amateur-radio

VK4MSL/BM under construction part 1

Lately I have been riding my bike … a lot. As in all the way into the Brisbane CBD and back again to my home at The Gap. Lately, my handheld, a trusty TH-F7E has also given up the ghost… previously this is what I used when bicycle mobile — had that thing on the handlebars, and a small mobile whip on the back (which I never bothered to tune).

The handheld worked well… even better when I had the headset interface working. However, SWR was high, and although my handheld never complained, my other radios did! With the handheld gone, I had to finally deal with this issue. The helmet that I previusly used too had also been replaced — with a new headset to go with it. Anyway… that’s a side issue.

The big problem I faced was how to get this antenna tuned. The antenna is a tunable whip, basically just a length of stainless steel with a suitable mounting at the base — you cut it to the length you need to achieve resonance. Pretty simple. c=fL, and since hopefully most of the energy is going to be on the surface of the metal, and the metal is not insulated, it’d be approximately 1/4 of the wavelength (L). But what wavelength??

The radio I’m replacing the handheld with, is an old one… a Yaesu FT-290R II, which is an all-mode 2m radio. Since I have an all-mode radio, it makes sense to set up the antenna for all the modes I am likely to use. I don’t know CW very well, flat out decoding it when sitting comfortably at my desk, let alone whilst peddling up hills, so I’ll leave that to people like LY2KW. SSB and FM however, are definitely on the money. This is where the band plans come into play.

Here in Australia… the SSB portion is down the low end between 144.100MHz and 144.320MHz. Then there’s an all mode allocation between 145.225 and 145.775MHz. Everything above 146.025MHz is FM. The highest I normally transmit on 2m is 147.500MHz, as there are repeater outputs above this. Since I don’t know CW, the lowest I’m likely to go is 144.100MHz. So in order to balance this, I split the difference and used that to choose my resonant frequency… which yields a frequency of 145.800MHz, which puts me just at the start of the satellite segment. Hopefully SWR won’t be too bad everywhere else.

So, back to the formula, c=fL. Plugging the info in, I got a wavelength of about 2.057 metres. So 1/4 of this … 514mm. Out with the hacksaw.

Plugging in the radio, I found my SWR was still appauling… well… I don’t know my actual SWR, but the radio was telling me it wasn’t happy with it! The power amp on the FT-290R II (I have the FL-2025 25W linear attached) throttles back when it sees a poor match — and this is shown on the S meter. Okay… maybe I mismeasured… well it turned out that I hadn’t taken into account the fact that there’s about 30mm of metal at the base of the antenna holding it in place… so off that 30mm came.

Tried again… still no good… doing an estimated 12W, which while respectable, it’s probably no good for my finals. Something else was the matter.

I did some probing around… yes, the shield of the coax was making good contact with the bicycle frame, but why was the SWR so high? I was hoping that the frame would make for a counterpoise radial. I knew it wouldn’t be a groundplane, but surely the frame being 1.5m long would count for something. As a hunch, I tried adding some lengths of copper wire to the antenna mount, connecting to the shield. Three of them extend 60 degrees apart, with one pointed straight out over the rear tyre. That brought the SWR down. So it seems although good contact was made, the aluminium frame made a bad counterpoise in other ways. The following is the new arrangement.

Now, with that solved… I turned my attention to the radio mounting. I don’t have any mounting brackets, nor am I likely to get any, so that method was out. I had invisaged mounting it on the handlebars… well… I’ve sorta achieved that. This will need fine tuning.

The FT-290 is intended as a portable rig, thus there is provisions there for a carry strap. I’ll have to get a more suitable one, but I managed to find one that fits, and at least straps the top to the handlebars. This does mean the back of the radio swings… I’ll have to sort that out before long, otherwise one old radio is going to get battered and bruised. The strap loops over the top of the handlebars and around the steering post. It looks okay for now, but I know I’ll need to tweak this.

The setup is almost complete… I still have to work on the mounting. It needs to be quick-release too so that I can take the radio with me when I chain the bike up. Another option may be just to sling it over my sholder. The battery sits nicely on the back luggage rack — a 9AH SLA battery, plenty of life for this radio.

