Hey there! I haven’t been posting for a really, really, long time now. Been so busyyy~ T^T
On one of the amateur radio chat groups I am in, someone asked if they could make use of a TV “bunny ears” antenna for 2m and 70 use.
I was initially skeptical of the idea, and was wondering if the idea would actually work. Of course, what better way to find out than to actually modify one? So I went out and bought myself one, and got to work.
After calculating what the lengths of each “ear” should be, trimming them and taking more measurements, I wound up with this VSWR graph:
I obtained a VSWR of 1.3 on VHF and 1.5 on UHF! That ain’t bad at all, for such a quick modification! =D
So I decided to test it out by making some contacts. I was able to reach the repeater from my place, which is about 11.47 km away.
I was also able to make a contact with another ham on UHF, simplex.
So the antenna seems to be working fine so far! But in this state…
the antenna probably won’t last that long in the elements. =P
Let us make this into a more proper build! With a little help from my dad <3, here’s what I came up with:
I then mounted the antenna out of the window.
This is the final measured VSWR plot:
VSWR 1.2 on both frequencies! ^^
As you can see, modifying/making an antenna suitable for use on the ham bands is not a difficult task. This antenna “build” cost me about $10 SGD. Maybe one should try building and experimenting with antennas, and see what results they get, and make a few contacts with people on the air! There are many resources like the “Practical Antenna Handbook” by Joseph Carr and the “ARRL Antenna Book” which are comprehensive and handy guides, even for beginners.
Hi there! I have been busy working on quite a number of projects, one of which is making my own Software Defined Radio (SDR) transmitters and receivers. My, it really is a paradigm shift! In this article, I will talk about FPGAs for use in SDRs, since that’s what I am working on now, and also, give a general overview on why I think SDRs will be the way to go.
Quoting from the ARRL’s website, “Software Defined Radio attempts to place much or most of the complex signal handling involved in communications receivers and transmitters into the digital (DSP) style.” Well, what this means is that parts of the signal processing chain, like filters and mixers, which are traditionally implemented as physical circuits, are now done digitally. That could be a computer doing the processing in software, or an FPGA chip that has been configured to do the processing.
I have a Terasic DE-0 Nano FPGA board, and it has a Cyclone IV FPGA chip (pictured below). As of now, I am working on getting simple things like AM and FM modulation to work. But then the question, why do I call it a paradigm shift?
When you get a new FPGA board, the FPGA chip does nothing. It isn’t configured for anything. If you want it to do anything, you must use a Hardware Description Language (HDL) to do so. A code example using a HDL called Verilog is pictured below:
Omg! Sooooo confusing right? And even if you have had experience with software programming languages, like C or C++, the transition to a HDL can be quite confusing. Why? The reason is simply, as the name suggests, is because it is a *Hardware Description* language! Unlike programming, where the computer does everything step by step and in order, a HDL actually describes what the hardware is supposed to do. A FPGA is versatile in the sense that a single chip could do many different things at once. One part of the chip could be responsible for signal processing, and another part of the chip could be meant to control stuff or do digital logic, and this happens concurrently.
Basically, if you want to do anything on the FPGA, like a mixer for example, you got to do it digitally. No more of the usual analog stuff like using diodes and transformers or Gilbert Cells. You have to think about things from a hardware design point of view, and more specifically, a digital hardware design point of view. You may need to know how things like a RF mixer work mathematically as well.
Older hams may be thinking, why should I do this, when I can do everything using traditional circuits, like ICs, transistors, or even tubes! Simply because of versatility! A SDR’s capability can be changed! You could configure different kinds of modulation schemes in a SDR, or implement filters in a SDR.
In fact, check out this awesome piece of software called GNU Radio!
With the appropriate hardware like a HackRF, USRP, or even a plain old RTL-SDR, you can drag and drop elements like filters and demodulators, and even get the software to plot out a frequency spectrum for you! SDRs are really versatile pieces of equipment. Even Icom is going down the road of SDRs with their IC-7300, which uses FPGAs in the design. Who knows? Maybe one day, you might be able to upgrade your radio’s capabilities by downloading an update!
In essence, I think that SDRs are great, and while the workings of a SDR may be a bit technical to some (including myself! =P), I think as hams, we should spend the time to learn about them, and share our knowledge with others, so that in the future, the workings of SDRs will be more accessible to all of us. That being said, I may post up examples using FPGAs and Verilog HDL.
Hi there! I’d like to share an experience I had when I was conducting RF measurements. A few weeks ago, an acquaintance passed me a MFJ 259B antenna/SWR analyzer. I took it back home, and played around with it.
