lots of force, and may need an hydraulic press.

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Higher current Axicom relays

Jim W0EB has finally received his >500 mW Axicom relays, after they got misrouted in the mail.

H plugged these into his V4 test bed built up on an aluminium open chassis. Sockets are mica filled 2mm thick with silver plated flat contacts. This leaves the relays 2mm above the PC board.

With the >500mW coils (i.e. with coils that have fewer turns on them) the hope was that this would lessen  the extent of inductive coupling between the switched portion of the circuit and the relay coils.  Unfortunately this does not appear to be the case.  Jim got almost exactly the same results as with the 400 mW relays mounted the same way.

40 meters is still just barely in spec with the 3rd harmonic being -43.8 dBC. The others are also in spec but not much better with 3rd harmonic varying from -46 dBC on 10 meters, -44.5 dBC on 12 & 15 meters , -44.7 dBC on 20 meters and -57.3 dBC on 80 meters.  60 meters wasn’t tested.

Raj VU2ZAP notes that it makes quite a difference between socketed an unsocketed replacements.  It is best not to use sockets for the relays, in order to get the relays as close as possible to the PCB.

Before and after shots of relay replacement as observed by Raj follow:

Before replacement
After replacement

Raj and Jim both note that there is variation between boards in terms of improvement in performance.  The reasons for this are not known.

The conclusions are:

  • relays help, and ubitx.net recommends them, but replacing relays with AXICOM relays may not always be sufficient to reduce “blow by” on some bands or modes
  • there is no point in spending more on Axicom relays with a higher coil current
  • the best option would be to replace the filter section altogether, and there are a number of ways of doing this.
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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.

Reference
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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:

SSSSSSShhhhh!!!!!!  THIS IS MY **SECRET** U.S.A. SOURCE…

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..

https://www.allelectronics.com/item/cry-12/12-mhz-crystal-hc18/u/1.html

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.

Reference
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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.

Reference
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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.
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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.”

Reference

 

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:

http://www.i1wqrlinkradio.com/antype/ch97/chiave50.htm

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

Conclusion

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.

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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.

Reference

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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 …

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