Charleston Hamfest 2015

Had another good time at the Charleston Hamfest today. There were a lot more vendors (including in the tailgate area) this year than last year. The day got off to a bit of a cool and frosty start, but warmed up nicely. Good sized crowd of people attending the hamfest. Here are the early birds. Even shortly after 8 there were a fair number of people browsing around the tables.

Early hamfesters at the 2015 Charleston Hamfest
Early hamfesters at the 2015 Charleston Hamfest.

Started off the day picking up a couple of items off the Free Stuff table: a 1971 edition of the Radio Amateur’s Handbook, and a CueCat.

1971 ARRL Handbook
1971 ARRL Handbook

I’m looking forward to going through the 1971 handbook. I think it will be cool to see what amateur radio was like when I was 1 year old.

CueCat
CueCat

I remember CueCats from my Apple //e days and always thought it would be cool to have one. Should be fun to play with this.

Browsing around the hamfest, I picked up an Ethernet shield, a big bag of resistors and some perfboard to mess around with.

Ethernet shield
Ethernet shield

The Ethernet shield will fit in nicely with the growing collection.

Bag of resistors
Bag of resistors

A big bag of 1/4 watt 1% tolerance resistors. I should be set for resistors for a good long time.

Perfboard
Perfboard

2 large plain perfboards, and 8 prototyping boards with solder pads. I like using the perfboard for quick prototyping and experimenting with circuits.

I was also able to find most of the things I’ll need to build a window pass-thru block for the antenna.

Bulkhead connectors and caps
Bulkhead connectors and caps

Two 3 inch SO-239 bulkhead connectors and some screw-on caps for the cable ends.

There were a lot of other things I wish I could have picked up too (isn’t that always the case?), but I’m pretty pleased with what I got today.

To top things off, I won one of the door prizes being given out too, a $50 ARRL gift certificate. Not sure what I’ll get with it, but it will be something good.

The VE testing session at the hamfest went very well. There were 16 people taking tests today, 9 of whom were taking the Tech (Element 2) test. Everybody left with either a new license or an upgrade, so very successful on all accounts.

Coils

Time to see if I can build myself a crystal radio set. Or two. Or three.

I decided rather than demonstrate building a radio at the Charleston Library’s DIYFest, it would be easier to bring in some examples of some simple crystal radios that could be built using stuff easily obtained around the house. Found three nice cylindrical objects and started winding magnet wire.

The inductance of an air wound coil, according to Section 2.12.1 of the 2014 ARRL Handbook, is approximately

L (μH) = d2n2/(18d + 40l)

where d is the coil diameter in inches, l is the coil length in inches and n is the number of turns. Converting it to use more sensible metric units, it becomes

L (μH) = d2n2/(45.72d + 101.6l)

where d and l are in centimeters.

The first coil is 182 turns around a toilet paper tube. d = 4.5 cm, l = 6.7 cm, n = 182, and L = 757 μH.

First coil - 182 turns, d=4.5cm l=6.7cm
First coil – 182 turns, d=4.5cm l=6.7cm

Coil #2 is 110 turns around the cardboard core of a packing tape roll. d = 8 cm, l = 4 cm, n = 110, and L = 1003 μH.

Coil #2 - 110 turns, d=8cm, l=4cm
Coil #2 – 110 turns, d=8cm, l=4cm

Coil #3 is 118 turns around an empty plastic pill bottle. d = 4.8 cm, l = 4.1 cm, n = 118, and L = 504 μH.

Coil #3, 118 turns, d=4.8cm, l=4.1cm
Coil #3, 118 turns, d=4.8cm, l=4.1cm

This is the basic schematic I’m planning on using.

radio schematic
radio schematic

I want the radios to be able to tune in the AM broadcast band (540-1710 kHz) so the capacitance I’ll need for coils #1 and #2 is around 10 pF and around 17 pF for coil #3.

Since I have three coils, I think I’ll make one with a variable inductor, one with a variable capacitor, and one with a variable inductor and capacitor.

I don’t have any high impedance earpieces (I’ve seen some builds say you can use a telephone handset though), so I might add an amplifier or impedance matching transformer and see if I can get regular headphones to work.