Bendix Radio – Cape Bruny

Was taking another visit down to the Cape Bruny (Tasmania) lighthouse a couple of weeks back. While there I tookĀ some time to capture the Bendix radio setup in the museum there. It’s a transmitter and receiver combo made up of:

  • Receiver: Bendix RA-10FA
  • Transmitter: Bendix TA-12D

These were WWII era and there are some snippets of info online (especially regarding the TA-12 series of transmitters). Also, they seem to pop up for sale on eBay and the like from time to time.

Enjoy the pics!

2200m Oscillator PCB Version

Thanks to Dale (VK1DSH) I now have a PCB version of my oscillator. Further, thanks to his input it also now has a few additional items to the circuit. It has a diode on the +ve supply input to protect against reverse polarity, etc. as well as better use of capacitors for the voltage regulator and to provide better bypassing.

I just built up the PCB and found the resulting waveform much cleaner as a result. Very nice.

Just out of curiosity, although I had it running on the table inside I went out to the shack to see if it's very small signal was being picked up by my grabber. Indeed it was – pleasant surprise (capture attached). Also seems to show that my grabber freq read-out is inline with my 'scope.

A big thanks goes out to Dale for his input and doing up a PCB for me – something I'm still to try my hand at (I've some photosensitive PCB material, but that's it).

Vk1is_2200m_osc_-_pcb-001Grabber-0150Newfile1_-_pcb

More on 2200m Oscillator

Last night I had a bit more of a play with my osciallator. I wanted to look at a few things:

  • The RF on the supply rail;
  • The use of an Rd capacitor in the oscillator circuit; and
  • Stability.

For RF on the supply rail I was reminded by both Dale (VK1DSH) and Dimitris (VK1SV) about the use of a bypass capacitor on the Vcc pins for ICs. I say reminded as I remember reading about these when I was playing with microcontrollers briefly, but had completely forgotten.

Being a best practice, I decided to follow suit and add them. So the slighly revised schematic is now – note C6 and C7:

Oscillator_schematic_-_rev_2

On the use of Rd in oscillator circuit, maybe it’d help if I first show what I’m talking about. In the below is a red box where often an Rd resistor is added:

Rd_loc

In the datasheet for the 74HC4060N they suggest trying a 2k2 resistor, but for my crystal this stopped oscillation. Indeed, testing last night showed 1k5 stopped oscillation and only once I went to 1k did it kick off. However, I found at 1k the waveform was a distorted sine wave and only once I got it down around 100R did things resume a reasonable sine wave. So in the end, I decided I’d do what many others before me have, and continue to leave it out.

But, an interesting exercise none the less, and first real good opportunity to play with my new resistance wheel.

Finally, I also wanted to see if this thing was stable. Not an overly detailed test, just something indicative. So, I hooked it up to the scope and ran it for just shy of two hours on the bench. In that time, the frequency counter on my ‘scope did not budge from 137.500kHz. So, at least to an accuracy of 1Hz I can say it seems pretty damn stable. šŸ˜‰

Oscillator for 2200m

Recently I was frustrated with the amount of time it took to get some simple 2.205 MHz and 2.185 MHz crystals from Futurlec – in the end it took some 8 weeks to only get a small number of what I was after (and only with a bit of poking and prodding). This is not uncommon with Futurlec as you’ll find if you search the ‘net, but the thing is there are very few suppliers of such options and these were needed as I was trying to build something like the VK1SV MEPT transmitter for 2200m. Indeed, to build a crystal based oscillator for 2200m there are very few options, so that made it more frustrating.

Anyway, that frustration led to me to come up with a modified design myself. I saw on element14 that they had an abundance of 22MHz crystals and they’re known for rapid postage – and free at that. Suddenly, the idea seemed feasible for a 137.5 kHz oscillator based on 22MHz crystals – which I found more readily accessible. Only problem was, I only knew the basic idea of using 4000 series CMOS chips as dividers, so I had a lot to learn to do it myself.

Within time though, I’d come up with this design:

Oscillator_schematic

 

As you can see, it’s based primarily around three items:
  • 74HC4060N 14-stage binary ripple counter with internal oscillator
  • 74HC4017N decade counter
  • 22 MHz Crystal

The idea here is simple, the 74HC4060N first provides a 22 MHz Inverter Oscillator with the 22 MHz crystal. This is then used as it’s input for which we derive the ouput on the Q3 pin to achieve a divide by 16. This giving us a frequency of 1.375 MHz. This is then feed into the 74HC4017N so that we can further divide it by 10 which gives us the final frequency of 137.5 kHz. (A total operation of 22 MHz divided by 160.)

The only other bit of the circuit is the power supply. This is contoled with a 78L05 to provide 5V regulated as required by the 74HC chips. This should be noted, as these are not the plain CMOS chips (CD series) as these would not be able to work at the 22MHz clock frequency – normally only workable up to 12 MHz, but in some cases only 1 MHz. As a result, they require a Vcc of 2 to 6 volts.

