Log Periodic ant
 

Hi All.....

I have just bought a Yaesu VR5000 receiver/scanner and while looking round
for a wideband antenna
came across the good old log periodic but what a price!!!, circa £200+......

I have a discone but it`s not a lot of use up around the 2gig area (or
anywhere else!!) so i thought i
might have a go at making my own, the magloops are working great as is the
QFH 137megs
weathersat ant, thanks to everyones assistance!.....

Is there any software around to design a log periodic to around
2400megs??.....i`ve found some but is there any better?..

Regards...

Lee......de G6ZSG....

Log Periodic ant
 

On Tue, 29 Aug 2006 07:43:15 GMT, "Lee"
wrote:

Hi All.....

I have just bought a Yaesu VR5000 receiver/scanner and while looking round
for a wideband antenna
came across the good old log periodic but what a price!!!, circa £200+......

I have a discone but it`s not a lot of use up around the 2gig area (or
anywhere else!!) so i thought i
might have a go at making my own, the magloops are working great as is the
QFH 137megs
weathersat ant, thanks to everyones assistance!.....

Is there any software around to design a log periodic to around
2400megs??.....i`ve found some but is there any better?..

Regards...

Lee......de G6ZSG....


Hi Lee,

Simply build a discone to the scale of 2GHz. The reason why the
present one might not work is that if it is designed for a very lower
base frequency, it is TOO BIG.

Discones can be as wide banded as over a 10:1 frequency range, but
this is for matching (certainly useful, but this does not necessarily
make it usable).

For instance, if you have a discone that has a low frequency design of
450 MHz and claims to go as high as 2GHz; then it may well match at
those frequency extremes. On the other hand, at 450 MHz it is looking
out at the horizon, BUT at 2GHz it is looking into the sky. The
higher the frequency, the more the most favored direction climbs up.
In other words it becomes deaf unless you want to hear an overhead
satellite. This may be the source of your disdain for your current
discone.

Build a Discone whose lowest frequency would be around 1.2 GHz.

How do you know what the lowest frequency is? Measure one of the
skirt elements (the skeletal part of the cone). It should be a
quarter wave of the lowest frequency of intended use. In this case,
1.2GHz, that would be 6 cm or so. The high end would then comfortably
range up to 2.4 GHz, match, and still look at the horizon.

Of course, the simple ground plane at 2GHz would work as well, unless
you really need widebandedness (and it would probably work even then).
Simply observe the simple geometry of keeping the element no longer
than 5/8ths wavelength at the HIGHEST frequency you plan to monitor.

If you still want a log periodic, simple scaling will always work.
However, remember that scaling must be applied to ALL physical
dimensions (this means element diameters too).

73's
Richard Clark, KB7QHC

Log Periodic ant
 

"Richard Clark" wrote in message
...
On Tue, 29 Aug 2006 07:43:15 GMT, "Lee"
wrote:

Hi All.....

I have just bought a Yaesu VR5000 receiver/scanner and while looking
round
for a wideband antenna
came across the good old log periodic but what a price!!!, circa
£200+......

I have a discone but it`s not a lot of use up around the 2gig area (or
anywhere else!!) so i thought i
might have a go at making my own, the magloops are working great as is
the
QFH 137megs
weathersat ant, thanks to everyones assistance!.....

Is there any software around to design a log periodic to around
2400megs??.....i`ve found some but is there any better?..

Regards...

Lee......de G6ZSG....


Hi Lee,

Hi Richard....

Simply build a discone to the scale of 2GHz. The reason why the
present one might not work is that if it is designed for a very lower
base frequency, it is TOO BIG.

Discones can be as wide banded as over a 10:1 frequency range, but
this is for matching (certainly useful, but this does not necessarily
make it usable).

For instance, if you have a discone that has a low frequency design of
450 MHz and claims to go as high as 2GHz; then it may well match at
those frequency extremes. On the other hand, at 450 MHz it is looking
out at the horizon, BUT at 2GHz it is looking into the sky. The
higher the frequency, the more the most favored direction climbs up.
In other words it becomes deaf unless you want to hear an overhead
satellite. This may be the source of your disdain for your current
discone.

Build a Discone whose lowest frequency would be around 1.2 GHz.

How do you know what the lowest frequency is? Measure one of the
skirt elements (the skeletal part of the cone). It should be a
quarter wave of the lowest frequency of intended use. In this case,
1.2GHz, that would be 6 cm or so. The high end would then comfortably
range up to 2.4 GHz, match, and still look at the horizon.

Of course, the simple ground plane at 2GHz would work as well, unless
you really need widebandedness (and it would probably work even then).
Simply observe the simple geometry of keeping the element no longer
than 5/8ths wavelength at the HIGHEST frequency you plan to monitor.

Yes, thanks for the info but i would like a little more gain than a standard
discone gives...going to a parabolic discone would be the alternate option
but a little too involved....

73...

Lee ....de G6ZSG...

Log Periodic ant
 

On Tue, 29 Aug 2006 11:27:10 GMT, "Lee"
wrote:

Yes, thanks for the info but i would like a little more gain than a standard
discone gives...going to a parabolic discone would be the alternate option
but a little too involved....

Parabolic discone??!!!?? Involved is not the word.

Hi Lee,

Everyone wants more gain. You don't say why and that may negate
wanting gain.

At the frequency you are desiring, performance is very much limited to
line of sight and even 2M handhelds with rubber duckies can work the
Space Shuttle an hundred miles up.