The last piece of the puzzle is one of operation. You notice the handmic slung over the handlebars. This is very temporary, as I hate taking my hands off the handlebars when riding. I replaced my helmet with another open face motorcycle helmet, similar to those worn by Australia Post delivery people. Yes, overkill on a bicycle, but the visor already has prooven useful in keeping rain or low branches out of one’s face (yet to see a bicycle helmet with one) and the peak keeps the sun off well (again, most bicycle helmets are lousy at this). It also makes it very easy to embed a headset.

The headset in this one is made up from two $10 computer headsets that broke. The microphone is off one, the speakers out of another. The audio quality is quite good, and shown in the photo below is what it looks like, with the adaptor for my mobile phone (Nokia 3310) attached. The next step is to make another adaptor for the FT-290, and suitable PTT arrangement. I’m thinking a toggle switch, since then I can flick it between transmit and receive without needing to hold a button down. VOX doesn’t appeal. 😉

The bike isn’t quite ready yet, but hopefully I’ll get something up and running this coming weekend. Then I shall be mobile once more. I shall report back once I have given this a try out proper.

What amateur radio is not…

Well, I really didn’t think I’d be writing a post like this.

This is following on from, and indirectly in reply to, an operator who decided to call in on the Australia-Wide Night-Owl and Insomnia net which is held every Friday night at 3595kHz.

Now, this net is pretty laid back… all are welcome. There are however, some things that just are not done on radio. Just as much as they are not done here on the internet. One of them, is to air dirty laundry on air.

Without going into detail… we had an operator call in from Victoria (a VK3V.. call, standard licensee) who then proceeded to make allegations about the off-air activities of another operator (VK2.., advanced licensee), in particular, the allegations involved claims of abusive phone calls and threats. The VK2 station responded pointing out some other misdemeanors allegedly purpotrated by the VK3 station, before (thankfully) moving on with the net. Thank heavens both had the decency to leave it there rather than tie up net time arguing.

Now, undoubtedly, the vast majority (me included) are not privy to all the information. They may be completely false, or there may be some truth to them. That isn’t for me to decide and does not concirn me. What I object to, is the usage of the amateur bands, as the platform for this kind of debate. It does not help any of the participants, or bystanders at all… and perhaps what both sides should realise here, is that by airing this material on-air, they are opening themselves up for a potential defamation case.

It is no different to me for instance, making similar allegations on this site… I could be sued for defamation. This is one of the reasons why I did not reveal the callsigns, or even the names of the guilty culprits. In the past, I recorded the net and provided it as a podcast (and had I done this, the recording would have been up for the world to hear)… but sadly the computer that I used for this is not operational at the moment. In any case, those who were listening, know to whom I refer.

I would ask that all people, who make use of radiocommunications services, whether it be amateur, citizen’s band, marine, airband or any other service out there… please bear this in mind. Your personal squabbles have no place on the air, as I for one (and likely countless others) am not interested in hearing them.

2009/09/26 by Redhatter (VK4MSL) Amateur Radio AWNOI Net Public Syndication Rants 0

Onwards and upwards

Well… three bits of news to share… I can’t be stuffed doing three separate posts however, so I’ll stuff all three into the one, it puts less load on the servers involved.

X.org working on Yeeloong

I managed to get X going on the Yeeloong within Gentoo… I’m currently battling problems with Python 2.6 not building, but at least X runs. I hope to get the necessary patches into my overlay shortly.

latest xorg-server ebuild works… you just need to add the loongson patch for version 1.6.0. This is already in my overlay, just needs updating.
xf86-video-siliconmotion needs a patch to detect the video RAM. This is due to the driver relying on some magic BIOS trickery which naturally doesn’t work on a BIOS-less RISC machine like the Yeeloong.
xorg.conf needs the LCD panel resolution specified … that is: Options “PanelSize” “1024×600” in the options for the siliconmotion driver.

