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

January 15, 2012

The Hunt for a Good Compromise Antenna

Filed under: ham radio — Cranky Sysadmin @ 11:10 pm

I’ve been tinkering with my homebrew slinky antenna and my G5RV lite which I picked up and Ham Radio Outlet. I’ve decided to abandon the slinky dipole. It will easily tune to everything from 80 meters to 6 meters, but there is always a lot of noise compared to the G5RV. Due to the fact that there are no trees on our lot and the fact that I don’t want questions from the neighbors, the highest I’ll be able to get an antenna is about 25 feet, the height of the house.

According to popular literature, this is a terrible situation for a dipole in the HF bands except maybe 10 meters. If I am honest with myself, I’ve read about people who have had great success with antennas atop their houses or in their attics. I don’t have a metal roof, so it’s probably a viable option. Even though what I have may be sufficient, I’ve been reading about alternative antennas. I’d like something easily portable and quick to set up anywhere with as little compromise on performance as possible. This lead me to reading about magnetic loop antennas. As you can see, there is lots of information about the building and operating of these fascinating devices. One of the more informative sites out there is run by AA5TB. It has a loop calculator so you can build a loop fairly close to resonance at the frequency you want.

All of these sites, and other sites which involve antenna research recommend that you have an antenna analyser so you can measure the performance of your antenna. One of the simpler ones, called the ‘tenna dipper, uses a simple oscillator, an impedance bridge, and an LED to tell you what frequency the antenna is resonant at. If you’re willing to take on a more complex project, ZL2PD describes an analog and a digital version of an antenna analyser. The digital one uses an 87C552 micro-controller, which I’m not at all familiar with. If I attempt that project, I’ll probably try to convert it to something Atmel flavored.

This line of study lead me to start building oscillators again because I really would like to have a small signal generator. One of the more interesting ones for its simplicity is described by Ashhar Farhan. VU2ESE from India. His site is a wealth of information on homebrewing on a tight budget. He is the designer of the BITX 20 20 meter transceiver, which is popular in the QRP (low power) crowd. I made a modification or two to his signal generator design to match the parts I have and soldered it together, ugly style. Below is the result.

ugly Hartley based VFO

Depending on the inductor, I can get a fairly broad range of clean looking frequencies. I still have a couple of stages to complete, and I have to put in a switch to move from band to band, but it’s looking pretty good.  This may not even be my final stab at a general purpose signal generator, but it sure is a lot of fun building this stuff.

 

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