26 Apr 19

Last night (Thursday 25th April 2019) I was about to head off to bed when I decided to go up into the shack and have a listen on the 20m band. Almost immediately after turning on the transceiver I heard R4FBH in Russia calling CQ with a big signal and I gave him a shout […]

25 Apr 19
This is an exciting day for me — we finally get to build some ham radio gear! To me, building gear is the big attraction of amateur radio as a hobby. Sure, it’s cool to buy a radio, even a cheap one, and be able to hit a repeater that you think is unreachable. Or on the other end of the money spectrum, using a Yaesu or Kenwood HF rig with a linear amp and big beam antenna to work someone in Antartica must be pretty cool, too. But neither of those feats require much in the way of electronics knowledge or skill, and at the end of the day, that’s why I got into amateur radio in the first place — to learn more about electronics. To get my homebrewer’s feet wet, I chose perhaps the simplest of ham radio projects: dummy loads. Every ham eventually needs a dummy load, which is basically a circuit that looks like an antenna to a transmitter but dissipates the energy as heat instead of radiating it an appreciable distance. They allow operators to test gear and make adjustments while staying legal on emission. Al Williams covered the basics of dummy loads a few years back in case you need a little more background. We’ll be building two dummy loads: a lower-power one specifically for my handy talkies (HTs) will be the subject of this article, while a bigger, oil-filled “cantenna” load for use with higher power transmitters will follow. Neither of my designs is original, of course; borrowing circuits from other hams is expected, after all. But I did put my own twist on each, and you should do the same thing. These builds are covered in depth on my Hackaday.io page, but join me below for the gist on a good one: the L’il Dummy. L’il Dummy As Al points out in the article linked above, a dummy load is just a resistive element that matches the characteristic impedance of the transmitter’s antenna connection. In almost every case, that’s going to be 50 ohms. The reason that the load needs to be as resistive as possible is that it needs to continue looking like a flat 50-ohm load no matter what frequency is applied to it. Any inductive or capacitive elements in the load will make it more reactive, changing the impedance as the input frequency changes. This could lead to RF power getting reflected back into the final amplifier transistors in the transmitter, possibly damaging them or destroying them altogether. Not what you’re looking for. That means our resistive elements need to be as non-inductive as possible. But, they also need to be able to dissipate a lot of power. The HT dummy load, which I’ve dubbed L’il Dummy, needs to handle the 5 to perhaps 8 watts an HT can output. Trouble is, power resistors in that range are often wirewound, and a coil of wire will have too much inductance. We’ll need to be clever in sourcing components. Looking down into L’il Dummy just before applying the torch. The RF Biscuit board is a handy little thing. The circuit for L’il Dummy is hardly worth a schematic – it’s just an SMA jack with a 50-ohm resistor across the outer ground and the inner conductor. I chose to build the circuit on an RF Biscuit board. This is an open-source design that enables all kinds of handy little RF circuits — attenuators, filters, and as in this case, dummy loads. The resistive element I chose was a thick-film SMT device capable of dissipating 35 watts – way more than enough for this job. That and an edge-mount SMA jack should have been all I needed to make a working dummy load. To my surprise, once I soldered the resistor to the RF Biscuit board, the dummy load was almost as good an antenna as the stock rubber ducky on my Baofeng HT. I was able to hit a local repeater through the dummy load without any issues. Clearly not a good design. To correct it, I put the whole thing into an enclosure made from 1″ copper pipe. Not cheap stuff, but not too bad, and I like the look of polished copper. Soldering the whole case together was a challenge that my big Weller soldering gun wasn’t up to, and trying to get everything heated up enough with a propane torch without overdoing the heat was a fun time. Testing on a Budget Now for the $50 question: does it work? I tested the resistance with a DMM and it comes out to just about 49 ohms, which is close enough in my book. But that’s DC resistance; what about impedance? I don’t have an antenna analyzer, so I trolled around and found a simple method for measuring impedance with only a function generator and an oscilloscope. My scope has a 20-MHz function generator built in, so I whipped up a quick test jig from a BNC jack and an SMA jack, connected in series through a leftover 1000-ohm resistor. L’il Dummy test setup. Measure the p-p voltage on each side of the series resistance connecting the function generator to the dummy, and do a little math. Applying a sine wave into the dummy load, measuring peak-to-peak voltages on each side of the resistance, and doing a little math is all that’s needed to characterize the impedance from 2.5 MHz to 20 MHz. The math is simple: with V1 being the voltage across the input, V2 being the voltage across the output, and Rref being the actual value of the series resistance, which I measured at 998 Ohms. And the results are pretty close to 50 Ohms, and flat across the tested band f (MHz) V1 (V p-p) V2 (V p-p) Z (ohms) 20.0 1.49 0.062 43.3 15.0 1.89 0.082 45.3 10.0 2.57 0.113 45.9 5.0 3.90 0.173 46.3 2.5 4.70 0.217 48.3 I wish I could measure it at VHF and UHF frequencies, but that will have to wait until I get a function generator that goes up to 400 MHz or so. I doubt very much that a $50 budget would cover that, though. Next Time I had intended to cover both L’il Dummy and its bigger, somewhat smarter brother in one article, but I still have some testing to do on Big Dummy. I’ll cover that next time, and after that we’ll move onto measuring the output of a cheap Chinese HT and perhaps building a filter to clean it up.
25 Apr 19

