Sent DB2 back: Homemade HDTV antenna idea?
 

On Wed, 5 Mar 2008 06:21:17 -0700, "Howard Lester"
wrote:

wrote

I commend you for your resourcefulness, but if you're gonna build for
the outside, why don't you take an ohmmeter down to the hardware store
or scrapyard and find something more durable than unshielded coaxial?

Who are you directing this to, and what good is an ohmmeter at a scrapyard
or hardware store?

Maybe you can trade the ohmmeter for an antenna!

(old joke - name five ways you can use a barometer to measure the
height of a building!)

J.

Sent DB2 back: Homemade HDTV antenna idea?
 

In article Jim Wilkins writes:
On Mar 4, 4:28=A0pm, wrote:

So..... I recognize that what I need is an OUTSIDE
antenna and one as high as possible.... matter of fact
these two parameter are more important then the
'design" of the antenna I gather?

Right now the active element of my HDTV antenna is two 6" aluminum
rods jammed into the ends of a short horizontal plastic tube. I
drilled and tapped through the inner ends of the rods for stainless
steel screws which hold the rods in and connect to the 300-75 Ohm
matching transformer. Come spring I'll redo it by mounting slightly
longer rods in opposite sides of a plastic box which will protect the
connections from the rain better.

It's actually a home-made replacement part for a Radio Shack UHF
antenna that was damaged in a storm.
Depending on whose formula I use, the end-to-end distance should be 15
to 16 inches. The original was 14.6" but it was made for older, higher
UHF channels.

I don't know what formulas you are using, but a good start is 468 feet/f
(in MHz). With that, you are not in *any* television band, as your antennas
are all in the 300-400 MHz range.

Why are you using an impedance transformer? Remember the impedance of
a dipole is about 73 ohms. Your coax is about the same. The 300:75 ohm
transformer just gives you a 4:1 SWR, probably resulting in a fair amount
of pickup by the coax as well as the antenna.

Alan
wa6azp

Sent DB2 back: Homemade HDTV antenna idea?
 

On Mar 5, 1:00*pm, (Alan) wrote:
In article Jim Wilkins writes:





On Mar 4, 4:28=A0pm, wrote:

So..... I recognize that what I need is an OUTSIDE
antenna and one as high as possible.... matter of fact
these two parameter are more important then the
'design" of the antenna I gather?

Right now the active element of my HDTV antenna is two 6" aluminum
rods jammed into the ends of a short horizontal plastic tube. I
drilled and tapped through the inner ends of the rods for stainless
steel screws which hold the rods in and connect to the 300-75 Ohm
matching transformer. Come spring I'll redo it by mounting slightly
longer rods in opposite sides of a plastic box which will protect the
connections from the rain better.

It's actually a home-made replacement part for a Radio Shack UHF
antenna that was damaged in a storm.
Depending on whose formula I use, the end-to-end distance should be 15
to 16 inches. The original was 14.6" but it was made for older, higher
UHF channels.

* I don't know what formulas you are using, but a good start is 468 feet/f
(in MHz). *

That, also this, set to Channel 59, which is currently the highest one
worth receiving here.
http://www.k7mem.150m.com/Electronic...vhf_quick.html

With that, you are not in *any* television band, as your antennas
are all in the 300-400 MHz range.

End to end = BOTH elements.

* Why are you using an impedance transformer? *Remember the impedance of
a dipole is about 73 ohms. *

Two 6" rods = open dipole, like the original. They are hex standoffs
because I didn't have any 3/8" aluminum rod or tube.


* * * * Alan
wa6azp-

Sent DB2 back: Homemade HDTV antenna idea?
 

On Mar 5, 1:51*am, wrote:
I commend you for your resourcefulness, but if you're gonna build
for
the outside, why don't you take an ohmmeter down to the hardware
store
or scrapyard and find something more durable than unshielded
coaxial?

What in the hell is unshielded coax? If it has no shield, it isn't
coax.

GG

Sent DB2 back: Homemade HDTV antenna idea?
 

