A Linear-Loaded Monopole antenna for hiking

There is a lot of information online about Linear-Loaded Dipoles, but I haven’t found anything at all about cutting a Linear-Loaded Dipole in half to create a Linear-Loaded Monopole worked against ground. The legendary L.B. Cebik (W4RNL, SK) published a design philosophy for an 80m Linear-Loaded Monopole, but it didn’t match what I had in mind. So I decided to build one for the purpose of experimentation. Maybe I could make it into a compact, lightweight antenna capable of rapid deployment while hiking – maybe.

What is Linear-Loading?

According to my search engine’s “Search Assist”, “Linear loading is a technique used in antenna design where a portion of the antenna wire is folded back on itself to reduce its overall length while maintaining good electrical performance. This method allows for a shorter antenna that can still operate effectively on the desired frequency.”

Sounds very simple doesn’t it? In the real world, where the RF hits the ether, it gets a little more complicated – especially when venturing outside the box. I could have made life nice and simple by building a Linear-Loaded Dipole; there are lots of designs available online that I could have used. But a dipole is too large for agile, rapid deployments; it needs a taller pole which, in turn, requires pegging into the ground and guy wires. I could use a tree limb for support, but only if suitable trees are available; often they are not. No, my requirement for a very simple hiking antenna implies a vertical antenna – a short vertical antenna.

Short antennas are easy to build; simply add a loading coil at the base and Bob’s your uncle. But that won’t qualify for my purposes. Short loaded antennas have a reduced radiation resistance and ohmic loss in the coil – they are inefficient. So how to shorten an antenna while maintaining efficiency? That’s where linear loading comes into play. A linear-loaded antenna is almost as efficient as a regular version.

How to build a Linear-Loaded Monopole?

It should have been “EZ-PZ”. Just take the dimensions from any of the online designs for a Linear-Loaded Dipole and cut them in half. That’s where I started. For a 20 meter antenna, a length of around 11 feet of window line, shorted at one end, is a good starting point. I hauled it up the mast in my newly glacier-free backyard, attached a counterpoise wire and started trimming. Between snips the resonant frequency was monitored on my RigExpert antenna analyzer. I use the term “resonant frequency” loosely in this context. The expected impedance of a quarter-wave vertical is around 37 ohms which implies there will be some reactive component to the impedance. I searched for a dip in SWR over a wide frequency range until it was possible to locate where the antenna was “resonant”.

So long John?

A low SWR in the region of the bottom end of the 20 meter band was the target, but the dip in the curve was below the bottom of the band – way below. I snipped and snipped until that dip fell where it was needed. Then the counterpoise length was adjusted until the lowest SWR was obtained. How long was my ladder line? A large pile of snipped ladder line lay on the grass beneath the pole. When I took the antenna down, laid it out on the ground and measured its length it was quite a surprise to see the ladder line radiator was only 8.67ft (2.64m) long. And the counterpoise length was 18ft (5.5m).

Jingo-la-ba!

Will it QSO? I fired a smidgen less than five watts into it and received a response from a station somewhere in the US with an encouraging signal report. Well, at least it “works”. But now came the next step. That pesky 18ft counterpoise had to go, to be replaced with the 2T2C (Tuned Tank Circuit Coupler) described in the last post.

A new challenge

The 2T2C ground coupler was directly connected to the ground side of the short coax feedline and a further wire was added to connect to a small capacitance plate on the ground. Life is complicated and then you die, so why do I insist on adding more complications? It’s called experimentation – experiment and learn! I learned. I learned that my choice of inductance and capacitance for the 2T2C resulted in impossibly sharp tuning of the ground circuit. The 2T2C needed a design modification to reduce the inductance and increase the capacitance. Spreadsheet modeling suggested this would make the 2T2C easier to adjust. I needed to confirm that before rebuilding the 2T2C, but how?

L-match innovation

The answer came in the form of a variable L-match that I built quite recently. It has switch selectable inductors and a variable capacitor. It could be adapted to fit this bill very nicely.

This idea was inspired by VK3YE who published a YouTube video about it some time ago. At one terminal of the L-match a connection is made to the BNC center conductor. At the other terminal, a connection is made to the shield side of the BNC. If you trace the signal path through the device it can be seen that the inductors and capacitor are in series. Now we have a Ground Tuning Unit (GTU) and can use binary selection of the inductances, together with rotating the variable capacitor, to determine the combination of inductance and capacitance for easiest tuning of the ground connection.

The inductances available on my L-match are 0.5, 1, 2, 4, 8 microhenries, allowing the inductance to be varied up to 15.5 microhenries in 0.5 microhenry increments. The variable capacitor is a 30-160pF polyvaricon.

Now, with the 8.67ft linear-loaded vertical erected and the “L-match GTU” making the ground connection via a capacitance plate on the ground, it was easy to select values that would allow smooth adjustment of the antenna SWR. It was found that 1 or 1.5 microhenries worked best. With these values selected the polyvaricon could be adjusted around mid-range to easily select best SWR.