Going back to the 22 MHz oscillator in the circuit, a couple of notes. The 4 ~ 40pf trimmer capacitor with the 100pF capacitor are to provide the required crystal load capacitance: CL = (C3 x C4) / (C3 + C4) + Cs. With those values – and using an arbitrary stray capacitance (Cs) of 5pF – it will cover a CL range of 8pF to 38pF – that should cover most crystals (mine required 18pF). Also of note in the inverter oscillator circuit is the missing Rd resistor (or Rs depending on your reference) to limit current to the crystal. With most circuits I see online this is ommited, but I think it could be interesting to try and figure out what this value shold be – as there are benefits. Initially I tried with 2k2 and that was far too large and meant no oscillation. But from calculations I guess you could try 75R, but I’m yet to experiment.

The other item to note is that I have observed there is a significant 22 MHz signal on the +ve rail (almost 1.5 Vpp). The above mentioned Rd resistor could help, but more so a decoupling network should ideally be added to the supply rail. Something I might consider if I have time to play with this circuit a bit more.

Anyway, even with these considerations this circuit works. Further, it also seems stable – although I’ve only run it for a few minutes and with no load. Oh, and don’t forget, the output from this is a square wave – so use appropriately. Here’s the output on my scope:

Scope_capture

All the parts you require can be fetched from element14, however if for some reason you struggle with acquiring the 22 MHz crystal there’s good news on that. Another reliable local supplier stocks them: Mini-Kits. And if needs be, I see that Futurlec have a 22 MHz can oscillator for just under $2.

Another option on the crystal front, is to consider using an 11 MHz crystal. But then the circuit is slightly different as you need to divide by 80. You could do this with two 74HC4017N chips (the first could divde by 8, the second by 10), however you’d also need to build the 22 MHz oscillator on it’s own to feed into it. But something to consider.

Anyway, here’s some photos from my resulting prototype:

Vk1is_2200m_osc-001Vk1is_2200m_osc-002Vk1is_2200m_osc-003Vk1is_2200m_osc-004Vk1is_2200m_osc-005Vk1is_2200m_osc-006

Finally, I’d like to send a thanks out to Dimitris (VK1SV). It was only a month or so back I had no idea about how you go about frequency division. I knew basically about frequency multipliers, but hadn’t even heard of frequency division. With his kind introduction to this theory I was able to produce the above and understand how it worked. Further, doing so also made me spend time to better understand inverter oscillators.

Thanks Dimitris! I may just get on 2200m one day soon.

 

 

 

VK1SV’s simple LF MEPT

Dimitris (VK1SV) has released an excellent page on a nice easy to build beacon (or even CW transmitter) for 2200m. Be a great project for anyone wanting to try 2200m I reckon. Wish this was available when I was starting! (I might still change track from the transverter and build something like this.)

Enjoy, and here’s Dimitris’ announcement from the “The DownUnder 136kHz Experimenters’ Group“:

Hello group,
Here is the description of my manned experimental propagation beacon:
http://people.physics.anu.edu.au/~dxt103/136/MEPT/
This is work in progress, I will be adding more details as time permits.
73, Dimitris

Receiving MF and Below

You'd like to start playing with MF or below; specifically you'd love to try the 'new' 2200m band in VK. A good place to start with this may be attempting to just receive a few of the active stations already up and running. Below I hope to give you some ideas on where you might start with this. After that, you can move onto TX – I'm still working on that.

First up, couple of things to keep in mind:
  • Your HF dipole probably has a balun in it that effectively acts as a bandpass filter with a passband between about 1.6MHz and 30MHz;
  • Your HAM transciever – even thought it claims to go down to the LF frequencies – is probably rather deaf down there;
  • You don't need a half wave dipole to start receiving – because obviously you're going to struggle with this if you're in suburbia; and
  • All these items though are easily addressed – in a number of ways.
To start with, I suggest considering your antenna. My first reception on the 600m band was actually with a typical HiFi AM loop antenna hooked up to my FT-817. The signal was weak, but I was very excited when I achieved this – considering prior to this I was attemptingĀ via my HF dipole and hearing nothing. With that success, I then quickly threw up a random wire antenna (now understanding the issues with balun's) and was amazed to see the increase in signal strength.

However, this was only receiving VK1 stations (either VK1SV or VK1DSH – can't remember which). This setup failed to receive any stations further afield. This was when I learn the need for some form of pre-amplification to overcome the rather deaf HF transciever.

Therefore, one option (that I didn't try – but am considering) is to simply use your random wire antenna connected to a dedicated pre-amp then straight into your transciever. This would be rather simple and fit in a small aluminium enclosure – maybe including an LPF for extra selectivity.

A slightly more involved option that I went down the path of was creating the rather common PA0RDT mini-whip. I recommend searching the web and read up on this, and take note of the various improvements one can make. But one issue with this, is that one of the key transistors (2n5109) is somewhat hard to find – or impossible. So normally it'sĀ substituted, and in my case I used a 2n2219.

