$300 mistake antenna, on multiple levels.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4VBB, Sep 10, 2019.

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  1. K4VBB

    K4VBB XML Subscriber QRZ Page

    Well, I've needed an antenna that would perform better on 80m than what I have for a while now, and this past weekend I decided that I'd take some of the spare wire and stuff and make myself an 80m dipole.

    I started out by making a truck that I could use to hoist both antennas up the mast. Then I used a bunch of spare wire I had laying around to cut my 80m dipole--I think it was 60' 4.5 inches or something like that. Got everything connected, got everything soldered, but when it came to attach the feedline, I found that I was missing coax that I 'thought' I had. I could not find it to save my life. (Watch -- It'll turn up in a week or so.) Since I didn't have the length of coax necessary to feed the antenna down to the base of the mast, my choices were to abandon the project or to find an alternative. Not one to give up, I decided that I'd think out of the box a little.

    I didn't have any coax. That was a fact. What I did have is a long spool (40' or so) of 450 Ohm Window line. So, I did what the desperate do and modeled the antenna in 4nec2 and determined that I'd need a feedline of 31' and 11" to get a match on 80. So I cut and attached the feedline. By this time it was way past midnight (probably closer to 3AM), but I decided that I'd at least hoist the antenna up next to my existing dipole so that I could tell myself that I got at least 'something' accomplished.

    Well, slogged out of bed on Sunday and hooked the antenna analyzer on it. Needless to say, it was long--this was expected, but it seemed WAY off from my model. It was WAY long, and resonant on something like 2.6xxxMHz. I expected to have to cut it a bit, but not that much. So I started cutting small portions on either side to start matching it up, but it seemed as if I would have to cut an extraordinary amount off of the antenna in order to get this thing to match up on the 80m band. However, due to an error on my part in operating my analyzer, I did notice that it had exceptional SWR on 40m. I checked out the impedance, made some minor adjustments, and voila -- I now have an unintended 40m "Frankentenna". It's not a doublet -- It's not a dipole -- it's a "Frankentenna".

    All looks good on the analyzer. 1.12 on the low side of 40m, and 1.8 on the high side of 40m. Not exactly what I was aiming for, but for a few hours work and ZERO cost involved (yet) I figured I'd at least made a decent 40m dipole.

    So, on to the shack I go. I hook up the feedline to the shack and proceed to place the analyzer onto the shack-end to see what's up. Looking good. And then things take a turn for the worse.

    Well, as they say, stupid is as stupid does, and I believe I qualify for the Stupid-of-the-Month award, if there is such a thing. Keep in mind that my analyzer is still hooked up to Frankentenna, which runs parallel to my existing antenna. The two are not separated by more than about 18".

    Right about this time I notice 5T5PA pop up on 40m. Been trying to work that station for a couple of weeks now with no success. I can hear him just fine. He doesn't appear to hear me at all. So I turn to my station and attempt to make contact. Again, no success. Some of you will immediately know what just happened, some of you may not.

    So I temporarily give up and turn back to looking at Frankentenna, but this time SWR is at 200, and impedance is at infinity. Checked all my connections both inside and outside the shack -- everything is solid, what the hell is going on?? Now it's time for selective interchange, and I hook up my existing antenna. Again, SWR is 200 and impedance is infinity. Battery indicator reads I have juice, but I figured I'd better check them anyway. I pop the lid on the analyzer and notice the distinct smell of burning electronics and immediately know what I had just done.

    For those of you who haven't guessed yet, let me ask you a question: What happens in a parallel wire when an alternating current is applied to the other parallel wire? Well, the voltage at the end of the feedline where my analyzer was connected was apparently enough to fry it.

    So in the end, I wound up with an antenna that seems to perform well on 40m, but that I really didn't need, that ultimately cost me the replacement value of my nice RigExpert Anenna Analyzer.
    K8KPO, 2E0CIT and K1LKP like this.
  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    Yup. Exactly why we don't let anyone transmit on our field outings until everyone has put away their analyzers.
    NH7RO likes this.
  3. KB1CKT

    KB1CKT Ham Member QRZ Page


    What I'm hearing is, don't design antennas at 3am. Which is probably true in other ways, 3am is a bad time to do anything other than sleep.

    Just what did you wind up with? I was thinking an extended zep (full wavelength dipole?) with a matching section, but I don't think the math works out here.
    NH7RO likes this.
  4. KB4MNG

    KB4MNG Ham Member QRZ Page

    gosh thats a long post..
  5. K4VBB

    K4VBB XML Subscriber QRZ Page

    I STILL can't figure out the math.

    Maybe a Double Extended Zepp? Now that you mention it, If I were to take some measurements I bet that's EXACTLY what I wound up with.
  6. K4VBB

    K4VBB XML Subscriber QRZ Page

    I SOOOO wish that was the case for me. I've been fighting the night-owl syndrome all my life. Left to my own devices I'd probably stay up 'till about 4AM and sleep 'till noon.
    KC2PMM and KL4BT like this.
  7. K4SAV

    K4SAV Ham Member QRZ Page

    You need a half wavelength of feedline to get a match on 80, about 133 ft. for the low end of 80 or about 105 ft for the high end. You need about 32 ft to get a match on 40.

    Jerry, K4SAV
    WB5YUZ likes this.
  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Let's go through the math for those who need a refresher:

    1/2 wave dipole including End Effect:
    468/3.5Mhz = 133.7 Feet

    468/7.0Mhz = 66.9 Feet
    WQ4G, WB5YUZ and NH7RO like this.
  9. N2SUB

    N2SUB Premium Subscriber QRZ Page

    I was told there would be no math. :mad:
    N4HFO, W9FL, KD4MOJ and 1 other person like this.
  10. K4SAV

    K4SAV Ham Member QRZ Page

    The math is OK but you can't match an 80 meter dipole on 40 with a half wavelength of feedline.

    Jerry, K4SAV
    WB5YUZ likes this.

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