Portable antenna 101:
Any portable radio antenna less than ¼-wavelength in length is nothing more than a radiating dummy load at best. 50 Ohms to keep the radio fat, dumb and happy.
FORMULA: 234/frequency in MHz = length in feet, or 2080/frequency in MHz = length in inches
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For 800 MHz,
2080 / 800 = 2.6 inches = a quarter wave whip.
A quarterwave whip over a proper groundplane exhibits a gain of Ø dB.
1 watt in, 1 watt radiated, life is good!
At 800 MHz, this is the little 3 inch whip with the YELLOW stripe cap you see most common.
At 800 MHz, you can also encounter two other models of longer antenna:
A coaxial ½-wavelength dipole. (this is the yellow cap whip, ~9 inches long, tapers halfway to skinny)
And an end-fed ½-wavelength whip. (this is the whip around 8 inches long, fat at the bottom for a matching coil and skinny at the top)
Both of these ½-wavelength whips exhibit a gain of 2 to 2.5 dB. Yes longer, but it beats not having coverage.
If you are using a helical whip on 800 MHz (1.5 inches or less), the LOSS FACTOR on these is -6 dB to -9 dB. For a shorter length, you are giving up 2/3’s of your transmit and receive range.
(Try explaining that to a Cop, and watch their heads explode !!!)
Broadband portable radio antennas for 700/800 MHz are similar in most cases, for this simple overview…
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Let’s jump down to VHF-Highband and UHF frequencies.
At VHF-HIGHBAND, a quarter wavelength whip is roughly 17 inches, depending on exact frequency, and offers a gain of Ø dB.
For most applications, the standard antenna is the helical whip (aka: rubber ducky) at a length of ~6 inches. Like at 800 MHz, the LOSS FACTOR on these shorter antennas is -6 dB to -9 dB.
Some people even opt for the “stubby” helical whip at VHF-HIGHBAND, and go with a 2 inch model. You can expect a -20 dB LOSS or worse with these.
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At UHF frequencies, a quarter wavelength whip is roughly 6-7 inches, depending on exact frequency and offers a gain of Ø dB
For some applications, the antenna of choice is the helical whip (aka: rubber ducky) at a length of ~3 inches. Like at VHF-HIGHBAND & 800 MHz, the LOSS FACTOR on these is -6 dB to -9 dB.
What is really bad with these antennas is that Public Safety users will be issued a Speaker-Microphone with remote antenna fed with a three-foot section of incredibly lossy RG-174 coax then attach a 3 inch helical whip.
The net result of this is a few watts, fed though seriously lossy coax, then connected to an inefficient helical antenna were only 1/3 of the effective radiated power is getting out.
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And we wonder why users complain about portable coverage?
My humble 2¢ worth....
