A lot of hams like to carry a VHF radio. Of course, nearly everyone wants to carry a phone. Now, thanks to the kv4p HT, you don’t have to carry both. The open-source device connects to your Android smartphone and turns it into a radio transceiver. You can build it yourself for about $35. Check out the video below.
The device uses an ESP32 and only transmits one watt, but it has lots of features like APRS and scanning.
If you are involved in any sort of radio transmission, you probably have at least heard of SWR or standing wave ratio. Most transmitters can measure it these days and most ham radio operators have tuners that measure it, also. But what are you measuring? [KI8R] points out that if your coax has loss — and what coax doesn’t? — you are probably getting an artificially low reading by measuring at the transmitter.
The reason is that most common SWR-measuring instruments pick up voltage. If you measure, for example, 10V going out and 1V going back, you’d assume some SWR from that. But suppose your coax loses half the voltage (just to make an obvious example; if your coax loses half the voltage, you need new coax).
Now, you really have 5V getting to your antenna, and it returns 2V. The loss will affect the return voltage just like the forward voltage. Reflecting 2V from 5 is a very different proposition from reflecting 1V out of 10!
The Braun TS2 radio was a state-of-the art tube set in 1956. Today it still looks great, but unsurprisingly, the one that [Manuel Caldeier] has needed a little tender loving care. The table radio had a distinct style for its day and push-buttons. However, the dial glass and the speaker grill needed replacement. Even more interesting, the radio has a troublesome selenium rectifier, giving him the perfect chance to try out his new selenium rectifier solid-state replacement.
The radio is as good-looking inside as it is outside. You can tell that this isn’t his first restoration, as he has several tricks to test things at different stages of the project.
While the radio looked good, it smelled of smoke, which required a big effort to clean. The dial glass was intact enough for him to duplicate it in a graphic program and print it on a transparent adhesive sticker. With a deep breath, he removed the original markings from the glass so he could add the sticker to it. That didn’t work because the label needed cutouts. So now he is waiting for a piece of acrylic that will have the art UV printed on it.
We want to see the next part as we imagine the radio sounds as good as it looks when it is working. If you want to know more about the rectifier replacement, we covered that earlier. Even years later, Braun would have a clean aesthetic.
If you need an amplifier, [Hans Rosenberg] has some advice. Don’t design your own; grab cheap and tiny RF amplifier modules and put them on a PCB that fits your needs. These are the grandchildren of the old mini circuits modules that were popular among hams and RF experimenters decades ago. However, these are cheap, simple, and tiny.
You only need a handful of components to make them work, and [Hans] shows you how to make the selection and what you need to think about when laying out the PC board. Check out the video below for a very detailed deep dive.
Old radios often had selenium rectifiers to convert AC to DC. The problem is that the old units, dating back to 1933, are prone to failure and to release dangerous chemicals like hydrogen selenide. [M Caldeira] has a new board made to fit a particular rectifier and also allows a varying voltage drop. The circuit consists of a few diodes, a MOSFET, and a pot for adjusting the voltage drop. An IRF840 MOSFET provides the adjustment.
Did it work? It did. The good news is that if it fails — which shouldn’t happen very often — it won’t release stinky and noxious fumes
We wondered if he should 3D print a fake case to make it look more the part. If you haven’t seen a real selenium rectifier, they were made of stacks of metal plates coated with bismuth or nickel. Then, a film of doped selenium was annealed to the surface to form cadmium selenide. Each plate could handle about 20 V and the more plates you used, the more reverse voltage the device could withstand.
Selenium was also found in old photocells. If you fancy replacing other parts of an old radio, you might consider a faux magic eye or even one of the main tubes.
For as useful as computers are in the modern ham shack, they also tend to be a strong source of unwanted radio frequency interference. Common wisdom says applying a few ferrite beads to things like Ethernet cables will help, but does that really work?
It surely appears to, for the most part at least, according to experiments done by [Ham Radio DX]. With a particular interest in lowering the noise floor for operations in the 2-meter band, his test setup consisted of a NanoVNA and a simple chunk of wire standing in for the twisted-pair conductors inside an Ethernet cable. The NanoVNA was set to sweep across the entire HF band and up into the VHF; various styles of ferrite were then added to the conductor and the frequency response observed. Simply clamping a single ferrite on the wire helped a little, with marginal improvement seen by adding one or two more ferrites. A much more dramatic improvement was seen by looping the conductor back through the ferrite for an additional turn, with diminishing returns at higher frequencies as more turns were added. The best performance seemed to come from two ferrites with two turns each, which gave 17 dB of suppression across the tested bandwidth.
The question then becomes: How do the ferrites affect Ethernet performance? [Ham Radio DX] tested that too, and it looks like good news there. Using a 30-meter-long Cat 5 cable and testing file transfer speed with iPerf, he found no measurable effect on throughput no matter what ferrites he added to the cable. In fact, some ferrites actually seemed to boost the file transfer speed slightly.
Ferrite beads for RFI suppression are nothing new, of course, but it’s nice to see a real-world test that tells you both how and where to apply them. The fact that you won’t be borking your connection is nice to know, too. Then again, maybe it’s not your Ethernet that’s causing the problem, in which case maybe you’ll need a little help from a thunderstorm to track down the issue. Continue reading “Ferrites Versus Ethernet In The Ham Shack”
[MIKROWAVE1] claims he’s not a radio repair guy, but he agreed to look at a malfunctioning Hallicrafters S-120 shortwave receiver. He lets us watch as he tries to get it in shape in the video below. You’ll see that one of his subscribers had done a great job restoring the radio, but it just didn’t work well.
Everything looked great including the restored parts, so it was a mystery why things wouldn’t work. However, every voltage measured was about 20V too low. Turns out that the series fuse resistor had changed value and was dropping too much voltage.
