Active element question -> Jeff?

HoHoHo

When I played with stacked dipoles, and if I remember correctly, the
length of the segments contributed to the gain of the system. As the
gain grew, the antenna became more 'focused' and narrowed the ...ahem...
beam which raised the sensitivity in terms of distant location. I used
this antenna on my ceiling for TV and had problems with one of them
stations that was further that 80 miles. I could make the antenna
segments longer which would pickup that far station, but then the other
closer ones would become noisy. I did this in 1/4 wave, then 1/2 wave
and finally full wave. Full wave would get the far one but not the
local, 1/2 would get all with noise, and 1/4 wave would get the local
ones but not the far one. I couldn't get those dang TV stations to move
their antennas in a line as they liked them spread out at about a 30
degree angle from where I was.

Does this apply to the element that sticks up in a cantenna? If we make
a full wave element, does the gain grow along with the element length as
we do the 1/4 multiples? We would also have to move the element the 1/4
multipler from the back of that antenna also wouldn't we? If this is
correct, this would help in dem long distance squirts wouldn't it? I
know that it would be harder to aim, but manageable.

Sorry, just Sat ramblings...

Oh, and just cause you all are so informative, I thought that I would
share this with ya:

The .gov showed up with the police at my doorstep claiming that I was
stealing cable. Apparently the cable installer guy that forced his way
into my house a few weeks before and into the living room; had a look at
the 23ga wire on the ceiling and thought that I was stealing cable and
reported it. Could have been because I realized that he did this and
got by my kid, and when I finaly got my wits, grabbed him and threw him
through the door. Bounced then skidded on the sidewalk! Definently a
guy with a ID ten T problem. Laughed about that one for a long time....
hmmm still do.

todh

Antenna gain comes from redistributing the radiated energy into a smaller
beamwidth.
The beamwidth of an antenna is proportional to (wavelength / antenna
length), so yes, a longer dipole will give a tighter beamwidth and a higher
gain than a shorter dipole.

As for the cantenna.... I think that this is acting as a resonant cavity
(and I'm not an expert on these!), but my guess is that the 1/4 wavelngth
between then antenna and the back of the can is critical and should not be
changed (so that the reflected wave is on phase with the forward wave). The
beamwidth from the cavity will be dependant upon the diameter of the
apeture.. BUT... I *think* that the cantenna dimensions have been chosen to
allow only the propogation modes that will end up in the direction that we
want, so I suspect these can't be altered significantly either.

Some groups have been experimenting with waveguides to get tight fan-shaped
beams. (google wireless waveguide).



"OldGuy" <(E-Mail Removed)> wrote in message
newsp6dnTTF3J2-olncRVn-(E-Mail Removed)...
> HoHoHo
>
> When I played with stacked dipoles, and if I remember correctly, the
> length of the segments contributed to the gain of the system. As the gain
> grew, the antenna became more 'focused' and narrowed the ...ahem... beam
> which raised the sensitivity in terms of distant location. I used this
> antenna on my ceiling for TV and had problems with one of them stations
> that was further that 80 miles. I could make the antenna segments longer
> which would pickup that far station, but then the other closer ones would
> become noisy. I did this in 1/4 wave, then 1/2 wave and finally full
> wave. Full wave would get the far one but not the local, 1/2 would get
> all with noise, and 1/4 wave would get the local ones but not the far one.
> I couldn't get those dang TV stations to move their antennas in a line as
> they liked them spread out at about a 30 degree angle from where I was.
>
> Does this apply to the element that sticks up in a cantenna? If we make a
> full wave element, does the gain grow along with the element length as we
> do the 1/4 multiples? We would also have to move the element the 1/4
> multipler from the back of that antenna also wouldn't we? If this is
> correct, this would help in dem long distance squirts wouldn't it? I know
> that it would be harder to aim, but manageable.
>
> Sorry, just Sat ramblings...
>
> Oh, and just cause you all are so informative, I thought that I would
> share this with ya:
>
> The .gov showed up with the police at my doorstep claiming that I was
> stealing cable. Apparently the cable installer guy that forced his way
> into my house a few weeks before and into the living room; had a look at
> the 23ga wire on the ceiling and thought that I was stealing cable and
> reported it. Could have been because I realized that he did this and got
> by my kid, and when I finaly got my wits, grabbed him and threw him
> through the door. Bounced then skidded on the sidewalk! Definently a
> guy with a ID ten T problem. Laughed about that one for a long time....
> hmmm still do.
>
> todh
>

On Sat, 18 Dec 2004 06:30:58 -0800, OldGuy
<(E-Mail Removed)> wrote:

>HoHoHo

Bah-humbug (T'is the season).

