----- Original Message -----
From: "Jim Lux" <firstname.lastname@example.org>
To: "K8RI on TowerTalk" <K8RI-on-TowerTalk@tm.net>;
Sent: Sunday, September 17, 2006 6:14 PM
Subject: Re: [TowerTalk] And now for something completely
> Come on, Roger, you need a better antenna.. Let's see.. f/10 at 2500mm
> means you've got 250mm aperture, at lambda of say 500 nm, or 500 lambda.
> So, on 20m, you only need an antenna with an aperture of, let's see, 10
> km. But... real hams work on top band, so you'd
So this translates into a boom length on my tri-bander to about the same
although we might have to add a few elements due to spacing. But given
about the same figures I'd need about a 10 km boom length or about 6.2
miles. That sorta adds a new meaning to "boom truss" doesn't it? <:-))
Let's see. Figuring it's center mounted for simplicity, the boom would
extend 3.1 miles or 5 km from the rotator. A typical rotator for a
tribander runs about 1 RPM while the bigger stuff is 1/2 RPM. So giving it
the benefit of the doubt and running 1/2 RPM Knowing the diameter is 10 km
and the circumference is pi * d the circumference is 31.14159 km or about
9.7 miles. So with the slower rotator "for the big stuff" the ends of the
boom have to travel 9.7 miles in 2 minutes or 4.8 6 miles in one minute. We
know a mile a minute is 60 mph so 60 * 4.8 gives us boom tip speeds of 292
mph or 471 km/h. Spose maybe we need to ramp up the rotator speed instead of
just starting to turn? Other wise we may get a bit of lag between what the
rotator does and what the ends of the antenna do. I guess this means a
suspension system and a bit heavier/stronger boom and elements are going to
be needed. OTOH...How many individual tri-banders would it take in a phased
array to get the same arpeture? I can see I may have to order a bit of extra
coax too. OTOH as I can only go 1500 watts PEP total into the antennas I
could get by with some very inexpensive milliwatt transmitters at each
antenna. Now the main thing left is how to account for path variations over
the entire array besides the phasing.
> need about 100 km. (hmm, you'd have trouble at Field day.. everything has
> to fit in a 1000 ft circle.) This is up in LOFAR antenna territory.
Oh! Wait...That means I'm going to have to prune the boom a bit and we've
dropped the boom tip speed down to a bit above 30 MPH at one rpm. I went
with the faster speed as we've shortened the boom so much.
Still with the phased array approach using one wavelength spacing between
antennas we could put up 14 + 1 across that 1000 foot circle. Too bad they
didn't make it a rectangle a 1000 feet on a side. The we could put up 15^2
or 225 tri-banders and it's only 6.66 watts to each at the feed point. So if
we use fiber optics with active phase delay to and from the main control
point the phasing becomes relatively simple and I don't need all that extra
Hmmm... It's not nearly so difficult to reach astronomical precision at HF
as I thought<:-)) Iwonder who I can get to construct the 225 way power
divider. I gave up trying to calculate how much coax it'd take let alone the
continuously variable phasing as the antennas are turned as with the fiber
it's just cut and fit with active delays. This stuff is getting simpler by
>>These are two different realms with far different requirments. Applying
>>far greater astronomical precision requirements to antenna alignment hurts
>>nothing, but gains nothing as well. OTOH is good experience<:-))
> I think everyone should try several alignment methods.. not just for their
> antenna, but just because it's fun and not particularly time consuming.
> Especially the "stick and shadow" approaches. It's easy to say, "mark
> when the shadow is shortest".. much tougher to actually do it.
Being able to determine *when* the shadow is shortest is an art. If we took
10 hams (myself included) I wonder how close we could each determine local
"noon" and how much variation we'd have. Personally, I don't think I'd do
very well as I've not done that sort of exercise in a long time.
Knowing the time of local noon should give us our longitude. How close can
you come to matching my GPS?
Now I think I'll just go watch the cartoons as soon as they unlock the door.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)
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