Finding the characteristic impedance of an antenna cable

May 24, 2020

I recently stumbled on a cool video that showed how to compute the impedance of an antenna cable. Unfortunately it’s almost five minutes long, so I thought I’d summarize:

  1. Measure the capacitance of the cable when it’s open (C) in farads (F).
  2. Short one end.
  3. Measure the inductance of the cable from the non-shorted end (L) in henrys (H).
  4. Impedance (Z) = √(L/C) in ohms (Ω).

It works for both coaxial cables and ladder line. Neat trick!

Building Signalink Cables

June 13, 2016

Many of us have sound card interfaces for our radios that use the standard RJ-45 plug on one end and a custom connector for the radio on the other. If you’ve got more than one radio, it’s sometimes possible to buy additional interface cables. That can get pricey, though – and depending on the connector on your radio, an interface cable might no longer be available.

For many rigs it’s possible to buy a connector that ends in bare wire fairly cheaply. I hit eBay and found a cheapie Kenwood connector for $2.49 (“4 Wire Speaker Mic Cable for Baofeng UV5R Kenwood TK-240”).

Kenwood style connector with bare ends

While holding one of these in my hand, I noticed that the individual wires in the radio cable were roughly the same diameter as the wires in cat-5 network cable.

Before doing anything else I wrote down which wires connected to which pins on the radio. All of the wires in my cable had different colors, which made identification a lot easier. Next, I determined which pin in the RJ-45 plug should be connected to which wire. This varies depending on the radio connector and sound card interface you use. In my case, green went to the 2.5mm plug tip aka speaker, red went to the 3.5mm ring aka mic, black went to the 3.5mm sleeve aka PTT, and white went to the 2.5mm sleeve aka ground. I found this Tigertronics page useful.

Close-up of connector and wire

After that, I cut the interface cable straight across with diagonal cutters. My cable came with an integrated strain relief, and I cut that off as well. Then I carefully removed a little more than half an inch (about 13mm) of the cable jacket, being careful not to nick the wires inside.

Cable together but before crimping

I arranged the wires in the correct order they’d need to be into the RJ-45 plug. The wires were solid core, so I was able to spread them more or less into position. Next I inserted the cable into the RJ-45 plug, being careful to slip each wire into the appropriate channel. One or two recalcitrant wires needed persuasion with a pin to find the right home.

Once all the wires were in their channels, I pushed hard on the cable to ensure all the wires were as far forward in the plug as they would go. At this point I crimped the RJ-45 plug. There are two nice things about an RJ-45 crimp: there’s no need to strip the wires (the plug bites down on them to make the connection), and the crimp forces part of the plug’s shell against the cable, which keeps it in place.

Then came the moment of truth: I tested continuity of each pin on the connector. Success!

Completed cable

The radio’s connectors were in the right place, and I had a professional-looking interface cable for a radio that needed it.

Adding ATtiny support to Arduino IDE 1.6.1

March 22, 2015

I recently upgrade from the 1.0.5 version of the Arduino IDE to 1.6.1. Things aren’t completely smooth – in particular, I was using Coding Badly’s cores for ATtiny85, ATtiny84 and ATtiny2313.

Unfortunately, those haven’t yet been updated for the new layout specified in the 1.6.x IDE. So instead I switched for now at least to the damellis cores (which support ATtiny85 and ATtiny84, but not ATtiny2313/4313).

Installing this is a matter of unzipping it in the sketchbook/hardware folder (on my Windows box in C:\Users\{my user}\Documents\Arduino).

In Windows cmd shell:
cd "C:\Users\{my user}\Documents\Arduino"
mkdir hardware
unzip c:\{whatever}\
cd hardware
move ..\attiny-ide-1.6.x\attiny .
cd ..
rmdir attiny-ide-1.6.x

At some point in the future either the Coding Badly cores will support IDE 1.6 (hopefully with the nice variants structure that the damellis cores use) or the damellis cores will do ATtinyX313s.

Foot Switch for the Insane

December 28, 2014

I’ve been looking for a foot switch for a while now to act as a PTT for a radio. Yesterday at a thrift store, I came across the Koino KH-8012:
SPST Foot switch with NEMA 1-15 plug

Do you see anything wrong with this picture? That is indeed a SPST foot switch with a NEMA 1-15 plug on the end. I can’t think of any reason that you would want to do that – the switch is rated at 15 A / 125 V AC (as well as 14 V DC), so it’s not like it was meant for a European destination where the corresponding socket wouldn’t be found in the wild.

The label is a lie too – it’s normally open, and conducts when closed. It is not SPDT: there are only two conductors coming out of the switch.

I’ve since rendered it safe and unable to short out household wiring by cutting off the NEMA 1-15 and adding a 1/4 inch jack instead.

All I can find about this switch is that it’s available from China and Vietnam, and costs USD $6.49 each when bought in large lots. Any idea what this was originally used for, or why on earth anyone would want to terminate it in a way that seems designed to blow fuses? Leave a comment.

