ad: SDRKits-1


Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0CIT, Sep 6, 2016.

ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: Left-3
ad: Subscribe
ad: L-MFJ
ad: FBNews-1
  1. 2E0CIT

    2E0CIT Ham Member QRZ Page

    Hi, I want to experiment by building an efficient loop antenna for UHF mobile reception. Whilst I've had fun building a bunch of antennas for HF and have mobile antennas for vhf/uhf ham radio, I would like some advice on building this 'learning project', not for radio, but for digital TV reception whilst mobile in my car! (I have various commercial 'mobile TV antennas' but want to experiment.)
    Compared to the mass of info on the web for HF antennas, I can't find much definitive info on the web for such UHF loops, so in desperation actually had to resort to reading a book! : HC Wright: 'Experimental Antenna Topics' in which he very briefly describes such a UHF antenna for TV reception:
    1. Loop diameter 1/3rd to 1/2 a wavelength, fed at its base by coax.
    2. The loop is connected to, and sits on top of a 1/4 wavelength of coax extending vertically above a metal ground plane which is connected to the coax shield at the point the coax emerges from the ground plane.
    Wright states that all dimensions are approximate and one has to experiment...... but life is short and I would appreciate input from experienced hams about accurate dimensions.
    UK digital TV has a centre frequency of 525 MHz, it's overwhelmingly horizontally polarised except in small 'shadow areas' served by low power transmitters. My trials so far show that distortion due to multipath whilst moving is not too much of a problem and reception can be good.
    I imagine loops follow the same rules across the radio spectrum but as mentioned, most specific info, matching, etc, is about HF loops above normal ground.
    Would choosing 1/3rd or 1/2 diameter be better?
    Would velocity factor have to be calculated into the '1/4 wavelength of coax?
    Would a 1/3rd wavelength dia loop have 100 ohms impedance and therefore more or less match the 75 ohm feed to the tv box?
    Would it have H polarisation?
    What else do I need to consider?

    Not having a UHF analyser or any modelling software within my ability to use it I would maybe build one for 70cm, test it with swr meter, etc, and then build one scaled-down for the TV frequency but I'm really interested to get some advice on this project. ( AND NO!.... I WON'T be watching Judge Judy on TV while I'm driving, in case I then have to meet her in real life! will be a back-seat TV for the family.)
    Thanks, 73 Jeff, 2E0CIT
    Last edited: Sep 6, 2016

Share This Page