Anyway, this provided me with an active antenna, thereby providing me both with a small antenna and also an amplifier to help overcome the deafness of my HF transciever.

However, a simpler antenna option (albeit slightly larger with a diameter of about 2.2m) may be this:

I really like the sound of it, and more so the minimal part count and simplicity of build. It may provide an easy alternative to the mini-whip. But, I've not built one so I can't say for sure.

In it's circuit though, I wonder about using it's amplifier hooked up to a random wire antenna as a nice simple option. Maybe something you may like to try, once you had the parts that build would probably be about one hour – including enclosure setup.

Finally, I've mentioned deaf transcievers above, and have mentioned the key to over coming this is through some form of pre-amplifier – well, other than getting yourself a more specific receiver. Keep in mind though, each transciever has different abilities here. The FT-817 is known to be rather deaf down low, and indeed I was amazed at the improvement I gained by using my IC-718 instead for LF (actually received my first ZL station on 600m as a result of that simple change). The FT-450 also get's mention on the web, with some reporting you have to make sure you turn off it's built-in pre-amp because otherwise it activates a filter that blocks LF – or I think that's the gist of what I read (use google to check).

So the key here is that if you'd like to start playing with LF, consider:
  • Building a dedicated antenna (maybe just a random end fed wire);
  • Building a basic pre-amp to help your transciever; or
  • Building an active antenna.
You don't need a lot of space for these, and you may be amazed what you start receiving – plus, good excuse to build some more gadgets hey. šŸ˜‰

2200m Transverter Progress

Finally, some real solid progress on my transverter for 137kHz. It wasn’t by my efforts alone, because at the end of the day this is still my first build of any kind of transmitting device – so there’s a lot to be learnt. Thanks goes out to Owen (VK1OD), Dale (VK1DSH) and Dimitris (VK1SV) for their help and tolerance of my spam.

G3xbm_transverter_v2-001

After completing the build a few weeks back, things just weren’t right. First and foremost, the output from the transverter was not a sine wave.

5-transverter_output

From here I ended up focusing where it all started way back at the LO and the attenuator for the input from the FT-817. Basically, the LO and RF going into the SBL-1 mixer were way high. General guidelines seem to be that for a balanced mixer you want an LO of 7dBm and an RF of 1dBm. In it’s original config RF was way up at 13.75dBm and LO was up around 9.65dBm.

From here I reduced the voltage to the LO module by a bit more and got the LO closer to 7dBm (slightly below) and built a new attenuator to offer 25.5dB as on low power the FT-817ND was actually feeding in 28.3dBm rather than the 27dBm (500mW) that is commonly believed. Even then though, the design only used a ~15dB attenuator.

So using the online pi network calculator and some nice big 1W resistors from Jaycar I had a nice new attenuator:

G3xbm_transverter_v2-003

From here, although things were better at the start fo the chain, the output was only slighly improved. So I started to focus on the PA module and eventually on the load inductor (L4) for the IRF510. Basically, seeing this was acting ultimately as an RFC I was feeling that it was letting too much of the fundamental frequency through and as a result the secondary harmonic (254kHz) was becoming too strong and destorting the output.

The design used 40t on a iron powder core (possibly a T-106-2, but there’s a bit of mis-information on the schematic), but this seemed to yield a rather low impedance at 137kHz. Seeing this was not being used for tuning etc, it was early on pointed out to me that using an iron powedered core could be a waste and that a ferrite would do. Well, now wanting to also increase the inducatance a ferrite core seemed the way to go.

After some trial and error, I settled on using an 18mm Jaycar Ferrite Toroid with 20t. This definitely showed improvement (as did the previous attempt with just 10t) and the result of the output waveform was:

Newfile6
G3xbm_transverter_v2-005G3xbm_transverter_v2-006

After this, my thinking led to was there enough load for the IRF to feed into (which in this design, comes in the form of the low pass filter – LPF) and indeed was the LPF doing a good enough job at filtering. After using a few calculators etc. it seemed that the simple 3 pole LPF didn’t really match the frequency and thereby was probably not providing sufficient filtering nor was providing a suitable load.

The plan then came about to replace the LPF with a 5-pole Chebyshev filter in the hope of getting the filter to bring the harmonics to a suitable level and provide a suitable load (hoping to increase the output power). For this I used this online calculator and came up with a design for a 5-pole cheby with cutoff frequency of 150kHz and hopefully a bit over 30dB of attenuation at the second harmonic.

G3xbm_transverter_v2-002G3xbm_transverter_v2-004

The benefits of this LPF were immediate. The resulting waveform was finally the long sort after smooth sine wave and power output even increased slightly. As a result, I’ve finally got a transverter for 2200m!

Newfile0

As you can see, the power is a bit low at only about 1.6W (not the 8W advertised) but it’s a start. From looking at the PA module it looks like it could definitely do with some work (bit of biasing and coupling/buffering at both the input to the PA and output) but I think now I might move onto trying to setup a usable antenna system for 2200m.