Mastodawn

EFHW matching: 49:1 Impedance Transformer or L-Network?

What is the best way to match the very high impedance of an End-Fed Half-Wave antenna to the 50 ohm impedance of a transceiver? There are various ways to do this but this week’s post is going to focus on just two – a 49:1 impedance transformer (or UNUN if you prefer) and an L-network.

We are dealing with QRP devices but the same issues arise with QRO devices. In fact some of the complexities may be exacerbated at higher power – especially core overheating.

49:1 impedance transformer

QRP 49:1 impedance transformer. Note the separate primary and secondary windings

This is by far the most widely used matching device but many claim it is inefficient. I have used an “Outside the Box” winding method that I have seen described as “Fuchs style”. The primary and secondary windings are entirely separate instead of being twisted together. This method isolates the windings and is said to prevent static from traveling back down the coax to damage the transceiver. But it also requires a separate 0.05WL counterpoise connected to the bottom of the secondary winding.

Pros

Broadband operation
Easy to construct
No calculations needed

Cons

Lower efficiency claimed
Can be used on even harmonics but the antenna is only a half-wave on its fundamental frequency
Potential for losses due to core overheating
Leakage flux due to poor coupling between windings
May require capacitance across primary and/or secondary to compensate

L-network

QRP L-network featuring both a variable inductor and variable capacitor

Some claim that an L-network is more efficient than an impedance transformer. While I don’t dispute the claim I would respond “show me the math”. An L-network is usually constructed from a fixed value serial inductor and a fixed value parallel capacitor (although there are other topologies depending on the matching parameters involved). I built one using a slug-tuned variable inductor and a ceramic trimmer capacitor.

Pros

Higher efficiency claimed
Easy to construct
Avoids complex issues with transformer cores and winding coupling

Cons

Single band only
Calculations required to establish correct values of L and C

The Ham Radio Outside the Box laboratory (a grand name for my basement workbench) has built many 49:1 impedance transformers for both QRP and QRO operation. The QRP units are deployed in backpack portable operations and the QRO units have seen service both in the field and in the home shack. Both the conventional “twisted” coupling method and separate windings have been used.

Which winding method is best?

One of the issues with 49:1 transformers is “leakage flux” which means not all of the energy in the primary winding is coupled to the secondary. The conventional winding method is to twist the first two turns of the primary and secondary together to improve coupling. The remaining turns are only coupled to the primary by the flux in the core. Furthermore, there is often a “crossover” turn to bring the far end of the secondary out on the opposite side of the core from the primary. This may further reduce the coupling efficiency.

An alternative method is to wind the secondary, without a crossover turn, around the core. The separate primary is then wound around the center of the secondary. Should the secondary be spread around the core, or closely spaced? Opinions vary on this. I now favor keeping the secondary turns closely spaced. The reason? A closely spaced secondary winding should improve inter-winding coupling and reduce leakage flux.

What about the turns ratio?

Should it be 49:1, 64:1 or …? There is an easy answer to that: just divide your antenna impedance by 50 and bingo, there’s your answer. Oh, but what is the impedance of your antenna, 2000 ohms, 2319.647 ohms, 3000 ohms? We don’t actually know and it may vary depending on how the antenna wire is erected (which for portable operators may be different every time). A ratio of 49:1 provides a good enough match to most every value of End-Fed Half-Wave (and multiples) we are likely to experience.

Or just build an L-network!

We have seen that 49:1 impedance transformers have many variables that impact efficiency. Leakage flux has been discussed so it is relevant to note that placing a small capacitor (typically 100pF) across the primary winding is recommended to somehow compensate. Conventional 49:1 transformers are wound as autotransformers, so we have a series inductor between the antenna and the radio, and a parallel capacitor. Doesn’t that sound very similar to one of the topologies of an L-network?

My initial experiments with building L-networks involved a fixed series coil and a parallel capacitance made from a short length of thin coax – like RG-174. I experienced the problem that the calculated values of L and C did not provide the best possible match to 50 ohms. I still needed a “touch-up” tuner to bring the SWR down to a safe level for my QRP Labs QMX transceiver. I realized that a field portable antenna was going to need slightly different component values depending on whether my temporary station was setup on exposed ancient bedrock, or over the moist ground at the edge of one of the Great Lakes. What I needed was an L-match “tuner”, i.e. an L-network with variable inductors and capacitors.

42 years ago …

A long, long time ago (42 years to be precise) I was a penniless SWL foraging for food in the forest – alright that’s an exaggeration, but I had a young family and couldn’t spare the cash to buy a decent shortwave receiver. A friend told me about a design in Practical Wireless magazine for a shortwave converter that would work with a regular domestic AM receiver. I had the components shipped over from the recommended UK suppliers and built the converter. It worked splendidly and I spent many happy hours listening to the busy shortwave bands. Then I became fabulously wealthy (i.e. I could at last afford shoes and to eat every day of the week), bought a real HF radio and the converter was relegated to the back of a closet.

