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

March 9, 2013

First Crack at a Simple Antenna Analyzer

Filed under: Electronics,ham radio — Cranky Sysadmin @ 8:32 pm

I’ve built a prototype of my first antenna analyzer. Pictures and some details below.

The prototype uses a hartley oscillator based signal generator lifted from here. The RF output is attached to a simple wheatstone bridge. I have the bridge attached to my G5RV lite antenna on the output, and to an oscilloscope where the meter would usually go (I don’t have a meter, so the o’scope will have to do). I’ve also attached a bench frequency counter. Below is the initial prototype in all of its ugly construction glory

First simple antenna analyzer

First simple antenna analyzer

As I sweep through the frequency range with that ugly old broadcast capacitor on the left, the AC voltage level shifts as the antenna hits its various resonance points (none of which appear in a ham band). Below is a picture of the output at a frequency which isn’t in resonance.

The antenna is not in resonance!

The antenna is not in resonance!

And here is a picture of the output when the antenna is in resonance.

Antenna at resonance.

Antenna at resonance.

I added a germanium diode and a high value capacitor to act as a detector. This allows me to read a DC voltage. Below is a detailed view of the prototype with the detector on the far right.

antenna analyzer detail after adding a germanium diode detector

antenna analyzer detail after adding a germanium diode detector

I have to decide how far I want to go with this analyzer before I move on. Currently, the oscillator has a range from 6.5mhz to 28.5mhz. I also wound an inductor which has a range from 1.6mhz to 7.6mhz. Some obvious improvements would be:

  • Add a band switch and switch in the right coil for the range I want to use.
  • Add a small frequency counter so I don’t have to occupy the bench counter.
  • Add some opamps and a meter so I don’t have to occupy the oscilloscope.
  • find an adequate way to power the device so I don’t occupy my bench power supply.
  • find a nice enclosure.

One of the advantages of the hartley oscillator over the DDS is that the hartley uses a lot less power. The DDS seems to use about 150ma. The hartley barely registers on my bench meter. This is a serious consideration for portable use. I like the wizbang features of the DDS, so I may use cognitive dissonance to pretend I don’t see the power issue.

 

No Comments »

No comments yet.

RSS feed for comments on this post.

Leave a comment

Powered by WordPress