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Glen-Martin 4.5 foot Mini-Tower Suggestions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NE2X, Sep 29, 2011.

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  1. NE2X

    NE2X Ham Member

    All...

    I always enjoy the responses, thus my situation and any idea are welcome..

    Have a Glen-Martin 4.5 Mini-Roof Tower. All Assembled and on the ground. Have a Yaesu G-450A Rotor but need to decide what size thrust bearing to buy...

    Theory is to mount Moseley 2 element TA-32 18 inches (1/1/2 feet) above the tower top. Then a 3 or 4 element 6 meter beam 5 feet above the TA', then a Cushcraft A-148-20T near the top of the mast, but a 3 foot piece of PVC will be slipped over the 1/1/4 mast and this will stop any vertical elemental interaction..

    So, GM 4.5 tower, then 10 foot 1 1/2 inch mast tapered to a 10 foot 1 1/4 mast. The 1 1/4 will be in the 1 1/2 mast by about 2 feet, so total mast length is 18 feet with about 3 feet into the router and coming out.

    Is this a reasonable scenario? I want the 2 meter cross-pol on the very top..And , can I buy another A148-20T and phase these 2 antennas?

    I was also looking at some mini-beams, but the TA-32 is the longest I can use here..

    Thanks for reading

    NE2X
     
  2. KF6ABU

    KF6ABU Ham Member

    That will surpass the maximum of 6sq ft wind based on Glen Martins website...

    I have a TH5-DX on about 15ft of mast on my RT-424. I have the mast guyed at 2 elevations with thrust bearings. My mast is over 2 inches.

    Your set up may work, but I wouldnt feel too comfortable with it.
     
  3. KB4QAA

    KB4QAA XML Subscriber

    PVC slipped over the mast to prevent vertical interaction?? HUH?
     
  4. KF6ABU

    KF6ABU Ham Member

    my guess was he ment he would extend the mast with PVC pipe and mount the antenna on the pvc extension portion.
     
  5. KF6ABU

    KF6ABU Ham Member

    Also remember you cannot just use lagbolts to your roof's plywood. It has to be connected to braced rafters/joices.
     
  6. NE2X

    NE2X Ham Member

    Yes, as for PVC, the Cushcraft is mounted on the top so the elements have no interaction...

    As for the lag bolts, custom 4 x 8 deck wood painted the color of the roof, 3/8ths threaded rod ( 16 inches) fed through the roof to another 4 x 8 And braced against 3 rafters Mine are 24 inches apart..
     
  7. N0AZZ

    N0AZZ Ham Member

    If it were mine I would use a mast pipe that was intended for the job and not jury rigged in multi-stages and a factory thrust bearing.
     
  8. WB2WIK

    WB2WIK Premium Subscriber

    I read this over and I'd recommend assembling all of it on the ground first, before installing it on the roof.

    That way you can see if it's all working the way you think it will. Even 3' of PVC tubing may not be strong enough for the A148-20T...PVC looks strong and feels strong until you put an antenna on it. When you tighten down the U-bolts to mount the antenna, it will compress the PVC and may never get "tight enough" to prevent undesired rotation.

    I also wouldn't "taper" the mast 1-1/2" to 1-1/4" at all. That's a weak point, and 1-1/4" mast unless it's very special isn't very strong, either.

    I have a G-M RT-832 (8' tall RT on 32" leg spacing) and use a 2" mast from rotator up to "everything." It's strong. It will be eleven years this coming November 1st and nothing has ever budged a millimeter since the day it was installed. I've snapped my fair share of 1-1/4" masts (just wind, blowing the load) and won't do that again.:eek:
     
  9. AH6RR

    AH6RR Ham Member

    I have a GM RT-424 also with lots of bracing under the rafters. I have a 10' 2 inch Schedule 80 alum. mast with a SteppIR 3 element with the 30/40M loop 2 feet above the tower and a 7 element Gulf-Alpha VHF at the top. I use a Yaesu GS-065 thrust bearing and it has been up for over 5 years now with no problems at all.
    I would not use a mast over 10 feet on this tower without some serious guying both to the tower and mast. Why do you not add the 6M elements to the boom of the HF beam some retuning might be needed but it will work just fine and use a separate feed line for it then put the 2 meter antenna at the top of the 10' mast. Oh and using the PVC will do no good because the metal will still be there it will just be covered in plastic. My 2 meter beam is mounted vertical and I have no problems with the metal mast whatsoever.

