Antenna question

Discussion in 'Amplitude Modulation' started by N2DTS, Jul 18, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I had tree's removed and trimmed and put up a nice full length 80 meter dipole with a 1.4 to 1 swr fed with about 70 feet of rg8x coax.
    Seems to work great.
    I can tune it up on 40 meters with the drake tuna and it seems to work fine that way.
    I also have the zero5 vertical (30 feet tall) roof mounted which also seems to work ok on 40.
    The question is: if I added a 40 meter section, or put up a resonant 40 meter dipole in about the same situation, would it work better then the 80 meter dipole on 40?
    7.3 Mhz, so I am not going to have a lot of loss in the coax.
    Would it be worth the trouble to add a 40 meter section to the 80 meter dipole?
  2. W2NBC

    W2NBC Ham Member QRZ Page

    It depends..

    Based on the height above ground (especially important on 40), your 75 meter "2 half-waves in phase", will exhibit some gain on certain lobes, lower angle of take-off, and more or less broadside directivity. That's great for calling "CQ" and reeling them in, but for a predictable pattern with a true and wider broadside pattern, the 40 meter resonant antenna would be the winner. You won't see a HUGE difference, but the resonant dipole with an unbalanced system (coax) means efficiency VS using a tuner (some loss) in the path.. Adding 40 meter elements to your 75 meter antenna (fan dipole) will obliviously require some pruning but may be worth the effort with your new tree situation!
    N6YW likes this.
  3. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Well the biggest thing you will find is you will no longer have the mismatch at 40m at the feedpoint with your RG-8X. The Drake seems to be able to tune it out for your transmitter, but you still have the mismatch at the feedpoint and all the associated losses on the feedline, both on transmit and receive. Adding a 40m section will eliminate all or most of that like Jeff @W2NBC mentioned.

    Once I get the house rebuilt after the fire, I'm going with a fan (80/40) for a time as well. Eventually, I'll go with a doublet and OWL.
  4. N2DTS

    N2DTS Ham Member QRZ Page

    The new location makes it a bit hard to add the 40 meter section.
    That is why I am trying to figure out if the signal will be vastly improved, or if it would be marginal.
    A longer antenna can have gain, no?
    Its the mismatch and swr on the coax that might cause problems, but does that matter much at 7 MHz?

    I thought I would toss a 40 meter dipole up as a test, but my path is blocked by a huge tree next door...
  5. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Exactly what I did here at home. Works great!
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    What am I missing here? Why not put up a Fan Dipole?
  7. N1BCG

    N1BCG Ham Member QRZ Page

    You didn't actually try that, right?

    More than half your TPO will be converted to heat. If you can live with that, then...
    Last edited: Jul 19, 2019
  8. N2DTS

    N2DTS Ham Member QRZ Page

    Nothing seems to get hot/warm.
    Where the antenna is now causes it to catch on the roof (badly) when I lower it and try and get it back up.
    The first one turned into a big ball of knots when I tried it.

    Once it cools off a bit I will have to look at my options.
    I only have one tree out front now..
  9. W2VW

    W2VW XML Subscriber QRZ Page

    P=V squared/R

    1500 watts gives you 274 volts peak and about 5.5 amperes of peak RF current into the shack end of the RG8x.

    A dipole cut for 80 meters can exhibit more than 10:1 SWR when used on 40 but let's take 10:1 for simplicity's sake.

    SWR is the ratio of minimum to maximum voltage or current on the feedline.

    Take that ratio and apply it to the peak numbers above.

    The feedline will have to be able to hold off 2,740 volts which it probably can but it will see peak currents of 55 amperes in certain areas.

    The duty cycle helps but at QRO you will destroy that cable.

    The easy solution is a parallel dipole.

    The smart solution is make the doublet a little longer and feed it with real balanced line and homebrew an antenna coupler.

    That way you can move away from the cheese grater in the AM ghetto, work 40 and even 160.

    RG8x does not have a good scent when it cooks.


    W2BTK and AG5CK like this.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    I only run 200 watts on 40, maybe 900 watts at peaks.
    I will have to see what I can do about it though....

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