A noise bridge for cheap antenna analysis

A noise bridge is a very simple circuit comprising a wide band noise generator (a conducting zener with some amplification) and a bridge arrangement with "known" variable resistance and capacitance on one arm and a socket for the unknown device.

There's a bit of extra capacitance thrown in on the unknown side so that the variable capacitor nulls half way.

The circuit I built is very simple, it's written up in "The Antenna Experimenter's Guide" by Peter Dodd, G3LDO (second edition), and documented here where it was available as a kit known as the G3ZOM noise bridge.

Ralph Klimek has a nice write up of his implementation as well.

A receiver is used as a single frequency detector to enable the finding of the null.



Noise bridges have a reputation for being a very simple and cheap way to do basic measurements on unknown impedance's such as antennas, particularly when you're shooting for 50R and no inductance or capacitance.

The noise source works right through HF and is still going strong at 30Mhz, I imagine it's quite useful at 6m as well.

Given that most of us have some sort of receiver to be the null detector this is a very easy instrument to build. The hardest component for me to find was a 220R potentiometer, in the end I ordered from RS Components yesterday and it arrived today.

All this is prompted by the fact that I'm trying to make an antenna for 1.8Mhz work and I've currently loaned out my MFJ antenna analyser. It's great that such a useful instrument can be so simple.

In use

Hooked up to my 160m inverted L which I'm sure needs inductance, the bridge appears to show that the antenna is neither inductive or capacitive and the impedance appears very low, but I don't get a convincing null perhaps due to all the broadcast band rf coming back down the line.

Merry Xmas Forest Coach Lines!

One of the nice things about our friendly local bus line is that every year they get you to write your name and address on the back of your ticket and enter it in the draw for a pudding.

I've never won before and this year we were determined to give it a red hot go. We collected bus tickets for the whole year, yes hundreds of them, we have a rubber stamp with "Pudding Contest" and my address on it.

Yesterday I started entering the contest and guess what was at the door today...



Yes we won! Already!

Anyhow, sincere thanks to Forest Coach Lines, not just for the pudding but really for all the friendly humorous and professional drivers (including Richard) that make it such a great company.

The other side of high definition TV

I've just returned from being a remote guest on a TV show. This is something I've done a few times in the past, but not since HD TV and things have changed...

The studio was all flat screen monitors and blue (or green) chroma-key backgrounds (I wore a light blue shirt that looked semi-transparent at first, so they had to change the background).

What amazed me was the care taken with the makeup for high definition. My face was thoroughly brushed with gunk, eyes lightened, even something on the lips! Then the makeup artist trimmed my ear and nose hair as, she explained, these show up on high definition camera and look distracting.

The other new technology, at least to me, was the tiny earphone which was wireless and is inserted completely into your ear. The lapel microphone was still wired and run under my shirt.

Talking back to the barrel of a big camera in response to disembodied questions in your ear is still a bit of a challenge, but I think it went OK.

If it turns up on line I'll post a link tomorrow.

Time Machine requires built in network interface

I'm using an Intel Mac Mini with a dead on-board ethernet port (lightning strike) and it seems I can't use Time Machine for backup.

"Time Machine could not be configured. The built-in network interface could not be found." - I'm on 10.5.6 by the way (the error was less informative before).

I can set up networking either wirelessly or with a USB ethernet dongle (the one for the Air seems to work these days), but Time Machine won't let me choose a backup volume.

Weird thing is that I can configure the on-board ethernet port, it has a mac address it just permanently looks to be unplugged.

The other interesting thing about this is that Time Machine works on a MacBook Air which only has wireless or USB networking.

Update

Got it working by doing this as root:

mv /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist.old

Rebooted, went to Network System Preferences, tidied up the network interfaces (deleted bluetooth, a serial device, and the Firewire device) and re-configured Time Machine.

Anyway, it's good to back on the Time Machine routine again.

Panda Cub - a boatanchor for the shack

The Panda Cub transmitter survived a 900Km trip in the boot of the car. Despite the rough handling and a bit of accumulated rust, it fired up just fine.

I used a 240V 50W lightbulb as a dummy load, (it has a cold resistance of 70 ohms, no idea what it goes to when hot), and it certainly puts out a little power.

There's not a lot of information available on the internet about the Panda Cub. There are a few mentions and mostly information about the larger sibling by the English Panda Radio corporation.

A circuit in my bad handwriting is available in the scans of the old Contronics humour magazine here, and here.

It's a very simple design with a single 807 in the final. If anyone could point me to a user manual I would be most grateful.