VK4MSL contactable via IRLP

I recently put my homebrew 2m vertical back up … this time, using the mounting brackets from my old 2.4GHz vertical, and mounting the thing up as high on the antenna mast as I can push it. The choke balun on the antenna is level with the TV antenna yagi, so most of the radiated power is well above the TV antenna.

With this, I am now not only kinda able to work previously impossible repeaters such as VK4RBS (Bayside/Alex Hills), but also VK4RSS at Ocean View. What’s so good about VK4RSS? Well, I’m tripping it with 500mW of power (therefore good access when using 5W)… and it happens to be accessible via IRLP as node 6215.

I can also be sporadically reached on EchoLink node 37 37 40.

Graduated at last

I did say there were three items in this bulletin. I finally received my academic transcript, confirming that I have formally completed my studies at QUT, graduating with the following qualifications…

Bachelor of Engineering (Electronics)
Bachelor of Information Technology (Software Engineering)

This is timely, right at the bottom of the employment market… but I can’t help that. Now begins the task of finding work in the Brisbane area. I’m still running to/from Laidley doing some work out there… which may turn into paid employment (I hope so anyway… costs me almost $8 a day with a student discount in transport… That’ll double to about $15 when that card expires).

If anyone’s looking for someone to assist, particularly in the telecommunications field (I have a soft spot for radio and embedded systems)… feel free to get in touch directly.

2009/07/26 by Redhatter (VK4MSL) Amateur Radio Gentoo Development Linux Development Public Syndication University 0

I’m Baaaaaaack…

It has been a while, but I can safely say I have returned. Not sure what the next step is… looking for paid employment I guess, but I have passed all subjects this semester, which should mean that I am now qualified in IT and Electrical Engineering as a graduate.

After the last exam, I could not run out of the building fast enough. 6.5 years of studies has certainly taken its toll on my mental state. Anyway… I wound up traveling northern and central NSW with my father and his girlfriend for the last fortnight — got back home yesterday. I am currently putting together some photos, and I’ll have a slide show ready for the next BOSQ meeting. I’ll put a link up to the photos when they’re done processing (the aging PIII 550MHz webserver here takes a while to resize over 500 photos, most 10Mpixel in size).

Where did we go? We camped at:

Dalmorton (abandoned settlement on the old Grafton-Glen Innes road) — overnight
Glen Innes — overnight
Bingara — overnight
Waa Gorge (pronounced “war”, part of Mt. Kaputar National Park… you’re not supposed to camp here, but it was late in the day, and the road in/out passes through private property with many gates) — overnight
Mt. Kaputar National Park — 3 nights
Coonabarabran — 2 nights
Port Macquarie — 3 nights
Dorrigo — overnight
Grafton — overnight
Brooms Head — overnight

In that time:

We explored a number of walking tracks at Mt. Kaputar, Dorrigo and Brooms Head.
Did some sight seeing at Port Macquarie and Coonabarabran.
Checked out the sandstone caves in the Pilliga State Reserve
Got bogged on a forestry road in state forest just north of Coffs Harbour (thanks go to the Clarence Valley State Emergency Service for pulling us out of that muddy mess)
HanoiCalc got a bit of work done — it works now.
I checked into three nets:
- Ipswich & District 80m Net (3.585MHz LSB) from Waa Gorge
- AWNOI Net (3.595MHz LSB) from Mt. Kaputar
- Coffs Harbour & District 2m Net (146.650MHz FM) whilst waiting for the SES to arrive

We learned:

Setting up the annex and awning on a hard-floor camper trailer for an overnight stay is a pain in the bum.
My camp stretcher doesn’t fit in the camper itself, and only barely fits in the annex.
Holden (or Zupps) decided to put a really low tow hitch on the back of my father’s car… meaning we had to either find rocks/blocks of wood/bricks to back the car’s back wheels on to, or dig a hole just near the jockey wheel in order to unhitch from the trailer
Just because a road is marked on a GPS or paper based map, does not mean that it is in drivable condition, nor does it necessarily mean the road’s actual route bares any resemblence to the marked route.
The NRMA do not assist people who are bogged, they refer you to the SES instead. (I hope some of the fees we’re paying are helping fund the SES for their troubles!)
My HF radio, which is normally very touchy on 10m… works fine on that band up in the higher altitudes — I suspect a temperature-related issue.

