On 5/12/2010 5:30 PM, Fredric T. Serota wrote:
> Thanks for responding. Actually, I have read and re-read "Up The Tower"
> several times. It actually doesn't give specifics about where on the mast to
> go. I don't understand why they sell 15' reinforced masts if they want you
> to put the beam right on top of the mast.
This brings up a point I think many may miss. The rotator should be
mounted DOWN IN THE TOWER! That's emphasis, not shouting.<:-)) IOW if
you have a 15' mast then all but 2' of it "in this case" should be
inside the tower to the rotator mounted on an accessory shelf.
> Only a collection of very light
> weight 2 meter and above antennas would qualify.
That is correct.
> Also not sure why they pick
> a tower height of 55' which is an odd height if comparing wavelengths of
> amateur bands,
It more than likely has to do with things not related to antennas or
even towers at all.
It's far less expensive to use standard stock lengths of material. IOW
tube, angle, and flat stock, comes in 20' lengths, but that would make
tower sections to ungainly to handle so they cut them in half. That's
why when you purchase UPS shippable tube, angle, or flat stock shipped
buy UPS or other non trucking carrier it's more expensive, but retailers
do it because it's the only way some customers can get the materials.
Yes they also sell it in 20' lengths, but better buy lots as shipping is
expensive.
Once they take those stock lengths and cut them in half they have to
make provisions for the tube or angle sections to fasten together. That
means overlap and in a few sections that can add substantially.
To use a different means of construction either wastes material of adds
labor. Take for instance the larger towers that use welded on, bolt
flanges. That's 6 of those, plus 18 or 24 nuts and bolts plus washers.
All of this adds up to odd sized hardware which seems expensive but
would look cheap if they made them to custom sizes. Take a look at and
the price of those really nice, heavy duty crank ups at Dayton. Now
there is both quality AND price.<:-))
> being more than a half wavelength at 20 and less than a half
> wavelength at 40.
>
> So I, too, am reading all I can and asking all I can. On this issue,
> actually, the ARRL Antenna Handbook makes the most sense going through the
> "moment" calculations. But they come to the conclusion that there is little
> difference between the top of the tower and 5' above.
>
That's one of those: sorta, kinda, true in *some* cases, but not for a
top mounted rotator (which is not recommended ). In that case the force
at the top of the tower is the wind load of the antenna *multiplied* by
the height above the top of the tower. IOW without the mast going down
through the tower an antenna presenting a 100# wind load @ 50 MPH will
present 700# to the top of the tower when mounted 7' above the top of
the tower with a top mounted rotator. Even down in the tower that
moment/arm is applied from the rotator to the thrust bearing at the top
of the tower. That distributes the wind load between the top of the
tower and the accessory plate. A good, substantial tower will distribute
that load across the entire length of tower from the rotator to top
thrust bearing. The calculation gets a bit more complicated for a less
than substantial tower or one that is loaded to, or beyond it's ratings.
Hope that helps.
73 and good luck,
Roger (K8RI)
> Fred
>
> -----Original Message-----
> From: Tommy Alderman [mailto:aldermant@windstream.net]
> Sent: Wednesday, May 12, 2010 5:17 PM
> To: fserota@msn.com
> Subject: FW: [TowerTalk] Best mast location?
>
> Sorry Fred. This got sent to Gene by mistake!
>
> 73,
>
> Tom - W4BQF
>
>
> -----Original Message-----
> From: Tommy Alderman [mailto:aldermant@windstream.net]
> Sent: Wednesday, May 12, 2010 4:04 PM
> To: 'Gene Smar'
> Subject: RE: [TowerTalk] Best mast location?
>
> Fred,
>
> I am just in the process of putting up my first tower/yagi after 59 years in
> the hobby. I seriously urge you to at least get the book 'Up The Tower' by
> K7LXC because it sounds, by your posting, that you really need to learn more
> about this process. I knew absolutely nothing about the process, but I have
> been reading most everything I could get my hands on to learn. And for sure,
> even after a year, I don't know enough yet. Because a tower says it will
> support 18 sq ft, really tells you nothing. You have to do what Gene has
> suggested and find out about the wind load. Because it says 18 sq ft, that
> does not mean it will support a 17 sq ft yagi. You have to consider wind
> loads, the mast& rotor wind load, etc. It is a rather involved process,
> although not complicated when you learn the subject. 'Up The Tower' provides
> you with an awful lot of the basic need-to-know information for us novice
> tower builders!
>
> 73 and good luck with your project.
>
> Tom - W4BQF
>
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Gene Smar
> Sent: Wednesday, May 12, 2010 3:12 PM
> To: Fredric T. Serota; towertalk@contesting.com
> Subject: Re: [TowerTalk] Best mast location?
>
> Fred:
>
> The 18 sqft rating of the tower is probably based on having the antenna
>
> load no more than 2 feet above the top of the tower. Check the tower
> manufacturer's specs for the derivation of this figure. Assuming that is
> the case, you would be overloading your TX455 by mounting the Yagi at seven
> feet up the mast.
>
> You also ought to be aware of the wind speed for which the tower is
> rated at 18 sqft. It might be only at 50 mph winds.
>
> Caveat Amateur!
>
>
> 73 de
> Gene Smar AD3F
>
>
> ----- Original Message -----
> From: "Fredric T. Serota"<fserota@msn.com>
> To:<towertalk@contesting.com>
> Sent: Wednesday, May 12, 2010 1:53 PM
> Subject: [TowerTalk] Best mast location?
>
>
>
>> Have a 55 ft tower rated at 18 sq ft (TX 455) a 15 ft mast and a 150 LB -
>> 17 sqft multiband beam (SteppIR DB36). Question is how far up the mast
>> should I mount the antenna? Would like to get to at least a half wave over
>> ground for 40 meters (60-62 ft) or 7 foot of mast.
>>
>>
>>
>> Fred K3BHX
>>
>> _______________________________________________
>>
>>
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