There is no such thing as a full-band mobile antenna on VHF!

There is no such thing as a full-band mobile quarter-wave antenna on VHF. Anyone who is trying to sell you this idea is being dishonest (or doesn't know what they are talking about).

For example you may be told (or shown) VHF 136-174MHz. In reality, the antenna would need to be tuned for a particular center frequency within this band and will only cover approximately 5MHz above and below that frequency. For example, an antenna like this would need tuned on 160MHz and would give a VSWR of less than 1:2 between 155 and 165MHz.

There may be specialist antennas out there that do indeed cover the VHF entire band, but performance (i.e. gain, radiation pattern, VSWR curve etc.) is significantly compromised to achieve this.

5/8 wavelength antennas may have slightly better badwidth but this is being masked by the loading coil. Yes, 5/8 wavelength antennas have got more gain but I'm only considering the quarter wave.

The Discone antenna may cover a few hundred MHz but would not be practical for mobile applications so I've excluded this.

Having a VSWR of less than 2:1 (or as close as possible to 1:1 on all transmit frequencies) is important, as a high VSWR will have a negative impact on radio performance and reliability.

The specifications, for the majority of Motorola's quarter wave mobile antennas, state that they only cover specific portions of the VHF band, because that is the inherit properties of a quarter wave antenna, and because these antennas require no tuning.

UHF (and upwards) is a little easier: that is because the wavelength doesn't change that much across the band (0,1m versus 0,53m) so the physics of making broadband antennas at those frequencies are a little easier in this sense.

8 comments:

  1. Does this mean all 1/4 wave VHF antennas need to be cut? And if so does cutting the ball off the end cause any problems?

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    Replies
    1. The Motorola mobile antennas (e.g. RAD4198A thru RAD4201A) do not require tuning and will cover the portion of the band for which they were intended (e.g. RAD4201A will cover 162-174MHz).

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  2. Some manufacturers will claim that their mobile quarter-wave covers the entire VHF band - this is simply not true and their antenna is in reality no better (in terms of operating bandwidth) than similar ones from Motorola.

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  3. When I say 0,1m versus 0,53m, I mean the change of wavelength across the two bands:

    λ at 136MHz ≈ 2,2m
    λ at 174MHz ≈ 1,7m
    Delta = 0,53

    λ at 403MHz ≈ 0,7m
    λ at 174MHz ≈ 0,6m
    Delta = 0,1

    ReplyDelete
  4. The Kathrein mobile antenna for VHF wide frequency coverage is known for decades, and it does a good job. SWR is overrated, a few 100mW loss do no harm, and this antenna with its match box has been the standard on uncountable local transport vehicles, buses, metros, trams, until the shark fin antennas became common.

    By the way, Motorola did the same. Look at the matching circuitry of MX3000, GP300 or GP340 radios. This is way those radios perform that great, compared to many 50 Ohms connector radios.

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  5. does anyone knows CPS setting in mototrbo for XIRP8668I to Configure radio device to send data to platform on port 6008. or any other desired port for GPS tracking, also do i need to connect my radio with mobile base or repeater for this or i can do it directly from my radio (XIRP868I).

    any help will be appreciated.

    BR//
    Vikasdadhwal@ebizsolutionsindia.com
    8448609855

    ReplyDelete
    Replies
    1. https://cwh050.blogspot.de/2015/08/radio-network-parameters-in-cps.html

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    2. https://cwh050.blogspot.de/2012/10/mototrbo-gps-in-nutshell.html

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