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Antennas for Aviation Radio and VSWR, part 2 (with thoughts on the human body antenna)

I'm talking about how to analyze the efficiency of coupling to an antenna. I'm picking up from yesterday's post. Today, I'll add a couple more antenna analysis for you to consider. First off, a short 18" extendible whip antenna. Jumping right in, the VSWR for this antenna looks like this:

The ratio of VSWR varies from 9.2 @ 118 mhz (left side of screen) down to 5.2 @ 138 mhz (right side of screen). In other words, this is not a great match. A lot of the received energy is simply being reflected back out into space. The loss varies between 44% and 64%.

An interesting thing happens if we couple a human body to the antenna: the VSWR ratio improves. Please note how my hand is touching the antenna.

The resultant VSWR ratio is now flat across the spectrum, with a value of 2.7 at all frequencies. What's going on here?

A small portion of the answer is that my body is being capacitively coupled to the antenna, and is in fact now part of the antenna being measured by my meter.

And so, I diverge slightly in my discussion and present this graph:

This graph shows that the human body can be a pretty nifty antenna. The upper right triangle shows that the body is working very well as a receiver antenna @ 90 mhz, which isn't too far from our aviation band. But this is a bunny trail. I credit the following article:

 Evaluation of Propagation Characteristics Using the Human Body as an Antenna Jingzhen Li 1, Zedong Nie 1,* ID, Yuhang Liu 1, Lei Wang 1 1 ID and Yang Hao2

The next antenna I wanted to analyze was a foldable Abbree antenna, designed for aviation use. With an unfolded length of 49", and a folded length of 18", it promises good reception combined with portability.

I hooked it up to the meter, and also used 2' square metal as a ground plane. It produced this graph, with apologies for the blurry focus:

The VSWR is excellent from 128 to 138 mhz (right side of screen), but approaches a ratio of 10 on the lower frequencies. Not great. Hmmm.

Conclusions on antennas, so far:

  1. My favorite antenna is my roof mounted J-Pole. It is ideal for long distance reception of aviation frequencies around 133 mhz.

  2. The car top antenna which my friend loaned to me is also excellent.

  3. Smaller antennas are more difficult for use as full bandwidth receivers.

  4. There's a better short antenna out there, I need to find it.

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