Over the years the built in modulation circuitry has been decommissioned and the unit must be modulated by plugging in a 30W audio amplifier drive through a socket in the back.

Also, the frequency multiplier that made this a multi-band transmitter isn't there anymore so this is now a 160m AM transmitter which is fine by me.

Dallas tells me that the metal covered 6AG7 valve has an "N" on it because it belongs to Nigel - let me know if you need it.

I'm modulating it with a 100W RMS amplifier (way more than enough) and it's looking good:

The trapizoid (audio on the X axis, RF on the Y axis) shows a phase problem, not sure how serious that is.

Would you believe I purchased this in the 80s for $10 at an auction. I count myself very lucky.

The next challenge is an antenna for 160m...

Antenna update

I've hammered an earth stake into the ground at the base of the neighbour's gum tree and run up an inverted L with a 10m vertical and 12m horizontal part.

A test transmission over the weekend sounds terrific on a transistor radio while I walk around the block but I don't think I'm getting out much more than that - no one responded to my calls during the Sunday morning VK2WI 160m callback.

No doubt more inductance is needed to make this work a bit better.

A visit with Dallas, VK3EB

A main objective of my recent road trip was to get back an old transmitter that I left behind. It's passed through several hands over the years and has been well looked after.

Most recently it's been in use by Dallas, VK3EB who I visited to pick it up.

Dallas has been doing some fantastic home brew radio construction, including this lovely antenna tuner:

He's been using the Panda Cub transmitter on 160m AM while he completes a fully home brew transmitter which is being constructed on a wooden bench:

If all goes well on the "bench of death" the magnificent dummy load will shine brightly:

So thanks Dallas for your hospitality and for taking such good care of the Panda Cub. I'm eternally grateful.

Internet on the road in Australia

I'm now back after a leisurely drive to Melbourne via the coast and back to Sydney via the inland route.

Along the way I was able to use my iPhone on Telstra everywhere I had a break. The plan includes free access to their hotspots which can be found at the McDonalds spread around all over the place.

When stopping for a night, I chose hotels advertising internet access and had a good experience. At Gundagai I stayed at the amusing Gabriel Motor Inn.

During the trip I chose motels with internet, one was $10 for 24 hours, the Gabriel was free for 2 hours (but it only counts time while you use it so that was fine for the night).

I think any motel that doesn't offer wireless internet these days is definitely losing business.

Anyhow, great to be back.

Driving through country Victoria

After staying overnight at Batemans Bay, I had a leisurely drive south with a break for a walk at Lakes Entrance.

Stopped off for lunch at the Berlin Cafe at Genoa where I met the wonderful Marian.

The cafe is on the river, it's decorated with flower boxes out the front and chooks in the garden.

While much of the menu is crossed out and many of the other items aren't actually on today, Marian was able to whip me up a huge hamburger with the lot in no time.

While she cooked I heard about the local politics surrounding the colour of the proposed public toilet.

After the cooking was done we sat outside, smoked our pipes (well, she smoked hers) and we had a very amusing conversation.

Marian has an amazing story including a war, truck driving, rescuing motor accident victims and running a country cafe.

Tonight I've stopped over at the Hacienda International motor inn at Sale.

The $80 room is clean and comfortable, decorated in a Spanish style (I presume) with exposed beam ceilings.

For $10 you get internet access and I notice they use a system called WifiDog where they give you a password that works for 24 hours and limits you to 100MB.

It worked very well when I first checked in but now it's peak time it has slowed and was difficult to get authenticated again.

Tomorrow it's off to Melbourne to catch up with friends and family.

Enjoying a leisurely drive Sydney to Melbourne

Currently on the road between Sydney and Melbourne. A couple of reasons for this involving transporting stuff each way.

I'm particularly looking forward to picking up an old "Panda Cub" Valve AM transmitter I left behind many years ago.

As I'm now on a Telstra iPhone plan, there is free access to their hotspots and each McDonalds seems to have one. (They send you an SMS token to your phone number to get on).

Connection seems very good and as long as I just stick to the coffee I'll keep my weight under control...

Spent last night at Batemans Bay in a roadside motel, the delux executive recently renovated room was pretty basic.

Strung a wire antenna up along the window which loaded up nicely on 40m and 20m, several stations heard but no chance of any contacts unfortunately. I have my FT-817 along for the ride.

The movie "Frequency" was playing on the cable TV which put me in the mood wonderfully. Recommended for anyone interested in Ham Radio and Time Travel.

Impressive NSW Home Brew meeting

We had the bi-monthly meeting at Dural on Sunday where the largest ever item brought for "show n tell" was supplied by Peter Stuart, VK2BEU - a 2002 Holden Combo XC he has skilfully converted to all electric drive.

