QRP (low power) RF stages – g3jkx

QRP, RF Stages

Why do QRP projects have to be so SMALL ? Why can’t the front panel be large with big knobs and switches even if the electronics are tiny?

If you are like me, most modern rigs are annoying because the knobs are too small and too close together. The radios that I grew up with, like the AR88, BC342 & 348, R1155 etc usually had a big weighted tuning knob which, when spun, would take you to the other end of the band in a hurry. There were also RF and AF gain controls, with integral on/off switch, a band-switch and a switch for the AGC. If you were lucky there was a crystal filter knob to twiddle as well. What a delight that was. A single crystal at the IF frequency with a variable capacitor to adjust the bandwidth. Cheap and cheerful and it worked well. Oh, yes, a BFO control too, for SSB and CW operating.

Do we really need anything better? I think not. Valve sets could also cope with high power adjacent transmitters really well, which modern sets have trouble doing. Valve PAs could also cope with a very big SWR too, without the ALC shutting everything down. Commonly,  modern rigs shut the PA right down at an SWR of 3:1. So tune up carefully before using.

What we need is High Q tuning in Rx RF stages, to reduce the bandwidth and keep out unwanted adjacent band rubbish. We don’t need wide band ‘receive everything’ sets. In other words , ‘amateur band only’, with an RF stage tuning control to peak up that weak signal. A classic example appears in the latest RadCom. This uses four 100pf ganged tuning capacitors with lots of inductances, just to tune across Top Band (1.85 to 2 MHz) Yes, it does mean re-tuning the thing as you go up and down the band, but that is necessary for peak performance. Narrower bandwidth also means less noise for the rest of the Rx to have to handle as well.

In the old days, a firm called Denco used to make a beautiful multi-band turret with a set of coils for each of the amateur bands, mounted on a multi-way rotary Yaxley switch. I used one of these to make a Q5er, which is basically an add-on, narrow band RF stage, to stick in front of the Rx. The result was amazing. Really easy listening to weak stations, previously very difficult to hear. Yes, it meant another couple of knobs to use but you soon got used to them and there were still less than 10 knobs to worry about. Some modern sets have dozens and have multiple uses for some, using  menus to alter other functions. Notice how, as the rigs get smaller, the instruction book gets increasingly larger! Any amateur confronted with an all bells and whistles rig has a lot of learning to do. It took me 6 months of hard DX operating to get really good use from my (1992) Kenwood TS690S. I bitterly regret selling it for a small, more modern thing, which shall be nameless.

Some new Amateurs (& old) do have difficulty coping with modern rigs. What is needed are sets with larger panels and knobs, to recreate the simplicity of yesteryear, but using modern components and techniques. When you can operate this set really well you could then add tailor-made modules such as a DSP unit, an FM demodulator maybe, a noise limiter, or an internal AMU. This stretches the cost out too and enables the operator to get used to one change at a time. Some kit sets are brilliant in this respect.

New hams don’t want to need BAs in ergonomics and computing to get on the air quickly with a decent signal.  The manufacturers will not give us what we need so it means building one yourself. The K1 and K2 kits are not cheap but they are superb. Easy to build and a delight to operate.

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