8ft stainless whip.....

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB4MNG, Sep 5, 2017.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I came across a forum where some guys mentioned they used strictly a 8 ft stainless whip mounted on a spring/ball as a mobile antenna. This was used on 40 with a very good antenna tuner and they had success. While I realize this is a compromise big time, just wondering if anyone has used this type of set up...
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    This can work only if the tuner is right next to the ball mount/antenna feedpoint and the interconnection from tuner to ball mount is a short piece of unshielded wire (not coax). The tuner needs an excellent electrical bond to the sheet metal of the vehicle as well.

    This won't work if the tuner is connected to the whip via coax, and you want the tuner right next to the base of the whip.

    I did this with an SGC230 remote tuner over 25 years ago, using a 9' SS CB whip on a Motorola ball mount on the fender of my Volvo station wagon. The tuner was bolted directly to the same sheet metal surface and the interconnection was about 3" of high voltage insulated hookup wire (not shielded). It worked okay. I found using any length of coax between the tuner and the whip, even inches, destroyed it all. Need HV hookup wire and a well insulated ball mount that can handle some serious voltage.:)
     
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  3. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    As WB2WIK mentions, it does work. In fact, it is how most HF radios
    in aircraft are based, a random wire and a tuner at the entry point.

    I would go with this route as well if I ran HF mobile.
     
    NH7RO likes this.
  4. KW1K

    KW1K Ham Member QRZ Page

    I used a 102" whip [Rat Shack] with a Hustler ball mount and an Icom AH-4 tuner mounted in trunk for years; worked great on all bands 40 - 10, including multi band DXCC. Never had an issue.
     
    NH7RO likes this.
  5. W5DXP

    W5DXP Ham Member QRZ Page

    A little math will show why that antenna must be connected directly to the tuner output terminals. EZNEC sez the feedpoint impedance of a 102" whip on 7.15 MHz is 1.5-j1042 ohms. Assuming 10 ohms of ground loss, the tuner must output 3 amps into an impedance of 11.5-j1042. That's more than 3000 volts. As Steve said, that will destroy most common coax.

    Here's a data point: The above configuration is ~14 dB down from a good screwdriver on 80m and the screwdriver is ~8dB down from a good dipole. That's about -24 dBD or about 0.4% efficient compared to a dipole.
     
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  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Nothing like a good screwdriver!
    upload_2017-9-5_11-18-18.jpeg
     
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  7. VK3YE

    VK3YE Ham Member QRZ Page

    I did similar experiments (but with 16ft/5m instead of 8ft/2.5m) when developing an effective antenna for pedestrian mobile (link below).

    The requirement is that it had to be multiband with a suitable impedance to be easily matched with a simple L-match right at the antenna base.

    A straight 5m was a full sized quarter wavelength on 14 MHz. It could also easily be matched on 10 MHz (approx 3/16 wavelength).

    I tried matching it on 7 MHz but I found that a centre loading coil was required. However results were excellent. 3.5 MHz was also possible but
    with poor performance. But then I was using low power on a band which is more suited to a low dipole for NVIS than a vertical.

    If we apply this logic to 8ft, we get operation without a loading coil for 15/12/10m. 17m might be doable without a loading coil. 14 MHz might need a loading coil but results should be good.

    Down at 7 MHz bandwidth will be tighter due to its electrically short length though a coupler right at the base could partly mitigate this. After all people have successfully used 8ft whips for 3.5 MHz mobile provided they're using 100w to compensate for lower efficiency.

    http://home.alphalink.com.au/~parkerp/projects/projwade.htm
     
  8. KB4MNG

    KB4MNG Ham Member QRZ Page

    Ok, now I'm getting the full picture. I read several combinations with the whip and mentioned tuner. Had no idea it had to be in that configuration.
     
  9. AI3V

    AI3V Ham Member QRZ Page

    Then the 5000v rms rating of rg213 will be more than adequate.

    Now just what toy autotuner can handle 3kv.......

    Rege
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    One problem with using coax, besides the BV rating, is the shunt capacitance it adds. A very short whip like this already has enormous capacitive reactance; adding coax to connect it just makes that worse.

    Since the tuner really needs to be very close to the antenna feedpoint anyway, no advantage to using coax when unshielded wire works fine, and if a couple of inches in the trunk is also radiating and part of the antenna, it doesn't really matter: The "outside" part of the antenna does almost all the radiating.
     
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