A caution!

There’s a gotcha with this technique. My L-match has a switch to connect the top end of the variable capacitor to either the input or output. This is used to enable fast selection of either high or low impedance antennas. Referring to the diagram above, if the switch (not shown) is set to connect the variable capacitor to the left side of the inductors, this technique will not work. The inductors will be out of circuit and only the variable capacitor will be in circuit.

Will it still QSO?

My low-band QMX was dug out of its field pack and hooked up to the revised antenna (8.67ft of vertical window line with the “L-match GTU” providing the “other half” of the antenna. Using the “Tune SWR” feature of the QMX, the best SWR of 1.36:1 was obtained by a very small adjustment of the variable capacitor in the L-match GTU. Then it was time to go hunting. My best contact was in the state of Arizona (the “Arid Zone”?) almost 3000km away from my station in Southern Ontario. Signal reports were 599 each way. My sent report was a genuine 599 suggesting the antenna has good ears. The 599 report I received may have been genuine or perhaps it was just a “contest report”. In any event a good solid contact was made. A second contact into North Carolina only yielded a 549 signal report, but perhaps the low angle radiation pattern favored longer distance contacts.

Notice that the L-match GTU has no RF current meter. I could perhaps have inserted my home brewed RF current meter in circuit, but it wasn’t really necessary. Adjusting the ground current also regulates the radiating element current. Simply adjusting for lowest SWR indication on the radio peaks the radiated energy.

For practical outdoor use while hiking through the woods and rapidly deploying the antenna in clearings, the L-match GTU will be replaced with a much smaller series L-C coupler (2T2C). A 13ft Crappie pole is used to support the antenna. It collapses to the perfect length for carrying inside a fishing pole bag (no surprise there then) and is very lightweight.

There’s another gotcha

When the current distribution on the antenna was viewed in EZNEC it was discovered that the current maximum is in the ground circuit instead of in the radiator. Just like any ground-mounted antenna, this can lead to ground losses and inefficiency. However, the primary design objective was not to seek a Nobel Prize in antenna physics, but to come up with a design that meets the objective of a rapid deployment, simple antenna for hiking through the woods. The Linear-Loaded Monopole may just meet that requirement, but I have other ideas to try first. Stay tuned.

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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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#SSB on the #QRPLabs #QMX+ 5W HF transceiver is just so unnecessarily painful. The built-in microphone doesn't work. Standard microphones don't work.

The very specific microphone capsule required goes for 21 € inc. shipping from Digikey. A finished microphone assembly by G7UFO is 41,85 € inc. shipping. The whole QMX+ kit is only 125 USD, on comparison.

I have revision 4 of the PCB. Really would have expected it to have been fixed to take any microphone by now, or come with the microphone.

My FT-891 has been retired from POTA service … why?

Following high level, wide-ranging, bilateral talks with senior management (XYL) a far-reaching, binding agreement was today handed down to me. The focus of the agreement is contained in the executive summary which reads: “ya got enough radios already”.

And it’s true

I own more radios than I really use. Nearly all of my radios are of the vintage variety. The sole exception is my QRP Labs QMX. The QMX is unique in that it is an SDR radio so it can be updated as needed. One day, perhaps, even the hardware will become obsolete although it is more likely to succumb to the fragility of its low-cost construction – or the indelicate treatment to which it is subjected in the rigorous outdoor environment where I like to operate.

Radios become obsolete quite quickly as technologies evolve. Many modern rigs incorporate what I like to call a “fish finder” – a waterfall display enabling an operator to “catch” another station with a simple tap on a touchscreen. Fantastic yes, but is it just “nice to have” or an essential convenience for modern operating?

Most of my own outdoor operations are related to POTA. Once out in the Big Blue Sky Shack, preferably far from the madding crowd, in a location accessible only via Shanks’ Pony (an old Scottish expression – “shanks” are legs), I set up my station, find an open frequency, call CQ and work the hunters until they quit coming. What level of technological sophistication does that require? Even a very basic, unsophisticated, boat anchor rig can accomplish that. Does it really require a “fish finder”? Well, a fish finder would be nice, but spinning the dial and listening up for active stations worked for many years. Wanna go high tech? Check the clusters on a mobile phone.

Technological advances can even be detrimental. Remember old tube rigs? They were robust (until the tubes needed replacing). Tube rig operators never had to obsess about SWR. High SWR in a modern radio can result in voltage peaks that can send delicate FET PA transistors to the semiconductor cemetery. Sure tubes (or “thermionic valves” as they are known in the Land of Hope and Glory) can be big and fragile. I remember, when I was a kid exploring the thrill of electromagnetrickery, I owned a receiver with tiny, wire-ended tubes. But this isn’t a post about boat anchor technology – no matter how our romanticized recollections of youth bring out fond memories of days gone by.

