70cm Yagi & 25' Quick Deploy Mast - Temporary Station

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KL4IU, Sep 22, 2018.

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  1. KL4IU

    KL4IU Ham Member QRZ Page

    I need some advice/guidance. I want to set up a 4-antenna array with some Yagis mounted on a 25' quick deploy mast from M2, run a short distance to a 300w amp and then into a Yaesu FT-991A. This is all on 70cm. Hopefully we can set up the mast and antenna outside and run coax thru an open window. This is going to be a quick set up, temporary operation and then tear down. I don't know if we will have much grounding resources, but likely could find a earth grounded pipe to clamp on to. The ground will likely be frozen and covered with snow when we set up. Lighting is not really a concern in our environment. The main concern is RF grounding. I understand a Yagi is a balanced antenna, but can there still be issues from not having good RF ground? I just don't know if we will encounter common mode current or any other RF interference from it. Does anyone have any good practical ideas on ways to protect or help minimize problems from RF grounding issues particular to a setup like this?
     
  2. KB0MNM

    KB0MNM Ham Member QRZ Page

    I did something very similar to what you describe with a Cushcraft Yagi ( dual band, 4 or 5 elements on both 2m & 70cm ) and a plumbing floor-flange mounted on plywood. My mini-van tire anchored the plywood and the floor flange supported a single section of TV mast, approx. 1.25: diameter. The flange bolts through the plywood would ( wood/would ) conduct, as would the braid of the LMR-400 cable shield. I would suggest that excess ( say 5 ft. ) cable be coiled near the base of your support to help combat common mode current ( el-cheapo choke might help at 2m, not sure at 70 cm ). Make sure the amp has a heavy braid clamped to the quick-deploy mast (if that is conductive). Do not forget that a ground-loop can be made by alternate paths, so if you add the braid- clamp it to the mast where you can. This may require removal of a non-conductive coating. Salt the ground in advance if you can. Saline water conducts much better than 'clean' ice. Unfortunately, salt also attracts deer and other critters- so you might find hunters in the area you want to use for parking.
     
    KL4IU likes this.
  3. KM3F

    KM3F Ham Member QRZ Page

    One large Umbrella stand weight.
    One fiber glass extension window cleaning pole.
    One adapting sleeve so pole can be mounted tight in the base weight.
    Light weight rope and 4 stakes.
    Mount needed to afix antennas to.
    One 12 volt DC rotor and control if needed.
    .
    Or, make a base frame to run a vehicle wheel onto.
    Provision base for two 10 foot x 1 inch conduit pipes.
    Whatever is needed for the antennas.
    Light weight rope and 4 stakes.
    .
    Both ways should be less than 15 minutes setup time once it's worked out and trial setup done.
    Good luck.,
     
    KL4IU likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You don't need any kind of "RF ground" with beams, and at 70cm unless your antennas and rig are laying on the ground, you'd never achieve one anyway.

    My "quick deploy" VHF/UHF setup involves a motor vehicle and you didn't say if you had one there or not. But if you have a vehicle at the station location, a "drive on" mount with a quick-up telescoping aluminum mast can get the beams up, in the air, and working in a few minutes.

    Drive onto the mast mount and park. Now you have an anchor for the mast. Add telescoping mast (from the trunk or wherever) and add it to the mount. Install beams (and rotator if necessary). Loosen mast clamps and telescope mast to 30-40 feet, and tighten clamps. Bingo.

    I've done this too many times to count. My drive-on mount and telescoping mast are made by "Wilburt" Co., who makes a lot of them for the military, commercial and industrial use. It all fits in the trunk of a car (or in my case, my van) and deploys very quickly but will support stacked VHF-UHF beams in a 50mph wind without any guying.
     
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  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    I made my own drive on mount. Takes about 30 minutes to drill the holes and assemble if you have everything.

    All you need is an aluminum plate, piece of unistrut, EMT collars, some knobs from a broken work light to tighten the mast inside the EMT collars, a couple of hose clamps to hold the EMT collars against the unistrut, some small strips of angle iron to bolt down the unistrut to the aluminum plate and some nuts and bolts.

    Next paint it with gray automotive primer, slap a DX Engineering sticker you get from a hamfest on it and you're done. :)

    IMAG0604.jpg

    IMAG0598.jpg


    30' aluminum telescopic mast I am using for my portable antennas:

    https://www.walmart.com/ip/Unger-Te...-Extension-Pole-System-6-30ft-Silver/51935117

    - or -

    https://www.walmart.com/ip/Unger-Op...Aluminum-Extension-Pole-Silver-Green/41134928


    Pole is pretty strong and will support a fairly long aluminum 20m HF antenna and even a 5 element VHF Yagi without attaching any guy lines to the pole at all. You could easily put bigger antennas on it if you use some Dacron rope guy lines.

    IMAG0314.jpg

    I can "casually" take this out of the car trunk and have it all up in the air in and ready to operate in around 5 - 8 minutes. Assembling and attaching the antenna to the pole takes the most amount of time. The drive on mount can be parked on with the car tire and pole fully raised into position in less than a minute.
     
    Last edited: Sep 30, 2018
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  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Heck, if I'm adding stickers I'd go for "Louis Vuitton" or "Versace" or something...available in trash bins behind all very expensive stores.:p
     
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  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    " The main concern is RF grounding. I understand a Yagi is a balanced antenna, but can there still be issues from not having good RF ground? I just don't know if we will encounter common mode current or any other RF interference from it. Does anyone have any good practical ideas on ways to protect or help minimize problems from RF grounding issues particular to a setup like this? "
    Is what you asked, and most folks told you how they did it with similar equipment. No one ( myself included ) had any particular ideas about why it might be necessary to provide an RF counterpoise* or ground, just ideas on how to accomplish that. As to the coax through the open window, I used a towel to cut down on the heat loss- that is, until I realized that the heat was not welcome after the work of putting up the mast the first time. So the towel would only be a kindness to an observer in the vehicle. It all depends on how cold your fingers get inside gloves...* If the vehicle that provides power to the amplifier is connected with a good ground, it will provide some amount of RF counterpoise. Since VHF and UHF communications are strongest at 'line of sight', a bigger concern is how high the other station antennas are as well as distance. Getting one's own antenna above the trees with a mobile setup like this is rare. Contacting others in a 20 mile radius- not so rare.
     
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  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    On 70cm, or even 2m for that matter, you'll never achieve an "RF earth ground" unless your equipment is all laying directly on the ground.

    Thankfully, it also isn't necessary. I don't know anyone who has a "ground" on 70cm, no matter what they're running.
     
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  9. KK6YAE

    KK6YAE Ham Member QRZ Page

    If you’re using a folded dipole style yagi, that has the driven element bonded to the boom and other elements at the zero volt point opposite the gap where the coax connects... Then you could do what the commercial world does, and strip off about an inch of the insulation a foot or two away from where your feedline connects to the antenna, and clamp/bond the coax shield to the mast, then daisy chain the mast to your car ground, or negative side of whatever power source you’re using. That’s the propper way to provide a cleaner return path for higher currents if you don’t want them finding their way back soley on the coax shield. More important would be to ensure that the coax you are using is rated for the amount of power you are driving the system with.
     
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  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    I do have a fairly good r.f. ground even as high as the 70 cm band. However, achieving that ground involved some aluminum flashing at least 10-inches wide all over the operating console that is connected to an outside chemical ground system with a 4-gauge wire less than 6-inches long.

    Glen, K9STH
     
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