A Ham with One Voltmeter Always Knows the Battery Voltage

Is this too high Hans?

There is a popular old saying about a man with one clock always knowing what time it is, but a man with two clocks is never sure. Well doesn’t that also apply to voltmeters and any other kind of meter in a ham’s kit bag?

The Ham Radio Outside the Box QRP field portable kit is powered by a Talentcell 3000mAh Li-Ion battery. It’s a nice little battery, but it has one rather annoying feature – an on/off switch. I am going to guess that the function of the switch is to prevent battery drain due to the LED capacity gauge on the battery. Problem is, I forget to switch the battery off when I have finished operating. Until recently the battery was buried deep in the bowels of my rapid deployment backpack radio kit – out of sight and out of mind.

To overcome this failing of the operator’s gray matter I connected one of those tiny LED voltmeters found at many hamfests, or online via the usual suspects. If the voltmeter is lit up, the battery is still turned on. It works fine business but how accurate is it? Should I care? It was just a quick and easy way to remind me to fish inside the radio box and turn off the battery when I’m done operating. Perhaps I should have used a simple LED instead.

During a recent outdoor operating session at a local park I found myself obsessing about the battery voltage. When I turn on the radio the tiny voltmeter shows a drop of one decivolt (cute word, means one tenth of a volt). Ok, the radio is drawing current so that’s to be expected. So what’s the problem?

Too much information?

The radio is the mighty but microscopic QMX from QRP Labs although these observations could equally apply to any other QRP radio with a restricted DC supply range. The QMX is a tiny little package that packs a powerful punch. It has proved it’s worth on a lot of POTA activations over the last year. The QMX has a small LCD screen that displays a lot of information – maybe too much in fact. Yes, some of the info displayed can be turned off in the user settings but who wants to keep fiddling with the settings when we could be pounding brass to get more contacts?

The paranoids are chasing me again!

The QMX tells me my frequency, signal strength on send and receive, ALC level, SWR and among other distracting data, the battery voltage. It can also decode and display the incoming CW but I turned that off. Call me paranoid, but if the radio can replace the need for the operator to copy CW, how long before a future firmware update incorporates AI and does the sending for me too?

Now here’s the problem: when this ham had just one voltmeter he was happy in the knowledge that all was well. But now there are two voltmeters – one on the QMX and another one whose job is to remind me to turn off the battery before packing up. The two voltmeters do not agree – which one is right? Are they both right but measuring the supply voltage in different places? Is it important?

There is much discussion in the QMXverse about the sensitivity of the PA transistors to excessive voltage, or high SWR, or both. In the world of big radios 12 volts means well alright, try to keep the DC supply voltage down below maybe 15 volts. Twelve volts, 13.8 volts, what’s the difference? But in QMXville, 12 volts means 12 volts. How strictly must we QMXheads adhere to 12 volts? No guarantees there and no hard and fast rule. Those four eensy-weensy BS170 transistors that pump power into the big, blue sky are as unpredictable as the weather.

Here comes my 19th nervous breakdown

So maybe you can understand why the obsession with the state of the battery. Which voltage is the one that is going to fry the finals – the voltage at the battery terminals, the voltage displayed on the QMX in receive mode, the voltage during transmit?

And … after sending out CQs for a half hour or so, the QMX display tells me the radio is no longer pumping out a full gallon. OMG, will the battery outlast the activation or will it die on me before I get “my ten”?

Don’t worry, be happy

Before “running for the shelter of mother’s little helper” anxiety can be overcome through understanding the discharge curve of a typical Li-Ion battery. But before we delve into that let’s talk about how to wrap our QMX baby in electronic swaddling clothes.

Note the “Goldilocks Zone” marked in orange and yellow.

A freshly charged 3S (3 cells in series) Li-Ion battery will have a voltage of 12.6 volts which represents 4.2 volts per cell. The nominal voltage is a little less at 11.1 volts or 3.7 volts per cell. As the battery discharges it will hold its voltage fairly steady in the Goldilocks Zone of 10.8 to 12 volts. Eventually, when the battery is nearly fully discharged, the voltage will drop precipitously and the internal BMS (Battery Management System) will shut it down to prevent damage due to over-discharging.

The only external protection needed is to curtail the excess voltage at the start of the discharge. This can be achieved by series diodes or (as I and others have adopted) a Buck Converter. The job of the buck converter is to limit the voltage to a preset value, for example 12.0 volts. When the battery voltage depletes to lower than the preset value the buck converter has no effect (except perhaps a very small voltage drop across the device). A buck converter is preferred over diodes because the latter will reduce the voltage by 0.6V per diode even in the Goldilocks Zone.