Repairing a MFJ-259B Antenna Analyzer

November 29, 2014

I’ve had an MFJ-259B antenna analyzer for a while, and for the most part it’s been pretty good. However, in the last few months I’ve seen it intermittently give me really high SWR as opposed to normal SWR.

Usually, that means there’s a break in a transmission line somewhere, but I kept seeing it on different lines. Curiously, it usually went away when I touched the antenna connector.

I wondered if I was adding capacitance or something to the system, but finally I realized it happened when the feedline cable pulled down on the analyzer. It was just a break between the antenna connector and the analyzer.

I took the analyzer apart, re-soldered the SO-239 and I was back in business.

Here’s what I learned when I took the antenna analyzer apart:

  1. Take the battery cover off first (two screws on the bottom)
  2. Next, unscrew both sides (four screws on each side)
  3. At this point, you’ll have access to the battery compartment. Take out the two top batteries and the two bottom batteries (don’t need to take out the rest).
  4. You’ll see four screws that hold the battery compartment to the analyzer. Actually, that’s a lie – only the two right-side screws hold the battery compartment to the analyzer. The left screws are screwed into Delrin insulators. Don’t unscrew the left screws or the insulators will drop off and you’ll have to look under the table for them. Just unscrew the right screws (top and bottom).
  5. At this point you can move the battery compartment to the side, and get easy access to the SO-239 connector. Don’t lose the lock washers that are under the screws.
  6. I suspect they used lead-free solder to solder the connector, which is more prone to cracking than 60/40. I upped the heat a little and mixed in some 60/40 solder to make it more durable.
  7. At this point you can put the 4 batteries back in and test with a dummy load and a good cable. I did this and verified my problems with mystery SWR were gone.
  8. Put things back together in the reverse order that you took them apart.

Editions of ARRL Hints and Kinks Books

February 14, 2014

I’ve been looking around the web for a list of the ARRL “Hints & Kinks for the Radio Amateur” editions. I wasn’t able to find one, so here’s what I know:

Edition Year Orig Price Preface
First volume (unlabeled) 1933 0.50
Volume Two 1937 0.50 Ross A. Hull (VK3JU operating as ARRL station W1MK/W1AW?)
Volume Three 1945 0.50 unsigned
Volume Four 1949 1.00 A. L. Budlong (W1BUD)
Volume Five 1954 1.00 A. L. Budlong (W1BUD)
Volume Six 1959 1.00 A. L. Budlong (W1BUD)
Volume Seven 1965 1.00 John Huntoon W1LVQ
8th Edition 1968 1.00 John Huntoon W1LVQ
9th Edition 1973? 1974? 2.00
Edition Year Orig Price Editor
10th Edition 1978 4.00 Stuart Leland W1JEC
11th Edition 1982 4.00 Charles L. Hutchinson K8CH, Stuart B. Leland W1JEC, Larry D. Wolfgang WA3VIL
12th Edition 1989 5.00 Charles L. Hutchinson K8CH, David Newkirk AK7M
13th Edition 1992 10.00 David Newkirk WJ1Z
14th Edition 1997 12.00 Robert Schetgen KU7G, David Newkirk W9VES
15th Edition 2000 12.00 Larry Wolfgang WR1B
16th Edition 2003 15.95 Steve Ford WB8IMY
17th Edition 2004 17.95 Dana G. Reed W1LC
18th Edition 2012 22.95 Steve Ford WB8IMY

All the editions from 10 on are the large format (8 x 11 inches) and have an editor; the editions from 1 through 9 are 6 x 9 inch and sometimes have the writer of the preface/forward listed. N4MW has info about the earlier editions as well as other classic ARRL publications.

Thanks also to James Smith for clarifications about three of the earlier editions.

Optoelectronics Cub battery replacement

January 5, 2014

I’m in the process of replacing the battery in an Optoelectronics Cub frequency counter I recently inherited. Unfortunately, the battery pack for this frequency counter (and the Optoelectronics Scout and apparently a bunch of others) is fixed in the device with double-stick tape and not really intended to be removed. A new one is available here: but that costs more than I want to pay, especially since it wears out after a while.

Instead, I decided to replace the battery pack with a battery holder that could hold NiMH batteries. The connector on the main board was about 5.3 mm wide and 4.4 mm high, with connectors about 2.5 mm apart. I think it’s a JST XH 2-pin connector.

According to the multimeter, the left pin is ground facing the socket. Unfortunately, this means the wires on my connector are the wrong colour – so I trimmed them short and soldered them to the correct wires on the battery holder with a quick bit of heat shrink.

Heat shrink on battery wires and JST connector

The no-load voltage across the connector is 10 V. Since the battery claims to have 4 NiCd cells, I’m guessing (hoping?) it’s 4 * 1.2 V cells in series. Because the battery pack is designed to be charged fairly continuously, I also hope that it’s a slow charger as described here so I can use NiMH without danger.

