446MHz Dipole and Balun help

Discussion in 'Wireless & RF Design' started by Andrew Leigh, Sep 9, 2013.

  1. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
    92
    2
    Hi,

    did a search but could not find anything.

    I recently I bought 2 Anytone 446MHz radio's for use while hunting. These perfomed well but had limitations which I understood to be aerial related, we were in a very dense area of bush surrounded by mountains.

    So after googling I found an aerial calulator site and have now produced a 1/2 wave dipole aerial from copper water pipe, each leg being 160mm long. From what I understand I need a Balun to balance impedance. Despite numerous searches I cannot seem to locate a very compact balun suitable for the task.

    Firstly do I need a balun with 50ohm coax?
    Secondly, of so, what are the specs (winding dope and ferrite core number) and how small can it be made i.e. can it fit into a 20mm diameter tube?

    Regards
    Andrew
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    A balun is probably not needed in your application. You can use an antenna analyzer to determine how good a match you have between the transmitter and the antenna. Adjusting the length of the dipole can put the Return Loss at its minimum, which means that most of the power should be radiated. Is the dipole vertical or horizontal? What power levels are you running, and how long is your coax?
     
  3. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
    92
    2
    Phew, new terratory for me here.

    Was hoping for the dipole to be horizontal on top of a hunting vehicle roof. Do not anticipate the coax being longer than 3m. Legal power is 0.5W but have programmed one emergency channel at 5W in case of a snake bite, shooting asccident etc. which would in my opinion justify the use of the additional power out in the middle of the sticks.

    Cheers
    Andrew
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    So a vehicle mounted dipole with horizontal polarization radiates best in the direction perpendicular to the axis of the antenna. Who is going to hear you on 446 Mhz with 5 watts. Are these unlicensed Part 15 devices? I think you might do better with a 5/8 wave vertical mounted on your roof. At least it will be omni-directional in case of emergency.

    You could also get a technician license and run 50-60 watts for extra peace of mind, you'd still have the problem of who is listening.
     
  5. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
    92
    2
    Hi,

    As you would have gathered I know zip about transmitting and radios so humour me please.

    The primary purpose of these radios was to communicate no further than 8km's in case something happened on my grandkids on their first hunt. Probably over cautious but that was the reason. I subsequently dscovered that they were very handy for general communication and not just in case of emergencies.

    I had bought a commercial dipole (430 to 470MHz) before we left and mounted it at the lodge at a height of 4m and connected to a 0.5W radio. I only discovered later that my radio was set at 5W (illegal here without a license), however this combination effectively gave a transmitting distance of 3.5 miles. We were transmitting, this in dense bush, and may have been better but that was the furthest we tested at.

    As it is illegal to transmit without a licence above 0.5W I would like to maximise on the antenna's to get the most out of the 0.5W.

    Will check out the 5/8th vertical.
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Andrew,at UHF frequencies the polarisation of the antenna (horizontal or vertical) is quite important,in that so-called "cross polarisation" between the two stations can reduce the received signal considerably.

    The easiest thing as far as fixing the "cross-polarisation" issue is to mount the dipole vertically---it may not be easy mechanically,though!

    Ideally,with your dipole,you should have a balun to go from unbalanced (your coax cable) to balanced (the antenna),but I wouldn't worry much about it.

    Your 446MHz allocation is just above the Amateur Radio "70cm" band,so if you want to try some antennas designed for vertical use,Google "70cm vertical antennas".

    Double check the licencing requirements for your radio,as some Administrations measure the output power as that radiated with a standard antenna,so you may be breaking the rules already---I hope not!

    In Oz,we are luckier,in that we have a UHF CB allocation around 477MHz with an allowed output power of 4 watts,or if you are really "going bush",you can get a HF radio on the VKS Network ( mainly used by 4WD clubs).

    Probably the ultimate,although horribly costly,is a Satphone.
     
  7. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
    92
    2
    This dipole at camp was vertical, we can make the dipoles on the car roof vertical but could snag in the bush. Perhaps a whippy type 70cm aerial will work.

    Will google 70cm aerials as well. Would really like to make an aerial, which will probably end up more expensive than buying one but there is nothing like acquiring new knowledge, understandign it and then putting it into practice.

    Cheers
     
  8. kumarverma97

    New Member

    Sep 6, 2013
    3
    1
    Great read here buddy. Thanks a ton.!!
     
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