Peter spoke about some of the things that re required to get a vehicle like this approved for road use. The batteries must be well anchored so they can't move (or fly) around.

There must be a functional window de-mister and he had to purchase heating elements to warm the air to meet this requirement.

This car is front wheel drive and the motor used is a 9" 22Kw, 120V device that draws a peak of 500A.

He's using Trojan wet cell batteries (16kWhrs) that allow him to drive to and from work - a daily commute of 35Kms with a 4.5 hour recharge time at each end.

It's been on the road since February 2008 and has travelled 10,000Kms.

The charger is built in so, as you see in the top photo, the car simply needs to be plugged in to the mains to recharge.

The main presentation of the day was a continuation of the "using valves" series by Stephen Lowing VK2BLQ.

On show were some more projects by Stephen and also a run through of the useful components that can be stripped from radio and TV chassis that can be sometime found on the curb during council cleanups.

This valve talk has prompted me to retrieve a "Panda Cub" transmitter I purchased many years ago that got left behind in Melbourne many years ago. I'm off to collect it next week and hope to get on air with a valve rig very soon.

Thanks to Dave VK3ASE and Dallas VK3EB for taking care of it! I last saw Dallas setting up a 160 portable station in a park.

UseCaseBench - Estimate software development

Mac software I wrote some years ago, most of the old links are broken, so here it is again.

This utility allows an analyst to capture the Actors, Use Cases, complexity, technical and environmental factors and it applies the "Use Case Points" methodology to provide an estimate of how long the project will take.

Information gathered about Actors and Use Cases, free form text, complexity and their association can be captured and then exported from the program into an HTML report or GraphViz Use Case diagram.

It's free. Enjoy.

Download and Documentation.

Search engine optimisation network disreputable practices

Last night my blog received some comment spam. These are comments that at a glance look genuine but they actually contain a link to some web site the author is trying to raise in search engine ranking by having more links to it.

Comment spam is deceptive and manipulative, if not actually illegal under the Spam act, it is certainly immoral.

The messages linked to Highlander Plumbing, a Sydney plumbing service. It seems that Kevin O'Kane has hired SEOWorks to "optimise" the traffic to his site.

Here's what SEOWorks say they do:

• Targeted keyword phrase research using SEO Works Keyword Tools to identify best SEO keyword phrases


• Deconstructing and reverse-engineering competitor website SEO strategies and developing SEO countermeasures for targeted keyword phrases


• Applying SEO Works research-based optimum meta-tag standards to all relevant pages


• SEO copy writing current on-page content to better target Search Engine spiders


• Click mapping and PageRank value redirection to critical pages


• Inclusion of SEO Works Proprietary XML Sitemap format to website core directory


• Using SEO Works database of 5,500 "friendly" web sites developed inbound link growth program from high quality government, university and authority sites


• Documentation and implementation of recommendations

I'm not sure which of these covers spamming blogs with fake comments. Perhaps that's the one about using the database of "friendly" web sites?

In the past I've only seen this done by the pornography industry, but now it appears SEOWorks my be using shady practices to try to game the search engines. I'm sure the plumbers don't know what's being done on their behalf, so I've written to them to let them know.

Update

I spoke with Kevin O'Kane at Highlander Plumbing, he was very polite and seemed genuinely appalled at the practice I described. He has received a couple of other complaints about it which he's passed on to SEOWorks.

The comment spam is coming from a blogger user called bhavani, as you can see it was just created this month and has had 37 profile views so a few people aside from me are looking in to this.

Baz Luhrmann is the Wizard of Oz

"Something tells me we're not in Cairns any more.."

Baz Luhrmann's Australia certainly is epic - it's more like two epics.

There's much to like about this film, the photography of the wonderful scenery in the Kimberly is fantastic.

The cast of familiar great Australian actors is terrific. The music is used skilfully to manipulate the mood. The visual effects are as convincing as we can have today (anything's possible).

So many good ideas are in there, it's overflowing. Any film that goes 2 hours 45 minutes is going to be draining, but there's a point in this movie where I lost my motivation to stay on. If he'd had an interval many wouldn't have returned.

Baz knows how to make a big Hollywood epic movie, and I admire the talent he's gathered. I loved the references to other great films, visually, musically, and even actual clips of the film in one case!

This isn't "The Wizard of Oz", I'm sure it will do very well, but I won't be hanging out to watch it again in ten years.

When it ends up on TV, my guess is that they'll split it over "two big nights" and they'll do all that what's coming up snippet stuff to drag people over to the second part.

Three stars.

