Backpacking with the QMX, a Pocket and a Pole

The QRP Labs QMX transceiver is a miracle of miniaturization. Sure, other rigs have achieved a similar form factor, but not with so many outstanding features built into such a tiny package. At this point I should acknowledge a very worthy exception to the rule – the Elecraft KH1. I don’t own a KH1 and probably never will for the simple reason that I could probably buy every product that QRP Labs produces for the same amount of money.

In my humble opinion the QMX is too small and here is why. First, there is no internal battery. DC power has to come from a external source. The current draw of the QMX (12V version) is a modest 80mA on receive and 700mA on transmit, so even a very small battery would be sufficient to power it for a long time.

Second, there is no internal tuner – or transmatch, as it should be called. A tuner selects a frequency, or band whereas a transmatch provides a low SWR to the transceiver even if the antenna has a high SWR. In this post, for simplicity, we’ll refer to transmatches by their common name – tuners. Incidentally, an automatic tuner is the ultimate deceiver; it might provide a 1:1 SWR to the radio but there is no way of telling what kind of mismatch it is dealing with unless you are already familiar with the antenna. Getting a “perfect match” does not make a bad antenna good, it only prevents the transmitter’s final transistors overheating. A dummy load presents a perfect match but doesn’t radiate very well at all.

Struggle, Struggle and Toil means Trouble!

It is sometimes possible to tell that an automatic “tuner” is struggling to match a bad antenna by the protracted sound of chattering relays. During an extended tuning cycle the transceiver may be exposed to a very high SWR for a long time – double-plus ungood! A manual tuner, on the other hand, makes it abundantly clear whether the antenna is good or not-so-good. For example, my Norcal BLT tuner incorporates a bridge circuit that has 2 functions. First, it protects the transceiver’s finals by never presenting an SWR greater than 2:1 during the tuning process. Second, a good match is indicated by a dimming of the tuning LED. A really good match extinguishes the LED completely.

So just always use a resonant antenna?

But, back to the QMX; an external tuner/transmatch is always required unless working with a resonant antenna. No problem right? Just always use resonant antennas and life is a bed of roses. But wait. Life ain’t that simple. A popular antenna is the broadband End-Fed Half Wave (EFHW). Let’s say we put up an EFHW for 40m; it will also be theoretically resonant on harmonically related bands. That means it will also provide some sort of match on 20m, 15m and 10m. “Some sort of match” may be 2:1 or even a little higher. This is because the bands are not perfectly harmonically related. Our antenna may be perfectly resonant on 40m but might need a tuner to provide a better match on the harmonics. A linked EFHW eliminates this problem, but only by compromising elsewhere. More on this later in the post.

Another popular antenna is the End-Fed Random Wire (EFRW). A random wire antenna is not resonant on any band – that is the nature of the beast. So why use a random wire? Because, with the aid of a tuner, it can be used on multiple bands, and not just harmonics like the broadband EFHW.

Some outdoor radio enthusiasts are content to operate inside their vehicle, or to spread out their gear on a picnic table. I prefer to operate away from the madding crowd in the back country, or at the side of a trail. Operating this way enhances the outdoor experience – and provides physical exercise, which I enjoy. The downside is that there are no picnic tables, no seats, in fact everything you need must be carried in to the operating site on your back. I carry a small folding camp chair upon which to park my derriere, and my operating table is an 8.5×11 inch modified clipboard that sits on my lap.

My “lap desk” must hold the radio (the QRP Labs QMX in this case), my CW key and a “Rite in the Rain” logging pad. Those 3 items fill the available space. There is no room for a battery or a tuner. So where to put the items that don’t fit on the lap desk? That required some innovation.

Incidentally, my key is the excellent Putikeeg paddle key. Its strong magnets hold it in place by attaching to a small steel plate attached to the lap desk.

Where to put the battery?

I currently have 4 options for powering the QMX:

• Bioenno 12V 12AH LFP
• Talentcell 6500mAH LFP
• DIY 3S1P LiIon battery
• 8xAA alkaline cell pack

The Bioenno 12V 12AH battery was purchased to power my current hog Yaesu FT-891. It does a great job in that role. Bioenno batteries are expensive but they are a good investment. However, this unit is overkill for the current miser QMX. I may eventually purchase a smaller Bioenno battery.

The Talentcell 6500mAH battery has been my main power source thus far. I was very pleased with it until it recently disappointed me. This unit is housed in a plastic enclosure with an on/off switch and LED bar graph voltage monitor. I have been relying on the LED bar graph voltage monitor to advise me when it is time to recharge. During a recent QSO the power out of this battery was insufficient to continue transmitting, but the bar graph still indicated full charge. I have set the QMX display to indicate my RF power output and it showed full power at the start of the QSO then almost nothing when I tried to transmit again.

I have to admit that I was using two series connected diodes to limit the voltage out of the battery. The QMX doesn’t like any voltage over 12 volts. I may have to add a shorting switch to the diodes.

On a positive note, the Talentcell battery is a convenient size and shape to slip inside a pocket with a short DC cable feeding the transceiver. It could also simply lay on the ground if needed.

