Low-gain antennas can be better than high-gain antennas in urban areas

Using a mag-mount antenna on a car roof as well as a trunk-lip mount, I compared the signal strength of the constant carrier on UHF (460 MHz) from a McDonald’s drive-thru system. The McDonald’s drive thru intercom leaves a constant carrier on the air, that even to a base station is only audible for maybe 2 km. The McDonald’s intercoms are typically licensed in the 460-469 MHz range, 100 mW ERP. I did this on a bit of a whim, and so I didn’t convert my AGC measurements (voltage proportional to signal strength) from my Kenwood TK-805 to dBm yet.

Test 1: Urban obstruction UHF diffraction

My test method was to pull up next to a long building two stories tall, so that I would tend to get signal from over the building without nearly as much signal from the sides of the building. Because I was about a kilometer from the transmitter, the building absorption would eliminate the remaining direct-path signal. That is, I should mostly be getting signal from over top the building.

I was a bit delighted to see that for every distance from the building up till I reached the end of the parking lot, the 1/4 wavelength antenna did at least as well, for the roof-mounted location. The 1/4 wave antenna on the roof did better than the 5/8 wave trunk lip mount, because the radiation center of the 1/4 wave antenna on the roof was higher and had an evenly surrounding groundplane.

Test 2: Rural UHF 5/8 wave vs 1/4 wave

Because I didn’t have a means to log the signal from the TK-805, the testing was subjective. Going down a straight road from the McDonald’s, with little nearby obstructions, the 5/8 wave antenna on the roof performed a little better; it filled in the “holes” of the picket-fencing signal a little bit more than the 1/4 wave antenna on the roof.

Future test

Log AGC voltage vs. time with ADC. A voltage-to-frequency converter would allow recording this signal on a laptop sound card, sampled at several kHz (sample rate-limited perhaps by the AGC time constant). Drive at constant speed, with noted start/stop landmarks in lieu of GPS tagging of measurements.

Convert AGC voltage to absolute RX dBm (may be slightly frequency dependent).

Conclusions

Despite lack of proper metrology, I feel more confident about recommending 1/4 wave antennas to customers who may have overhead height limitations (garage door) and especially for those who work in forested or urban areas. In flat rural areas, it is of course expected that a 5/8 wavelength antenna on the roof will out-perform the 1/4 wavelength antenna on the roof by a couple dB, by the design of the antenna.

Why then are high gain antennas so popular on 800/900 MHz, in locales where you see 1/4 wavelength antennas at VHF and UHF? Because at 800 MHz the reflections from buildings and objects is stronger than at VHF and UHF. The thought at 800 MHz is to take advantage of multipath with dual receive antennas–even on advanced SMR sites, not just cellular. I see this effect at 800 MHz even with yagi antennas–sometimes pointing the yagi off-boresight to the tower gives a better signal, where there is not a line of sight path (NLOS).

Leave a Comment