I found that I needed to measure the impedances of antennas and build some matching networks. Thus, I pulled the analyzer out and tried to do some measurements. Having some experience with a VNA like the Anritsu SiteMaster (two-port measurement mode), I decided to do a check by doing an open, short and load (50 ohms) test. I found something interesting during the test:
Really? A VSWR reading of greater than 25:1? On a 50 ohm load? Surely something is wrong here! After turning the knobs and changing the frequency bands, the reading dropped to 1:1 VSWR from HF to VHF. I was also convinced that the R and X values were inconsistent with the VSWR values. Why? I was measuring a load (RF matching transformer), and got these results:
At 13.71 MHz, the VSWR reading is 2.2, and the R+jX reading is 13+j24 (I assume a positive (inductive) value here. The analyzer doesn’t tell you what kind of reactive component the load is). Really? Let us verify this by plotting the point on the Smith Chart:
I have turned the VSWR circles on, and as you can see, the VSWR reading should be approximately 5:1 for a reading of 13+j24. The analyzer’s VSWR and impedance readings are not consistent with each other.
Now, I do not claim that the quality of the products manufactured by MFJ Enterprises are not good. And yes, if I wanted a very accurate reading, I should be using a better tool like a proper VNA, something like an Anritsu or Rohde And Schwarz VNA. Of course, you get what you pay for. But this article is about why checking and calibrating your equipment is important.
Hey there! Have not posted for a long time! (Omigosh! This is like the first post of the year!) This post isn’t going to be long, though =P
First, I got myself a new rig to replace my Kenwood TS-930S. I got myself this:
Nice radio eh? This radio originally belonged to another operator. I repaired it for him as the radio was found to not be able to receive anything, including the strongest of signals. I repaired it, and sent it back to him. He then found that it works, but he wanted to settle for a radio that was “simpler and easier” to use, and that he wanted to sell it. So I bought it from him (cheap; like $700 SGD) and now it is mine! =P
The receive quality is really very good on this unit. You can be suffering QRM/QRN, and you can easily improve the situation by adjusting the DSP and audio side filters, and you can least copy the other station. Anyway, having such a fine radio, I decided I’d do what I wanted to do, but was lazy to do; PSK31 operations!
I managed to make a few contacts; here’s one from Thailand:
And another one from Slovenia:
So that’s basically some of my PSK31 activities. It is sometimes fascinating, that one can encode digital signals into a tone, and transmit digital transmissions all around the world! It’s in a way, like an old version of the internet! (Remember dial-up connections and all? =P)
Before I go, if you have a Facebook account, and reside in Singapore, you might want to check out this new Facebook group, Ham Radio SG! They focus on amateur radio operations in Singapore. Click here to check it out!
Hi there! If you reside in Singapore, you would be aware that at Sungei Road, lies a flea market, also known as the “Thieves Market”. Here’s how it looks like:
Taken from Wikipedia article: “Sungei Road”
Well I like to drop by there time to time. Who knows? One might find something quite interesting over there. That was the case for me when I made an interesting find over a month ago.
Being someone who is fascinated by electronics and radio, I would lookout for devices such as valve amplifiers and such. At the time, I was in need of a cheap microwave oven, so that I could re-use the high voltage transformer for a valve amplifier project. I scoured around, looking for one, but I didn’t find one (at least not for cheap! =P). Disappointed, I left the area. But as I did, my eyes lit up! Something interesting caught my attention! What could it be? This:
A Kenwood TS-670 Quad Bander! I knew I just had to grab it, and so I did (Impulse buying… That’s why I am broke! XD). The unit is in a very good shape when I got it. For something that could have been lying around in the flea market for at least a few days, it was relatively clean, scratch free and dust free, even inside!
Taking it to Blazer (Radio store), Mr Phua let me use his power supply, and we did the smoke test. It worked! I took it back home and verified that it could receive and transmit. I noticed though, that the dial is a little on the fritz now. I can get the frequency to go up, but when I spin the dial the opposite direction, the frequency still increments! Well, it is a small problem, and I’ll fix it when I have the time.
What surprised me is that the seller provided me with the instruction manual (albeit in Japanese), schematic diagrams and an external SWR/power meter to go along with it. All for $80 SGD. I will probably take it out on field day or one of our QSO nights.
Hi there! I have been busy and haven’t got the chance to post anything for quite a bit. Well, I’d like to share my observations that I made when I was experimenting with RF dummy loads. Loads and impedance matching itself is very interesting, but quite a broad field on their own, so more on that in future articles.
Well, a dummy load is meant to allow transmitters to perform on-air tests, simulating an antenna, without radiating much power out. A dummy load is essentially, made out of resistors. As you know, a perfectly matched dummy load would have its impedance match the transmitter’s. An ideal transmitter for amateur radio would have an output impedance of 50 ohms, with no imaginary component (50 + j0).
I have been asked questions like: “Hey! Can I use wire-wound resistors for use as an RF dummy load?”. So, to answer that, I’ll be showing you my observations with various RF dummy loads. Let us start with a commercial RF dummy load. Pictured here is one that I use for quick tests.
And here’s the corresponding measurement data:
I’m sorry for the lower resolution on this image. For most of the amateur frequencies, say up to 500 MHz, the VSWR is lower than 1.5:1. That’s quite okay! Now let us look at another load. This time, it was constructed by Mr Jeff, out of axial resistors. Here’s it is:
And of course, the measurement data.
