Stacking 70cm Antennae

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by MM6ECO, Mar 3, 2017.

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  1. MM6ECO

    MM6ECO Ham Member QRZ Page

    Hi Guys!

    I had my foundation licence since 2010 and really enjoying ham radio. Recently I have been enjoying 2M and 70cms a lot more. I think that if we don't use it we will lose it especially when it comes to the 70cm band here in the UK/

    My plan is this! Firstly get on air on UHF as much as possible!

    I`m lucky to have two options of stacking 70cm antennas. See what you think?

    I have two small 2 element beams HB9CV with the beehive capacitor in them for matching.

    I will soon have two 10 element beams for 70cm Diamond type with the DIAMOND SS430 70cms Phasing Harness. I just need to get myself the final 10 element to get this project complete.

    I have been able to DX on 70cm here in Scotland to the south where I have a good take off! With 10 watts power I have managed to get as far as wales so far and I`m only 30m above sea level... This was just using the 10 element beam on the horizontal position in the garden at about 8m high...

    My question is this!

    Is there any difficulties in co-phasing antennas I should be aware about and is there any decent websites with information about how to stack UHF antennae properly? I just don`t seen to hear anything locally in my area that has stacked their beams. Is there a certain distance between the beams I should work out or just move the antenna equally away from each other until I get a good standing wave ratio?

    Would love to hear others experiences on vertical and horizontal stacking of two beams co=-phased?
    Even 4 if you have managed to do that?

    Thanks for all your comments in advance...
    .mm6eco.
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

  3. W1VT

    W1VT Ham Member QRZ Page

    KK5JY likes this.
  4. KK5JY

    KK5JY Ham Member QRZ Page

    I would also recommend that you do some modeling of the antennas you intend to stack. You can move them in the model by small increments and see that effect on pattern and impedance. 4NEC2 is free, and EZNEC is cheaper than a stack of UHF antennas.
     
  5. AI3V

    AI3V Ham Member QRZ Page

    Errors in the phasing harness can be tricky to spot.

    Two ways to stack beams above one another, and side by side.

    Unless the phasing harness is perfectly symmetrical the radiation pattern of the antennas will shift off of bore site.

    If the antennas are above one another, the pattern will be shifted above or below the horizon. Unless you vary elevation angle of array you will never know you are not on the peak signal.

    Now, if the antennas are side by side, the error will show up as a peak off azimuth when you rotate the antennas.

    Also, this effect is frequency sensitive, as you increase frequency, the beam shifts more.

    Rege

    I have always had the best luck by assembling antenna arrays a piece at a time and comparing each "improvement" by measuring the expected gain with the "attenuator substitution method"

    Also, MEASURE EVERY piece of equipment for insertion loss and vswr. Every coax, meter, splitter etc.

    KNOW, don't guess.

    And water is the enemy in antennas, but you know about rain.... :)
     
    Last edited: Mar 3, 2017
    AK5B likes this.
  6. KM3F

    KM3F Ham Member QRZ Page

    I have two stacked arrays, one for 1.25 and 1 for 2m.
    You can never predict the far field additions due to the many variables.
    Stacking can be by coax cable or power splitter.
    I have done the Coax type and works ok. But for DX the loss is greater in the coax type due to the cable loses and the SWR you have in that type.
    Stacking distance has a large effect on total pattern, especially side lobes.
    Stacking for max gain does not always produce small side lobes. This is not always a determent to weak signal operation if you have no interference to deal with from other directions.
    Stacking distance is usually about 1 wave length or more apart.
    Both antenna need to be feed in the same phase to add in the far field focal point area.
    That means you mount them the same at both locations with and not for just for the convenience of attaching the coax.
    The splitter type feed can offer a bit less loss because the loss from inherent SWR of the cable type is greatly reduced.
    The splitter type is made to offer a short section of about 75 ohms connected to a Tee for the feed.
    This allows you to feed the antennas with any length of 50 ohm cable to the splitter.
    The 50m ohm feed from each antenna run through the 75 ohm section results in 50 ohms at he Tee feed point.
    Sounds a bit funny but two 50 ohm antenna is 25 ohms if you don't use the splitter with this ability to convert back to 50 ohms at the splitter feed point.
    My 2m splitter is made from Brass tubeing, soldered to connectors each end and of course air insulated.
    The assembly was assembled on a Lathe to keep concentricity.
    Testing was done with 50 ohm non inductive resistor on each end.
    Feedimpedance results looked at with a Comet CAA 500 MkII show a flat match from 135 to 153 mhz.
    Each half of the splitter measured separately shows an R of about 63 ohms.
    The bottom line in stacking is you must have enough higher impedance to work with and do a transformation back to 50 so the antennas and the feed point are nearly the same.
    You get a good education on handlng impedance transformations and the pleasure of the end results.
    BTW, no modeling, just understanding of the basics along with some calculations.
    Good luck.
     
    Last edited: Mar 3, 2017
  7. KM3F

    KM3F Ham Member QRZ Page

    70 cm stacking might be in the future but 1.25 is the high interest now for double points in our contests.
    I built an all mode 222, 180 watt station that is mainly used for local FM and contests. It's the only one around except at contest time.
    God luck.
     
  8. W1VT

    W1VT Ham Member QRZ Page

    My best contesting on 23 cm was done with two 6 ft boom loop Yagis, one above the other on one side of an H frame. I used a power divider made out of aluminum and brass tubing and two equal lengths of 50 ohm Heliax between the power divider and the Yagis. I would have made the multiplier top list for the contest but they didn't include QRP portable stations. :confused:
     
  9. VK2JKN

    VK2JKN Ham Member QRZ Page

    Has anyone got a set of measurements for 70cm , I'm tearing my hair out ... instead of reminiscing about days gone past ...lol lol
     

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