    Good Luck
    Roland AH6RR
     
  10. NE2X

    NE2X Ham Member

    Roland,
    I like your input and it has me implementing your idea....1 thing. The PVC is a "sleeve" and slides over (not all the way) to the mast...The 2 Meter x-pol is on this. And yes, I have used PVC with masts before and I see no issues with this.

    Total mast length is 12 feet and will be aluminum now...9 feet above the thrust bearing...
     
  11. W9GB

    W9GB Ham Member

    Interaction? I don't think so, I would stick with aluminum or galvinized steel masts for this Glen Martin tower.

    I have seen PVC used, I actually used it as a cheap boom material for a 2-mter yagi in 1970s -- but I would NEVER recommend for MAST usage,
    especially with the wind load that Cushcraft will present.

    There are a couple of YouTube videos of the OM picking up what is left of his antenna array -- after the PVC mast failed in high winds.

    w9gb
     
  12. NE2X

    NE2X Ham Member

    So your saying there there is no interaction/effect of the aluminum mast passing through the X-pol's antennas vertical elements? And if so, it is negligible?

    Hmmm this goes against my engineering background of the mast acting as a parasitic element and skewing/distorting the vertical sides pattern E/H Plane
     
  13. N0AZZ

    N0AZZ Ham Member

    I think your being penny wise and pound foolish use a proper mast do it once and be done with it. I have 3 antennas mounted on my 15' mast with 3' into the tower so 12' out the top. Closest to the tower is a Mosley Pro-67-C3 10-40m 24' boom next is a 6 element 6m yagi 24' boom and on the top now is a 33' 2m yagi horz before that I had a 13B2 mounted vert on the top.

    All of these have amps to them 6-160m 1000w 2m 200w all the time they have been up never had a problem or interaction with any of the other antennas or the mast. All antennas are made to be mounted on a mast you need to watch what side you mount them on. All antennas mounted on this tower have 1.5:1 VSWR or less on all bands.
     
    Last edited: Sep 30, 2011
  14. W9GB

    W9GB Ham Member

    Yes, negligible.

    Unless the instructions have changed,
    I do not remember Cushcraft/MFJ making specific comments (instructions) for using a non-conductive (plastic or fiberglass) mast with this antenna.

    There are some AMSAT operators that use fiberglass horizontal booms with their Az-El rotators, but this is often due to specific antennas being used.

    w9gb
     
  15. WB2WIK

    WB2WIK Premium Subscriber

    I've installed bunches of these antennas.

    If you do everything exactly right you can maintain optimum performance; but that includes not only a non-conductive mast but also routing the coaxial feedlines back behind the reflector element and letting them drop down to below the rotator from that point ("swinging in the wind"), and not routing the feedlines down the boom and then down the mast -- since if you do that, might as well use a fully metallic mast all the way: The coax cables also form an undesired parasitic element in the middle of the antenna.

    No purpose in using a non-conductive mast unless you also route the cables properly.
     
  16. NE2X

    NE2X Ham Member

    You mean actually following the boom back and going over the last reflector and then dropping the coax down? To where it is 'swinging in the wind"? So, the mast is not used at all to secure the coax in any way?

    And am I correct in using a non-conductive mast with the concern that the vertical side will have some skewing with a metal/conductive mast?

    Does anyone have any pictures of this as I could "grasp" what this looks like. I think I have an idea..
     
  17. WB2WIK

    WB2WIK Premium Subscriber

    I did publish pictures of how to do that in my "VHF" column in CQ magazine years ago, but I'd have to go search for that...it's been years.

    A picture shouldn't really be required, though.