Consider the downside of gain: you are a tourist at the Grand Canyon
and you put a telephoto lens on your camera (high gain). When you get
home and show your pictures to friends, they might ask why you went to
the Grand Canyon but took pictures only of the river bed at the
bottom, or a cactus on the ridge. Too much gain for the intended use
which was to show the ENTIRE canyon in one shot. This would take a
wide-angle lens (very low gain).

Anyway, simple scaling works. Just research the model of antenna that
gives you the gain characteristics you want, and scale to your band of
interest. The downside here is you may find that you have designed
yourself into a corner because you cannot find the right stock that
supports the dimensions. For a log periodic, this might not matter so
much, and especially for a receiver, but latitude is not generous.

73's
Richard Clark, KB7QHC

Log Periodic ant
 

"Richard Clark" wrote in message
...
On Tue, 29 Aug 2006 11:27:10 GMT, "Lee"
wrote:

Yes, thanks for the info but i would like a little more gain than a
standard
discone gives...going to a parabolic discone would be the alternate
option
but a little too involved....

Parabolic discone??!!!?? Involved is not the word.

Hi Lee,

Everyone wants more gain. You don't say why and that may negate
wanting gain.

At the frequency you are desiring, performance is very much limited to
line of sight and even 2M handhelds with rubber duckies can work the
Space Shuttle an hundred miles up.

Consider the downside of gain: you are a tourist at the Grand Canyon
and you put a telephoto lens on your camera (high gain). When you get
home and show your pictures to friends, they might ask why you went to
the Grand Canyon but took pictures only of the river bed at the
bottom, or a cactus on the ridge. Too much gain for the intended use
which was to show the ENTIRE canyon in one shot. This would take a
wide-angle lens (very low gain).

I have a wide angle lens (low gain) it`s known as a discone !!!
freq range to 3000megs, supposedly 0db ref dipole that has heard one
or two weak signals just outside its range....now i need a telephoto lens
to make out what it is.......a wideband antenna with gain -- a log
periodic!!!!

With respect Richard, i have a scanning radio receiver that scans up to
2500megs
- that`s a lot of coverage which requires more gain the higher you go....a
discone
just doesn`t cut it gainwise so i need a wideband antenna with more
directional
gain to work alongside the discone...a yagi with one meg bandwidth isn`t
much use
either...imagine how many i would have lying around!!! besides, i know how
to
design those....... :-)


Anyway, simple scaling works. Just research the model of antenna that
gives you the gain characteristics you want, and scale to your band of
interest. The downside here is you may find that you have designed
yourself into a corner because you cannot find the right stock that
supports the dimensions. For a log periodic, this might not matter so
much, and especially for a receiver, but latitude is not generous.


If i knew how to scale or re-engineer an existing design, i wouldn`t be on
this NG
asking how to do it ...

Thanks for the advice Richard ......

73...

Lee......G6ZSG......

Log Periodic ant
 

On Wed, 30 Aug 2006 06:19:09 GMT, "Lee"
wrote:

I have a wide angle lens (low gain) it`s known as a discone !!!
freq range to 3000megs,

Hi Lee,

Simply because it has a "frequency range to 3000megs" does not mean it
is usable to 3000megs - only that it matches to 3000megs. Matching
and usability are not strictly related.

supposedly 0db ref dipole that has heard one
or two weak signals just outside its range

Antennas don't have "ranges." You may experience poor performance due
to the combination of transmit power, path loss, and receiver
sensitivity, but none of this has anything to do with the antenna
(unless its poor construction adds loss). If you have too much path
loss, not enough transmit power, or poor receive sensitivity, then,
yes, a gain antenna will make up for those problems (as long as S+N/N
is sufficient in the end) - but this still does not confer a range
specification to an antenna.

This would be whole lot simpler if you simply told us the model number
of this discone.

If i knew how to scale or re-engineer an existing design, i wouldn`t be on
this NG
asking how to do it ...

You don't offer enough parameters like frequency span, gain, F/B, how
long a feedline, what kind of feedline (lot of potential loss there)
for someone to whip out a design here, and that wouldn't be a modest
enterprise if you did. Scaling is the easiest solution, perhaps you
should buy a log periodic. Google would be another solution, your
problem would not be unique - would it?

73's
Richard Clark, KB7QHC

Log Periodic ant
 

"Richard Clark" wrote in message
...
On Wed, 30 Aug 2006 06:19:09 GMT, "Lee"
wrote:

SNIP!!!!

Richard, all i want is design software to build a LPA!!!! do you or
don`t you know of any?????

Lee.....G6ZSG.......

Log Periodic ant
 

On Wed, 30 Aug 2006 07:04:08 GMT, "Lee"
wrote:
Richard, all i want is design software to build a LPA!!!! do you or
don`t you know of any?????

EZNEC

73's
Richard Clark, KB7QHC

Log Periodic ant
 

"Richard Clark" wrote in message
...
On Wed, 30 Aug 2006 07:04:08 GMT, "Lee"
wrote:
Richard, all i want is design software to build a LPA!!!! do you or
don`t you know of any?????

EZNEC

Why didn`t you say that in the first place??

Thankyou...

G6ZSG...


73's
Richard Clark, KB7QHC

Log Periodic ant
 

On Tue, 29 Aug 2006 07:43:15 GMT, "Lee"
wrote:


I have just bought a Yaesu VR5000 receiver/scanner and while looking round
for a wideband antenna
....

Scanners are notorious for a lack of front end selectivity and
resultant high intermodulation products / noise.

Sometimes they depend on narrow band antennas to supplement their
meagre front end selectivity.

In the absense of external front end selectivity, you may find that
the realisable sensitivity is quite a deal higher (tens of dB worse)
than specification, depending on your own RF environment.

Owen
--