The point of the story is that I was able to scavenge that converter for the components I needed to build an L-match for an End-Fed Half-Wave antenna. The inductor shown in the picture above is wound over an adjustable slug-type ferrite core of unknown mix. The capacitor is a ceramic trimmer with a couple of fixed ceramic capacitors in parallel to bring its value into the range that was needed. The only comment I can make on the efficiency of that unknown core is that it didn’t get hot (or even warm) after an extended period of transmitting at 5 watts. Tuning is quite sharp but I was able to get a 1.5:1 SWR from my Shortened Sloping End-Fed Half-Wave antenna (see last week’s post). I probably could have obtained an even lower SWR by adjusting the length of the high Q top section of the SSEFHW.

QSO’s?

As a recent convert to L-networks I have only made enough QSOs to be countable on fingers and toes. On the other hand, over the years, I have made thousands of QSOs with a 49:1 impedance transformer. Both the devices shown in the pictures above accompany me on every field portable outing so I have options and can compare their performance.

Does it matter, really?

Sometimes I give my head a shake and tell myself to put the physics textbooks back on the shelf and just enjoy the experience of being out in the Big Blue Sky Shack with my radio. At other times, after calling CQ ’til the cows come home and getting no responses, I ponder the question of whether my antenna is doing its job or, as sailors used to say, is idly “swinging the lead”.

What are your experiences with either impedance transformers/UNUNs or L-networks? Your opinions are very welcome either by adding a comment below, or if you prefer, by email (QRZ.com).

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#AmateurRadio #Antennas #Counterpoise #OutdoorOps #Portable #QMX

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Ham Radio Outside the Box

Ham Radio Outside the Box Apr 16

I KISSed my Antenna – Here’s Why …

Many years ago I learned about a design technique called KISS. It was an acronym for “Keep It Sweet and Simple”. Somewhere along life’s journey I started seeing the acronym change to the rather offensive “Keep It Simple Stupid” which I entirely dislike. There are many sound reasons for simplifying a design but none of them imply a lack of intelligence on the part of the designer. Designs evolve and, in the process, become very complicated to the point where the probability of failure becomes critical.

A case in point is the Saturn V rocket that first took astronauts to the Moon. If I recall correctly, the Saturn V had something like 10 million components. If each component had been designed so that it had a 1 in 10 million chance of failure, the rocket would probably have failed at every launch. The reason is straightforward – failure probabilities in a complex machine are additive.

Every mistake is a learning opportunity. The designers of early rockets were certainly not stupid, but their rockets exploded on the launchpad, or during the early phases of launch. Even when rocketry had advanced sufficiently to repeatedly land men on the Moon, terrible disasters still happened.

There is a temptation to be so focused on an objective that errors slip into the design. Reviewing my own antennas, many of which have been featured in this blog, and the feedback I have received from some very knowledgeable Ham Radio Outside the Box followers, caused me to stop and re-think my designs. Every antenna discussed here in this blog has worked, meaning I have personally made contacts with each and every one of them. But, at the same time, some of the designs had flaws caused by too narrow a focus on end objectives. So, I made a decision to adopt a KISS approach.

These are my principle objectives based on my personal interest in operating backpack portable out in the Big Blue Sky Shack:

Rapid deployment – an antenna must be ready to transmit as quickly as possible upon arrival at an operating site.
Field expedient – An antenna must be specifically designed and constructed for temporary field operation. This necessarily implies that efficiency is not the prime objective. A “compromise” antenna is acceptable if it works well enough to make contacts.
A small footprint – the entire station should occupy as small a footprint as possible, keeping in mind that many operating sites are in public spaces where other people may be present.
Ham-made, meaning I don’t buy commercial antennas. I prefer antennas I have constructed myself. It saves money and allows more scope for experimentation.
Stealthy – the mission objective is to make contacts, not educate curious people passing by. Ham radio equipment may look suspicious to some people; better to look inconspicuous and not be noticed.
Self-contained – the antenna must not be dependent on anything I didn’t bring with me. This includes trees, vehicles or any other kind of antenna support.
Everything must fit in, or on, a backpack that is sufficiently lightweight to be hiked into a remote operating site, or transported using a wheeled cart.
Ancillary equipment, any chairs, tables, shelter, spare cables, spare battery, water and food must be part of the backpack portable package.

Softly, softly, catchee monkey

The “Stealthy” objective may sound unfriendly but it is very practical, especially when working a pile-up as is often the case with POTA. If somebody stops to ask questions I give them a very simple, but polite explanation. Usually they are just curious with no particular interest in ham radio. I was set up in a local park recently and had just completed a POTA activation when an official Ontario Parks vehicle pulled up in front of me. A young park warden got out and came over to ask me what I was doing. She told me she saw my big whip antenna and wondered what it was for and seemed interested when I told her I was contacting people by radio using Morse Code. She asked me how long I had been doing this. I was tempted to reply “oh, only about a half hour” but I overcame my frivolous inner self and replied “25 years”.