As you can see, the VSWR is at 3.76:1 on the UHF band. The performance is not as great as the commercial dummy load.
At the start of the year, I asked myself a question: “Why can’t we use wire-wound resistors for a RF dummy load?”. After all, wire wound resistors have high power ratings, and a few could easily dissipate 100 watts from a transmitter. Being curious, I decided to build one myself. Here’s my completed load:
And here is the measurement data:
Totally not good! The vector network analyzer doesn’t lie! The load performs very terribly! Not a good idea to begin with. Why is that so?
The name itself tells us why! Wire-wound! You see, they are made by coiling a resistive wire! It behaves like an inductive load! A little computer model shows similar (kind of) results:
Just a tiny, tiny amount of inductance is enough to make the VSWR very high, especially on the VHF and UHF band. Just so you know, this is a simulated sweep from 1 – 500 MHz. Granted, this may not be a very accurate model but it does show that reactances do matter in the realm of RF electronics.
So, it has been more than a year since I have become an amateur radio operator! Looking back, it certainly has been a year of learning, experimentation and fun, of course!
So far, I have:
Helped organize 3 QSO nights (articles coming soon!)
Played around with different antennas like a TV yagis
Helped with some aspects of setting up the VHF repeater (Ex: Antenna tuning)
Talked about amateur radio in a school and at the Singapore Maker Faire
There are some of the things I have done. It certainly has been interesting and fun doing all these. As you know, amateur radio is a hobby about experimentation and learning, so what do I plan to do in the next few months?
One thing I plan to do is to really, learn more about RF electronics! I’d like to improve my knowledge on RF electronics so that I can understand more on how radio communication and their electronics work, especially since I would be looking at building a SSB transceiver from scratch, but I would be looking at power amplifiers first.
This year, I would also be looking at antenna designs and how they work, and specifically, try to turn my TV Yagi into a dual-band antenna (Not gonna be that simple, haha =P).
Finally, I might also fool around with moon-bounce communications. Of course, I am not going to do it alone. A few months back, I met a person named Roland Turner at the SARTS meeting, and he expresses interest in moon-bounce communications, especially with lower power levels (<25 watts). If I could, I’d like to work with him on the subject. Oh! Check out his Facebook page on his moon-bounce experiments here!
That’s all from me for now, here’s to a fun and fruitful year of learning and experimentation ahead!
Hi there! This year, a group of members from SARTS, including myself, took part in the Singapore Maker Faire and set up a booth there. We set up 2 booths, one being an EchoLink booth, and the other, being an outdoor HF station.
What made this event special was that this year’s Singapore Maker Faire is now a full scale Maker Faire event, and also, some of my classmates and I did this as part of a school project. I certainly do hope that they learned something of value from this event. I would like to say, many, many, many thanks to them for helping out with manning the booth.
Here are some photos taken from the event:
My classmates and friend engaging visitors:
I met this visitor in the picture above 2 years ago, when we were participating in a 24 hour programming competition. It was nice to see him again, and it certainly was nice that he could still recognize me after 2 years.
This visitor, Mr Lim Khuan is a licensed operator, who came to visit our booth. Thanks for coming! Oh and don’t worry! I am still trying to fix your radio! =P
Testing the antenna with a radio:
Well, it has been fun, and we’ll certainly do it again next year, if we can.
Hi everyone! Our VHF repeater has now been moved to its permanent location at Dover. It’s a better location as there are almost no obstructions in the areas surrounding the repeater.
The repeater was placed at Mr Jeff’s office, in the Bukit Batok Industrial area.
The repeater was not sensitive to weak signals, and it was hard to make contacts on the repeater, so Mr Jeff went back to the office on Christmas 2014 to tune the duplexer, and try to solve the issue.
The issue was due to site noise around the location, and the antenna was blocked by the building. There was a measured 14dB of antenna noise in the area. As a result, not much could have been done.
One day, Mr Jaya, 9V1AI decided that he would want to take the repeater back and install it at his place in Dover (We intended to install it there anyway).
Since the repeater was now in a new location, Mr Jeff sent out emails to the members of the SARTS, asking them to test the repeater from different locations. Quite a number of members, including myself, helped out with the testing of the repeater from different locations, and I have done a presentation of the VHF repeater testing in May this year.
Here are the slides that was used in the presentation: Here!
In any case, if you want to help test or use the repeater, here are the details:
You transmit on: 145.025 MHz
Repeater re-transmits on 145.625 MHz
CTCSS tone: 156.7 Hz
Please do leave comments about your testing experience, and help us test the repeater out!
Hi! Didn’t post for quite a while. Anyway, I gave a presentation at my previous school, Christ Church Secondary. We connected a handy-talkie to Echolink using a audio interface board which I presented during the March meeting at SARTS. Link to materials here. Here are some photos of the presentation.