That makes the job of the buck converter very easy. With reference to the discharge diagram above, the buck converter only plays a role during the first 10% of the battery discharge cycle. A Talentcell 3000mAh battery has a nominal capacity of 36Wh. So the excess voltage is only a problem during the first 3.6Wh of operations. In theory that represents about three quarters of an hour of operation at 5 watts, although my own experience is that the battery voltage drops more quickly. I suspect this may be due to improper charging. Although I have been using the charger supplied by Talentcell, I have been leaving the load (i.e. the buck converter) connected. This creates a small “parasitic load” which may confuse the charger.

Is battery voltage regulation really necessary?

Are the radio’s final transistors in jeopardy at 12.6 volts? Some users have reported using an unregulated 12.6 volts with no damage to the radio. Perhaps the magic smoke is waiting for a later time to be released – who can tell? It’s a gamble.

What is the impact of operating the radio at 11.1 instead of 12.0 volts?

When I received my QMX a year ago, it came with a test sheet from the factory. The lab results showed my radio produces 4.8 watts on 20m with a 12 volt DC supply. Since the output power is proportional to the square of the voltage we can calculate the expected output power at any supply voltage. A quick and dirty back-of-an-envelope calculation suggests that at 10.8 volts (at the end of the yellow/orange Goldilocks zone in the diagram) the expected power output will have dropped to 3.9 watts. Let’s round that number to 4 watts.

So, after discharging the battery until it is almost fully depleted we can expect to lose less than 1 watt of output power. Even that could be compensated for by replacing the buck converter with a buck/boost converter. A buck/boost converter will ensure a constant voltage right up to the point where the BMS turns off the lights and says goodnight.

Is it really necessary to become paranoid about battery life, or output power? Before reaching for the “little yellow pill” prescribed by Messrs Jagger and Richards, maybe we should just relax and enjoy playing radio out in the Big Blue Sky Shack. It doesn’t matter if we run out of battery power, or our puny signal gets eaten by the D-layer on its round trip into space because QRP is such fun (isn’t it?).

Addendum: I have recently made a couple of small changes to my kit to overcome the issues identified above. First, I relocated the battery to make the on/off switch more accessible and allow the load to be disconnected during charging. Second, I eliminated the tiny voltmeter since I have convinced myself not to become paranoid about the supply voltage. Third, I have invested in a CC/CV (Constant Current/Constant Voltage) bench power supply to charge the battery. Now I can see the changes in charge voltage and current and verify that the battery is indeed reaching full charge.

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No “tip-jar”, “buy me a coffee”, Patreon, or Amazon links here. I enjoy my hobby and I enjoy writing about it. If you would like to support this blog please follow/subscribe using the link at the bottom of my home page, or like, comment (links at the bottom of each post), repost or share links to my posts on social media. If you would like to email me directly you will find my email address on my QRZ.com page. Thank you!

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A Simple Antenna that is Omnidirectional, Directional and NVIS?

Our winter weather may have a few weeks to run yet, but a relatively warm spell gave me the opportunity to get out into the Big Blue Sky Shack to try out another antenna idea. Destination: MacGregor Point Provincial Park on the Ontario shore of mighty Lake Huron. The shore ice still stretched quite a long way out onto the lake in the direction of Michigan, about 100 miles away and a cold wind was blowing in off the lake. Not perfect weather for outdoor operations – but good enough.

Purpose: to find out whether a simple idea could turn a humble vertical whip antenna into something more versatile. Could this be used as a directional antenna to focus a signal into a desired target area? Could it even be used as a cloud burner to shoot a Near Vertical Incidence Skywave (NVIS) signal straight up to the F2 layer for strong local coverage? I decided to find out.

The antenna was actually not quite a simple vertical, but close. It was the Ham Radio Outside the Box Coil-Loaded End-Fed Half-Wave (CLEFHW). Its advantage over a quarter-wave vertical is that no separate counterpoise wire is required – just a short length (about 18 inches) of coax terminated in a 1:1 unun.

This was also the first outing for a new ham-made radio backpack. The radio is a QRP Labs QMX (low band), built into a steel 30cal ammo box along with a Talentcell 3000mAh Li-Ion battery, Drox buck converter (to keep the voltage down to 12 volts – the QMX gets unhappy with excessive supply voltage). The Putikeeg paddle key has strong magnets on its base that lock it into place on the steel ammo box which keeps my keying from getting too erratic!

A second identical ammo box sits below the first one and contains all the spare parts that might be needed during an outdoor ops session (standby battery, spare cables, connectors etc).

Both boxes sit on a custom aluminum frame, secured by 1-inch webbing straps. The whole pack is carried by means of a set of 2-inch webbing shoulder and waist straps. In use the radio and key sit at just the right height when the operator is perched on a camping stool so no table is needed.

