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I don't usually get this lucky....

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W7UUU, Sep 15, 2021.

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  1. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Three years ago I put up a doublet fed with 450-Ohm window line. 150 feet long, with the center just shy of 50 feet high. All in the clear - the window line ran at a slope right into the shack to a balanced tuner. It was my only antenna for some time - and did OK on pretty much all bands - it's what I had.

    But I now have a tower with a TA-33MW, a resonant Vee on 80 and 40, with a final Vee coming soon for 30... all cut to frequency and coax fed.

    So what to do with that doublet? It's virtually impossible to change in length or height without just starting all over - it's not on a pully system.

    Turn it into a Marconi "T" antenna! I shorted the ladder line and tied onto the "+" side of a 1:1 current balun. The "-" side of the balun attaches to a DXE radial plate. There's an 8' ground rod at that junction, and a 75' #8 ground wire that runs up to the tower and ties into the tower/shack/power panel "system ground" - all grounds bonded.

    I used my old radial setup from my previous QTH - a big roll of welded chicken wire, spiked to the ground and connected to the DXE plate. I FULLY plan to add a bunch of radials soon - this was just my initial fiddling.

    So imagine my surprise when I swept it with my AA-600 analyzer to see a nice deep, clear dip smack dab centered between 1800 and 1900 KHz... the sweet spot of the 160m band!!

    SWR of the analyzer confirms with the FTDX at roughly 1.5:1 to 1.7:1 across the whole 1800-1900 range (I don't care about above 1900 but it will tune there if needed). Noise right now (02:37 UTC) is decent - S5 to S6 - FAR better than when I was in the city, with noise of S9+10 at all hours. Even the KPA1500 is totally happy, with the internal tuner bypassed.

    What are the odds???

    I found a "Marconi T" calculator online and sure enough, were I to build one, I'd be wanting my dimensions to be pretty much what I have now.

    All I need to do now is add a lot more radials to improve the ground efficiency.

    I'm pretty happy!! Winter 160 contest season is fast approaching and I think I'll be fairly ready.

    I just can't believe I lucked into virtually perfect dimensions right off the bat

    Dave
    W7UUU


    scope reading 160 antenna 2.PNG
     
    WD4ELG, K0UO, KI5WW and 6 others like this.
  2. KE0NSK

    KE0NSK Ham Member QRZ Page

    I'd be extra cautious the next few weeks, your luck may have just run out.
     
    K0UO and KI5WW like this.
  3. N7VGO

    N7VGO Ham Member QRZ Page

    You know some times when you don't expect it, everything comes together. When I work as a Electronic Tech, if it worked I just walked away!
     
    WD4ELG, W7UUU and WG7X like this.
  4. WA7F

    WA7F Premium Subscriber QRZ Page

    David, you have already got your bad luck out of the way. It’s time to enjoy some good luck.
     
    WB5YUZ and W7UUU like this.
  5. N4UFO

    N4UFO Ham Member QRZ Page

    SWEEET!!!! Reminds me of when I had up a 10' tripod and then a not completely extended 30' push up mast to a mini-beam. The whole thing was bolted to railroad ties and insulated from ground. I got to looking at the height plus the boom (top loading) and wondered about the resonance... Sure enough, I hooked up three above ground radials to a ground rod at the base, connected an analyzer and the whole thing was resonant right around 7 Mhz with a very wide SWR curve due to the pipe/tripod width! My 40m antenna at the time was very poor and narrow banded, so I laid out 32 quarter wavelength radials around it and had the most awesome 40m DXing antenna ever!

    beam mast tripod.JPG

    So, yeah... I feel your excitement! So awesome to stumble across something you didn't know you had! I bet that will be absolutely awesome on 160m. Enough vertical to get out well on 160m. If you work FT8 maybe I will see you this season! I will be on their trolling for a few more states I need from the new QTH and any DXCC I haven't worked yet. :)

    73, nice find! Kevin N4UFO
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yankee Doodle put a feather in his cap and called it A Marconi.:)
     
    W7JMP, 2E0CIT, M0AGP and 1 other person like this.
  7. K7JOE

    K7JOE Subscriber QRZ Page

    nice going... sometimes luck is on our side !
    Might be fun to see if you can put a coil at the base and resonate it on 630M band too... because.... why not !
     
    K0UO likes this.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Dave, pity you did not measure the actual R where jX=~0. That would give us an idea of your ground loss resistance.
     
    K0UO likes this.
  9. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Here's what my AA-600 tells me... one reading at the balun, the other in the shack. The yellow captions label each accordingly.

    Dave
    W7UUU

    apic1.png

    apic2.png
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Dave, it wasn't clear in your initial posting. Did you shorten the original run of window line so that it runs vertically from the ground rod/balun to the center of the the 150ft wire? If so, how long do you estimate that the vertical run of window line is after the mod?

    I created a model of a Marconi-T ( "shorted-at-the-bottom" 2 parallel conductors window line 50 ft long running vertically to two co-linear horizontal 75ft wires at 50ft agl) over average dirt. It resonates a bit below the 160m band, which is consistent with what you measured at 1.85MHz (+j40), making me question how long the vertical window line really is?

    I also learned something. A grounded Marconi with a single vertical conductor connected to the center of a horizontal 150 ft wire at 50ft agl (a real T) is a different proposition to what I describe in the paragraph above (using two closely-spaced parallel vertical conductors where the 150ft wire is broken into two sections at the middle and the parallel vertical wires are shorted together at the bottom.

    Give me an idea of the window line wires spacing?
    Are the doublet wires bare or insulated?
     

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