The DIY 3S1P LiIon battery was constructed from recovered 18650 Lithium Ion cells. I have a collection of these cells. Some were removed from an old laptop battery pack bought at a charity store for three dollars. Another source is single cell power banks available from dollar stores. BMS (Battery Management System) circuit boards were purchased from Amazon. I have not been able to source a good CCCV (Constant Current, Constant Voltage) charger so I have to dismantle the pack and charge the cells individually. Improper charging of Lithium Ion cells can result in overheating and even fire. I use a Hammond aluminum enclosure to physically protect the cells. Rough handling can also result in fire. This is not my favorite battery because of the inherent danger of Lithium Ion technology.

The 8xAA alkaline cell pack was an experiment – and a failure. It is small enough and light enough to fit underneath the QMX on my lap desk, but the Energizer Max AA alkaline cells have a sharp voltage decline (as seen in the accompanying image) that make them unsuitable for this use case. I will reserve this pack for bench use to power the QMX for settings adjustments.

Where to put an antenna tuner?

Having established that an antenna tuner is sometimes necessary, the question of where to put the darn thing arises. My solution is a trekking pole. Strictly speaking it is a modified aluminum ski pole.

I am currently carrying three antenna options inside my backpack. You never know whether suitable trees will be available for hanging antenna wires at the end of a hike, so I carry a vertical whip just in case. Here are my options:

• Linked End-Fed Half Wave (EFHW) for 20m, 30m, 40m with 49:1 matching transformer
• End Fed Random Wire (EFRW) – various lengths with Norcal BLT transmatch
• Extended raised whip with shortened radials suitable for setting up where a small footprint is necessary.

The EFHW is probably my best option during a field portable operation. It is probably the most efficient. To economize on wire (and hence weight) I chose a linked antenna. The entire wire is resonant on 40m. There are two links along the wire; if the first link is opened, the antenna will be resonant on 20m. If the first link is closed and the second link is opened, the antenna will be resonant on 30m. This required careful tuning of each band in the outback (out in the back yard).

Typically, an EFHW can be used on multiple bands without the complication of opening and closing links. BUT … as noted earlier in this post, an EFHW resonant on 40m will not be exactly resonant on its harmonics and will require a tuner. Also remember that an EFHW is only a half-wave on one band. In the case of a 40m EFHW, it will be a full-wave on 20m and the 30m band is not even a harmonic at all. I chose 20m, 30m and 40m because those are all supported by my low-band version of the QMX (and by my other QRP rig – the Hendricks PFR-3). The downside of a linked antenna is that it has to be lowered to adjust the links.

What about the pole? Oh yes, the tiny 49:1 transformer needed to match the high impedance of an end-fed half wave sits atop the pole which supports the weight of the wire. It is connected to the QMX by a short length of RG-174 coax. I tried direct connection to the transceiver, but the tiny little QMX wouldn’t stay on the lap desk if the tree supporting the antenna swayed in the wind.

The End Fed Random Wire (EFRW) always requires a tuner – for any band. But, it offers the convenience of multi-band operation without the need to lower the antenna wire to adjust links. I had two options for a tuner. First, the Norcal BLT (Balanced Line Tuner) with a switch to adapt it for unbalanced (coax) use. The BLT is a Z-match. I don’t really like Z-matches, they are persnickety devices that can sometimes be very frustrating to use. But, they do provide a simple, compact method for matching almost any wire. I prefer to use L-matches which provide a more positive experience in my opinion. I have a compact L-match which incorporates a “Rainbow Tuner” circuit that lights a combination of LEDs to indicate SWR, but it requires power. The BLT is a passive device. The BLT attaches to the top of the pole as shown in the accompanying image.

The Extended raised whip with shortened radials has been described in previous posts. It is based on the principal that if a quarter-wave ground plane antenna’s vertical element is made longer, and the radials are made shorter, than a quarter wave, the antenna will perform the same but occupy a smaller footprint on the ground. Sometimes it is necessary to economize on horizontal footprint where available space is restricted.

I expanded this idea by electrically extending the length of the vertical radiating element with a variable loading inductance. The whip is 18.5ft when fully extended. The variable loading inductance was extracted from a “Miracle Whip” antenna – it is an excellent piece of Canadian engineering, but unfortunately the company that made them is no longer in business. I have been able to tune the whip on 20m, 30m, 40m and even 80m. For 40m and up I use four 8ft radials; 80m requires longer radials. The rotary variable inductor is mounted in a rugged Hammond aluminum enclosure supported by the same pole.

Other Antenna Options?

I love to experiment with antennas. I keep an emergency dipole (among other things) in an EDC (Every Day Carry) bag in my truck – just in case I forget to bring an antenna on a trip – it has happened! I have also had success with a VP2E (Vertically Polarized 2- Element) – similar to an off-center fed full-wave dipole. If you have other suggestions let me know in the comments.

Backpacking with the QMX, a pocket (for the battery) and a pole (for sticking in the ground to support accessories that won’t fit on the lap desk) has been a challenge to ingenuity and improvisation. A whole multi-band station that fits in a backpack and can be set up anywhere. There is much else that goes into the backpack, but that is a story for another post. The pole serves as a trekking pole while en route to an operating site. And if I meet a bear along the trail I can always shake my pole at it – that should see it off 😉

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