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Ways to measure frequency response

 
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Lord Phat
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Joined: 30 May 2015
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Location: Philadelphia, PA

PostPosted: Sat May 30, 2015 3:48 pm    Post subject: Ways to measure frequency response Reply with quote

Hi, I am building a fractal antenna for ham use and I have modeled it in 4nec2. Frequency response pattern looks interesting and useful but I want to validate it experimentally.

Is there a way to do this myself with a frequency generator or do I have to use some kind of lab service? Thanks.
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helix
Antenna Theory Regular


Joined: 29 Jan 2015
Posts: 64

PostPosted: Sun May 31, 2015 2:16 pm    Post subject: Reply with quote

Fractal designs are mostly associated with electrically small antennas. Is your design electrically small (less than about 0.3 wavelengths in extent)?

Also "frequency response" and "pattern" are distinct concepts. For an electrically small antenna, the pattern is generally similar to a dipole/torroid/donut due to theoretical constraints, and there is usually no great need to confirm this.

For an electrically small antenna, the frequency response is dominated by the antenna input impedance, which changes very rapidly with frequency, producing significant mismatch loss outside of a narrow bandwidth. This can be characterized with a vector network analyzer, a VSWR meter, or a slotted line.

The only other issue impacting realized gain is Ohmic efficiency, which for an electric dipole will generally be high, say greater than 90%. Modeling will give you an idea what it is. To measure this experimentally you can use a Wheeler cap, but you will need an accurate way to measure the input impedance in this case (see above).


Or, you can send your antenna off to a lab and they will characterize it for you, but this is not likely an option for you due to cost.
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