On 25 Apr 2009 06:33:45 +0100, Gordon wrote:
This seems to be the only news group devoted to the topic of antennas.
(and the origins of the universe). So maybe you guys can help me
with this.
Cosmology is often easier to understand than the antenna designs.
I have been experimenting with fabricating a decent
indoor DTV antenna.
Please define "decent". You know, the numbers.
How big? (max allowed length, width, height)
How much bandwidth (VHF, UHF, or both?)
Minimum (average) gain?
Mounting limitations?
Omni or directional? Rotator?
I'm also partial to NEC2 models of creative designs. They often show
problems that are not obvious until after the antenna is built. If
you're going to design your own, spend the time building a model.
I have tried several things.
You seem to have done your homework. Also see:
http://www.hdtvprimer.com/ANTENNAS/comparing.html
Some are indoor antennas, which should give you a clue as to what to
expect.
What I have
now is two bowties spaced about 7 inches apart on a wood dowel.
A peanut butter jar with rocks in it makes up the base.
2 element yagi like design?
It seems to give pretty good performance. It could be better.
But I am wondering about a few things.
1) Right now all the digital transmissions are in the UHF band.
but in 6 weeks, three stations will move back to their VHF
assignments in the VHF high band.
How well can I expect my antenna to work in that frequency band?
Dunno. I haven't seen any dimensions. Plug the current design into
an NEC model and see what comes out at VHF frequencies. My guess is
that it will be worse than hideous. You would do better with
adjustable rabbit ears.
2) I have not trimmed or dressed the twin lead from the bowties.
I understand that twin-lead can act as antennas. So what
is the best way to manage the twinlead? Cut it back? I noticed
that when i laid the leads together, in an attempt to make a
neat assembly, I got signal cancelation.
Lose the twin lead. The problem with balance line is that any
mechanical imbalance (i.e. proximity to walls, objects, people, etc)
will unbalance the line, causing some radiation. Such radiation shows
up as loss, something you can barely tolerate with such a minimal
antenna. If your design is setup for a 300 ohm match, install a balun
and run 75 ohm RG-6/u coax instead.
3) Any suggestions to improve this design?
Improve how? More gain? More bandwidth? Smaller? Numbers please.
4) Any suggestions for alternitive DIY designs?
Yes, but you may find them rather unconventional.
I once built a log periodic antenna on a sheet of plywood, using #14
AWG house wire and two parallel wires for the transmission line.
Elements were arranged on both sides of the plywood and secured with a
staple gun. This was for a ladyfriend, who was stuck with severe
limitations on outside antenna installations. I hung it from the
ceiling, which was the only place suitable for the monstrosity. 4-6dBi
gain from 60 to 800MHz (no channel 2). Where were a few ghosts, but
it worked much better than the rabbit ears and UHF loop it replaced.
However, it only lasted about a month. The landlord found out and
demanded its removal while muttering something about attracting flying
saucers and alien visitors. Oh well.
Another TV antenna I built took advantage of the chicken wire buried
under the stucco outside of the house. The chicken wire acted as the
antenna reflector, which was conveniently arranged in the right
direction. The driven elements were 4ea phased bow-tie dipoles,
hidden behind a decorative wood grating. It didn't work as well as I
predicted, but good enough to get decent OTA TV reception.
If you have a suitable window facing the right direction, you can do
something with aluminum foil tape. I use aluminum duct tape cut down
to size for this purpose. For electrical connections, copper stained
glass tape soldered to the coax cable. Any antenna with horizontal
polarization will work. You won't get much gain, but the outside
window location will somewhat compensate for the lack of gain by
offering a less cluttered line of sight to the TV xmitter.
Anyway, use your imagination, but also run the numbers and do a model.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558