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Aerial pole on house

ChrisW - 22/1/12 at 12:31 PM

I'm planning on installing a pole on the gable of my house in order to mount a Ubiquiti long-range wifi aerial. I need to gain some height above the top of the roof in order to clear my neighbour's roof which is right in the way of the signal.

I'm using Airgrid M5's with the larger (higher gain) reflectors which aren't amazingly balanced (ie the pole mount is well behind the centre of gravity) and aren't exactly light either at just under 2kg.

Link to data sheet: http://dl.ubnt.com/ag5_datasheet.pdf

Obviously the higher I can get the better, but I need to strike a good balance between stability, wind resistance, etc. The link is 11.5km so pointing accuracy needs to be high.

(I'm also tempted to put a HAM vhf/uhf vertical on the top too, but that's another discussion)

So, plan is to use a 2" pole and some 'T&K' mounts secured into the gable with those expanding metal wall anchor bolts.

T&K's for the initiated are: http://www.alltrade.co.uk/product/W0555

What I can't get my head around is, for a fixed height of the dish, does the distance between the T and the K make any difference to the stability?

Let's assume minimum distance between the T and K is 50cm, and the dish is 2m above the T. Would increasing the lenght of the pole and moving the T and K to 1m apart make any difference to the stability at the top?

I can see that at very small distances it would make a difference - for example if the separation was 6" - but when the distance is substantial already will and increase make any real world difference?

Also, another question, the choice seems to be 2" 14g Alu pole or 2" 16g steel. I'm assuming the alu will be lighter, but is the (thinner and cheaper) steel stronger?

Chris

splitrivet - 22/1/12 at 12:58 PM

The larger the distancs between the T and K the better as the pole will use the upper t as a pivot as per a lever so obviously the greater the distance the less the pole loading on the k bracket will be.
Always cap the top of the pole as they tend to rust inside out if using steel galv. Steel isnt that much stronger compared to ali as you think, most steel will fail eventually.
Cheers,
Bob

ChrisW - 22/1/12 at 01:20 PM

quote:
Originally posted by splitrivet
The larger the distancs between the T and K the better as the pole will use the upper t as a pivot as per a lever so obviously the greater the distance the less the pole loading on the k bracket will be.

Thanks for that, Bob.

I can see the theory, my question was more related to whether there was a situation of diminishing returns and at what sort of order of magnitude of distance between the fixings it occured.

My thinking is that there must come a point where the movement due to the pole flexing between the T and the dish is significantly larger than the movement arising due to the fixings, such that any increase in the distance between the T & K creates no real difference in the rigiditiy of the whole structure.

Perhaps I'm overestimating the amount a 2" alu mast will flex, but I've certainly seen TV aerials wobbling about on roofs in moderate winds, so I'm assuming a top-heavy assembly such as what I'm proposing will be worsely affected?

What I can see, of course, is that the further apart the T and K the less strain that will be put on the wall.

Chris

HowardB - 22/1/12 at 01:45 PM

as an amateur with a few odd balanced antenna installs, I'd make the following recommendations.

Use approx 3:1 as the ratio, so for 3m above top bracket, have 1m between.
Use plastic sheath bolts rather than steel expanding ones, far less likely to crack the brick work.
Wrap the cables in a spiral down the pole, will help to shed vorticies and thereby reduce vibration.
Ensure that the nuts are nylocks, or use locking nuts too.
Seal cable joints with self amalg tape and overwrap with PVC tape to keep the UV off it.

Oh and be safe up there, don't do a Rod Hull!

wilkingj - 22/1/12 at 01:51 PM

Ally TV pole, and the T/K brackets as far apart as you can (will depend on the overall height)

Bigger issue is to use the best Low loss co-axial cable you can. The last thing you need is to get a decent aerial up at agood height, and then lose all that signal in the Co-AX. Just looked at the antenna. At 5Ghz, this is even MORE so!!!!

The type of co-ax will be dependent on the connectors you are using. Namely, you cant get some connectors in the size needed for the co-ax cable size.
Check the spec of the cable at 5Ghz, ans many will only give figures for 100Mhz and 1Ghz. 5Ghz is a lot higher, and the loss figures will rise rapidly.

I would put the transmitter in the loft as near to the pole and as high as possible to shorten the cable length.

Dont forget to fuse the socket you use to power all this, especially if you tap it off the lighting circuit ( usually the only mains available in a loft!)
A 2" ally pole should be OK, and also support a short Dual Band Amateur antenna. I would use Westflex cable for the Ham aerial if you have a long-ish run, you will need to get the Conenctors to fit the Westflex as its a non std cable - (Westlake in Holdsworthy Devon is your man)
You could use UR67 and std connectors. Personally at UHF I will not use std PL259 connectors and stick to N type. Lower loss and constant impedance, and more moisture restitant. (check what you have on the end of the antenna first!)

A steel pole will rust, and is not an option I would even think about.

What you are proposing on a 2" ally mast is not excessive.
This is what my mate had up for 3 years, and only bent the pole after a sustained 65Mph winds. Look at the amount of antennae, Your small antenna is smaller than either of his Panel antennae on there, and the Big HF beam is a LOT LOT bigger.
This was a std TV pole, and has withstood a lot. But the recent winds were just a bit too much.
He should have used a 2" ally scaffold pole (I do).
You wont need more than a 2" pole for that antenna and a Co-Linear on top.

Make sure you completely seal ALL the connectors using Self amalgamating tape, and the put a couple of layers of PVC tape over that, as SA tape will suffer in the open weather. Trust me on this, I have been doing this for over 30 years!

The pole is bent over at about 100 degrees! But didnt break. Just flattened and keeled over. And Yes that is an OWL on the mast as well. It helps keeps the birds from defacating allover his patio. It also hoots during the day at given and random intervals. But not at night.

Brian antenna after 65MPH Wind

Hope this helps.

splitrivet - 22/1/12 at 02:22 PM

The only other thing I'd add to what the guys have said is drill the top bracket a few courses down from the top of the brick work and drill midway through the brick not into the course.
Cheers,
Bob

HowardB - 22/1/12 at 03:54 PM

I concur, all good advice well presented,...

ChrisW - 22/1/12 at 07:01 PM

Thanks Gents.

That 3:1 ratio is the thing I was looking for. I'm thinking a 6m (20ft) alu pole with 4m above the T and 2m below. Also, having the T and K far apart means any error in the positioning of the brackets is reduced.

The 5GHz stuff isn't a problem with coax as the 'radio' is built into the dish feeder. Only ethernet (aka baseband in amateur radio speak) comes down the pole, so regular CAT5 is fine. It was a nightmare back in the day before these ruggedised radios were invented. Even at 2.4GHz we'd be using feeders made from LMR400 cable (similar physically to RG213) to keep the losses down, plugged into tiny SMA connectors and the like on radio cards. It made for some horrible installations!

For the VHF/UHF I have some long lengths of URM67 taken from an (indoor, so not weathered) military installation which will do the job nicely I think. Losses as follows:

100MHz - 6.2dB
200MHz - 9.8dB
400MHz - 14.9dB

(all per 100m)

My run length will be <20m, so maybe 1.6dB for VHF and 3.2dB at UHF, or something like that. Not too excessive I don't think.

Chris