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Heat shrink & Antenna Propagation

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Rickyrb101

Rickyrb101

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Hi just a brief query, does anyone know if the propagation range of a helical antenna would be adversly effected by the addition of heat-shrink for protection from a pcb??

Im having a reduction in received power and think that maybe having an insulator semi-between the coils is causing this but my antenna theory is not too hot atm and im not sure of the theory to investigate this however practically it seems to cause 5-10dBm reduction from my test point.

Thanks for you interest!!!
 

Beware of black. Not always, but color black and resistive carbon are common partners.

A quick test: Place a piece of shrink tubing in microwave oven. Run it for a few seconds to see if it gets warm. Progressively increase microwave oven ON time to see if it gets hot.

Microwave oven is 2.4 GHz but it is good general test for RF dissipative properties.
 
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Most heat shrinking tubes are either made from PE or PVC plastic material. They come in different colours, and the black one can be expected to be coloured by dyes rather than carbon. There are possibly special conductive types for ESD protection, but not commonly used. PE is a perfect RF dielectricum, also used in coaxial cables, PVC is only good for low frequency.
 
From my experience the most troublesome color for EM is red, due to contain of iron oxides.
Anyway, 10dB reduction only from color type looks too much for me. Be sure that the heat-shrink didn't affect slightly the dimensions of the antenna. Or changed the initial distance from the PCB, where the antenna was tuned.
 
My experience with small helicals is that they are very sensitive to the winding spacing/shape. If you distort it, it moves off frequency. Are you simply mesuring gain at 1 frequency, or are you doing a swept measurment. Maybe the heatshrink added no loss, but moved the resonant frequency down.
 
Just remember any antenna covered by any sort of plastic/fibreglass radome is going to detune that antenna.
increased capacitive dielectric (antenna to air) increase in capacitance = decrease in resonant freq of the antenna
so the antenna (element/s) will need to be shortened to bring the antenna back into resonance on the required freq.


Dave
 
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