Getting harder to talk to humans

This morning I wanted to do something related to banking on the web site when a box popped up offering to connect me to someone who could help, keyboard to keyboard.

I explained what I wanted and it was no help at all, pretty soon I realised it was just an expert system - it was clever enough to realise that it hadn't helped me and apologise.

A couple of weeks a go our BigPond cable modem connection went down, the light on the box was flashing meaning there's no carrier.

I did the usual, power cycled the modem, checked the cables etc but clearly there was no signal from the street. So I rang them...

An extremely tedious interactive voice response system led me step by tedious step through silly things like re-booting my computer (which is pretty irrelevant given that the modem is plugged in to a router). At each stage I had to tell it when I'd done that.

In the end it said to power cycle the modem and say "continue" when the light was on solidly again, which of course never happened. There was no way out of this loop!

I felt like a rat in a maze.

I hung up and rang back. Right away the system said "we notice you've just rung back" and offered to put me through to a human!

The human told me I could have got out by saying "Operator" at any point, but you can't tell that by calling. The human also said that yes, there's an outage in your area.

Surely if they're smart enough to know that I just rang back, they could also figure out that there's a known problem in my area rather than torturing me for 15 minutes luring me into a trap?

Using an iPhone 2.2 as a 3G modem

I'm doing a demo at a site that has a totally locked down Windows network so I need a wireless internet connection... Until now, my iPhone has not been jailbroken, but I finally cracked, and it works very well.

Here's the steps:

• Grab a copy of QuickPWN


• Run it and follow the on-screen instructions


• (When iTunes pops up, or says things, just ignore it but leave it running)


• Run Cydia and let it update itself


• Under commercial applications is PdaNet, install it (you get a demo period)


• On your laptop, create a WiFi network


• On the iPhone, go to settings, Wi-Fi and join the network you created


• Run PDANet, let it sit for a few seconds to figure things out


• On the laptop, use the internet, it works!


• Leaving PDANet running, press the power button on the iPhone to save power, it keeps running


• When you've finished using it, go to PDANet and turn off Wi-Fi router and exit

This is all much smoother than I read it used to be, starting proxies from a command line etc.

While it's running you get a status display.

On the tethered computer it feels pretty snappy. I used OsSpeedTest, and measured the download of a 3MB image at 169 KB/s which is what they suggest I should get on an ADSL 1 connection.

As I said, it feels snappy, which is often a result of good DNS and low latency as much as raw speed.

Naturally, this whole exercise was an experiment and I wouldn't dream of breaching any terms of use.

PDANet for iPhone has a 14 day trial of "full mode" which proxies all traffic, after that the free version just does HTTP. The full version costs $29.

Update

My iPhone has been acting a little strangely. Three times it has kind of crashed as an email notification comes in. In one case it dropped a call and I couldn't call back until I'd rebooted it which took more than just turning it off.

So I can't say if it's the jailbreaking or if version 2.2 is less stable than 2.1. I've done a restore back to 2.2 native from Apple and we'll see how that goes.

Newstopia - Australia's Daily Show?

Just watched Newstopia on SBS. It's a great show, deeply funny, surreal, including the bonus ads in the breaks.

Aside from running it at 11pm, SBS supports the show in it's web site search engine as you can see above.

Shaun Micallef is a genius and it's great to see the hand of Doug McLeod in there as well.

It's hard to avoid comparisons with the Jon Stewart hosted "Daily Show". Both shows bounce off the news of the day, and that's a rich source.

This program should be on in prime time.

Insulators for dipole experiments

For quick and dirty dipoles I've been using bits of plastic such as old film containers and bits from the kitchen but they're not very strong and not good under strain.

There's a purveyor of plastics near by and I dropped in looking for small squares of perspex(tm) that would suit my application. I was amazed to see that they had a wall display full of suitable squares:

Of course they are samples that are given out to architects. The lady was a little puzzled when I asked how much they are, I got the impression she'd have given me a few for free but asked for a few dollars for ten.

Here's what I made:

And here's the back:

I clearly need to learn how to drill straight and proper strain relief for the dipole wires but this is perfect for my "quickie" temporary antennas.

Simple RF Sweep generator and plotter

Having recently manually plotted SWR for a couple of antennas I got thinking about how this might be automated.

As a proof of concept I hooked an Arduino up to a DDS, fed the RF through a little tuned circuit into an RF probe. The DC voltage from the RF probe goes back into an analog to digital port of the Arduino.

The program sends the current frequency and the A/D value out the serial port where I capture it and then plot with gnuplot. The graph looks like this:



And it's pretty consistent from run to run.

