[question] 2m & 6m separation distance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4LMM, Aug 6, 2017.

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  1. N4LMM

    N4LMM Ham Member QRZ Page

    Greetings,

    I was talking with a local fella about separation distance between horizontals yagis (2m & 6m) and the number we came up with was 38 inches. How critical is that space between these two yagi?

    Right now, I have a gulf alpha 7x7 2m and a M2 6m3 through a glen martin 450 on a 10 foot 1.5'' emt pipe. Roughly 2.5 feet of mast sits in the tower through a thrust bearing, which leaves 7.5ft above.

    I got the damn satellite bug again, and want to add sat capability to the shack once more. So my idea is to remove the vertical elements off the gulf alpha, and slide it down above the 6m3, and then put the elevation rotor module of the G5500 on the top of the mast.

    My concern is the weight of the ELV rotor, which is around 11 lbs. The tower legs are secured to the beams with 3 lags each, but I wonder if ELV rotor + M2 LEO Pack + crossboom is going to be too much top weight for the pipe. The idea I had to mitigate would be to cut the 10 foot mast down some.

    So if I add more weight to the top of the mast, and cut the pipe down from 10ft to 6ft, it seems I may not have room for good separation between the 2m and 6m yagi. Yet, if I don't cut, would I run the risk of folding the pipe in a wind storm?

    What do you think? Should I cut the mast down a few feet, leave it as is, or upgrade the pipe (rotor + bearing would support 2'' pipe)

    Below is a terrible illustration of the current config and the proposed change.

    [​IMG]


    Thanks for your insights in advance.

    73 -Lou
     
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    The further away you install the antenna away from the rotor, the more stress the antenna on top places on the rotor.

    Think of the antennas installed on top as a cork screw handle. And think of the mast as the cork screw shaft. The bigger the handle on top and the longer the cork screw shaft is, the easier it's going to be to torque the cork below out of the bottle.

    In terms of antenna separation, it's getting a little crowded up there in the proposed part of the drawing. I think the GA 7 should probably be installed on another separate mast elsewhere.

    I didn't look everything up yet, but I am wondering about the additional wind loading all the additional antennas places on the G5500 rotor below. The rotor is rated at 10 sq, ft. I am not sure what some of the other antennas on top are exactly, so I can't tell you anything about their wind load ratings / specifications, but I think that would be the first order of business to figure out.
     
    Last edited: Aug 6, 2017
    AJ5J likes this.
  3. N4LMM

    N4LMM Ham Member QRZ Page

    Thanks for the reply. The GA is the Gulf Alpha 7 x 7 on a 8 foot boom. (documentation on this one is scarce, I got it third hand)

    The M2 satellite antennas are 436CP16 and 2MCP8A

    It sounds like, given the weight, height, and windloading, that I might be better off keeping everything as low as possible and dump the 7 element 2m yagi.
     
  4. KJ4TX

    KJ4TX Premium Subscriber QRZ Page

    Before you dump it, why not test with it first. If it's easy enough for you to get to the mast, run some tx tests with just the antenna you are interested the most in. If you have digital set up so you can send a signal out and watch how well you hit monitoring station on PSK Reporter. Then add the 2nd antenna and try the tests again to see if they are off or pretty consistent on which monitoring stations you hit and what the db value is given.

    If you have pretty easy access you should be able to do all of this in an hour or two before propagation changes and affects your results between antenna tests. I have a 4 element Arrow 6 meter yagi right above my rotor and about 2 feet above that a 9 element M2 2 meter yagi and have for a number of years and I've not noticed very much interaction between them and I've done a lot of tests using JT65 while watching PSK Reporter and seeing what monitor stations I can hit. It may be that you will find that having both antennas outweighs any interaction there is between the two antennas.

    I know technically you are suppose to wait until winter and bad weather to do antenna work, but in this case you can probably sneak it in without getting caught. ;)

    Mike
     
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  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    Are these the 6m PSK monitoring stations you tested your antennas against from your location in Oregon?

    Untitled.jpg

    Assuming their antennas are even pointing in your direction, perhaps it might take longer than an hour or two between antenna tests?
     
    Last edited: Aug 6, 2017
  6. KA2BPP

    KA2BPP Premium Subscriber QRZ Page

    This should give you some insight as to how close you can mount the 6 and 2 mtr antennas. I would upgrade the mast
    to 1.9 dia 1/4 inch thick wall aluminum pipe. EMT is heavy to begin with. I have a 23 foot section to go in my tower.
    if and when I get to putting it up.
     

    Attached Files:

  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd ditch the 1.5" EMT also. It's not intended to be a structural support for anything.

    Although this may vary, most EMT I've seen isn't even seamless (extruded), it's rolled and has a brazed seam. That's fine for its intended purpose, which is to carry insulated wires inside, but as a "mast" it leaves a lot to be desired.
     
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  8. N4LMM

    N4LMM Ham Member QRZ Page

    Thanks for the tips. I'm equipped for WSJT and could use FT8 for potential measurements. There are plenty of those monitors up lately.

    Thank you. Going through it now.

    The tower / roof is easily accessible, although I'd need a pal to help pull the mast stack in and out. Suppose it is easy enough to start real early in the morning with several configs.

    The 2'' is definitely heavy. You know when I went shopping for a pipe, all I could find at the box stores were 1.25, 1.5, 2 and 3'' emt conduit. Suppose I'd have to order something from DXE
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You probably don't have to order from DXE as "shipping" for anything longer than six feet can get very expensive.

    If you have a local metal yard, and most places do, try asking for 1.75" extruded tubing with .125" wall thickness (2" O.D. -- but tubing goes by I.D., not O.D.) and it usually comes in 12' lengths. Very expensive to ship, but very cheap to pick it up and put it on roof racks, and just drive it home.

    That's what I do, although for big antennas I use the 1.5" I.D. stuff with .250" wall thickness, it's called "aluminum hollowbar" and it's 6061T6 alloy. Very, very strong but not heavy at all.
     
  10. KJ4TX

    KJ4TX Premium Subscriber QRZ Page

    Yes, they are, although I did a lot of my testing on my 6 meter yagi when 6 meter had a lot of JT monitor stations across the US. I would step my yagi from due north to due south (and even west to test how signal was getting out the back side).

    I also did this on HF to test my parallel HF dipole to see which was it's strongest direction and which it's weakest. Even now, I still do this because I like to chase balloons and try to get one just a tiny bit further than the last one. Who needs contacts when I have balloons to play with. :D

    Mike
     
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