Crossmember on tower

[GUNSABLAZIN]

I have a tilt up tower similar to some I've seen on this site. Made from 2" pipe.
I'm putting a crossmember about 3/4 of the way up to mount two wire j-poles that are inside pvc pipe.
If the crossmember is between 3-4 feet long each antenna will be approx. 2 feet from the mast. Will it affect the transmit/receive? I've seen antennas mounted this way before.

 

[mmckenna]

What frequencies are you running?

Putting them too close can reflect some power, also make them slightly directional. I forget what the rule is about spacing, I want to say 1/4 wavelength, but I could be wrong.

 

[jonwienke]

1/4-wavelength is the ideal spacing for directionality.

 

[GUNSABLAZIN]

2 meter, 70cm., 1.25 meter, and commercial bands throughout this range.

 

[mmckenna]

1/4-wavelength is the ideal spacing for directionality.

Ah, that was it.

2 meter, 70cm., 1.25 meter, and commercial bands throughout this range.

So, 1/4 wave on 2 meters is about 19 inches, so you want something more than that. or it will be directional to some amount. I'd shoot for 1/2 wavelength if you can. that would be 40 inches on VHF, 12 inches on 70cm.

Keep in mind that a little bit of directionality wouldn't necessarily be a bad thing. It might work to your advantage.

 

[GUNSABLAZIN]

Direction?

If it becomes directional, which direction is the strong side? I'm guessing 90 degrees from the pole, both directions?? Yes?

 

[jonwienke]

The tower will block some signal, so reception will be stronger in the opposite direction.

 

[GUNSABLAZIN]

Thanks for all the info guys! Much appreciated.

 

[cmdrwill]

I have a tilt up tower similar to some I've seen on this site. Made from 2" pipe.
I'm putting a crossmember about 3/4 of the way up to mount two wire j-poles that are inside pvc pipe.
If the crossmember is between 3-4 feet long each antenna will be approx. 2 feet from the mast. Will it affect the transmit/receive? I've seen antennas mounted this way before.

Those 'wire' so-called j-pole antennas will go bonkers next to any metal, like sidemounted on a mast or tower. Anything metal close to them destroys not only their pattern but SWR also.

 

[GUNSABLAZIN]

Oh boy. What now?

 

[cmdrwill]

Side mounted dipole, one for VHF and one for UHF.

 

[GUNSABLAZIN]

For now I'm not going with new antennas.
Suppose I raise the crossmember to the top of the mast? At that point there will be an A-99 for 10 meters between the two wire j-poles. 2 ft. clearance between all antennas. Way less metal anyway.???

 

[KC4RAF]

cmdrwill is absolutely correct about " j-pole antennas will go bonkers next to any metal". They don't like ANY metal close to them. They need a long distance between them.

 

[GUNSABLAZIN]

Okay. Here's the plan. Since I'm dealing with an easy mount/up/down (tilt up mast), I'm going to try mounting them anyway. If it sucks I'll remove them and do something else. I wasn't really ready for this setback.

Thanks for the info though Guys.

 

[jonwienke]

2 feet isn't enough mast separation for VHF, unless you want a directional pattern. 5 feet would be better. Especially if you're running J-poles.