Mobile Antenna Gain – Part 1

When choosing a mobile antenna, is more gain better? Maybe, though practically, radiation efficiency and radiation pattern are probably more important. But this is about antenna gain.

Let’s start with ISED regulations in SRSP-500

6.3 Radiated Power Limits

6.3.1 Mobile Systems

Normally, the maximum power for base stations shall be 125 watts Effective Radiated Power (ERP) and

30 watts ERP for mobile stations. However, in all cases, the ERP shall be limited to that which is

necessary to providing a balanced radio system (mobile talk-back capability) over the desired service

area or the required level of service as governed by system requirements.

Let’s grind through some theory.

Measuring radiated power in a real-world situation is not practical. It requires measuring the RF field around the vehicle the antenna is mounted on. Where the antenna is mounted, the type of vehicle, etc., all influence the results. We (and ISED) need a simpler method to approximate it. Calculated ERP.

Starting with our goal of 30 watts ERP or 44.8dBm.

Let’s assume you have a 50W VHF radio operating at 155MHz. That rounds out to 47.0dBm.

The installation consists of an NMO mount on a fender bracket with 17ft of RG58. Your cable loss at 155 MHz is 1.1dB and add another 0.1dB loss for the connector. The loss of the NMO assembly is rarely stated, but assuming it’s similar to a coaxial connector, we’ll add another 0.1dB of loss. That gives us a total of 1.3dB loss.

Doing the quick math: 47.0dBm – 1.3dB loss = 45.7dBm available to the antenna.

If you use a 1/4wave antenna (0dBd gain) cut to be a perfect match at 155MHz, that works out to about 37watts ERP. Practically, that’s not enough of a discrepancy to be a concern, especially considering the less than ideal efficiency of a fender mount (or even a roof mount), and chances are that your antenna is not a perfect match. Too long or too short, the antenna becomes reactive and reflects some power back to the radio. Chances are, your real ERP is less than the regulation 30watts.

But what if you chose to use a longer antenna? 1/2wave and 5/8wave antennas will give you between 2 and 3dBd. Some manufacturers make claims of higher gain. On occasion, they will even list gain in dBi (dB over isotropic). For VHF and UHF systems, we use dBd (dB over dipole). 0dBd = 2.15dBi. Chances are if you see a “5dB” gain antenna, it’s 5dBi or a nominal 3dBd gain antenna.

With that in mind, switching to a 3dBd gain antenna: 47.0dBm (transmit power) – 1.3dB loss + 3dBd antenna gain = 48.7dBm or about 74W ERP.

Technically, that is in contravention of ISED regulations. To keep things legal, you would have to reduce your radio output power to a nominal [ working backward 44.8dBm target ERP + 1.3dB – 3dBd = 43.1dBm ] 20W tx power.

If you want to skip the theory…start here

As a rule of thumb, if your VHF radio installation requirements favour a longer antenna with gain, you should probably reduce your transmit power to the 20 Watt range. If you are running a 1/4 wave antenna, running 40-50 watts will be fine in most instances.

Caution: If the mobile will be operated in a metropolitan area, especially in the international coordination zone, there is less regulatory latitude. In some instances, ISED may require you to maintain a lower ERP as a condition of your license.

So why bother with an antenna with gain? Think ‘system’.

There are several practical reasons to use an antenna with gain. The most important is that gain also works in receive mode.

Consider a signal between two identical analog radios with 0.25uV sensitivity (12dB sinad). That works out to about -119dBm.

Scenario #1: 50W TX Power into a 1/4 wave with 0dBd gain and 1.3dB of losses. Our ERP is 45.7dBm. That gives up a maximum propagation path loss of 45.7dBm – (-119dBm) = 164.7dB

Scenario #2: 20W TX Power into a 5/8 wave with 3dBd gain and 1.3dB of losses. Our ERP is still nominally 45.7dBm. That gives up a maximum propagation path loss of 45.7dBm – (-119dBm) + 3dBd rx antenna gain = 167.7dB.

The 3dB of additional margin isn’t much, but on a lonely back road, it could make all the difference. On top of that, you reduce stress on your radio and reduce current consumption.

There is another important factor in choosing the antenna, one that is easier to visualize than calculate. A 1/4 wave antenna mounted in the center of the roof with no obstructions will radiate a signal very differently from the same antenna mounted on a fender bracket. The lopsided ground plane skews the radiation pattern toward the mass of the vehicle. However, that mass, namely the cab inhibits the signal in as much as a 90deg arc. It will still work, but the path loss may be substantially higher. In a well-designed repeater-based system, it may not matter. Ripping down a logging road, it could make all the difference. Having an antenna that extends above the cab doesn’t solve the problem, but it helps.

The two-way radio world is one of compromise. Nowhere is this as true as in-vehicle antenna installations. We’ve only scratched the surface of antenna radiation pattern issues and all but ignore the matter of radiation efficiency. In practice, the difference between a longer ‘gain’ antenna and a 1/4 wave is minimal in most situations. Reliable communications systems have a built-in margin for fade, equipment degradation, and installation compromise. Of course, that’s not always possible. My rule of thumb? In the bush, save your radio, use a bigger antenna when practical.

More on radiation patterns and efficiency in part 2.