Here's the lashed up prototype:

The Arduino sketch is:

// Control a AD9851 DDS based on the good work of others including:
// Mike Bowthorpe, http://www.ladyada.net/rant/2007/02/cotw-ltc6903/ and
// http://www.geocities.com/leon_heller/dds.html
// This code by Peter Marks http://marxy.org

#define DDS_CLOCK 180000000

byte LOAD = 8; 
byte CLOCK = 9; 
byte DATA = 10;
byte LED = 13;

#define MHZ 1000000

void setup()
{
  pinMode (DATA, OUTPUT); // sets pin 10 as OUPUT
  pinMode (CLOCK, OUTPUT); // sets pin 9 as OUTPUT
  pinMode (LOAD, OUTPUT); // sets pin 8 as OUTPUT
  pinMode (LED, OUTPUT);
  
  Serial.begin(9600);
}

void loop()
{
  // Do a frequency sweep in Hz
  for(unsigned long freq = 1 * MHZ; freq < 10 * MHZ; freq += 1 * MHZ / 4)
  {
    sendFrequency(freq);
    delay(200);
    Serial.print(freq / MHZ, DEC);
    Serial.print(".");
    Serial.print(freq - ((freq / MHZ) * MHZ), DEC);
    Serial.print("\t");
    int value = analogRead(0);
    Serial.println(value, DEC);
  }
  Serial.println("----------");
}

void sendFrequency(unsigned long frequency)
{
  unsigned long tuning_word = (frequency * pow(2, 32)) / DDS_CLOCK;
  digitalWrite (LOAD, LOW); // take load pin low

  for(int i = 0; i < 32; i++)
  {
    if ((tuning_word & 1) == 1)
      outOne();
    else
      outZero();
    tuning_word = tuning_word >> 1;
  }
  byte_out(0x09);

  digitalWrite (LOAD, HIGH); // Take load pin high again
}

void byte_out(unsigned char byte)
{
  int i;

  for (i = 0; i < 8; i++)
  {
    if ((byte & 1) == 1)
      outOne();
    else
      outZero();
    byte = byte >> 1;
  }
}

void outOne()
{
  digitalWrite(CLOCK, LOW);
  digitalWrite(DATA, HIGH);
  digitalWrite(CLOCK, HIGH);
  digitalWrite(DATA, LOW);
}

void outZero()
{
  digitalWrite(CLOCK, LOW);
  digitalWrite(DATA, LOW);
  digitalWrite(CLOCK, HIGH);
}

It's just a start and I'm thinking you could make a simple antenna analyser by building a little SWR bridge. Looks like the miniVNA is something like this.

6m Delta Loop antenna

There was some activity on 6m this morning, some sort of contest, and so I did a quick web search and came across this simple design for a 6m Delta Loop antenna by DL5DBM, Anwar Von Sroka.

My construction used PVC pipe and it sits on a balcony on the first floor.

Following the instructions in DL5DBM's design yielded an antenna that resonated beautifully at 49Mhz, low SWR and impedance of 50 ohms. The 99cm of 75 ohm coax does a great job of matching. I assume this is a quarter wave transformer but I'm not clear on how the length is derived.

This is one of the most satisfying simple antennas I've constructed - in terms of resonating nicely at about the right spot.

I've cut down the length of the main loop to bring it up to the 6m band but the resonance isn't as pronounced, not sure if the matching coax needs to be tweaked as well.

I heard VK2AWX portable on 50.152 USB and the VK2WI Sunday broadcast from Dural on 52.525 FM is a solid S3 here I was acknowledged in the very brief call-back (so I'm getting out) but didn't get a signal report. Dural is about 35km away.

Just had a contact with VK2KFJ, Steve who was mobile and I can report that this antenna is working very nicely for me. The current set up is purely experimental and will need some work to survive the weather.

Boat suppliers - the antenna rigger's friend

Should have thought of this before... I've been rigging up wire antennas using venetian blind cord and crummy pulleys available at the hardware store.

Yesterday I went to an excellent boat supply shop, Whitworth's, and was able to source nice pulleys and nylon cord that's the right size for it so it won't slip off the track and jam.

I was talking knots with Rob VK2ATC on 20m and he mentioned the bowline which is an excellent knot. (I've been using a round turn with two half hitches).

Whitworth's, and no doubt other boating suppliers, had other useful things such as self-amalgamating tape (you stretch it, then wrap on itself, and it bonds to itself making a water tight seal), heat shrink tubing, and all sorts of useful stainless steel fittings.

Although there's expensive stuff in there, the pulleys and cord were cheaper than the rubbish I've been using.