Today (Thursday 25th April 2019) is an extremely important day on the Australian calendar.  It is ANZAC Day, a National day of remembrance for Australia and New Zealand.  It commemorates all Australians and New Zealanders who have served and died in all wars, conflicts and peacekeeping operations.  It is also just one of three days […]

25 Apr 19

Thursday evening 25 April 2019 the impulse after walking a league was to fire-up my Yaesu-Musen FRG-7 communications receiver to listen to a new shortwave station while resting my dogs. Was pleasantly surprised after slowly turning the dial to hear some oldies on 5085 khz at 2045 cdt here in Iowa from WTWW transmitting on […]

23 Apr 19
N3MRA's Amateur Radio Blog

On one hand, I was not very active on amateur radio this week.  On the other hand, I spent a bunch of time updating my logbooks.  I was able to submit my NM QSO Party log on time, and updated DX4Win as well. I did check into the Albuquerque SCAT Net a few times, mostly […]

22 Apr 19

Our thanks go to Rob in the Netherlands for the link below. I know a number of people whose wallets are bulging awaiting the arrival of this radio. Below we’ve a small video by Rob K8RR and Mark W8BBQ for you to watch. Thanks to Rob, continued great Easter holidays everyone. Link: https://youtu.be/FeRwenHs5_w All Rights […]

20 Apr 19
Get out of the Radio Shack and Live Life

Good Friday 19 April 2019. Tom M1EYP in the UK  has organised at Good Friday DX 30m CW S2S QSO Party, in addition Tom has invited 40m SSB participants.  I have considered my options and decided to activate Mt Stromlo VK1/AC-043 for the first time this year, in doing so I will gain a single […]

19 Apr 19

Hello Operators First of all, yesterday was World Amateur Radio Day. So I’ll start by hoping everyone had a wonderful World Amateur Radio Day. Next I’d like to leave you with a few things to ponder. Test your gear One of the most important things you can do from a ham radio and/or preparedness perspective, […]

18 Apr 19

The Yaesu engineer who put the brochure together obviously has not used the radio I tested the following current draw figures Yes, it is very high at 5w due to the finals inefficiency at low outputs but I can rock it up to 80 watts and add 2  units to my signal for those receiving […]

17 Apr 19

What if mankind faced an event they may have not have ever seen coming? An event that would bring humans back into the dark ages. I am not talking about the black plague, resulting in the deaths of an estimated 75 to 200 million people or a new pandemic.

16 Apr 19
Market Research Report

A walkie-talkie (more formally known as a handheld transceiver, or HT) is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola. Similar designs were created for other armed forces, and after the war, […]

16 Apr 19
Dean's Chase Logs

Initial target – Wooster, Wayne County OH, 1330 Eastern.Starting point – Shanor-Northvue, Butler County PA, 0900 Eastern.Successful intercept – 3.5 Miles southwest of Shelby OH, 1630 Eastern.Associates – NEOChasers group, Jonny GlessnerCounties traversed – Pennsylvania; Butler, Lawrence, Mercer. Ohio – Trumbull, Mahoning, Portage, Summit, Medina, Wayne, Holmes, Knox, Ashland, Richland.Financial Cost – $84.00 (Rounded) Tech […]