Back to the future

I bought my Yaesu FT-891 a few short years ago based on recommendations I read online. I wanted a radio that would pack a punch and make getting QSOs from a campsite almost a sure thing. I remember self-spotting on the POTA website with the comment “100 watts!”. I wanted to attract hunters who wouldn’t have to struggle to hear me. A hundred watts for a CW signal is equivalent to AM broadcast signal strength compared to SSB. Alright, I exaggerate, but it quickly occurred to me that a QRP CW signal into an efficient antenna would get the job done equally well. Since that time I have rarely strayed from QRP – or sometimes QROp (20 watts for a 1 S-unit signal boost) when conditions are bad.

The Yaesu FT-891 is an interesting radio. It’s compact format makes it easy to carry into the field, but also has a downside. Small radios bury most of their impressive set of features in layers of menus. And the FT-891 has an impressive set of features. Audio bandwidth can be set as wide as the mighty Mississippi or as tight as Scrooge’s purse strings. Zero beating the other station’s frequency can be accomplished in a single button press if the “ZIN” function is programmed to one of the A, B, C buttons on the front panel. Then pressing the <F> key repeatedly brings up another four layers of menus. These menus allow the operator to select and adjust other levels of IF filtering like “APF – Audio Peak Filter”, “CNT – Contour”, “SFT – IF shift”, “IPO – Intercept Point Optimization” and “NCH – Notch”.

Filters can get you into trouble

During one POTA activation I recall hearing a hunter respond to me, but his signal sounded like a series of atmospheric clicks. “What the heck is that?” I remember thinking. “Is he testing whether I can copy railroad code?” (I can’t). I set my RIT (Receiver Incremental Tuning) a little off frequency and suddenly his signal was perfectly clear. The problem was my filter was set too narrow. Responding to an activator a little away from zero beat is a technique often used to stand out in a pile-up; it works in SSB too. Since my filter was too narrow the hunter’s signal was just outside my passband. Some CW operators are able to use the filter between their ears to separate a signal in a busy band. It takes some concentration – more than I have.

Tis a gift to be simple

All those menus are such fun to play with while working a pile-up out in the back country with mosquitoes, deer flies and other winged pestilences trying to have lunch on the back of your neck. As you swing an arm wildly to crush the airborne assault your CW key crashes to the forest floor and the contacts fill with wet sand and soil turning dits and dahs into incomprehensible gibberish. The hunters have fled and you are back to sending CQ trying to entice them to return. Wouldn’t it be nice to have a simple radio instead?

It get’s worse

It’s fine and dandy reducing a QRO rig’s power to peanut level. On the FT-891 it’s a simple matter of holding the <F> key down for 2 seconds, rotating the Multifunction knob to select section 16 of the Mariana Trench level menus, then choosing which of the six HF power sub-menus to adjust. Then click the Multifunction knob again, rotate it to the desired power output, from 5 watts to 100 watts; click the <F> key again and in no time at all you’re all set.

But there’s still a problem. The FT-891, like many other QRO radios adjusted for low power, still sucks power out of your battery like a camel filling its hump before a trek across the desert. The FT-891 draws over 5 amps even when the output power is wound down to 5 watts. By contrast, the QMX and other QRP radios can run when powered by a tiny 9 volt alkaline battery.

Big eyes, small wallet

Twenty years ago I worked for a few months in a Toronto establishment known to local hams as “the candy store”. Every day I had every one of the big Japanese manufacturers radios to play with. The staff were encouraged to become familiar with all the radios on display so that we could offer expert advice to customers. My big wide eyes fell on one particular radio from Yaesu; it was the FT-897 – a radio that I began to covet but couldn’t afford to buy at that time. I left that employment to start my own business and after a while I had the funds to buy that rig.

The Yaesu FT-897 is a big and chunky, yet rugged looking QRO radio that was intended for use in the field. I used it as a base station radio instead and it served that purpose until quite recently. It is old-tech now but maybe that’s a bit of an advantage. You see, it is relatively simple compared to the later FT-891. The FT-897 was introduced before IF filtering was widely available to the low budget ham market. Instead it has audio frequency filtering – accessible from the front panel – that works remarkably well. I can narrow the CW receive bandwidth down to 60Hz (danger of missing calls), 120Hz or 240Hz very easily without diving deep into a menu system.

The FT-897 does not integrate very well with common programs like FLrig and FLdigi. Too few functions can be controlled with CAT commands – unlike the newer FT-891. So the momentous decision was made to switch the two radios. Who cares if it takes a lot of clicks, twiddles and turns to select a desired feature on the FT-891 if the clicks, twiddles and turns are replaced with on-screen slider controls? The FT-891 is now my shack radio and I am very happy with it in this role. My XYL is equally happy that we don’t have to have another full and frank discussion about my urge to deplete our retirement savings to buy yet another whizz-bang box of tricks that will only keep me happy until the next whizz-bang box appears.

And the old FT-897? Is that going to be my regular portable rig now? Sorry old fella, you’re still a little hungry on battery amps. Maybe field day, perhaps.

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.

#amateurRadio #CW #FT891 #FT897 #OutdoorOps #POTA #QMX