The Cranky Sysadmin A world of technology, fun, and ignorant rants.

January 31, 2012

Lots of Little Radio Projects

Filed under: ham radio — Cranky Sysadmin @ 7:55 pm

I’ve recently done a lot of small but interesting projects. I should probably break each out into its own small article, but I’m lazy, so you get to see a mishmash.

First up, I’ve been studying oscillators and ugly construction again (or still). Below is a Hartley oscillator based on a J-FET and a tapped coil with a straw as a coil form. I need to learn to make the circuit more compact and to lay it out better. I try to make things pretty, but that’s probably a lost cause.

ugly hartley but it works

One of my projects which is moving along very slowly is building a magnetic loop antenna. One of the problems is that the trombone capacitor I want to build is difficult to find parts for. I finally found some appropriate parts at the big home improvement center near by. What I really needed was a pipe which fits inside a dielectric which fits inside another pipe. The picture attempts to show some part numbers of the stuff I found in case someone else is having similar difficulties. The store sells 2 foot lengths of these items, so I don’t need to over-buy. The pipes are; 3/4 inch type M copper pipe, 1/2″ ID poly pipe (unidentified plastic), and 1/2″ type L copper pipe inside that. I have no idea what type of plastic “poly pipe” is, but it doesn’t appear to be PVC. I slid the pipes all together and the capacitance was about 370pf, so it must have a really good dielectric constant. I’ll have to find the strength next.

What is "poly pipe", anyway?

I found possibly the best appointed antique shop in my area called “Paul Martin Antiques” in Haverhill, MA. They have a huge warehouse with three floors of stuff. I bought an unidentified FADA Radio tube AM/shortwave there. After fixing some bad connections and replacing a capacitor, the thing works fine. It’s also a beautiful radio. My original plan was to part it out, but I can’t bring myself to wreck such a nice looking radio. This looks a lot like the Fada L67T but not quite. I believe it’s from around 1939 or 1940.

Art Deco... Nice.

Not so art deco, but still nice.

I realised recently that I never built a crystal radio as a kid, and not since then either. I remember my dad and grandfather talking about them a lot. It’s a simple project, so I built one this past weekend. The coil form was a piece of 2″ PVC pipe. The wire was 26 awg magnet wire. The diode was an old germanium diode I found among my grandfather’s parts, but a modern silicon diode may work fine too. The air variable capacitor is from an old TV tuner. It’s not needed, but can give you some extra flexibility in tuning. If you don’t have an air variable, you can steal the plastic variable cap from a cheap AM/FM radio. As a side note, I couldn’t get any kind of speaker to work, even the smallest peizo element didn’t produce any sound. A cheap ear phone worked fine though. The only station I could pick up was the spanish station a few miles away.

Layout. The other side of the board is a ground plane.

Schematic. I really need to get more symbols.

Alternative variable capacitor.

My last project was getting my G5RV lite antenna off the privacy fence and up higher in the air. My “delivery system” was a slingshot, a fishing pole, and some dacron cord. The transmission line goes up to the peak of the roof. The ends of the antenna go to nearby trees now. The improvement in reception is amazing. It is also much easier to tune the antenna. I just need to get up the gumption to transmit more then, “antenna test, AB1PX”.

antenna deployment system.

If you squint, you can almost see the wires of the antenna.


1 Comment »

  1. Greetings,

    Your poly pipe is high density poly ethylene (HDPE) made for high pressure purposes. It’s a good choice for a trombone cap as HDPE has a fairly low loss tangent and is only _slightly_ less efficient than teflon (and a million times more efficient than pvc!) A search for pe3608 will tell you more than you’ll probably ever want to know :)

    Comment by Dave — February 21, 2012 @ 2:31 am

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