2 meter antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KM4ZLO, Oct 30, 2018.

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  1. KM4ZLO

    KM4ZLO Ham Member QRZ Page

    i am still new to all of this so hope this question does not sound stupid, but if i have a 2 meter antenna can i transmit on 70cm using that same antenna.
     
    KX4O likes this.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Depends on exactly what antenna it is.

    What is the antenna, specifically?
     
  3. AG6QR

    AG6QR Subscriber QRZ Page

    Since the 440 band is 3 times the frequency of the 2m band, they are harmonically related and a basic antenna resonant on 2m will usually be resonant, or nearly so, on 440. This applies to simple antennas like dipoles, ground planes, J-poles and similar. It does not necesarily apply to antennas with loading coils or similar devices to change the frequency response.

    But resonance doesn't mean it'll be effective. A simple half-wave vertical radiator at 2m becomes a 3/2 wave radiator at 440. A 3/2 wave vertical radiator sends most of its energy into lobes angled up at 45 degrees and down at 45 degrees. This probably isn't where you want to send your signal. It will work reasonably well for a short, clear path, though. And it won't harm your transmitter.

    You'll get a much more effective antenna at 440 with something that is designed to radiate toward the horizon. Many dual band antennas have a small coil designed to act as a trap to make the antenna appear shorter on 440.
     
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  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It's imperative to know exactly what the antenna is.
     
  5. KM4ZLO

    KM4ZLO Ham Member QRZ Page

    I HAVE TWO DIFFERENT ANTENNAS THAT HAVE BEEN GIVEN TO ME BY A FRIEND. THE FIRST ONE IS A HOMEBREW 2 METER GROUND PLANE. THE SECONDED IS A OPEK MODEL VH-1210 VHF MAGNETIC MOUNT COMMU ANTENNA WITH FREQ 136-174MHZ.
     
  6. KA0GKT

    KA0GKT Ham Member QRZ Page

    The 1/4-wave ground plane will accept power on 70 cm, but the antenna will be a cloud-warmer with a null toward the horizon.
     
  7. KV4JW

    KV4JW Ham Member QRZ Page

    It's a loaded question. CAN you technically transmit into the 1/4 wave ground plane with *some* result? Yes. The SWR may not be optimal though, and antenna efficiency probably won't be either. Remember, a 50 ohm dummy load is a perfect match, but makes a poor antenna.

    Will it work as good as a designed for the purpose "dual band" antenna? No, it won't. Will it work? Yes, however poorly. Why not build your own 70cm antenna that would work much better? Same basic principle as the home made two meter antenna you have, the lengths of everything are just different. It would be guaranteed to perform better.
     
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    For FM, especially for accessing a repeater, a 2-meter ground plane antenna will work fine on 70 cm. Yes, the angle of radiation is somewhat higher on 70 cm. However, there is approximately a 3 dB gain on 70 cm and the signal still gets into most repeaters at least as well as a 70 cm ground plane if not slightly better.

    Years ago, my junior year at Georgia Tech, when I was working as a technician at the Atlanta, Georgia, Motorola Service Station, one of the major customers was a taxi company operating in the 450 MHz segment. For some reason, we ran out of replacement quarter-wave "stingers" used for the mobile antennas. So, until we got in a replacement shipment, we started installing 150 MHz quarter-wave stingers on those mobile units when they came in with a damaged antenna.

    The base station was located on the highest building in the City of Atlanta. The mobiles transmitted on 1-frequency and the base station transmitted on another frequency with the mobiles not being able to hear each other. It was soon discovered that the mobiles with the 3/4ths wavelength antennas were performing much better than those mobiles with the original quarter-wave antennas. The way the system worked was that the base station would put out a call for a cab near a certain location and the cab with the strongest signal would come through and then get the "fare". Those taxis with the longer antenna were consistently getting the fares even when the driver was farther away from the base station than the other drivers.

    Of course, when the drivers discovered the fact that the longer antenna equipped vehicles were getting more fares, the drivers, with the shorter antennas, started damaging their antennas so that they could come to the radio shop and have the longer antenna installed. When the replacement stock of short "stingers" came in, the owner, of the taxi company, dictated replacing all of those cabs with the longer antennas with the shorter antenna to again put all of the drivers on equal footing.

    Glen, K9STH
     
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