I just got a 100W HF radio and I am new at this. What is the cheapest antenna I can hook up without burning up my radio or shocking myself?
The cheapest antenna you can set up is a speaker wire dipole. A spool of speaker wire long enough to split the conductors and make a center fed half wave dipole ought to cost around ten dollars US (or less). You can get useful radiation with supports made from cheap stud lumber from the home improvement store (8-10 dollars per support to get 5 meters off ground -- low for an HF dipole, but it will work). Insulators can be made from bungee cord or zip ties (since the poles aren't conductive).
The only thing you have to spend real money on is a suitable length of 50 ohm coaxial cable and an adapter to connect the antenna wires. You most likely don't even need a balun with a center fed dipole.
This close to the ground, you won't get the very low takeoff angle for DX, but you'll get plenty of ground wave contacts -- when 40 and 80 m are behaving a little (gray line or at night) you should be able to cover a few hundred miles, anyway.
Hello and welcome to the hobby!
Generally speaking, it depends on your conditions and area available. Assuming it's not a problem (e.g. you live in a suburb area and have a backyard or are willing to travel to the nearest forest) the simplest, cheapest and quite efficient antenna you can build is a dipole / inverted-V. There are a few ways to build one. I believe there is little sense to describe them here because there is a lot of information on this topic online and in the literature as well. I would like only to leave a few hints here:
- A 10-meters long fishing rod is a good and cheap mast for a 7Mhz or 14Mhz dipole. Use three ropes and screwdrivers to make guys.
- A 1:1 balun made of 8 turns of RG58 coax on FT240-31 or FT240-43 ferrite core will improve the performance of the antenna.
- You can build a multiband antenna, so called fan dipole, if you connect multiple arms for each band
- "ARRL Handbook" and "ARRL Antenna Handbook" are great source of information on antennas.
Wire antenna are usually the easiest for beginners. The standard is a dipole with 1/4 wavelengths on each side connected in the center to your coax. Another easy antenna is a end fed half wave. It requires a bit more equipment as you need a transformer to match the high impedance.
You can buy or make your antennas. Home brew antennas are a popular way to get started in making equipment instead of buying. While there are many web resources with information on making EFHW antenna, here is good presentation that is relatively recent, http://gnarc.org/wp-content/uploads/The-End-Fed-Half-Wave-Antenna.pdf
I made a 40 meter end fed half wave antenna with a 49:1 transformer with 2 type 43 cores. I have made over 2500 contacts using it, primarily digital modes like FT8 and FT4, using mostly 40 meter, a few 30 meter, some 20 meter,a couple of 17 meter and 15 meter.