>Beyond that, it's time to think tower of some kind.
Yes. I believe someone suggested a rooftop tower, which would certainly be a
much better solution than the mast. But there are some important
considerations. First and foremost, a rooftop tower needs to be anchored
properly, and that often means reinforcement of the underlying roof trusses
and the use of long bolts and other hardware. If you're going to put bolts
through the roofing, you're going to have to consider waterproofing issues
as well. Guys may be required and the height of your mast above the tower
may be limited. It seems to me that a professional engineering opinion would
be a worthwhile investment to prevent potentially serious damage to your
house. If you plan to resell the house in the next few years, you might have
to spend money on restoring the roof to its original appearance and
condition.
In my opinion, the cost, effort and risks of doing all this suggest that a
house-bracketed or guyed tower might be a better solution, even though it
will require pouring a concrete base (which might not please the next owner,
either.) Structural integrity of the house is still an issue for bracketed
towers, at the bracketing point and at any rooftop guying points that may be
required due to height of the tower above the roof.
One of the big issues with rooftop and house-bracketed towers that I don't
recall seeing discussed here is the potential for serious RFI and
interaction with metallic housing materials. About ten years ago, at another
QTH, I had a 6-foot Rohn roof tower with a TA-33M and HD-73 rotor mounted on
a short mast. Even so close to the roof, the beam had acceptable SWR and
directional gain. F/B wasn't spectacular, but the antenna was vastly
superior to my old R5 mutiband vertical. Unfortunately, even at low power
lots of RF got into the stereo, the phone, the TV, etc. Running QRO was not
possible without disrupting family life. The problem was probably a
combination of proximity to the internal house wiring passing through the
attic, proximity to the main electrical feed, and proximity to the coax
feedline that ran along the roof and down the side of the house back to the
shack. The beam also exhibited significant changes in SWR depending on the
direction it was aimed. I'm sure this was largely due to proximity to the
feedline (in some orientations it was parallel to the beam), but SWR may
also have been affected by the electrical wiring and large amounts of
aluminum roof flashing nearby.
If you do an installation with the beam close to the roof, it is advisable
to route the coax directly through the roof, so it will be perpendicular to
the beam for as long a run as possible. Even if you do this, proximity to
electrical wiring may still be a problem. I have a friend who had a TH5 on a
50-foot bracketed and guyed tower on his house. The coax came all the way
down the tower. But the house was a three-story New England farmhouse with a
fairly high roof, probably 30' or more. Whenever my friend was running QRO
with the beam pointed in the direction of the roof, RF got into everything,
causing severe disruption in his shack and all over the house. No doubt, it
was getting into the electric wiring in the attic or the main feed attached
at the roof.
In my opinion, if you do a rooftop or house-bracketed installation, or any
installation in close proximity to your house, you need to get the beam well
above the roof. Remember that the power diminishes with the square of the
distance, whether horizontal or vertical. I would imagine that a half
wavelength on the lowest band would be the minumum. That's 30 feet on 14 Mhz
and 60 feet (!) if you have a 7 MHz add-on kit.
If you have room on your property, the best thing you can do to combat RFI
is get the antennas as far away from houses and wires as possible. We have
absolutely no RFI problems in my house, now that the antennas are 260' away!
You have to spend some money on low-loss coax, but it's worth it to avoid
all those complaints from the XYL during ER...
73, Dick, WC1M
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