And another consideration; CW ops have a stealth advantage over phone ops – our operation is silent if we wear headphones – or ear buds which look less suspicious. If they can’t hear me they are more likely to pass on by. “He must be tracking wildlife, or something, best not to disturb him”.

It’s okay to be an ambassador for the hobby, that’s what Field Day is for. If it’s a quiet day on the bands I might be happy to have a nice conversation with a passer-by, but when the ether is overflowing with chasers and hunters the focus is on the mission’s prime directive.

We gotta get out of this place

There are many reasons for wanting a rapidly deployable portable rig. Out in the great outdoors the weather can change suddenly necessitating a fast teardown of antenna and radio. In a public space – such as a park – other people may gather in close proximity to our operation creating a disturbance or becoming susceptible to tripping over wires or being electrically excited by the high voltage at the end of a wire. In the backcountry there is also the possibility of a representative of the ursus americanus community paying us a visit. For these reasons, among others, having a portable rig that can be set up, or moved, in a couple of minutes is a great advantage.

All these factors led me to build and deploy many of the antennas described in this blog. One in particular has led to a lot of discussion – the Coil-Loaded End-Fed Half-Wave (CLEFHW). This antenna comprises an 18.5ft telescopic stainless steel whip with a small loading coil at the base and a very short (0.05 wavelength) counterpoise.

What is “Electrical Length”

I made the claim that the loading coil changes the physical length from 18.5ft to an electrical length of a half wavelength on the 20m band. The choice of words is very important here. The physical length is measured in feet but the electrical length is measured in wavelengths.

Wikipedia defines electrical length thus:

In electrical engineering, electrical length is a dimensionless parameter equal to the physical length of an electrical conductor such as a cable or wire, divided by the wavelength of alternating current at a given frequency traveling through the conductor. In other words, it is the length of the conductor measured in wavelengths.

The purpose of the design was to eliminate long radial wires laying on the ground. A very short length of coax terminated in a common mode current choke acts as sufficient counterpoise. I wrote once before about a nice lady who stopped by to inquire, in a friendly manner, what I was doing. I was using a different antenna at the time and cautioned her to be careful of the wires on the ground. She responded by entertaining me with a little dance as she attempted to avoid stepping on them. Wires on ground in public spaces – ungood!

Another design objective of the CLEFHW was to be integral with a self-contained backpack kit occupying a ground footprint of only a couple of square feet. The backpack rig is its own operating table, so dump it on the ground, erect self-contained antenna, transmit. If I didn’t have worn out knees I wouldn’t even need a chair, but I did have to add an ingenious collapsible plastic stool to the kit.

Time for confessions

Now it’s mea culpa time … while the CLEFHW has performed successfully in more than one POTA activation and numerous casual QSOs, it does have a couple of design flaws. First, the biggest and baddest. A full-size vertical EFHW has a current maximum point half way up the antenna, far away from the power gobbling greedy green ground. But the CLEFHW cheats; it is not a physical half wavelength long, it is an electrical half wavelength long so the current maximum point remains at the base of the antenna. That results in high radiation around about head height. I checked with the ARRL RF exposure calculator and found that shouldn’t fry too many brain cells while operating at QRP levels. Proximity to ground also increases power loss. I mitigated this loss by raising the base of the antenna to about 1 meter above the terra firma. A few hundred milliwatts may still slip away to warm the worms, but heck, that’s the fun of QRP, so they say.

Another, shall we call it flaw, is the double impedance conversion. An email correspondent whose views I much respect advised me to think of loading coils as impedance matching devices. Using that way of thinking the CLEFHW’s loading coil converts the impedance of the whip to an R+jX value resembling that of a full-length EFHW. The 49:1 impedance transformer then reduces the impedance down close to 50+j0. There is almost certainly some inefficiency in that double transition. But, one of the design parameters already listed calls for Field Expedience above efficiency so I guess there is no free ride here. As somebody else once said: “every antenna is a compromise”.

The big, overriding objective of the CLEFHW design was to work as an integral element of a backpackable, rapid deployment portable ham radio kit. Despite some unusual quirks in its conception it gets the job done. Is it overly complex? Does the bizarre double impedance conversion cause more chaos than a monkey in a china shop? Should I abandon the design based on its assault on sound antenna physics? I seriously considered scrapping the project in favor of a Sweet and Simple tunable whip, but I am uncertain whether that would be an improvement.

The direction in which I am heading at the moment is to replace the 49:1 transformer with an L-match “tuner”, that is capable of dealing with the vagaries of terrain I experience in my itinerant portable operations. Despite widespread opinions describing End-Fed Half-Wave antennas in less than flattering language, the advantages for a portable operator outweigh any negatives so future endeavors will remain on that course.

As always, your feedback is much appreciated.

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#AmateurRadio #Antennas #CLEFHW #Counterpoise #CW #OutdoorOps #Portable #POTA