Received Signal Strength Expectations using a Whip Antenna with an Un-UnAuthor: R.J.Edwards G4FGQ © 26th January 2004
An un-un is an unbalanced-to-unbalanced transformer. In this program it is assumed to be 100% efficient. The un-un impedance ratio is selectable, either 1:1 or 9:1 with impedance step-down from antenna to coax line. No other means of matching the antenna to the coax line impedance is used.
This program roughly analyses the performance of a vertical antenna + un-un + a coaxial line + an optional preselector + a vague type of receiver. The antenna may also be sloping, or of inverted-L type, or vehicle-mounted. Total length of antenna is limited automatically to 1/3rd wavelength.
A preselector consists of a single tuned circuit which may provide an element of impedance matching to maximise signals at the receiver input.
For given field strength in the vicinity of the antenna the voltage is computed at the coax input, coax output, and at the receiver input. The only accurate calculations are at the ends of the coax when the receiver is a communications type with a well-defined input resistance. As a guide, when the input to a 50-ohm receiver is 50 microvolts then the signal strength S-meter reads S9.
Circuit impedances used in calculations are displayed for interest. The type of coaxial line used is assumed to be small-diameter like RG-58. Low capacitance, 70-100 ohm types are preferred but the difference in performance is negligible. A short unscreened wire performs well except that it may pick up local noise.
As expected, on average, antennas short compared with a wavelength result in reduced receiver signal volts but this may be obscured by resonances between the antenna, coax line and the uncertain line terminating impedance.
When antenna height exceeds about 1/5 wavelengths, receiver input volts becomes independent of height and length. So this program is useable to crudely predict signal strengths received on "random long wire" antennas. Coaxial line lengths longer than a few metres are not critical and quite long lengths can be used.
Signal usability depends on signal/noise ratios in the vicinity of the antenna. The following comments apply to a speech-frequency receiver bandwidth.
Man-made noise levels vary greatly with frequency and environment. From several hundred microvolts per metre at VLF in city centres down to 5uV/metre at 2 MHz in open city suburbs. Above 10 MHz, in sparsely populated rural regions, noise falls to atmospheric static levels at about 0.1-1.0 uV/metre depending on day or night, season of the year, climate, and geographic latitude.
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