* the outside, why don't you take an ohmmeter down to the hardware
store

An ohmmeter won't help, except to show that the coax isn't open or
shorted. The "75 Ohms" is the ratio of voltage to current and it's
controlled by the distributed series inductance and parallel
capacitance.

Sent DB2 back: Homemade HDTV antenna idea?
 

In article Jim Wilkins writes:
On Mar 5, 1:00=A0pm, (Alan) wrote:
Jim Wilkins writes:

It's actually a home-made replacement part for a Radio Shack UHF
antenna that was damaged in a storm.
Depending on whose formula I use, the end-to-end distance should be 15
to 16 inches. The original was 14.6" but it was made for older, higher
UHF channels.

I don't know what formulas you are using, but a good start is 468 feet=
/f
(in MHz).

That, also this, set to Channel 59, which is currently the highest one
worth receiving here.
http://www.k7mem.150m.com/Electronic...vhf_quick.html

Actually, you should look at:

http://www.k7mem.150m.com/Electronic...as/dipole.html

which gives the formula 468/f. The page you give is for building yagis,
and the reaction between the elements makes the lengths different than
for dipoles.

With that, you are not in *any* television band, as your antennas
are all in the 300-400 MHz range.

End to end = BOTH elements.

Yes, exactly, as in the web page I referred to.

Why are you using an impedance transformer?
Remember the impedance of a dipole is about 73 ohms.

Two 6" rods = open dipole, like the original. They are hex standoffs
because I didn't have any 3/8" aluminum rod or tube.

I was asking about why you used the 300 ohm to 75 ohm impedance
transformer. You have 75 ohm cable, and an approximately 75 ohm
antenna -- the transformer just makes things bad.

Two 6 inch rods adds up to 12 inches, not 15 inches.

Alan

Sent DB2 back: Homemade HDTV antenna idea?
 

John McGaw wrote:

Of course, a thinking person would have taken the DB-2 antenna an placed it
in a suitable location outside and used it as originally intended and
probably had a good signal with little effort, no experimentation, and no
return necessary. To quote the maker: "...the DB2 was originally designed
for outdoor use..."

http://www.antennasdirect.com/DB2_Indoor_antenna.html

A "thinking" person would send it back and get the DB4
or something for high VHF bands

Sent DB2 back: Homemade HDTV antenna idea?
 

"Alan" wrote

Two 6 inch rods adds up to 12 inches, not 15 inches.

He's probably got the two rods spaced 3" apart. A *little* knowledge can be
a dangerous thing! ;-)

In my earlier proposed "experimental" wire dipole, I suggested a 12" one,
never remembering what frequencies UHF TV occupies. So for 300 - 400 MHz, it
should have been measured to be about 16" +/- overall for 350 MHz.

Sent DB2 back: Homemade HDTV antenna idea?
 

On Mar 5, 6:21*pm, "Howard Lester" wrote:
"Alan" * wrote

*Two 6 inch rods adds up to 12 inches, not 15 inches.

He's probably got the two rods spaced 3" apart. A *little*
knowledge can be
a dangerous thing! ;-)

In my earlier proposed "experimental" wire dipole, I suggested a
12" one,
never remembering what frequencies UHF TV occupies. So for 300 -
400 MHz, it
should have been measured to be about 16" +/- overall for 350 MHz.

I found this a while back in HDTV Magazine. Definitely check 'page 2
rev 2'

http://www.prism.gatech.edu/~wn17/

GG
GG

Sent DB2 back: Homemade HDTV antenna idea?
 

"G-squared" wrote

I found this a while back in HDTV Magazine. Definitely check 'page 2
rev 2'

http://www.prism.gatech.edu/~wn17/

THAT is hysterical, and incredibly ingenious. It probably works a little
better than a dipole, huh? ;-)

I wonder if that'd qualify as an acceptable rooftop tv antenna in a
subdivision ruled by a homeowner's association (per the FCC ruling years
ago). Put a rotator on it and....