Common dipole antenna dimensions for Amateur Radio HF bands

When building your own antennas, you can either look up a design or dimensions online or from book, or you can use a common formula. 468/MHz is commonly used to calculate the total length of a wire dipole in feet. I had a few dimensions jotted down for reference, but it’s easy to stick the formula into a spreadsheet and just enter the freqs you’re interested in. For example, here’s a Google Doc with the common HF bands:

I hope I’ve got these right, but if not, drop me a comment below.

Here’s a link direct to the Google Sheet if you’d like to bookmark it:

In progress: Setting up an onscreen keyboard for the 7″ touchscreen on the Raspberry Pi

I just received one of these very cool 7″ touchscreens for the Raspberry Pi:

I have the default logon prompt when my Pi starts up, so first challenge is, how do you logon without a real keyboard attached, without having to disable the logon? There are a number of onscreen keyboards available – based on this thread I installed florence:

sudo apt-get install florence

and then edited /etc/lightdm/lightdm-gtk-greeter.conf adding ~a11y; for the Accessibility icon on the logon screen, and keyboard=florence so it appears in the menu.

The keyboard now appears, but as soon as I press a key it disappears. From this thread, installing at-spi2-core appears to fix the issue:

sudo apt-get install at-spi2-core

This seems to fix the keybaord not disappearing, but not having any luck getting characters to appear in the username/password fields. Still some investigation to do on this one.

Taking screenshots on Raspbian / Raspberry Pi

Install scrot (if not already installed): sudo apt-get install scrot

To pick a window, run ‘scrot -s’ and then click on the window that you want to capture. Screenshot is saved in your home dir by default.


AngularJS form validation and ng-messages

I’ve been working on a simple example AngularJS app uses ng-messages. Code so far is on my github here.

This is part of my ongoing activity learning AngularJS. You can see my other posts and progress so far here.

To get up to speed with ngMessages, there’s a great tutorial here, and online docs here.