Are External ATUs Better?

VIDEO

Internal antenna tuning units (ATUs) built into modern HF transceivers are convenient, but there are still very good reasons to consider an external antenna matching unit. Having used an LDG AT-100 Pro M2 for several years, I’ll walk through what external ATUs bring to the bench and the shack, when they make sense, and some practical tips for getting the best from one.

External ATUs offer advantages that internal ATUs often cannot match. The main benefits are:

• Greater matching range — larger components and design flexibility let external units handle much more challenging antenna/load combinations.
• Extra features — built-in forward/VSWR meters, accurate LED PEP meters and two-way antenna switching are common.
• Flexibility — the ability to add baluns or ununs on the output to feed balanced line or end-fed wires, and manual controls for fine tuning.
• Durability and longevity — external ATUs are independent of changing transceiver models and tend to be long-lived like power supplies.

The LDG range is a good illustration of the typical external ATU feature set. The AT-100 Pro M2 I’ve used is representative.

•  Coverage: 160 metres through 6 metres.
• Power rating: 125 W SSB, 100 W CW and 75 W digital (FT8). LDG also offer AT-200 and AT-600 variants with higher power handling.
• Front panel: seven multi-function controls including tune and function keys, LED bar-graph forward power and VSWR meters.
• Rear panel: two antenna inputs selectable from the front, earth terminal, transceiver input and a DC input (designed for 11–16 V DC).
• Construction: neat internal layout with relays, capacitors and inductors arranged for an L-network match topology.

External ATUs like the AT-100 Pro M2 offer semi-automatic tuning (press tune while supplying a small amount of carrier, typically 5 W) and fully automatic modes. In fully automatic mode the unit will sense a rise in VSWR and retune itself. The unit also memorises tuning settings by frequency, so a previously used frequency retunes almost instantly.

LED bar-graphs are very effective for measuring peak envelope power (PEP) and give a clear visual indication of power peaks far more crisply than many analogue meters. The forward power meter is switchable to a low-power scale (for example 12.5 W full scale) for accurate tuning on QRP levels.

Many LDG external ATUs provide front-panel buttons to step the capacitor and inductor values up or down. This manual trimming is useful when you move a little off the tuned frequency and want to quickly bring VSWR back down without a full re-tune. There is also a control to reverse the L-network (swap series and shunt positions) which increases the range of reactances the unit can match.

LDG external ATUs are designed for unbalanced coax. If you need to feed balanced line or an end-fed wire you simply add a balun or unun at the ATU output:

• For balanced line: use a 1:1 or 4:1 balun on the output.
• For end-fed wires: use a 4:1 or 9:1 unun as appropriate for the feed arrangement.

Note that many ATUs use a balun on the output rather than providing a true balanced tuning network internally. Adding the correct balun or unun is both effective and straightforward.

An interesting effect of resonating a poorly behaving antenna with an ATU is the tuned response the system can show. With the ATU bypassed the antenna system may look broad and messy on an analyser. Once the ATU matches the feed to the transceiver, you often see a pronounced dip at the tuned frequency and good rejection either side, effectively producing a front-end band-pass filtering effect. This can help reduce off-frequency noise and unwanted signals entering the receiver.

External ATUs generally outperform internal ATUs when faced with very high reactances or difficult matches. Compared to fully manual tuners, automatic external units are faster and often just as effective in practice. While a seasoned operator using a manual tuner can achieve excellent matches, automatic units save time and remove a lot of fiddling—especially when hopping around the band or changing antennas.

• Handles more severe mismatches than many internal ATUs.
• Gives accurate power and VSWR readings at the matching point.
• Two-antenna switching provides extra convenience for rigs with a single antenna jack.
• Memory and automatic re-tuning save time in real use.

An external ATU is a good fit for you if any of the following apply:

• You use non-resonant or multi-band wire antennas (end-feds, random wires).
• You need to feed balanced line or want the option to do so using an external balun.
• Your transceiver has only one antenna socket and you want an easy way to switch between two antennas.
• You want fast, reliable retuning and clear PEP/VSWR metering while operating at modest power levels.
• You want a unit that is independent of transceiver changes and will last many years.

1.  Low-power tuning: Use about 5 W carrier (or the radio's tune function) when initiating a tune—this is faster and kinder to the ATU relays.
2.  Use the memory: Take advantage of the ATU’s memory so previously used frequencies tune almost instantly.
3. Choose the right balun/unun: Match the balun/unun ratio to your antenna type when feeding balanced line or end-feds.
4. Watch the ratings: Verify the ATU’s power rating for your typical modes—SSB, CW and digital modes may have different safe limits.
5. Bypass when needed: Tap the tune button briefly if you want to bypass the ATU and run direct coax for antenna testing or for a permanent resonant feed.

An external ATU is not essential for every station, but it is a valuable tool for many. It gives increased matching capability, useful front-end metering, dual-antenna switching and the flexibility to feed balanced or end-fed systems with the addition of a balun or unun. For operators who run non-resonant antennas, need quick retuning or want a robust, transceiver-independent solution, an external ATU is often a worthwhile investment.

Enjoy experimenting with your antennas and matching arrangements—sometimes small changes and the right tools unlock much better performance from familiar wire and coax setups.

For more information please visit our online store or alternatively contact us and our team will be happy to assist you!