What now? Well… as I say, I’ve got to find some employment somewhere. I now officially become “unemployed” according to the damn lies^W^Wstatistics. Potential employers in the Brisbane area, should contact me directly.

This also means I should have some time to dedicate towards Gentoo. My last attempt at stage builds got sidetracked by a need to study and also hit technical issues (something in glibc’s build kept hard-locking boxes).

Also on the agenda here is a proper port of Gentoo to the Lemote Yeeloong. The little netbook has been running well under Debian, with Gentoo sitting in a chroot environment… now that I’m no longer using the machine for daily studies, I think the time is ripe to start looking into reloading the machine. Zhang Le did a great job incorporating Lemote’s patches into a mirror of the Linux/MIPS git tree, which I’ve been using to build my kernels… 2.6.30-rc4 has been quite stable.

I’ve also been looking at the ARRL handbook, with the view of upgrading my license to the Advanced level. Then I’ll be paying for a 5-year license before the ACMA/WIA decide to up the fees again.

So, much to do, and a mountain of bugs in Bugzilla with my name on them… Ohh joy.

2009/07/06 by Redhatter (VK4MSL) Amateur Radio Gentoo Development Linux Development Nature Public Syndication University 3

Circuit simulation

At university, I’m currently studying a subject entitled “RF & Applied Electromagnetics”, and at the moment, we’re looking at the issue of electromagnetic compatibility.

One thing that has come up both here, and browsing through the ARRL Amateur Radio handbook (I have the 1975 edition) is that above all, circuit layout has some of the most profound effects on how a circuit behaves.

What I did not realise, is just how low in frequency these issues show up. The physical construction of many electronic components often means that, far from being a “pure” component, they in fact exhibit both resistive and reactive components. I understood this occurred a lot in the VHF and UHF… which is a good reason why many homebrew radios stop in the lower HF. These effects in fact, start to become visible as low as 14MHz.

This got me thinking… the typical circuit design workflow is some variant of this:

Draw up the schematic in a schematic capture frontend
Feed this into the circuit analysis package, inspect the simulation results.
If unsatisfied, tweak schematic then go back to step 2. Else feed into PCB layout package.
Lay out the PCB
Get PCB manufactured and build circuit.
Do testing, tweaking component values… maybe go back to step 3.

It seems to me… the workflow should be more like the following:

Draw up the schematic
Lay out the PCB
Feed the PCB layout and circuit schematic into the simulator, inspect results.
If results are unsatisfactory, tweak layout and schematic, else get PCB manufactured and build the circut.
Test & tweak

One criticism I have of gEDA, it seems to treat the schematic and the PCB as being two different things. I find it awkward to move from gschem to PCB and back again when designing a circuit. I also haven’t figured out simulation as yet.

I’d say the behaviour of the circuit has more to do with the way the board is layed out, than with just the components alone, and as such, I feel a modern EDA system should reflect this. Qucs does seem to be approaching what is going on, in that you can simulate a stripline or similar PCB object in your circut. I think it has possibly the greatest promise, but alas, it can’t yet do PCB layouts itself. Time will tell I guess.

The commercial suites… well, I’ve used Electronics Workbench for years… I really must say I find the simulation in that much more intuitive than working with SPICE or Qucs… there is perhaps something worth looking into with its interface. Multisim is more like traditional SPICE… but still retains some of the old feel of EWB. However, all of them seem to simulate the schematic… which is fine… but you do miss out on a lot of what goes on in the circuit.

I’ve been getting used to Qucs… and so far, I quite like it. Once again though, we’re looking at a simulation of an ideal circuit, under ideal conditions. That is… pass a 100MHz sinusoid through a circuit: the resistors remain resistive, the capacitors store charge as they should and the inductors continue to add inertia. In the real world, many resistors and capacitors become inductive in nature, and inductors look more like capacitors, depending on the physical construction of the component. Mutual inductance, and capacitive coupling are also problems in the real world.