Why the military look?

Well, a couple of reasons there. First, I actually like the appearance of military style radio gear. Probably nostalgia because I was first introduced to ham radio in the 1960s and the first “amateur” radio I saw was a converted WW2 surplus No.19 Wireless Set. But second, and more importantly, the military and I have similar objectives – we both need rugged gear that can withstand the rigors of rough handling out in the field. Snow, mud, wind and rain all be damned – comms must continue regardless <smile>.

The canvas parachute bag at the front contains a selection of coax cables, as well as other wire antenna options.

The radio box at the top can be sealed by replacing the detachable lid. It has a rubber gasket to keep out the elements when the radio is not in use.

The radio box can be removed from the pack frame quickly and easily. I keep a wire bail for picnic table operation, although that luxury is a rare occurrence for me.

Orienting the antenna

The whip and loading coil are attached to the pack frame by means of an aluminum bracket with a 3/8×24 to SO-239 adapter. I wish they made a 3/8×24 to BNC adapter; instead I made up a short cable with a PL-259 on one end and a BNC on the other.

The bracket is the secret to the antenna’s versatility. As you can see in the picture, the pack frame has curved shoulders. By mounting the bracket on the straight portion of the pack frame, the whip remains vertical and vertical whip antennas have an omnidirectional radiation pattern.

Now, if the bracket is mounted on the curved shoulder of the pack frame the whip becomes oriented at an angle. As we shall explore in a moment, this creates a major lobe in the radiation pattern in a direction away from where the whip is pointing.

But doesn’t the weight of a leaning 18.5ft whip cause the whole pack to topple? Actually no. It was discovered that the weight of the two steel ammo boxes and contents are sufficient to counteract any potential gravitational instability. In fact during the field trial on the shore of Lake Huron the whole pack remained entirely stable, which is vital for this operator who cannot operate a set of paddles properly unless they are very securely mounted.

It is not necessary to set the antenna bracket too high on the curve of the pack frame because the whip itself is quite flexible which enhances the lean angle.

To operate in NVIS mode all we have to do is raise the bracket a little higher on the curve of the pack frame so that the top section of the whip lays almost horizontal a few feet above the ground. This method has been used on vehicles by the military so I have to credit them as the originator of the idea. It probably won’t perform as well as a low dipole, but it benefits from being self-supporting and quick to deploy.

How did the directional antenna perform?

The Huron shore trial tested the directional properties of the antenna. The wind coming off the lake was a little too cold for a long operating session and besides I had to find a small corner of the operating area that was sheltered and clear of snow and the vast expanse of thick mud created by the early spring thaw. So, the test was focused on checking the performance of the whip oriented as a sloper. A sloper is a simple, well-established way of getting directionality out of an antenna, but is usually achieved with a wire antenna. This unique version of that method gets the same effect with an entirely self-contained whip antenna in a rapid deployment portable radio pack.

A simple antenna such as this could not be expected to rival a Yagi-Uda beam but it does exhibit a very pronounced directional radiation pattern as EZNEC reveals in detail.

The elevation pattern shows a strong low angle lobe in the direction opposite to the lean of the whip. This should produce good DX results when the propagation conditions are favorable.

If we look at the azimuth propagation we can see that it is almost omnidirectional at low angles. The front/back is only about 2dB which is less than half an S-unit.

The real power of this antenna orientation can be seen when we examine the azimuth propagation at higher angles. In the third image we can see the radiation pattern at 60 degrees elevation. The front/back is now at around 13dB which is approximately 2 S-units.

60 degrees elevation is almost in NVIS territory and should provide excellent propagation over quite a wide area.

NB: For simplicity, these results were modeled using a full-length EFHW on 20m. If anybody wishes to model the exact configuration please note that the base loading coil is 6.6 microhenries and the whip is 18.5ft long. I chose not to go this route because the curve of a sloping telescopic whip is unpredictable (especially in the wind).

Could a puny 3.5 watt signal into a compromised whip antenna cut the mustard? On the principle that you can work DX with a wet noodle on the right day, then yes. Propagation conditions were moderate with a K-index of 3 on the day of the trial, but among my other contacts I did work a station in North Dakota (from my QTH is southern Ontario). That’s a distance of a little less than 2000km; not outstanding but encouraging.

Help support HamRadioOutsidetheBox

No “tip-jar”, “buy me a coffee”, Patreon, or Amazon links here. I enjoy my hobby and I enjoy writing about it. If you would like to support this blog please follow/subscribe using the link at the bottom of my home page, or like, comment (links at the bottom of each post), repost or share links to my posts on social media. If you would like to email me directly you will find my email address on my QRZ.com page. Thank you!

The following copyright notice applies to all content on this blog.

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.