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Author Topic: Height or gain?  (Read 552 times)
KI4BUM
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« on: May 13, 2004, 03:13:27 PM »

Which would you pick?

Put a commercial 2meter 7 or 8 dBd vertical antenna in your attic (stick built house with siding and normal shingle roof)

   or  

A home built 2m 5/8 Wave vertical ground plane antenna in a tree 20-30 feet higher than the attic

Thanks!
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WB2WIK
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« Reply #1 on: May 13, 2004, 04:40:23 PM »

Depends on the tree, and how far away it is from the shack -- which will determine line loss and overall viability.

A "7 or 8 dB gain vertical" for 2m has to be *very* tall, and I'm surprised it would fit in an attic.  The RFP Stationmaster commercial repeater omni antenna, which has more gain than any amateur 2m omni I've ever seen, has about 5.8 dB gain and is 20 feet tall, overall.  It's impossible to achieve 6 dB omni gain with a single antenna at 146 MHz without occupying that amount of vertical space.

WB2WIK/6
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KI4BUM
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« Reply #2 on: May 13, 2004, 04:59:17 PM »

ahh....good points.

more details:

The tree would probably add an extra 50-75 feet of cable. It's an old growth tree with probably a good 10 to 15 foot base (eg: would take two people to put their arms around it)

I forgot to actually check if an antenna of that dB gain would actually fit. You are correct Smiley The biggest one I can find that will fit the attic is 5 dB.

Given the new data what would you pick?


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WB6BYU
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« Reply #3 on: May 13, 2004, 05:23:30 PM »

I suspect that the antenna on the tree would work much
better than one in the attic.

First, a 5dB omnidirectional vertical is still pretty
tall:  it takes something around 8' to get 3dB over a
dipole.  Unless you have a REALLY tall attic, you
probably will end up closer to 0dBd - that is, a vertical
dipole in some form or another (J-pole, ground plane,
etc.)  The marketing department could never admit that
their antenna had no gain over a dipole, but that is
probably the case.  Basically any antenna about 3' to
4' tall will fall in this category.  I've built home-
made dipoles and ground planes in the attic before and
they work quite well.

On the other hand, I wouldn't use the 5/8 wave whip in
the tree, either.  Again, a J-pole, ground plane, or
other "dipole equivalent" is probably the best choice
if you are limiting the height for visual aesthetics.
Otherwise a commercial antenna in the 6- to 8- range
will give more gain while still being reasonably
practical to install.  (The 5/8 wave whip does give gain
over a quarter wave whip when used over an infinite
ground plane.  But a car roof doesn't come close to
being infinite, let alone anything you might want to
put up at the top of a tree.)

Do use good quality coax.  The improvement due to height
is probably still worthwhile if you use RG-213 coax,
but will be even more as you use better feedlines to
reduce the loss.

Height gain really does make a big difference at VHF.
If your attic was at 100' in the air, adding an extra
20 - 30 feet probably wouldn't make a lot of difference
(especially on the local repeaters.)  But if your attic
is anywhere below 30 or 40' above the ground, I'm sure
you will notice a lot better reception with the added
height.
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KI4BUM
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« Reply #4 on: May 13, 2004, 05:38:25 PM »

I have a really tall attic Smiley  8 foot is doable. Only probably with a tall attic is that it means I give up width which means I can't install the most basic of Yagis.

Sounds like the tree is a viable option.

Now I have to figure out how to attach a vertical antenna to a tree that's out of reach of pretty much anything but a bucket truck.  Any ideas?

I know I can probably 'sling shot' a line up to a high branch and then 'pull' the antenna up..but do people just leave the coax dangling? Doesn't seem right. But I dont see how I can attach the coax to the tree at 75' up Smiley

That and I have to figure out how to attach a loop to the top of the antenna to hang it.

Stupid CC&R's. Wasn't a ham when I bought this house.
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KD5JFT
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« Reply #5 on: May 14, 2004, 03:52:08 PM »

We get some severe lightning storms in my area.  To keep me on the air during such storms, I installed some antennas in the attic.  I have two small ground plane (2M/70cm) dual band attic antennas.  One is kinda home-made, the other is a commercial (MFJ) antenna.  Both work well for local repeaters and short range simplex.  I also have two outside verticals for longer range (GP15 6M/2M/70cm triband hooked to a Yaesu FT-726 and GP6 2M/70cm dual band hooked to a Yaesu FT-847).  I also have a dual band 3 element yagi for the FT-726.  I am still working on better antennas for the 847.  The small yagi will fit in an attic, but I have it about 10 ft above the roof.  If all you are trying to do is local work, the attic will be a good start.  If you need more, start with the attic antenna and then work on getting the outside stuff up.  I wish you luck and let me know how the tree antenna goes.  I have three good sized trees that would make nice tower substitutes.

Most of my stuff is concentrated on 50Mhz and above as I am just a lowly no-code tech.  I do have a G5RVjr in the attic also, but intend to move it outside also.
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WB6BYU
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« Reply #6 on: May 14, 2004, 04:39:37 PM »

Actually I've put yagis in the attic before.  Works
particularly well if they are fixed in a particular
direction so rotation isn't necessary.  You can use
#8 aluminum wire and PVC pipe to build them since they
are protected from the weather - this makes them
cheap enough that you can have two or three on a switch
for your favorite directions.

Hmm...  perhaps I should add a caveat to my previous
post:  the tree probably will give better performance,
but the attic may give ADEQUATE performance.  And if it
is good enough, there is no need to bother making it
better!

Clearly the method of mounting an antenna in the tree
will depend on the type of tree.  Straight-trunked
conifers like pines are much easier than the more
free-form shapes like oaks, for example.  If you have
a central trunk I'd suggest mounting the antenna on the
end of a length of TV antenna mast then hoisting the
mast vertically up along the trunk and lashing it into
position where the antenna just clears the top of the
tree.  This may require a bucket truck or someone to
climb the tree - again, depends on local factors.  I've
seen several installations where they just climbed the
tree and topped it where the trunk was a few inches
thick and bolted on a stub mast using lag screws,
through bolts, or muffler clamps.
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