2M 5/8 groundplane - 70CM use?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KV4JW, Dec 2, 2018.

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  1. KV4JW

    KV4JW Ham Member QRZ Page

    I have a MFJ-1750 2 Meter 5/8 groundplane antenna. It's a very good performer for me, and I hate to replace it, but..

    I have a need for a dual band QTH antenna now.

    Is there a way that I can modify my antenna to be used on 70CM / 2M before I look into building or buying another one?
    Last edited: Dec 2, 2018
  2. K7JEM

    K7JEM Ham Member QRZ Page

    AJ5J likes this.
  3. AJ5J

    AJ5J Ham Member QRZ Page

    Build a dual-band fan GP with 10 pieces of wire (5 for 2m,5 for 70cm) and a SO-239 silver-plated panel mount. Two bands, one coax; good match on both bands. The radiator will resemble a thin V soldered carefully to the center pin; slope the radials around 45 degrees around the edge...

    Stick it up high in the sky as you can and it will knock your socks off---or hit all the repeaters you need, at least (in a nutshell).:)

    Save or sell the 5/8th wave if you no longer need.


    KC8VWM and AC8UN like this.
  4. KV4JW

    KV4JW Ham Member QRZ Page

    I'd planned on sticking it on top of my rooftop 20' mast that will support my G5RV. That will put the antenna at about 35' above ground. My 5/8 2M GP is able to tickle repeaters about 90 miles away right now, and it's about 10' AGL ( live on a bit of a ridge)..

    AJ5J likes this.
  5. KV4JW

    KV4JW Ham Member QRZ Page

  6. KV4JW

    KV4JW Ham Member QRZ Page

    If I was to purchase an antenna, what do you guys recommend? I've been looking at the DIamond X30 and X50, and the Comet GP-1. Ideally, I'd like to stay in the 3-4' tall range. The antenna will be installed approximately 35' AGL and fed with LMR-400 (Glad I kept this from my CB days!)

    I'd like low angle radiation on VHF - where I'm up on a ridge I don't have much in the way of obstacles. I like being able to hit some distant repeaters when I can. I feel this will be rather important as I'm going to be taking a power output hit when I swap out radios. My IC2300H has full power of 65W. The radio I'll be replacing it with is 50W.

    The UHF repeaters I'm interested in are local and have good coverage anyway, I can get into them with an HT.

    So I'll be losing 15W in high power, adding a longer run of coax (however better coax), and adding about 20' to the height of the antenna.

    I understand a small 1/4 wave would have more of a "bubble" type radiation pattern - and where I'm not in a valley, or where the antenna won't be near AGL, I'm afraid I'd be wasting power at steeper takeoff angles.
    Last edited: Dec 2, 2018
  7. K7JEM

    K7JEM Ham Member QRZ Page

    Don't concern yourself with the takeoff angle at VHF. For any antenna under about 20' in length, it just won't be an issue.
    AJ5J and KV4JW like this.
  8. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    Just build a pair of ground planes. That's what I did. They work well. Those are power lines in the background.
  9. AJ6O

    AJ6O Ham Member QRZ Page

    The higher the gain antenna you buy will give you more of a pancake shape radiation pattern. If there are any mountains around you, you may want a lower gain antenna that gives more of higher, and lower radiation take off to hit repeater in the mountains or in the valley below. I have the "X50" antenna which is not high, but moderate gain, and it works great for me. It is only around 6 feet tall which is not bad for a dual band antenna.
    KV4JW likes this.
  10. KV4JW

    KV4JW Ham Member QRZ Page

    I think I'll probably go with a X50. The gain figures are attractive. I'd love to build my own, at this point I just don't really have the time though.

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