With today’s modern hardware, I figure the computing overheads to model how a physical circuit would behave based on the layout, is certainly a lot more feasible than it was when SPICE was first conceived. SPICE was good for its time… but with the modern world requiring a higher level of engineering for its electronics, I think it has had its day now.

2009/05/11 by Redhatter (VK4MSL) Amateur Radio Public Syndication Thinktank University 0

Samuel Morse’s Birthday

Did anyone happen to visit Google today? Those who haven’t… if it isn’t too late, have a squiz now. Update: Google have reverted back to their old logo… the one they had up is pictured below.

Yeah… I saw it and said… WTF… then it hit me what I was looking at. You’d think with the number of times I’ve heard CW on the air, I’d have recognised it straight off.

Anyway… happy birthday to you Samuel, and thank-you for providing us with this wonderful low-bandwidth mode that still sees use today. One of these days, I’ll actually get around to having a QSO on this mode. 😉

– – • • • • • • – – • • – • • – • – – – – – • • • – – • – • • • • – – – • • • • – • •

2009/04/27 by Redhatter (VK4MSL) Amateur Radio Public Syndication 1

Embedded development

Lately I’ve been looking around at microcontrollers.

With my work with Eze Corp, I’ve gotten quite accustomed to the MSP430. It’s a nice little MCU, with a very low power drain, gcc port and is in general, quite easy to drive. At my workplace, I use a Windows XP machine to do development under MSYS, using a MSP FET-430UIF JTAG device for programming and debugging. This is not my preferred mode of development, I’d much rather be doing it on a Linux host (or at least something that properly understands POSIX)… but it’s tolerable.

For my home projects though… this isn’t an option. I do not own a Windows computer any more, and in any case, I dislike working with it at my workplace — to hell with using it in my recreation! (The reasons why are best saved for another rant at some other time. No, I’m not a free-software hippie, I have other reasons.)

There are a number of MCUs available that I’m aware of. The biggest hurdle with many of them is the requirement of some piece of hardware which is either proprietary, expensive and/or unavailable. I like the MCUs that provide a JTAG interface, since it’s a well defined protocol that allows real-time debugging of the device as well as programming. JTAG for what its worth is little more than an SPI bus at the hardware level, it’s what’s layered on top of this that differs between devices.

JTAG can be implemented via GPIO. Since a computer parallel port is merely a specialised form of GPIO port, this enables people to wire up a JTAG port to a parallel port, and bit-bang the signals. There’s a couple of issues with this approach:

Parallel ports (I mean real ones) are getting harder to come by, especially on portable computers (understandable on netbooks, but I don’t see why full-size laptops don’t have them…anyway)
Most USB Parallel cables only have the smarts to drive a printer, they don’t allow arbitrary bit-banging of data. (e.g. the PL2305. And try explaining the difference to a salesperson…)
USB introduces a 1~2msec latency… which stuffs up timing at high speed.

I’ve seen some programmers (for PIC and AVR chips) that use RS-232. Now these are usually specialised devices for a given range of MCUs… that’s fine… but they suffer the same problem as the parallel-port JTAG cables… that is, most modern computers don’t have RS-232 anymore for some silly reason (as I say, understandable on netbooks… but are they saying they can’t cram a mini-din8 somewhere on a 15″ laptop?!)… and the USB latency screws timing up (I suspect this is the issue with the PL2303-based adapters we have here).

That leaves me with native USB devices.

A lot of the ones used by ARM devs are based on a FTDI chip which offers UART (RS-232) and SPI… so you get a serial console and a JTAG in one unit. Nice… In addition, there’s a open framework for debugging with them, OpenOCD. I don’t have the knowledge necessary to port this across to other MCUs… I guess one hope is that a MSP430 guru might contribute an OpenOCD port, but in the short term this isn’t an option.

For many ARM devices, there’s the J-link developed by Segger. I’ve mentioned this one in past posts. For MSP430, TI provide their MSP FET430UIF. Both of these devices are on the nose from a usability point of view. Both require you to use proprietary software in order to access their device. This is fine if you’re on a supported platform, and the tool is well maintained.

