How to connect balanced-line ant. tuner to random-length pole + radials using coax?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4VDI, Mar 14, 2021.

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  1. N4VDI

    N4VDI Ham Member QRZ Page

    Scenario:
    • Repurposed vertical metal pole, reasonably insulated from the ground and raised about a foot above it, that's approximately 22 feet long... probably a little too long to use directly for 20 meters (14.07-14.08MHz in particular) without a tuner or loading coil.
    • Limited radials... one north-south pair 180 degrees apart that could conceivably be up to 40 feet long each way, plus a few more non-symmetric ne-sw and se-nw radials that are 5-10 feet long (5-7 feet on the western side, 5-10 feet on the eastern side).
    • Yaesu FT-1000MP transceiver with built-in antenna tuner
    • Palstar BT1500A balanced-line antenna tuner, modified to use a pair of 75-ohm RG11 coax cables as the transmission line.
    OK, now assume that I'm standing by the new antenna... I have one terminal screw for the transmission line's hot wire, another terminal screw connected to the radials & ground pole for the transmission line's ground, and I'm holding the pair of 75 ohm coax cables being used to carry the balanced signal in my hand. Just what have I gotten myself into, and what do I need to do to transform the ends of those two cables into something I can connect to the pair of aforementioned terminal screws?

    Do I just need to do something like:
    • Connect the braids together, and possibly to ground
    • connect one cable's center conductor to the pole's terminal screw
    • connect the other cable's center conductor to the radial/ground terminal screw
    ... or do I need something more complicated, that possibly involves making coils, a balun/unun, etc?

    Now, consider a potentially simpler alternative... just running a RG-8U cable from the radio to the same pair of terminals, connecting its center directly to the pole's terminal screw, and connecting its braid directly to the radial/ground terminal screw, and letting the FT-1000MP's internal automatic antenna tuner sort things out. Is it actually up to the task, or would a ~22 foot pole be too detuned and require that I make something like a loading coil to get it closer to what it needs to be for the FT-1000MP's tuner to finish the job?

    Put another way... is the Palstar tuner a golden opportunity dropped into my lap, or a ghastly complication that would needlessly make the task a lot harder to pull off?
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Why dont you just connect a short jumper wire from one of the tuner's balanced outputs to the chassis grounding screw, and then use the other balanced output to feed a coax cable that runs to the vertical, either 50 or 70 Ohm, the shield of which is also connected to the tuner's grounding screw?

    There is absolutely no reason to "export" the balanced output all the way to antenna base just to "ground" one side of the "balanced pair" at the antenna. You might as well "unbalance" the tuner right at the tuner... Bottom line is that this tuner is not ideal for this job.

    The tuner built-into your transceiver probably doesn't have enough range to "sort out" your non-resonant vertical.

    ps - a 22ft vertical standing on just two diametrically opposite 40ft radials, elevated 1ft has a feedpoint impedance of 198 - j85 Ohms at 14.1MHz. If fed with 50 Ohm coax, that puts a Swr of ~4.7 on the coax.

    Can your in-built tuner handle a Swr of 4? Another idea would be to put a 1:4 transformer (50:200 Ohm unun) at the base of the vertical. That would guarantee that the in-built tuner could handle it with ease, or you could even leave the tuner turned off.
     
    Last edited: Mar 14, 2021
  3. K8JD

    K8JD Ham Member QRZ Page

    you got a balanced line antenna tuner to take advantge of balanced line's low loss potential, why not used a balanced antenna again ?
     
    K1LKP likes this.
  4. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

    JOHN,

    CONGRATS,
    ===
    U

    HITTING THE-nail_on_the_head NEW.jpg

    PS. HOW IS THAT RASCAL UMBRA DOING ?
     
  5. WA9UAA

    WA9UAA Ham Member QRZ Page

    The internals of the tuner expect to see a balanced antenna, put a 1:1 balun on the output of the tuner. Connect the balanced terminals of the tuner and balun together and use the coax connection on the balun box for the coax.
    73,
    Rob
     
  6. N4VDI

    N4VDI Ham Member QRZ Page

    Mostly, because an unused ~22 foot vertical pole is what I have at my disposal to repurpose as a quick & dirty 20-meter antenna.

    I'm actually rebuilding my dad's antenna farm. His former antennas were destroyed by Hurricane Irma. Between age (80), demands made on his time by my mom's declining health, the poor recent state of HF propagation (esp a year or two ago), and a degree of nihilism arising from the destruction of his antennas every few years by hurricanes, he's been off the air since then. I'm staying with them for a few weeks while my mom recovers from hip surgery, and have taken it upon myself to get him back on the air again and put in some quality father-son bonding time.

    I built him a cute triband antenna last week (30m+40m+80m) using a 5' mast and trio of Hustler mobile radiators, but he still misses being able to work 20m (historically, his favorite band). The 22-foot pole in question was formerly his tilt-up mast... except the roofers screwed up after Irma, and the new gutters are a little too wide and get in the way of tilting it down, so it's now basically useless for anything besides being a 22 foot vertical metal pole.

    Ergo, the reason why I have all this uber-expensive gear at my disposal that I don't really know how to use (or even why much of it is useful), but a minimal antenna situation.

    I can't ask HIM too much about it in advance, because he's overwhelmed with my mom at this point. The key to my success so far has been to get everything done as little mini-projects that I can start and be far enough along by the time he notices to decide that he might as well let me finish it.

    Insofar as cabling goes, there's already a pair of 75-ohm RG11 cables and a 50-ohm RG8u cable running from the radio through the wall to the pole I'm planning to use as a 20m antenna. I just don't know what needs to be done at the antenna end if I use the pair of cables with the balanced tuner.

    In theory, there are also four poles about 6' high 75 feet apart (two at the rear corners of the lot, plus two more about halfway back) that were formerly used to anchor guy wires. I suppose I could use them to make a straight dipole up to 75' long, a "L" shaped dipole up to 150 feet long, or a "U" shaped dipole up to 225 feet long... but from what I've read, a dipole there would kind of suck due to being so low (and the rear portion is near and parallel to power lines, so it would pick up noise from them).
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I downloaded the manual and looked at the schematic of the tuner. I see no reason that yet another balun is needed between the tuner and the coax as long as you ground the other side. It is, after all a "floating" tuner....

    Jeff, since you have two coax cables running to the antenna, just use one of them...

    Minor nit: When running two parallel coaxes to a balanced load, the total length of paralleled coaxes should be the minimum possible, usually just a few feet to get from the tuner through the building wall, at which point you should convert to window line or open-wire line. Running paralleled coaxes for tens of feet completely negates any loss advantage that parallel feeders might have brought to the party; you might as well as have run a single coax in the first place...
     

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