    Yes, you run the coax not towards the center of the boom, but towards the back (reflector) end of the boom, tape it or tie it off to the boom behind the reflector, and let the coax drop straight down several feet. Pull coax back to roof tower below rotator and secure there.

    That's the only way to keep the feedline out of the radiation plane of a single, center-mounted, vertically polarized beam. There isn't any other way to do it, and it does matter -- the coax is the same as a "mast."

    Three other ways to accomplish this involve different kinds of installations:

    1. Use an end-mount beam (available up to about 7 elements on 2m, usually), where the entire beam mounts to one side of the mast, with the mast behind it. Then, no problem running the coax down the mast;

    2. Use two vertically polarized beams, one on each side of the mast, separated by a cross-bar. For the -20T antenna, that cross boom would have to be at least six feet long, with identical antennas mounted at opposite ends. There is strong field cancellation in the middle between two beams like this, so you can run the coax down the cross bar and then down the mast without any interaction;

    3. Use a single vertically polarized beam, offset mounted on a standoff bracket to one side of the mast. It needs to be offset about three feet to be completely "clear." Then you can run the coax down the standoff arm and down the main mast without interaction.

    Another thing that "works" is to modify the way you install the 20T: Instead of installing it on the mast so 10L are horizontal and 10L are vertical, twist the antenna 45 degrees so that no elements are horizontal or vertical, but all are 45 degrees offset from those planes; then, you can stagger feed them with a 90 degree phase shift network (1/4-wavelength of coax connected between the two driven element feedpoints, using a "T" connector on one of the feedpoints for attachment of the main transmission line down to the station) and create circular polarization which is neither horizontal nor vertical. The vertical mast won't interfere with that, at all.

    That solution is theoretically 3 dB "down" from vertical polarization when working a vertical station, and 3 dB "down" from horizontal polarization when working a horizontal station, but in my experience over years of trying this, that 3 dB pretty much disappears over any kind of tropo (over the horizon) path, and the loss just isn't there. You could lose 20 dB or more if you ever need to work another circularly polarized station that has the opposite circular polarity! But that's not likely to happen as hardly anyone runs circular except some satellite and moonbounce guys whose antennas are aimed for those purposes.
     
  18. NE2X

    NE2X Ham Member

    I have also thought of "creating" a CP antenna by doing the offset, but the 3 db is something I am trying to gain here, not lose due to mis-pol of the end station..I have used CP LHCp/RHCO @ 1.2 ghz for some fairly long paths in Yemen. As the CP helps as part of the link went over water...

    Just remember, there are 2 coaxial lines. Seems to make sense to use a switch and only have 1 piece of RG-213/u instead of 2..I can see the added weight "pulling" on the end of the mast...

    Hmmm..

    Also, I was looking at the hex beam (5 bands) but it is made where you cant install anything above it..Does anyone make a Hex beam or some quad where I can mount a VHF antenna above it? Or do I stay with a 2 element tri band band? Even a 2 element 20 meter Moxon looks enticing..

    keeping 20 meters and 2 meters as my primary bands..

    Thanks for the input..
     
  19. WB2WIK

    WB2WIK Premium Subscriber

    I've found CP helps on tropo paths that are even over land. It's kind of amazing how well it works to reduce fading on way-over-the-horizon tropo VHF-UHF paths. I never noticed "losing" anything at all.

    Not if you set them up for CP: Then, there's one coaxial line.

    A remote switch that has no loss on 2m and is also weatherproof is a bit daunting; not impossible, of course, just daunting. It will either be expensive, or perhaps a lucky surplus find. I use Transco 28V motorized switches for this, and they are weatherproof and they have absolutely no insertion loss at all up to 1 GHz or so, but "new" they're horrendously expensive, so for me it was "surplus or nothing." Luckily, I have found them surplus, sometimes for only about $100. They work great, but almost all of the surplus ones are 28Vdc so it involves building a little power supply. And of course a control line to run it (just two wires).
     
  20. NE2X

    NE2X Ham Member

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