For the J-link… well, if you’re not on Windows… forget it, that’s all they support. The advice from them is to just run a Windows computer purely to run their GDB proxy software (which costs too I might add).

TI’s tool is theoretically a little better for support… the userspace drivers are proprietary freeware… but again, if you’re not using Windows or Linux on an i386 PC, forget it. More to the point, the Linux version doesn’t seem to get updated as often — so the only real option is Windows. Bad luck for me and my Yeeloong.

For what it’s worth, I do not understand why these things need to be proprietary… it’s a flipping programming cable. Just moves data from one end to the other. If they need to load firmware into the RAM of the chip being programmed, fine… but why can’t this be an ELF binary or something that just gets picked up by OpenOCD (or equivalent) and stuffed down the wire?

Devices like the Rabbit Semiconductor series of microcontrollers drive me insane with their non-standard nonsense… they decided to chuck many years of C history down the gurgler and invent their own “wannabe C”. #include <foo.h>? Nah… don’t be silly, we’ll just stuff everything into .lib files and #use them. Urgh!

I’d love to know what low-end microcontrollers exist that don’t exhibit the proprietary nonsense that is outlined above. 8 or 16 bits wide… I like ARM, but I’m not needing something that flashy. I like the low current drain of MSP430, and the fact they use gcc… which I guess pushes me in the direction of AVR and maybe PIC, but I’d be interested in other options. Cheap is important too, I don’t have a lot of cash for expensive tools or chips.

The other thing in the favour of AVR/PIC MCUs, is that there’s a local shop that I can buy them from — no need to order on-line. Farnell and RS have a wide variety, but I have to purchase from their website, which is awkward for me.

The applications I’d have in mind? Well, one idea is a more modern version of my headlamp… some of these MCUs draw less power than the 74HC14 chip used in that project (a few mA in standby mode) and would offer more functionality. Other applications include small controllers for amateur radio projects (not SDR though).

I’m really interested to know what controllers are out there, which are open-source friendly. Once I have a clear picture of what’s on offer, I can look more closely at my applications.

2009/04/18 by Redhatter (VK4MSL) Amateur Radio Public Syndication Rants Thinktank 2

Ultra-Low Bitrate Speex

I just stumbled across this article, discussing the issues behind digital voice encoding at low bitrates. The use of AMBE in the D-Star protocol really does get up my nose, at best it’s distasteful, at worst, harmful.

I’ve been doing some experimentation with Speex… so far my experimentations have been with bitrates above 12Kbps. Above this, it is quite intelligible. This is helped with pre and post filtering with the following filter:

Filter for ultra-low quality Speex

It’s not ideal… it means things do sound rather flat, but it’s similar to existing filters employed on existing SSB and FM transmission systems. On SSB, around 2.2kHz is the maximum upper frequency… and it tends to drop out around 300Hz, give or take a little bit. That filter could do with some fine tuning, but I find for low-bitrate Speex, using it to filter the audio before encoding, and after decoding, helps to reduce the distortion of the codec. Some fine-tuning could allow for better fidelity.

It is interesting to hear though, that DVSI even cuts corners though in an effort to lift perceived clarity by trying to reproduce the bottom end.

A good low-bitrate encoder benefits everyone… VoIP becomes even cheaper when you can send the same quality audio in fewer bits, and the benefits for HF radio communications would be immeasurable.

I guess though, it comes back to the old saying:

The superior man understands what is right; the inferior man understands what will sell. — Confucius

2009/02/23 by Redhatter (VK4MSL) Amateur Radio Public Syndication Thinktank 1

AWNOI

Australia-Wide Night-Owl and Insomnia Net

The AWNOI net is a weekly evening net that starts at 10:00PM UTC+10 (or 9:30PM UTC+10 when daylight savings time is in effect) and continues until the last station falls asleep. This recording is produced by myself using my HF set (Kenwood TS-120S), Yaesu FC-700 tuner and G5RV-like outdoor antenna. My QTH is at The Gap, Brisbane, QLD. Most of the stations participating, bar 3, are VK4 stations (two VK2s and one VK1).

If you’re a licensed amateur with equipment for 80m communications, feel free to join us at any time.

