Mast size for antenna

[SIMON11]

Here I am asking advice again!
I soon want to be putting a 2m 70cm antenna on my chimney stack. The antenna is about 2.4m long. What I am not so sure about is the thickness of the mast and the material to use. I want it to be mounted on a 10ft mast but I don't know what thickness I should use or what material. Alloy or steel. The winds here reach about 70-80mph and I am open to the winds off the sea.
Any suggestions please. The brackets that came with the antenna go up to 1.5 inches. I suppose this is what I should use but steel or alloy?

I almost forgot. What is the best coax to use for about a 50ft run?

 

[cgriggs]

I'm sure others will have different opinions but here goes. I just put up an antenna of my own, and I used a 10' piece of conduit straight from Home Depot. I would suggest 1 1/4" sizing. Good Luck

 

[Al42]

Simon, Aluminum (aluminium to you ) is stronger than steel for equivalent weight, but you want something that can withstand the wind load, so I wouldn't depend on either a brick chimney. A few kilos of wind load (70 mph isn't a spring breeze) with an arm a little longer than 5 meters is quite a bit of torque. Whatever mounting you use, I'd guy it at the top of the mast. That way you move the strain off the mast mount (or whatever the mast is mounted to) and on to the guy anchors, which can be much more secure.

 

[RISC777]

I totally agree with Al42, guy whatever mast you decide on using. I have wind to deal with and *had* to guy my main mast (20' on top of a flat roofed 15' tall building). A trip to a local hardware for some 1/4" cable, some base rings (flat plate with a ring to run one end of the cable through), a clamp and stainless rings on the mast for the other end of the cables, some clamps with those ... wht do you call them ... sleeves that go through the ring and the cables goes 'around' them making the loop. Turnbuckles for adjusting are an option also.

 

[SIMON11]

Thanks for the comments. I hadn't thought about the chimney, it is only about 5ft tall and thin.
I think I can forget about the guy ropes also as my neighbours wouldn't be very appreciative. I didn't think a tall thin antenna would have attracted that much wind pressure. I have a television aerial up there which has moved in the wind but I would have thought that would be more likely to than my scanner aerial. I was thinking more of the mast bending rather than the safety of the chimney.
I am wondering if it might be o/k if I only have 5ft extending above the chimney and put two good lashings several feet apart down the chimney stack. Would this reduce the effect on the chimney much. Otherwise I might have to think about bracketing it to the gable end of the house but this would be a bit more unsightly. I may though just leave it at chimney height as it will still be an improvement from being in the loft.

 

[RISC777]

Had some 18 gauge mast bend at the bottom (20') with a 1.8 lb. antenna at the top. On the other hand 10' clamped to the soffit on my house has not bent and it's not guyed. They do make what's called a z-mount for chimneys, spreads the load.

 

[MacombMonitor]

I totally agree with Al42, guy whatever mast you decide on using. I have wind to deal with and *had* to guy my main mast (20' on top of a flat roofed 15' tall building). A trip to a local hardware for some 1/4" cable, some base rings (flat plate with a ring to run one end of the cable through), a clamp and stainless rings on the mast for the other end of the cables, some clamps with those ... wht do you call them ... sleeves that go through the ring and the cables goes 'around' them making the loop. Turnbuckles for adjusting are an option also.

1/4" cable? That's a little heavy isn't it?

 

[MacombMonitor]

How about an antenna tripod? They're very common in the U.S., and typically come in 3 FT, and 5 FT models. They are not expensive either. Just make sure you dope up the mounting bolts that enter the roof so you don't have water leaks.

 

[RISC777]

1/4" cable? That's a little heavy isn't it?

It's actually very light, blows around in the breeze until it's tightened up. The tripod's is/are good ideas also. Next one is going to have a base mount that holds the butt end of the mast connected to the roof, a 3' three-legged tripod, and some guy wire. The tripod comes with a 'patch kit' to plug any holes in the ceiling.

 

[Al42]

I am wondering if it might be o/k if I only have 5ft extending above the chimney and put two good lashings several feet apart down the chimney stack. Would this reduce the effect on the chimney much.

The shortening from 10 feet to 5 feet will halve the load, but 20 lashings won't change it one bit.

The load is the force at the end of the arm times the length of the arm - it's measured in foot-pounds or meter-kilograms or any other length-force combination. A given antenna in a given wind will produce a given force, so the only thing you can control is the length of the arm (mast).

If the lashings are insufficient, they'll give with enough force and the whole thing will come crashing down. If they're sufficient, the chimney will be the thing that comes down. Something has to be the weak point - nothing is infinitely strong. But if the chimney is stronger than the strongest force the mast will put on it (chimneys usually aren't very strong for any but compressive forces) it'll survive. I wouldn't lash it, though. If you're determined to trust it with the load, use a chimney mount - they're specifically designed to hole a 1-1/4" to 2" mast to a chimney. Put the 2 parts as far apart vertically as you can - one a course or two of bricks down from the top of the chimney (under the border course, if there's a border), the other at the roof line.

 

[Tweekerbob]

Simon:

To answer your initial question, I would go with Beldin 9913 or LMR-400 for your coax. This is at least twice as expensive as the rg-58 you currently run, but is a worthwhile investment if you intend to "hear all that you can hear".

