Pages: [1]   Go Down

Author Topic: Tower Mounted Vertical Question  (Read 427 times)

KB2SIN

  • Member
  • Posts: 31
Tower Mounted Vertical Question
« on: March 19, 2023, 10:24:15 AM »

I am looking for constructive input on a project I am contemplating.

I have a tower with a Cushcraft MA5B mounted on top. I also have a Cushcraft R8 Vertical that currently isn't being used. I would like to mount the R8 off the side of the tower using a 4ft standoff and off the ground 10-12ft. My question is this.... Would this work being so close to the tower? Thoughts? Concerns? Idea's?

The reason I am wanting to use the R8 is my MA5B doesn't tune on the 6 or 40 meter bands.

Thank you,
de Kb2sin
Bill J
Logged

WB6BYU

  • Member
  • Posts: 20894
    • Practical Antennas
Re: Tower Mounted Vertical Question
« Reply #1 on: March 19, 2023, 11:06:12 AM »

On 6m you probably would be better off with a
dipole side-mounted on the tower.  If you are
running FM, then the tower will act as a reflector
and give a bit of gain when mounted within 4’
or so from the tower.

For 6m SSB/CW and other weak signal work,
a horizontal dipole will be much better than the R8.


On 40m I would expect that the tower would
mess up the tuning on the R8, which is finicky
enough to start with.  The spacing (in wavelengths)
is much smaller on 40m than on 6m.  You probably
would have much better results with a simple
wire dipole or loop hanging off the tower.

Or, depending on the tower height, you may be
able to use a 1/4 wave wire vertical on 40m,
or even load up the tower as a vertical.

KL7CW

  • Member
  • Posts: 1161
Re: Tower Mounted Vertical Question
« Reply #2 on: March 19, 2023, 11:46:04 AM »

Give us more info and we can help you.  How high is your tower ??.  Is it a crank up or something like a Rohn fixed tower you can climb ?.  If it is at least something like 30 or 40 feet, as BYU suggested, probably a dipole or inverted V with the apex near the top would be easy, and possibly better than a vertical, unless you have room for lots of radials, buried, or on the grass at least 20 feet long.  If the grounded tower/beam combo is not 1/4 wave resonant near 40 meters then a vertical wire suspended out 2 to 4 feet from the tower fed against the radial field may give you OK DX performance, but not great for local or regional (within a state or two) performance where the dipole would probably be better.  If the tower is say 25 feet, the beam will top load it and it may be resonant perhaps close to 40 meters.  If the tower sections are electrically well bonded (not a crank up), then the tower could probably be easily loaded....they call it shunt feeding and performance should be equivalent to a stand alone vertical.  I have shunt fed most of my many dozens of verticals for nearly 70 years.  They should be resonant at least not much above 40 meters, but lengths longer than 1/4 wave resonance (34 feet), can still be shunt fed just fine, and may work even better.  If you shunt feed a tower run ALL your RF coax cables, rotator cables, etc., inside of the tower and exit at ground level, and ideally bury them for some distance toward the shack.  This alone decouples much RF from the cables, but additional choking, especially if you run high power might help.  Read up on this and then we can give you some more info.  If a vertical is surrounded by buildings and has a compromised radial field, the dipole higher up may often work better.     Rick  KL7CW
Logged

KL7CW

  • Member
  • Posts: 1161
Re: Tower Mounted Vertical Question
« Reply #3 on: March 20, 2023, 11:00:03 AM »

I had some more thoughts on your situation.  If you want vertical polarization on 6 meters, you could mount a quarter wave 6 meter insulated mast immediately above the beam boom.  The boom, beam elements, and tower would "probably" act "kind of" like a traditional ground plane counterpoise. The radiator could even be something like a bicycle or snow marker cut a bit longer than 1/4 wavelength, then run a wire up the fiberglass rod and cut the wire for lowest SWR on 6 meters.  Now I know this is not a "proper" ground plane for your vertical, BUT it will probably work, and most important it will give you a significant height advantage over a lower side mounted vertical dipole.  The extra height could easily give you a few S units advantage, so if you lose a dB or so with the imperfect untuned counterpoise and probably not a perfect 50 ohm match, probably you will have a net gain.  You can always place a simple common mode choke right near the feed point of the vertical on the coax, which will "force" more of the current out onto the boom and elements, rather than possibly running down the outside of the coax.  KL7CW
  If you want to use a side mounted vertical dipole on the tower for some reason, just google something like side mounted vertical on tower or pole....you will find lots of patterns for various spaces.  I have side mounted VHF/UHF verticals on towers and poles, and just find a radiation pattern you like and space it that far out.  Some patterns are nearly omnidirectional over at least 180 degrees, other spaces are a somewhat sharper pattern in one direction.  The patterns you find will be for different frequencies, but you can scale them for 6 meters.   Good luck   Rick  KL7CW
Logged

KH6AQ

  • Member
  • Posts: 9290
Re: Tower Mounted Vertical Question
« Reply #4 on: March 20, 2023, 03:56:56 PM »

Yes, that is too close to the tower. If you tell me the tower height I will run some quick simulations to confirm or deny this.

Cushcraft R8   https://www.dxengineering.com/parts/csh-r8
Logged

WS7X

  • Member
  • Posts: 58
Re: Tower Mounted Vertical Question
« Reply #5 on: March 21, 2023, 08:44:46 AM »

 Yes it will work.  But only very poorly! And will have terrible swr!.   Mount your R8 on a post completely away from your tower!  It will work much better. 
Logged

AA5MT

  • Member
  • Posts: 445
Re: Tower Mounted Vertical Question
« Reply #6 on: March 22, 2023, 07:05:01 AM »

I mounted a R8 about 40 feet away from a 50 foot  tv antenna tower.  It worked fine except on 40 meters, which had about a 7 swr.  I could not figure it out.  Finally, thinking it must be reflections from the tower, I moved the antenna about 30 feet further away.  The swr cleared up.

Tom
Logged
Pages: [1]   Go Up