HF Amplifiers for Portable Use

The most popular radios used for lightweight portable operating tend to be quite low power devices. It is certainly the case that in most situations power levels of less that 10 Watts will give plenty of contacts. However, when conditions are poor or if you are a habitual HF SSB user, higher power can often be welcome. There are potential disadvantages though. Not only will your radio (or radio plus amplifier) weigh more, you will also need a bigger battery. You may even need to upgrade your antenna system to cope with the higher power. The "sweet-spot" for higher power portable operating seems to be between 20 and 50 Watts. Any less and the improvement over 5 Watts is not significant, any more and the extra weight you need to carry starts to increase rapidly.

Get a higher power radio?

The most common way to run more power is to use a radio such as a Yaesu FT857. This is the most popular 100 Watt portable radio and is a solid performer. It's over twice the weight of its little brother the FT817 (2kg vs 1kg), draws more current on receive and obviously draws a lot more current on transmit! While you might think you can make some savings by running it at a lower power, beware because the efficiency of the radio falls rapidly as the power is reduced. At 5 Watts output an FT857D draws 4 amps compared to 2 Amps for an FT817.

Use a separate power amplifier?

Another way forward is to use a separate HF amplifier with your existing low power radio. This is a flexible solution and is favoured by many portable operators. It does have some "down sides" though. Separate amplifiers can be designed to reduce their size and weight and can weigh less than the amplifier section in a standard 100 W radio. that may sound surprising but an outboard amplifier can be designed to work at an intermediate power (say 50 watts) which gives a big improvement in signal strength without needing as much (heavy) heat-sinking as a built in 100 W pa. An an outboard amplifier you can further reduce weight by covering less bands and sizing the heatsink for intermittent use rather than sizing it to survive an RTTY contest at 100 Watts.Down sides are more to carry and more leads to forget or to fail.

A popular commercial amplifier was the Tokyo Hy-Power HL45B. Not the smallest or the lightest, it did seem to be a good performer. It was expensive though and Tokyo Hy-Power no longer exists so it's a secondhand purchase only these days.

A reasonably priced option seems to be the Chinese 45 Watt MX-P50M. This amplifier has been around for quite a long while in several forms. It is designed specifically as a companion to the FT817 and if you want to use it with other radios you may need to modify the leads to get the right switching. I have an older one which works as largely as advertised. It's cheap for a reason of course; the build quality is not the best and the output filtering is a bit of a compromise with just four low-pass filters covering from 3.5-29 MHz. My early version was not very linear; no problem for CW and usable on SSB but just not great. This seems to have been addressed on the current version. The heat-sink on the current design is quite small making it a poor choice for 29 MHz FM users. Despite that, at the price it's worth a go. It is marketed on eBay at around Â£125 including shipping from the Far East.

An even cheaper option is to use one of the lower power amplifiers from RM ITALY.Their amplifiers are cheap and are unlikely to meet the require specifications for unwanted emissions. However, the KL-203 can be made into quite effective an CW amplifier for the higher HF bands (20m and up) with the addition of suitable low-pass filtering. Operation down to 40 m is possible but requires some modification to the amplifier's output transformer. The KL-203 is not linear and its power output rating is hopelessly optimistic but for the price you get something that can be modified.

KL203 revealed

If you are capable of building a kit there are several options. The HF Packer is a good amplifier that has been around for a long time (15 years?). Virgil K5OOR provides good support and with an amplifier that is currently at Version 4 you should be assured of good performance. A feature that I especially like with this amplifier is its use of an internal boost switched-mode power supply. This means the amplifier works well even when your battery is sagging. This is a great feature.

The rise in popularity of low power software defined radios has spawned a number of interesting HF amplifier kits in the 20-50 Watt range. Unfortunately these seem to come and go with small production runs. The Hardrock-50 is one such kit which seems to get good reviews and might stick around for a while.

Of course, this article would not be complete without mentioning the Elecraft KXPA100. Unlike the others, this is a 100 watt amplifier and is available as a kit or built. With solid state TX/RX switching and an internal ATU option it's in a different league to the other products. As you might expect, you do pay a significant premium for these and many other features. Interestingly Elecraft describe the KXPA100 as being for "desk top and mobile use" perhaps suggesting that lightweight portable was not envisaged. The KXPA100 weighs in at over 3 kg (with the internal ATU) so I would agree that while it might be useful for some sorts of portable use, backpacking isn't one of them.

Kits are not essential and it's quite possible to design and build your own amplifier (or use someone else's design). Don't expect this to save you any money though, and if you factor in your time it will probably be the most costly option. The fun and learning to be gained are priceless though.

73 Richard G3CWI

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References

MX-P50M Technical Review by John GM4SLV
HF Packer Information
KL-203 Information
Hardrock-50 Technical review by Phil AD5X
Elecraft KXPA100
Self-build
Homebrew Portable Amplifier design and build by John GM8OTI
Excellent resources and ideas by Robert M0RZF

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What's new @SOTABEAMS ?

Powerpole to automotive socket adapter

One morning I noticed someone searching for "cigarette lighter" on my website. After puzzling why someone would search for such I thing I guessed that maybe they were searching for a Powerpole to Automotive plug lead. I didn't sell them at the time so I have had some made up. And to my surprise, the person who was searching contacted me to say they were indeed what he needed!

New products being developed

We are busy working on several more complex products. This has required a considerable investment in test equipment.

This is our RF test bench. Two spectrum analysers for different purposes.

I hope that we will have some interesting new products for you before the summer.

As usual, I have lots more ideas for products so keep checking the website!

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73 Richard G3CWI

On the web

A good resource for electronics engineers is the informative site Radio and Electronics edited by Ian Poole. He also runs a sister site aimed at radio amateurs called (perhaps rather confusingly) Electronics and Radio. Ian is a professional RF and electronics engineer - and was my boss when I was a young design engineer at Racal Electronics in the 1980s.

Out and About

Out and About is an online newsletter dedicated to portable operating. Although its focus is on VK5, there is much useful information of a general nature about portable operating. Worth a look!

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