>When I played with stacked dipoles, and if I remember correctly, the
>length of the segments contributed to the gain of the system. As the
>gain grew, the antenna became more 'focused' and narrowed the ...ahem...
>beam which raised the sensitivity in terms of distant location.

Not exactly. A simple 1/4 wave per element dipole has a rather wide
donut shaped pattern. Gain is about 2dBi. -3dB beamwidth is about
150 degrees. When I elongate the elements to 1/2 wave each, the
pattern looks like a cloverleaf donut. There's no lobe perpendicular
to the length of the antenna wire, so you'll get crappy reception.
Enlarge the antenna to 3/4 wave per element, and the center lobe comes
back, but the two cloverleafs are still there. The "gain" of the
center lobe is slightly more than it was with the 1/4 wave dipole, but
not by much. (I'm too lazy to run the numbers to get the exact gains
and beamwidths).

>I used
>this antenna on my ceiling for TV and had problems with one of them
>stations that was further that 80 miles. I could make the antenna
>segments longer which would pickup that far station, but then the other
>closer ones would become noisy. I did this in 1/4 wave, then 1/2 wave
>and finally full wave. Full wave would get the far one but not the
>local, 1/2 would get all with noise, and 1/4 wave would get the local
>ones but not the far one. I couldn't get those dang TV stations to move
>their antennas in a line as they liked them spread out at about a 30
>degree angle from where I was.

I think you were moving the lobes around, not the gains. TV goes from
54Mhz to 700MHz. Trying to build a single antenna that covers all
that range is not a trivial exercise. No matter how well your antenna
pattern looks on one frequency, chances are high that it will not be
useable on another frequency.

>Does this apply to the element that sticks up in a cantenna?

No. The can antennas work on a completely different principle.

>If we make
>a full wave element, does the gain grow along with the element length as
>we do the 1/4 multiples?

No. The gain of the cantenna does not increase much with the diameter
of the can. There is a minimum diameter (waveguide cutoff frequency)
but above that diamter, the gain is fairly constant. Expanded large
enough, it would start to resemble a monopole over an infinite
reflector.

>We would also have to move the element the 1/4
>multipler from the back of that antenna also wouldn't we? If this is
>correct, this would help in dem long distance squirts wouldn't it? I
>know that it would be harder to aim, but manageable.

Sorry. I can't tell what you're trying to build here. Making the
antenna twice as big does *NOT* increase the gain 3dB. It mangles the
tuning frequency, changes the impedance, changes the pattern, and
generally mess things up.

Need a project? Download the latest 4NEC2 antenna modelling program.
I'll send you my latest semi-functional can antenna generating
spreadsheet, which will generate an NEC2 "deck" for any size cantenna.
Play with the dimensions and watch the pattern, VSWR, and gain change
all over the place.

>The .gov showed up with the police at my doorstep claiming that I was
>stealing cable. Apparently the cable installer guy that forced his way
>into my house a few weeks before and into the living room; had a look at
>the 23ga wire on the ceiling and thought that I was stealing cable and
>reported it. Could have been because I realized that he did this and
>got by my kid, and when I finaly got my wits, grabbed him and threw him
>through the door. Bounced then skidded on the sidewalk! Definently a
>guy with a ID ten T problem. Laughed about that one for a long time....
>hmmm still do.

Yeah, I know how it works. The day after I installed some antennas on
my roof, was when the neighbors showed up with crappy TV reception
complaints. I had a difficult time convincing one of them that I had
no effect on cable TV.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558