This recording was made 2009-01-30 and is available as Ogg/Speex, Ogg/Vorbis or MPEG4/AAC codecs.
A short highlights recording is also available, in MP3, Ogg/Vorbis or Speex.
Continue reading…

2009/01/31 by Redhatter (VK4MSL) Amateur Radio AWNOI Net Public Syndication 0

The puzzle of handheld radio antennas

Well, lately I’ve been tinkering around with antennas again for use whilst mobile. By mobile, I mean public transport mobile, and pedestrian mobile… I don’t own a car.

Most handheld radios come with rubber ducky antennas. My Kenwood TH-F7E is no exception. Rubber ducky “dummy loads” as I call them, are quite dismal performers against an efficient and resonant dipole — however they are usually much more convenient size-wise than a resonant antenna. It’s a similar story for mobile HF antennas… they’re a negative gain antenna.

Naturally, the way I’ve improved it, is to construct my own. So far, I’ve built a number of these antennas. The first version was a bit of stainless steel fencing wire with a SMA connector soldered to the botto and a few turns of a coil. It was about 50cm long… roughly cut, so possibly not that well tuned… but it performed quite well… allowing me to reach repeaters up to 80km away on 2m with minimal line-of-sight. The SMA->antenna interface proved to be a fragile component however.

A later version attempted to be a half-wave 2m, and could extend (fold out) to become a full-wave 70cm antenna. This again, had similar mechanical issues to its predicessor, but performed excellent otherwise.

It was pointed out to me that trying to match a full-wave antenna is asking a bit much of the power-amp in my radio, and that an odd multiple of 1/4 wavelength was better.

Thus, the third revision, I made using some surplus solid core electrical copper cable, and a BNC connector, was constructed at a length of 52cm. 52cm was calculated according to the rule v=f?… the frequencies I had in mind were 146.500MHz (2m FM calling frequency) and 439MHz (near to the input frequency of most 70cm repeaters). 52cm was calculated approximately as being 1/4 wavelength on 2m, and 3/4 wavelength on 70cm.

This antenna performed exceptionally well, and with the BNC connector, showed less mechanical problems than its predicessors. It did however, put a lot of strain on the BNC->SMA adaptors I was using, and I had to be extra careful with the antennas length.

Using MMANA-GAL, I tried modelling this antenna, just to see how well it infact should work in theory. Well I was right… about 6dBi gain on 70cm… it wasn’t bad at all! However, SWR was through the roof, 1:several hundred thousand.

I’ve since found a long forgotten RC car remote that operated on 40MHz, which had a 40cm long antenna. I’ve stretched the base coil out a bit to make it 42cm and added a SMA connector to the bottom of it… I may have to extend it a bit, but this seems like a closer match to what I’m looking for… but my modelling of it is just as dismal in terms of SWR.

Now, as far as improving SWR… the ways I know of to fix this problem are:

To make the antenna a dipole, effectively doubling its length
Adding a ground plane or counterpoise radial
Adding a matching section (like a J-pole)
Using some sort of matching network

The thing that has me curious, is the rubber ducky antenna. Now granted, I know those things do not radiate well. They must however be doing something to keep the finals from blowing. My understanding is that they’re little more than a spring in a plastic jacket… I can’t see how that is meant to match to the 50? source impedance.

The antenna design I use is apparently very similar to one used by Motorola for some of their professional radio antennas, according to this post. Now I’m not sure how 19.5″ is arrived at — it doesn’t fit with the maths I used above, there is something I’ve missed. Surely though, an antenna of this type would have the same impedance matching issues as the ones I’ve designed. Either there’s other magic involved, or the finals in many handheld radios are more hardy than I thought.

At some stage I might borrow a SWR meter or antenna analyser for VHF/UHF… it’d be interesting to see just how far off the mark I am. I haven’t blown my finals…yet. The radio seems to still have plenty of life in it. I’d be interested to know however, some of the background on this topic. There is something I am missing, and I can’t quite put my finger on it.

2009/01/05 by Redhatter (VK4MSL) Amateur Radio Public Syndication 0

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