Battery pack connected for testing

Fitting the battery connector into the frequency counter requires a little work. First, it needs to go face down, so the curvature of the batteries matches the curvature of the case. Next, at least with the one I have, there’s a wire that goes across the connector from one side to the other. This needs to be at the bottom… which means the wires have to be run down the middle of the connector. Finally, the components on the PCB stick a little too far down. I had to remove some of the plastic on the left and right.

I cut too many notches in the battery pack

I used a Dremel with a sanding bit to make room down the back:
Use a Dremel to grind down the back

Battery pack trimmed and in position

Putting back together required a little squishing, and I didn’t feel comfortable tightening the screws at the bottom that held the board down all the way. For now they’re just a little loose… a better option might be to get a few small washers under the board.

Useful parts:

  • Battery holder Radio Shack 2700391 ($2.49)
  • Connector pins eBay Micro JST 2.5 XH 2-Pin Connector with Wire Male Female x 10 set ($6)

Standard for 12 volt DC Edison sockets

September 2, 2013

I recently decided to create a 12 V DC lamp for emergency lighting. I started with a 120 V AC desk lamp from a big box store ($5.99) and added one of the 2W 12V DC bulbs that use the Edison E26/E27 socket.

Of course, I needed to remove the existing NEMA 1 plug and replace it with something that can’t be plugged into household current. I chose PowerPole connectors. That brought up the question: what should be the positive terminal and what should be the negative?

I did a lot of searching in RV and boat wiring, but couldn’t find anything regarding polarity of Edison E26 / E27 sockets. So in the end I just went with what seemed sensible. I wired what used to be connected to the wide blade of the 120 V plug (neutral) to negative, and what used to be connected to the narrow blade to the positive terminal.

This means that in the E26 / E27 socket, the brass projection ends up connected to the + terminal and the shell ends up connected to the – terminal.

That seems to make sense to me – but if anyone knows a better standard I’d love to hear about it.

Useful resources for filter design

March 30, 2013

Some useful links I’ve found for designing passive RF filters:

Repairing a Samsung LN32A450

December 12, 2012

I own a Samsung LN32A450 TV set, which has been fairly good so far… until last weekend, when it didn’t turn on. Instead, the power LED flashed steadily at about one flash every 500 ms.

It seems that my TV set suffered from the capacitor plague. A bunch of the Samwha capacitors swelled up and were no longer to spec. I could order a replacement board (the board is BN44-00214A available from A little searching showed that others had successfully revived their power boards just by replacing the capacitors.

First, I had to open the TV set. This involved removing 16 screws (all the same size) from the back. I discoverd I had to be careful with the ones marked “S” – those four also hold the TV stand in place, so I wanted to remove them last. Also, there’s a screw on the back panel below the “EX-Link” connector, and another one on the back underneath the A/V 2 inputs.

Once I did that I could slide the back off (face down, since the support was unscrewed). The power board is the one in the middle. There are five connectors to disconnect from there: two at the upper left, one at the upper right, and two power connectors that go to the lower right. In my case they stayed in the right position once I removed them because there was tape holding them to the flat panel.

After that, I removed 6 small screws that held the power board in. I didn’t need to remove any of the standoffs – they’re just there to push the board away from the flat panel.

When I investigated the board, I could see the telltale swelling of capacitors that indicated problems. I saw problems with four capacitors:

CB852 in the middle right of the board: 2200 uF 10V
CW856 near the top left: 470 uF 25V
CW858 right below CW856: 680 uF 25V
CM868 right below CW858: 680 uF 35V

Some people reported success with using Radio Shack replacement capacitors. I was a little nervous about that – these caps are all rated to 105 degrees C and have high ripple current tolerance, and I didn’t want to swap in something I’d just have to replace later. I ended up getting replacements from Digi-Key. They have a minimum $25 order (otherwise they charge you $5 for handling). Luckily I had some other stuff to buy at the same time.

I replaced them with the following:

CB852: Panasonic EEU-FC1A222L (Digi-Key part P11189-ND)
CW856: Panasonic EEU-FM1E471 (Digi-Key part P12388-ND)
CW858: Panasonic EEU-FM1E681 (Digi-Key part P12390-ND)
CM868: Panasonic EEU-FM1V681 (Digi-Key part P12417-ND)

The replacement for CB852 was about 1 cm taller than the original part, and that made it the tallest part on the board. I was a little worried about that, but there seemed to be enough clearance that it didn’t cause a problem.

These capacitors are electrolytic and have a polarity, so I had to replace them the same way ’round that the originals were. Luckily, on my circuit board all the negative terminals were marked with a white semicircle underneath the capacitor.

After that, I plugged the cables back into the board (not forgetting the single-pin green ground cable). Then I put the 6 screws back.

Next I put the cover back on and put the 16 screws back in. I started with the base “S” screws, then the top three, then kind of haphazardly put the rest in the right places.

Then I powered up. Success! The TV came on and was as good as it was before this happened.

A few links I found useful: