Best metal to use for boom to mast plates

The following discussion about the best metal to use for boom to mast plates took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 6-Jan-2004 KJ7F wrote:
While my antennas are on the ground this winter for repair I have noticed that the boom to mast plates are bent on all four antennas. The current plates are made out of 3/16" aluminum. I would like to replace the plates before the antennas go back up in the air this spring. Should I use thicker aluminum for the plates or can I use something stronger like steel or will that cause problems with the antennas?

On 6-Jan-2004 WA1ECF wrote:
Hello Jerry,

I always avoid using dissimilar metals as much as possible, so steel plates are out. Even galvi plates are out.

How did the plates get bent/deformed ? over torquing the u-bolts ? Did you use a saddle with the u-bolts ?

Are the booms slipping on the mast ? If not, reuse the existing plates but invert them to bend them the other way.

Use SS U-bolts with a full coverage saddle for maximum grip. Like the HARBACH u-bolts now sold by DX engineering.

Be sure to grease the threads on the u-bolts and use a short stubby wrench to limit torquing.

The greasing will allow you to take things apart in 5 years, without breaking something.

On 6-Jan-2004 KE2N wrote:
My advice - DO NOT grease the bolts. Use proper anti-sieze compound (messy though it might be). I made the mistake of greasing one installation and had a LOT of things come loose 3-6 months later.

On 6-Jan-2004 K1FO wrote:
I would not use steel unless it is galvanized. There are many different aluminum alloys, and then there is temper of the alloy. Something like 6061-T6 or 6062-T6 (hard temper) aluminum plate 1/4" thick should work OK.

It's even better if you use some sort of u-bolt saddle.

However, if you really want to keep your yagis aligned with minimal problems, the key is contact surface area. On my rear mount yagis, this was a big problem, so what I did was to machine up saddle blocks out of aluminum bar. The saddle has a half circle machined into the block that is the same diameter as the mast. With this block the rear mount yagis are held in place with only a single U-bolt.

These blocks would not be cheap, but you should be able to find some local machine shop that can make them for you. Unfortunately we can't post photos in Moon-Net, but if anyone is interested I can e-mail photos.

On 6-Jan-2004 K2TXB wrote:
> It's even better if you use some sort of u-bolt saddle. 
> However, if you really want to keep your yagis aligned with 
> minimal problems, > the key is contact surface area. On my rear mount yagis, this was a big 
> problem,  so what I did was to machine up saddle blocks out of aluminum 
> bar. The saddle  has a half circle machined into the block that is the same 
> diameter as the  mast. With this block the rear mount yagis are held in place with only a single U-bolt.

I have done this too. It helped that I had my own machine shop at the time. There is another trick if you want to make absolutely sure the antennas can't slip. Make the U-Bolts out of all-thread. They will never slip but it is very difficult to get the antennas aligned right in the first place. The antennas have to be held in perfect alignment and the bolts tightened perfectly equally or they will end up "out". If this happens, you can't just loosen the bolts a little and adjust, the bolts have to be loosened completely in order to turn the antenna in them. Then you start all over again...

On 6-Jan-2004 GM4ISM wrote:
My 2 pennyworth

An alternitive that I have used (and has been in service without dismanteling since 1986) is to use a scaffolding clamp. The type I use is cast aluminum and not the folded steel type. The Boom of my 6m Yagi (in my case 1" square ally) is held in the scaffolding clamp by the rubber insert of a waveguide hanger. The rubber insert is the full width of the clamp and had to be tightened up hard (there is a tendency for the boom to rotate in the clamp as this happens) The result is an antenna that has never gone out of alignment on the stub pole, but is able to buffer strong gusts of wind. There is a small ammount of play in the rubber that seems to have protected that boom from the fatigue probles I have observerd in the past, and of course there is no electrolytic corrosion. I have not earthed the antenna to the mast, but have not noticed any detrimental effect. (All feeders are earthed as they ender the building HI.

On 6-Jan-2004 KJ7F wrote:
Thanks for the huge response to the boom to mast plate question. I'm in the process of having new plates made up using 1/4" thick hard temper aluminum plate. I may have them made 1/2" wider to add just a bit more strength to them while I'm at it.

To answer some of the questions posed for those interested...

Yes, I used the proper saddles on the u-bolts. The bending was past the u-bolt and I believe caused by the torque on the antenna and not over tightening the bolts. Some of the plates were bent inwards and others outwards.

I don't think that the antennas were turning on the vertical risers but that is a possibility. While the antennas are down I decided to switch from round to square tubing to make it easier to align the antennas so that they are all square to the frame. Bent plates were an inconvenience with the round pipe but a killer with the square.

If you check out the picture of my array while it was still up in the air you can clearly see that the top left yagi is tweaked out of alignment and pointing outwards. This is the same plate that is bent outwards. The picture was taken about 2 years ago and the array looked pretty much the same when I took it down in November to work on it.


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