Also, be sure to properly weatherproof your outside connections (where the connector attaches to the antenna). This is essential to protecting that investment in your coax; you won't have to replace it every year

Be sure to buy quality connectors and SOLDER them on. If you are unsure how to solder a pl-259, do a google search and several pretty good tutorials should pop up. Buy a couple of extra feet of cable and get a number of cheapie connectors to practice with. Once you're condident in your connector soldering skills, go at it with the good quality connector, check for coninuity and that there are no shorts across the conenctor, then properly waterproof the outside connection (google this, many different ideas out there) and you should be in business. Since you are on the side of that hill, try and get it as high as you can, but be mindful of the windloading problem you have with those tropical storm sized winds.

HTH,

Ryan

 

[RISC777]

So, opinions on this type of chimney mount ($20) by GC/Waldom...

" Heavy duty chimney z mount. Fits mast up to 1 3/4" diameter. Comes complete with 2-18' lengths of strapping and necessary hardware. "

 

[kb2vxa]

Hi Simon and readers,

It is my considered opinion that first you must inspect the chimney for cracks, loose mortar and signs of general weakness before you go any farther. If it appears secure go ahead and use it, that antenna and mast present very little weight and wind load and I haven't heard about any ice storms in the UK lately. Just keep the bands far apart as not to apply too much leverage to a small section of the structure. You have a choice of heavy wall aluminium electrical tubing or standard thin wall steel TV antenna mast, both are strong enough to resist buckling with that small an antenna on only a 10' section and the uppermost bracket at the 4' mark up the mast. I think you can visualize that, shoving it up an extra foot or two will buy you no improvment in signal.

Now if you live in a row house with a chimney put up in the Middle Ages you may consider the other suggestions, very sound ones I might add. FYI, my antenna is about the same and it's up on 15' of thin wall steel TV mast, 12' is above the uppermost mounting clamp. Here at the Jersey Shore our winter gales are legendary and it's been up for 5 years without complaint. Of course a fire escape is a bit sturdier than a chimney so that's your prime consideration, the mast is secondary.

 

[SIMON11]

Hi Simon and readers,

It is my considered opinion that first you must inspect the chimney for cracks, loose mortar and signs of general weakness before you go any farther. If it appears secure go ahead and use it, that antenna and mast present very little weight and wind load and I haven't heard about any ice storms in the UK lately. Just keep the bands far apart as not to apply too much leverage to a small section of the structure. You have a choice of heavy wall aluminium electrical tubing or standard thin wall steel TV antenna mast, both are strong enough to resist buckling with that small an antenna on only a 10' section and the uppermost bracket at the 4' mark up the mast. I think you can visualize that, shoving it up an extra foot or two will buy you no improvment in signal.

Now if you live in a row house with a chimney put up in the Middle Ages you may consider the other suggestions, very sound ones I might add. FYI, my antenna is about the same and it's up on 15' of thin wall steel TV mast, 12' is above the uppermost mounting clamp. Here at the Jersey Shore our winter gales are legendary and it's been up for 5 years without complaint. Of course a fire escape is a bit sturdier than a chimney so that's your prime consideration, the mast is secondary.

Thanks for this input.
I must admit that I thought the wind load would not be that excessive on an antenna as thin as this one. Especially by keeping the bands a long way apart. Simple leverage laws tell me that it is better to share the load along the height of the stack and length of the mast. It is a post war brick chimney and not one of the old stone built. There is no sign of any real wear & tear either. I think I will go ahead with this as soon as the weather is a little warmer. This will give me that extra bit of signal strength I am after.
Thanks to Tweekerbob I now know which coax to purchase and I am quite adept with soldered connections.
Thanks to everybody for all your input it is much appreciated. It is interesting to see all the varying opinions and tips and then to make a decision. Right or wrong.

 

[Randall]

How about an antenna tripod? They're very common in the U.S., and typically come in 3 FT, and 5 FT models. They are not expensive either. Just make sure you dope up the mounting bolts that enter the roof so you don't have water leaks.

i prefer the tripod mounts to chimney mounts , one reason is the smoke from the chimney will corrode your antenna faster.

 

[SIMON11]

i prefer the tripod mounts to chimney mounts , one reason is the smoke from the chimney will corrode your antenna faster.

Surely these are only good for flat roofs. Mine has a 60 degree angle and is slate.

 

[RISC777]

Surely these are only good for flat roofs. Mine has a 60 degree angle and is slate.

Why not do a wall mount? 6.5" clearance out from wall, up to 1-3/4" mast OD (lag the clamps into the wall), $8 for two brackets/clamps. Or 12" clearance, $14.50. Just a thought.

 

[SIMON11]

Why not do a wall mount? 6.5" clearance out from wall, up to 1-3/4" mast OD (lag the clamps into the wall), $8 for two brackets/clamps. Or 12" clearance, $14.50. Just a thought.

I have thought of doing this, but it would be neater and less noticable on the chimney stack. It would save me about a 15ft run of coax though.

 

[fuzzymoto]

On an older CB antenna I did we were worried about the wind loads on the chimney/antenna and if it would damage the chimney. To solve that problem we took 4 pieces of heavy galvanized angle iron and put them on all 4 corners of the chimney from the roof line to the top of the chimney (about 2-feet each in my case). Then we put the chimney straps for the antenna around them. It was very secure and the chimney never moved.

 

[Al42]

Surely these are only good for flat roofs.

I had one on the peak of a peaked roof for over 30 years.

Mine has a 60 degree angle and is slate.

It's the last part that's the problem. You'd have to drill through the slate, and sealing the holes up afterward would be a mess on slate. But the angle's no problem.