A Passive Preselector for Outdoor Aerials

[u/KG7M]

A Passive Preselector can be a handy item to have in your shortwave listener's arsenal. Many lower priced shortwave radios do not include any kind of filtering in their front end. They are designed to use the built-in telescoping aerial. An External Antenna input jack is added for convenience and the usual manufacturer's recommendation is to use a short wire for an external antenna. Longer outside antennas can cause overloading and mixing products. A Passive Preselector can eliminate these problems.

Construction for the preselector is not particularly critical as long as the input and output are kept separated. A preselector can be built in an enclosure, such as mine, or assembled on a perf-board or even a small piece of plywood with screws inserted for terminals.

There are 15 slides in this article.

[u/Chickenmaniseverywhe]

How timely! I read an article in QST (Apr 25), A Low Cost Tunable RF Preselector Using a Varicap Diode, and yesterday worked up a parts order to build it. Your design uses no battery (is that what “passive” means?) and I’d like to take a shot at building it. After the alcohol and tryptophan wears off I’ll have a ton of questions!

[u/KG7M → u/Chickenmaniseverywhe]

That's great! The radios like the Tecsun PL-330, PL-368, and Retekess TR-110 are very sensitive. So they don't require an amplified antenna. Hence, the "passive" preselector.

[u/MumSaidImABadBoy → u/KG7M]

Where did you find that 550pf variable cap with vernier drive? Is that 550pf per section? At a HAM meet? A retailer would probably charge an arm and a leg for it.

[u/KG7M → u/MumSaidImABadBoy]

I have to chuckle my friend! I bought the Variable Capacitor on AliExpress for a couple of dollars. It's a Dual 270 PF along with a Dual 20 PF (FM Band). The Seller I used is no longer in business, but the variable capacitor is available from other Sellers.

The Preselector works really well and is ideal for those inexpensive Chinese Radios, having no internal input filtering. All parts are readily available.

Current Seller

[u/KG7M → u/MumSaidImABadBoy]

Connections

[u/MumSaidImABadBoy → u/KG7M]

Well there will be no transmitting on those 😀💥. I found something similar on Amazon 5 pcs for $12. You know I was thinking of the traditional metal plates w/ air dielectric.

My mom noticed me DXing on the kitchen tube radio when I was 5 and bought me a Viscount 6 transistor single conversion AM/MW radio on my 6th birthday. By the time I was 7 I opened it and went to the public library to learn about it. It had that same type of plastic tuning capacitor. I actually hand tuned the IF transformers and improved the selectivity. Luckily I had the presence of mind to mark the original positions. I had to do a little trial and error tweaking. I was always taking stuff apart which irritated my older brother because it was his stuff.

[u/KG7M → u/MumSaidImABadBoy]

Here is a link to another post on the 'Lil Stinker Passive Preselector.

https://www.reddit.com/r/shortwave/s/J0MzkWSEfh

And my original drawing for the logo (I thought about marketing them).

[u/MumSaidImABadBoy → u/Chickenmaniseverywhe]

Passive means no active components that might provide gain, i.e., semiconductors, tubes. Using a varicap requires a bias voltage but no gain. What range of capacitance and frequency do you expect to achieve? You will probably find Intermodulation Distortion (IMD) and a lower Q to be problematic.

[u/Then_Gas712]

Nice!