crazy idea for 450 ohm ladder line....

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4RRX, Feb 17, 2016.

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  1. K4RRX

    K4RRX Ham Member QRZ Page

    Dear Forum readers / contributors.....

    I have an interesting, yet possibly crazy, idea for using 450 ohm ladder line and would like to poll the expertise of amateurs in the forums here.

    Preface: I have been licensed for only 4 + years and have only been "active" for about 1 of those years. I have done a little experimenting with wire antennas, home made verticals, feed line, and such. I have a fair amount of failures that have lead to enlightening successes with antennas which gives me more confidence to try new (to me) things in the realm of antennas.

    Here comes the crazy idea.....
    I am currently tip toeing back into the radio world and have become very interested in vhf packet mode. I have a Raspberry Pi, TNC-PI, and an older Icom IC-228H radio. This is connected to a rather disgraceful piece of RG-8X to a home built DBJ-2 design vertical vhf antenna about 55' in the air, supported by rope on a tree limb.

    I have recently come across some short (20'-40') pieces of LMR-600 but don't want to use too many "splice" connectors if I can avoid it. However, I have plenty of 450 ohm ladder line and was curious how to implement it's use with my current antenna. I know it's possible to merely solder a PL-259 or N type connector on the antenna end of the ladder line, but am curious as to the dangers of doing the same on the radio end.

    I understand that 50 ohms to 450 ohms is an issue and can usually be resolved with a balun on the antenna end, or by means of a tuner with a balun built in. Yet, I can find no definitive information on either a vhf tuner with such termination, nor the possible high current dangers with using a standard balun right at the radio.

    So, do any of you fine folks out there have any helpful information about my current crazy idea? I am looking for mainly safety issues because experimenting can usually answer the functionality questions in most things amateur radio.

    Thank you all in advance!
     
  2. W1VT

    W1VT Ham Member QRZ Page

    A half wave coaxial balun made out of 75 or 50 ohm coax will get your from 50 to 200 ohms balanced. A quarter wave matching section of 300 ohm line will transform that to 450 ohms. As a bonus that may also work on 70 cm--the 3rd harmonic.

    If you really want to be clever you can lower the impedance of 450 ohm line by narrowing the spacing with a wood jig and a propane torch.

    Zack W1VT
     
  3. K4RRX

    K4RRX Ham Member QRZ Page


    OK, I think I understand what you're getting at. Basically, this would be a choke with a matching section to cover both bases with one build.... I can definitely handle that. I will look up some more info from the ARRL book about matching sections and go with that. Ladder line has such little loss (dB wise) that it would be a shame not to use it....
     
  4. W1VT

    W1VT Ham Member QRZ Page

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Or...you could cut the ladder line to be an even integer of 1/2 wavelengths long, electrically, for the frequency you'll use and then just use a simple 1 to 1 balun at the transmitter end.

    1/2 WL in most 450 Ohm window line I've tested, which is really closer to 400 Ohms, is 38 inches at the center of the 2m ham band. That can vary a bit, and of course if your antenna is up 55 feet, plus some more line to get it to the rig, you might need 22 or 23 half wavelengths of line to reach. But 1/2-WL of any impedance transmission line replicates its termination impedance at the source, so if carefully adjusted for length, you don't need any impedance transformation at all.

    I wouldn't use the big LMR cable for this, especially without support. That cable is large and heavy enough it's normally used on towers and secured to the tower leg...it won't self-support, hanging in free space, terribly well and was never designed for that.

    Remember that using balanced line you need to keep it away from conductors from one end to the other, and its loss will increase when it gets wet, more so if it gets ice coated at which point it can have more loss than coax.

    For a run of less than a couple of hundred feet, I wouldn't bother with it; but for really long runs, or for HF applications where its terminating mismatch might be very high and likely feeding a higher-impedance antenna, it has advantages.

    100' of RG8X on 2m usually only has about 4 to 4.5 dB loss, which really isn't that much.
     
  6. AA7QQ

    AA7QQ Ham Member QRZ Page

    If you can alter the antenna for 75 ohm impedance, you can use RG6 CATV coax. At the radio end, you can just deal with the 1.5:1 mismatch or build a balun for it to change 75-50 ohms. RG6 quad shield is cheap & rather low loss.

    Ed
     
  7. KH6AQ

    KH6AQ Ham Member QRZ Page

    As you've seen there are many ways to skin this cat. Here's another way to go from 50 ohms unbalanced to 450 ohms balanced:

    1. A halfwave balun made of 50 ohm coax to convert 50 ohms unbalanced to 200 ohms balanced. RG-58 can be used.
    2. A 1/4 wavelength of 300 ohm line to convert 200 ohms balanced to 450 ohms balanced. 300 ohm TV twin lead can be used.

    One of these 50 ohm unbalanced to 450 ohm balanced devices is used at both ends of the 450 ohm ladder line.
     
  8. K4RRX

    K4RRX Ham Member QRZ Page

    Wow!
    First, I am thoroughly pleased and impressed with how deep this rabbit hole of knowledge goes on the QRZ forums! Thank you all who have replied to my post.
    Second, I have taken all of this knowledge and logged it in my notebook that I keep for antenna building so it can be used later instead of forgotten.
    Third, I will start my experimenting this weekend as it looks to be fair weather in the forecast and I will report my findings immediately after.
    This website, sure does put elmer-ing on a whole new pedestal!
     
    AA7QQ likes this.
  9. K4RRX

    K4RRX Ham Member QRZ Page

    OK, here goes the results.....
    First of all, thanks to everyone who lent me their knowledge, it is priceless
    Second of all, the results of using ladder line for VHF did not work for me at all.

    Problems in list form:
    1. Matching at the antenna end was quite simple, but the radio end proved to be disturbingly finicky
    2. Safety Safety Safety! If the antenna and ladder line aren't matched just right, which in my case it wasn't the first 7 times I tried, please wear welding gloves! I am absolutely useless when it comes to antenna modeling software and my math is sometimes off, so high current at radio end on two occasions caused quite the scare!
    3. Just because this didn't work out IN MY SCENARIO, doesn't mean it can't work with different conditions.
    4. I will keep these notes and results in my notebook for use at another location just to try and see what happens. Like field day maybe!

    Just for reference, after the matching "devices" were in place at each end of the 450 ohm feed line, SWR according to my trusty dusty Bird meter showed 2.2:1 at 144.0 mhz, 1.9:1 at 146.0 mhz, and 3.7:1 at 148.0 mhz. I think if I had some clamp on beads somewhere in the system testing would have yielded different results too.

    I enjoyed this experiment very much, and learned quite a few things. I sure hope other new amateurs find this information useful and can expound on it with there own "crazy idea" for a weekend with amateur radio!!!!
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Hams have been experimenters since the earliest days and the old saying, 'If it was a good idea, everybody would be doing it' comes to mind.
     

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