Dave take a look at SGC's website http://www.sgcworld.com/ . Look under
"technical" and they have
some ideas on stealth antennas. Wire bend around, not as efficient, but works
73 Larry K1ZW
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
Sent: Thursday, September 09, 2010 10:11 AM
To: Dave Tipton, W5DMT
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] Attic Based Antennas
Dave Tipton, W5DMT wrote:
> I'm looking for ideas for an antenna that could be used on 75M with
> reasonable results, can be mounted in an attic of a small townhouse. I'm
> guessing total dimensions won't be more than 40 x 20 with a 6 to 8 foot
> heigh in the middle.
>
>
>
What is "reasonable" <grin>
OK.. a full sized dipole would be about 130 feet long, and you've got a
maximum of about 30-40, or 1/4 the size ..
The feedpoint impedance of a 40 foot dipole at 4 MHz is going to be
pretty reactive and the resistive part will be small, so something is
going to have to "match" it. Whether it's some sort of "within the
antenna" loading or a LC network at the feedpoint, you're pretty much
stuck with it.
I would look at something that looks like the short vertical dipoles
with "T-bar" capacity hats, except turned on its side.
In a lot of ways, this is like a mobile rig.. One simple way to do this
is to basically string up the dipole (perhaps with some capacity hat
wires) and put a SGC or LDG autotuner at the feedpoint. Use 2.4" 31 mix
cores to decouple the feedline.
THings you want to be aware of mounting an antenna like this in the attic:
RFI - hundreds of watts + close to wiring = high possibility of
interference; and conversely, you'll be close to stuff that radiates
noise. Since you're not going to have a full size antenna anyway, you
can move the wires around to keep them away from vulnerable/noisy stuff
(nearfield drops off as third power of distance, so even a small move
helps a lot... moving from a foot away to 5 feet away is a huge change)
Fire - short antenna = high voltage at the ends = sparks = light your
attic on fire. Think about where the ends of the wires are, and how they
are insulated.
Narrow band - It's going to be narrow band.. however, if you use a
tuning network at the feedpoint to get "in the ballpark", you'll reduce
a lot of the line losses if you use a tuner at the rig. Even better is
a tuner at the feedpoint.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|